Chapter 9 PDF - Patterns of Inheritance

CHAPTER 9 Figure 9-1 | Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart) Introduction...

CHAPTER 9 Figure 9-1 | Experimenting with thousands of garden peas, Mendel uncovered the fundamentals of genetics. (credit: modification of work by Jerry Kirkhart) Introduction Genetics is the study of heredity. Johann Gregor Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well un- derstood. Mendel selected a simple biological system and conducted methodical, quantitative analyses using large sample sizes. Because of Mendel’s work, the fundamental principles of heredity were revealed. We now know that genes, carried on chromosomes, are the basic functional units of heredity with the ability to be replicated, expressed, or mutated. Today, the postulates put forth by Mendel form the basis of classical, or Mendelian, genetics. Not all genes are transmitted from parents to offspring according to Mendelian genetics, but Men- del’s experiments serve as an excellent starting point for thinking about inheritance. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 2 14 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  PATTERNS OF INHERITANCE CHAPTER OUTLINE 9.1 | Mendel’s Experiments......................................... 216 9.2 | Laws of Inheritance........................................... 219 9.3 | Extensions of the Laws of Inheritance.............................. 225 Key Terms...................................................... 234 Chapter Summary................................................ 236 D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 2 15 2 16 C H A P T E R 9 : P atterns of I nheritance 9.1 | MENDEL’S EXPERIMENTS By the end of this section, you will be able to do the following: Explain the scientific reasons for the success of Mendel’s experimental work Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles In 1856, Johann Gregor Mendel (1822–1884) began a decade-long research pursuit involv- ing inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with con- venient characteristics that is used to study a specific biological phenomenon to gain un- derstanding to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local natural history society. He demon- strated that traits are transmitted faithfully from parents to offspring in specific patterns. Mendel’s Crosses Mendel’s seminal work was accomplished us- ing the garden pea, Pisum sativum, to study in- heritance. This species naturally self-fertilizes, meaning that pollen encounters ova within the same flower. The flower petals remain sealed tightly until pollination is completed to pre- vent the pollination of other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoid- ed the appearance of unexpected traits in off- spring that might occur if the plants were not true breeding. The garden pea also grows to Figu r e 9 -2 | M o n o hy b r i d C r o s s maturity within one season, meaning that sev- eral generations could be evaluated over a rel- Mendel’s process for performing crosses in- atively short time. Finally, large quantities of cluded examining flower color. garden peas could be cultivated simultaneous- ly, allowing Mendel to conclude that his results did not come about simply by chance. Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 2 17 Plants used in first-generation crosses were called P, or parental generation, plants (Figure 9-2). Mendel collected the seeds produced by the P plants that resulted from each cross and grew them the following season. These offspring were called the F1, or the first filial (filial = daughter or son), generation. Once Mendel examined the characteristics in the F1 gen- eration of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F1 plants to produce the F2, or second filial, generation. Mendel’s experiments extended beyond the F2 generation to the F3 generation, F4 generation, and so on, but it was the ratio of characteristics in the P, F1, and F2 generations that were the most intriguing and became the basis of Mendel’s postulates. Garden Pea Characteristics Revealed the Basics of Heredity In his 1865 publication, Mendel reported the results of his crosses involving seven differ- ent characteristics, each with two contrasting traits. A trait is defined as a variation in the physical appearance of a heritable characteristic. The characteristics included plant height, seed texture, seed color, flower color, pea-pod size, pea-pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white and violet. To fully examine each characteristic, Mendel generated large numbers of F1 and F2 plants and reported results from thousands of F2 plants. What results did Mendel find in his crosses for flower color? First, Mendel confirmed that he was using plants that bred true for white or violet flower color. Irrespective of the number of generations that Mendel examined, all self-crossed offspring of parents with white flow- ers had white flowers, and all self-crossed offspring of parents with violet flowers had vio- let flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical. This was an important check to make sure that the two varieties of pea plants only differed with respect to one trait, flower color. Once these validations were complete, Mendel applied the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F1 hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait had completely disappeared in the F1 generation. Importantly, Mendel did not stop his experimentation there. He allowed the F1 plants to self-fertilize and found that 705 plants in the F2 generation had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers to one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained approximately the same ratio irrespective of which parent—male or female—contributed which trait. For the other six characteristics that Mendel examined, the F1 and F2 generations behaved in the same way that they behaved for flower color. One of the two traits would disappear completely from the F1 generation, only to reappear in the F2 generation at a ratio of roughly 3:1 (Figure 9-3). D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 2 18 C H A P T E R 9 : P atterns of I nheritance Fig u r e 9 - 3 | M e n d e l’s Pe a Plan t C h ar ac e r i s t i c s Mendel identified seven pea plant characteristics. Upon compiling his results for many thousands of plants, Mendel concluded that the charac- teristics could be divided into dominant and recessive traits. Dominant traits are those that are inherited unchanged in a hybridization. Recessive traits disappear in the offspring of a hybridization. The recessive trait does, however, reappear in the progeny of the hybrid off- spring. An example of a dominant trait is the violet-colored flower trait. For this same charac- teristic (flower color), white-colored flowers are a recessive trait. The fact that the recessive trait reappeared in the F2 generation meant that the traits remained separate (and were not blended) in the plants of the F1 generation. Mendel proposed that this was because the plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of their two copies to their offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic, or that it included one domi- nant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 2 19 9.2 | LAWS OF INHERITANCE By the end of this section, you will be able to do the following: Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems Use a Punnett square to calculate the expected proportions of genotypes and phenotypes in a monohybrid cross Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis Explain the purpose and methods of a test cross The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. Mendel deduced from his results that each individual had two discrete copies of the characteristic that are passed individually to offspring. We now call those two copies genes, which are carried on chromosomes. The reason we have two cop- ies of each gene is that we inherit one from each parent. Recall that in meiosis, homologous chromosomes are separated out into haploid gametes. This separation, or segregation, of the homologous chromosomes means that each parent contributes only one copy of each gene to their offspring. The offspring are formed when that gamete unites with one from another parent, and the two copies of each gene are restored. For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. For ex- ample, one individual may carry a copy of the gene that codes for a white flower color and a different copy of the gene that codes for violet flower color. Gene variants are called alleles. Mendel examined the inheritance of genes with just two allele forms, but it is common to en- counter more than two alleles for any given gene in a natural population. Phenotypes and Genotypes Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup (which alleles it has) is called its gen- otype. Mendel’s hybridization experiments demonstrate the difference between phenotype and genotype. For example, the phenotypes that Mendel observed in his crosses between pea plants with differing traits are connected to the genotypes of the plants in the P, F1, and F2 gen- erations. We will use a second trait that Mendel investigated, seed color, as an example. Seed color is governed by a single gene with two alleles. The yellow-seed allele is dominant, and the green-seed allele is recessive. When true-breeding plants were cross-fertilized, in which one parent had yellow seeds and one had green seeds, all of the F1 hybrid offspring had yellow seeds. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow seeds. However, we know that the allele donated by the parent with green seeds was not simply lost because it reappeared in some of the F2 offspring (Figure 9-4). Therefore, the F1 plants must have been genotypically different from the parent with yellow seeds. The P plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous for a gene have two identical alleles, one on each of their homologous chromosomes. The genotype is often written as YY or yy, for which each letter represents one of the two alleles in the genotype. The dominant allele is D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 220 C H A P T E R 9 : P atterns of I nheritance capitalized, and the recessive allele is lower case. The letter used for the gene (seed color in this case) is usually related to the dominant trait (yellow allele, in this case, or “Y”). Men- del’s parental pea plants al- ways bred true because all gametes carried the same al- lele. When P plants with con- trasting traits were cross-fer- tilized, all of the offspring were heterozygous for the con- trasting trait, meaning their genotype had different alleles for the gene being examined. For example, the F1 yellow plants that received a Y allele from their yellow parent and a y allele from their green parent had the genotype Yy. Pheno- types are physical expressions of traits that are transmitted by alleles. Capital letters rep- resent dominant alleles, and lowercase letters represent re- Figu r e 9 - 4 | M o n o hy b r i d C r o s s cessive alleles. The phenotypic ratios are the ratios of visible In the P generation, pea plants that are true-breeding for the dom- characteristics. The genotypic inant yellow phenotype are crossed with plants with the recessive ratios are the ratios of gene green phenotype. This cross produces F1 heterozygotes with a yel- combinations in the offspring, low phenotype. Punnett square analysis can be used to predict the and these are not always dis- genotypes of the F2 generation. tinguishable in the pheno- types. Law of Dominance Our discussion of homozygous and heterozygous organisms brings us to why the F1 hetero- zygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. For a gene that is expressed in a dominant and recessive pattern, ho- mozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype), and the recessive phenotype will only be ob- served in homozygous recessive individuals (Table 9-1). Table 9-1 | Correspondence between Genotype and Phenotype for a Dominant-Recessive Characteristic HOMOZYGOUS HETEROZYGOUS HOMOZYGOUS Genotype YY Yy yy Phenotype yellow yellow green D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 221 Mendel’s law of dominance states that in a heterozygote, one trait will conceal the presence of another trait for the same characteristic. For example, when crossing true-breeding vio- let-flowered plants with true-breeding white-flowered plants, all of the offspring were vio- let-flowered, even though they all had one allele for violet and one allele for white. Rather than both alleles contributing to a phenotype, the dominant allele will be expressed exclu- sively. The recessive allele will remain latent but will be transmitted to offspring in the same manner as that by which the dominant allele is transmitted. The recessive trait will only be expressed by offspring that do not have a dominant allele to mask it (Figure 9-5). Monohybrid Cross and the Punnett Square When fertilization occurs between two true-breeding parents that have different traits for a single characteristic, the process is called a monohybrid cross, and the resulting offspring are called monohybrids. Mendel per- formed seven types of monohybrid crosses, each involving contrasting traits for different characteristics. Out of these crosses, all of the F1 offspring had the phenotype of one parent, Figu r e 9 - 5 | A lb ini s m and the F2 offspring had a 3:1 phenotypic ratio. On the basis of these results, Mendel postulat- The allele for albinism, expressed here in hu- ed that each parent in the monohybrid cross mans, is recessive. Both of this child’s parents contributed one of two paired unit factors to carried the recessive allele. each offspring, and every possible combina- tion of unit factors was equally likely. The results of Mendel’s research can be explained in terms of probabilities, which are math- ematical measures of likelihood. The probability of an event is calculated by the number of times the event occurs divided by the total number of opportunities for the event to occur. A probability of one (100 percent) for some event indicates that it is guaranteed to occur, whereas a probability of zero (0 percent) indicates that it is guaranteed to not occur, and a probability of 0.5 (50 percent) means it has an equal chance of occurring or not occurring. To demonstrate this with a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds. A Punnett square is useful for determining probabilities because it is drawn to predict all possible outcomes of all possible random fertilization events and their expected frequencies. Figure 9-7 shows a Punnett square for a cross between a plant with yellow peas and one with green peas. To prepare a Punnett square, all possible combinations of the parental alleles (the genotypes of the gametes) are listed along the top (for one parent) and side (for the other par- ent) of a grid. The combinations of egg and sperm gametes are then made in the boxes in the D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 222 C H A P T E R 9 : P atterns of I nheritance table on the basis of which alleles are combining. Each box then represents the diploid gen- otype of a zygote, or fertilized egg. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant and reces- sive) is known, the phenotypic ratios can be inferred as well. For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one geno- type is possible in the F1 offspring. All offspring have the genotype Yy and have yellow seeds. When the F1 offspring are crossed with each other, each has an equal probability of contrib- uting either a Y or a y to the F2 offspring. The result is a 1 in 4 (25 percent) probability of both parents contributing a Y, resulting in an offspring with a yellow phenotype; a 25 percent probability of parent A contributing a Y and parent B a y, resulting in offspring with a yellow phenotype; a 25 percent probability of parent A contributing a y and parent B a Y, also re- sulting in a yellow phenotype; and a (25 percent) probability of both parents contributing a y, resulting in a green phenotype. When counting all four possible outcomes, there is a 3 in 4 probability of offspring having the yellow phenotype and a 1 in 4 probability of offspring having the green phenotype. This explains why the results of Mendel’s F2 generation occurred in a 3:1 phenotypic ratio. Using large numbers of crosses, Mendel was able to calculate prob- abilities, found that they fit the model of inheritance, and used these to predict the outcomes of other crosses. Law of Segregation Observing that true-breeding pea plants with contrasting traits gave rise to F1 generations that all expressed the dominant trait and F2 generations that expressed the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of segregation. This law states that paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor. For the F2 gener- ation of a monohybrid cross, the following three possible combinations of genotypes re- sult: homozygous dominant, heterozygous, or homozygous recessive. Because heterozy- gotes could arise from two dif- ferent pathways (receiving one dominant and one recessive allele from either parent), and because heterozygotes and ho- mozygous dominant individu- Figu r e 9 - 6 | Fi r s t Di v i si o n o f M e i o si s als are phenotypically identi- cal, the law supports Mendel’s The first division in meiosis is shown. observed 3:1 phenotypic ratio. The equal segregation of al- leles is the reason we can ap- ply the Punnett square to accurately predict the offspring of parents with known genotypes. The physical basis of Mendel’s law of segregation is the first division of meiosis in which the homologous chromosomes with their different versions of each gene are segregated into daughter nuclei (Figure 9-6). D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 223 Test Cross Beyond predicting the offspring of a cross between known homozygous or heterozygous par- ents, Mendel also developed a way to determine whether an organism that expressed a dom- inant trait was a heterozygote or a homozygote. Called the test cross, this technique is still used by plant and animal breeders. In a test cross, the dominant-expressing organism is crossed with an organism that is homozygous recessive for the same characteristic. If the dominant-expressing organism is a homozygote, then all F1 offspring will be heterozygotes expressing the dominant trait (Figure 9-7). Alternatively, if the dominant-expressing organ- ism is a heterozygote, the F1 offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes (Figure 9-7). The test cross further validates Mendel’s postulate that pairs of unit factors segregate equally. Fig u r e 9 -7 | T h e Te s t C r o s s A test cross can be performed to determine whether an organism expressing a dominant trait is a homozygote or a heterozygote. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 2 24 C H A P T E R 9 : P atterns of I nheritance ART CONNECTION In pea plants, purple flowers (P) are dominant to white (p), and yellow peas (Y ) are dominant to green (y). What are the possible genotypes and phenotypes for a cross between PpYY and ppYy pea plants? How many squares would you need to complete a Punnett square analysis of this cross? Figure 9-8 | A dihybrid cross in pea plants involves the genes for seed color and texture. The P cross produces F1 offspring that are all heterozygous for both characteristics. The resulting 9:3:3:1 F2 phenotypic ratio is obtained using a Punnett square. Law of Independent Assortment Mendel’s law of independent assortment states that genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur. Independent assortment of genes can be illustrated by the dihybrid cross, a cross between two true-breeding parents that express different traits for two characteristics. Consider the characteristics of seed color and seed texture for two pea plants, one that has wrinkled, green seeds (rryy) and another that has round, yellow seeds (RRYY). Because each parent is homozygous, the law of segregation indicates that the gam- etes for the wrinkled–green plant all are ry, and the gametes for the round–yellow plant are all RY. Therefore, the F1 generation of offspring all are RrYy (Figure 9-8). The gametes produced by the F1 individuals must have one allele from each of the two genes. For example, a gamete could get an R allele for the seed shape gene and either a Y or a y allele for the seed color gene. It cannot get both an R and an r allele; each gamete can have only one allele per gene. The law of independent assortment states that a gamete into which an r allele is sorted would be equally likely to contain either a Y or a y allele. Thus, there are four equally likely gametes that can be formed when the RrYy heterozygote is self-crossed, as follows: RY, rY, Ry, and ry. Arranging these gametes along the top and left of a 4 × 4 Punnett square (Figure 9-8) gives us 16 equally likely genotypic combinations. From these genotypes, we find a phenotypic ratio of 9 round–yellow:3 round–green:3 wrinkled–yellow:1 wrinkled– green (Figure 9-8). These are the offspring ratios we would expect, assuming we performed the crosses with a large enough sample size. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 225 The physical basis for the law of independent assortment also lies in meiosis I, in which the different homologous pairs line up in random orientations. Each gamete can contain any com- bination of paternal and maternal chromosomes (and therefore the genes on them) because the orientation of tetrads on the metaphase plane is random (Figure 9-9). Fig u r e 9 - 9 | L aw o f I n d e p e n d e n t A s s o r t m e n t The random segregation into daughter nuclei that happens during the first division in meiosis can lead to a variety of possible genetic arrangements. 9.3 | EXTENSIONS OF THE LAWS OF INHERITANCE By the end of this section, you will be able to do the following: Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, multiple alleles, and sex linkage from the results of crosses Explain the effect of linkage and recombination on gamete genotypes Mendel studied traits with only one mode of inheritance in pea plants. The inheritance of the traits he studied all followed the relatively simple pattern of dominant and recessive alleles for a single characteristic. There are several important modes of inheritance, discovered after Mendel’s work, that do not follow the dominant and recessive, single-gene model. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 226 C H A P T E R 9 : P atterns of I nheritance Alternatives to Dominant/Recessive Behavior Mendel’s experiments with pea plants suggested that 1) two types of alleles exist for every gene; 2) alleles maintain their integrity in each generation (no blending); and 3) in the pres- ence of the dominant allele, the recessive allele is hidden, with no contribution to the phe- notype. Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as carriers. Since then, genetic studies in other organisms have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelian genetics. Incomplete Dominance Mendel’s results, demonstrating that traits are inherited as dominant and recessive pairs, con- tradicted the view at that time that offspring exhibited a blend of their parents’ traits. How- ever, the heterozygote phenotype occasionally does appear to be intermediate between the two parents. For example, in the snapdragon, Antirrhinum majus (Figure 9-10), a cross be- tween a homozygous parent with white flow- ers (CWCW) and a homozygous parent with red flowers (CRCR) will produce offspring with pink flowers (CRCW). Since neither allele is domi- nant, we need to use different allele notation. In this case the “C” represents the locus, and the R or W in superscript represents the allele (CR for red, CW for white). This pattern of inheri- tance is described as incomplete dominance, meaning that one of the alleles appears in the phenotype in the heterozygote, but not to the Figu r e 9 -10 | I n c o m p l e t e exclusion of the other, which can also be seen. Do m in an c e The allele for red flowers is incompletely dom- inant over the allele for white flowers. Howev- These pink flowers of a heterozygote snapdrag- er, the results of a heterozygote self-cross can on result from incomplete dominance.  (credit: still be predicted, just as with Mendelian dom- “storebukkebruse”/Flickr) inant and recessive crosses. In this case, the genotypic ratio would be 1 CRCR:2 CRCW:1 CWCW, and the phenotypic ratio would be 1:2:1 for red:pink:white. The basis for the intermediate color in the heterozygote is simply that the pigment produced by the red allele (anthocyanin) is diluted in the heterozygote and therefore appears pink because of the white background of the flower petals. Incomplete dominance is common in mammals, including humans. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 227 Codominance A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote. An example of codominance occurs in the ABO blood groups of humans. The A and B alleles are expressed in the form of “A” or “B” molecules present on the surface of red blood cells. Homozygotes (IAIA and IBIB) ex- press either the A or the B phenotype, and heterozygotes (IAIB) express both phenotypes equally. The IAIB individual has blood type AB. In a self-cross between heterozygotes express- ing a codominant trait, the three possible offspring genotypes are phenotypically distinct. However, the 1:2:1 genotypic ratio characteristic of a Men- delian monohybrid cross still applies (Figure 9-11). The dis- tinction between incomplete dominance and codominance lies in the phenotype of the heterozygotes. With incom- plete dominance, the heterozy- gote phenotype is something in between the phenotype of either homozygote. With co- dominant behavior, the het- erozygotes express BOTH phe- notypes. Multiple Alleles Mendel implied that only two alleles, one dominant and one recessive, could exist for a giv- en gene. We now know that this is an oversimplification. Figu r e 9 -11 | C o d o m in an c e in Bl o o d Ty p e s Although individual humans (and all diploid organisms) This Punnet square shows an AB/AB blood type cross. can only have two alleles for a given gene, multiple alleles may exist at the population level, such that many combinations of two alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype in the natural population as the wild type (often abbreviated “+”). All other phenotypes or genotypes are considered variants (mutants) of this typical form, meaning they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele. An example of multiple alleles is the ABO blood-type system in humans. In this case, there are three common alleles circulating in the population. The IA allele codes for A molecules on the red blood cells, the IB allele codes for B molecules on the surface of red blood cells, and the i allele codes for no molecules on the red blood cells. In this case, the IA and IB alleles are co- dominant with each other and are both dominant over the i allele. Although there are three D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 228 C H A P T E R 9 : P atterns of I nheritance alleles present in a population, each individual only gets two of the alleles from their par- ents. This produces the geno- types and phenotypes shown in Figure 9-12. Notice that in- stead of three genotypes, there are six different genotypes when there are three alleles. The number of possible phe- notypes depends on the domi- nance relationships between the three alleles. Sex-Linked Traits In humans, as well as in many other animals and some Figu r e 9 -12 | I n h e r i t an c e o f A BO Bl o o d Ty p e s plants, the sex of the individual is determined by sex chromo- Inheritance of the ABO blood system in humans is shown. somes—one pair of non-ho- mologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or auto- somes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains fewer genes. When a gene being examined is present on the X, but not the Y, chromosome, it is X-linked. Eye color in Drosophila, the common fruit fly, was the first X-linked trait to be identified. Like humans, Drosophila males have an XY chromo- some pair, and females are XX. In flies, the wild-type eye color is red (XW) and is dominant to white eye color (Xw) (Figure 9-13). Males are said to be hemizy- gous, in that they have only one allele for any X-linked characteristic. Hemizygosity makes descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack the white gene on the Y chromosome; that is, their genotype can only be XWY or XwY. In Figu r e 9 -13 | X- L in ke d contrast, females have two allele copies of this gene and can be XWXW, XWXw, or XwXw. I n h e r i t an c e In an X-linked cross, the genotypes of F1 and In Drosophila, the gene for eye color is located F2 offspring depend on whether the recessive on the X chromosome. Red eye color is wild- trait was expressed by the male or the female type and is dominant to white eye color. in the P generation. With respect to Drosophila D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 229 eye color, when the P male expresses the white-eye phenotype and the female is homozygous for the red-eyed allele, all members of the F1 generation exhibit red eyes (Figure 9-14). The F1 females are heterozygous (XWXw), and the males are all XWY, having received their X chro- mosome from the homozygous dominant P female and their Y chromosome from the P male. A subsequent cross between the XWXw female and the XWY male would produce only red-eyed females (with XWXW or XWXw genotypes) and both red and white-eyed males (with XWY or XwY genotypes). Now, consider a cross between a homozygous white-eyed female and a male with red eyes. The F1 generation would exhibit only heterozygous red-eyed females (XWXw) and only white-eyed males (XwY). ART CONNECTION What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color? Figure 9-14 | Crosses involving sex-linked traits often give rise to different phenotypes for the different sexes of offspring, as is the case for this cross involving red and white eye color in Drosophila. In the diagram, w is the white-eye mutant allele and W is the wild-type, red-eye allele. Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to 100 percent of her male offspring because the males will receive the Y chromosome from the male parent. In humans, the alleles for certain conditions (some color-blindness, hemophilia, and muscular dystro- phy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, X-linked traits appear more frequently in males than females. In many groups of organisms with sex chromosomes, the sex with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. The sex chro- mosomes in birds are designated “Z” or “W.” The males have two Z chromosomes, whereas the females have a Z and a W. In this case, sex-linked traits will be more likely to appear in the female. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 230 C H A P T E R 9 : P atterns of I nheritance Human Sex-Linked Disorders Sex-linkage studies in Morgan’s laboratory provided the fundamentals for understanding X-linked recessive disorders in humans, which include red-green color blindness, and Types A and B hemophilia. Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disor- ders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild- type allele, they are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due to the inactiva- tion of the dominant allele located on one of the X chro- mosomes. However, female carriers can contribute the trait to their sons, result- ing in the son exhibiting the trait, or they can contribute the recessive allele to their daughters, resulting in the daughters being carriers of the trait (Figure 9-15). Al- Figu r e 9 -15 | X- L in ke d Di s o r d e r though some Y-linked disor- ders exist, typically they are The son of a woman who is a carrier of a recessive X-linked dis- associated with infertility order will have a 50 percent chance of being affected. A daughter in males and are therefore will not be affected, but she will have a 50 percent chance of being not transmitted to subse- a carrier like her mother. quent generations. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 231 Linked Genes Violate the Law of Independent Assortment Although all of Mendel’s pea plant characteristics behaved according to the law of inde- pendent assortment, we now know that some allele combinations are not inherited inde- pendently of each other. Genes that are located on different chromosomes will always sort independently. However, each chromosome contains hun- dreds or thousands of genes, organized linearly on chromo- somes like beads on a string. The segregation of alleles into gametes can be influenced by linkage, in which genes that are located physically close to each other on the same chro- mosome are more likely to be inherited as a pair. However, because of the process of re- combination, or “crossover,” it is possible for two genes on the same chromosome to be- have independently, or as if they are not linked. To under- stand this, let us consider the biological basis of gene linkage and recombination. Homologous chromosomes possess the same genes in the same order, though the specific alleles of the gene can be dif- ferent on each of the two chro- mosomes. Recall that during prophase I of meiosis, homol- ogous chromosomes synapse, with like genes on the homo- logs aligning with each other. At this stage, segments of ho- Figu r e 9 -16 | C r o s s ove r/ Re c o m b in at i o n mologous chromosomes ex- change linear segments of ge- The process of crossover, or recombination, occurs when two ho- netic material (Figure 9-16). mologous chromosomes align and exchange a segment of genetic This process is called recom- material. bination, or crossover, and it is a common genetic process. Because the genes are aligned during recombination, the gene order is not altered. Instead, the result of recombination is that maternal and paternal alleles are combined onto the same chromosome. Across a given chromosome, several recombination events may occur, causing extensive shuffling of alleles. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 232 C H A P T E R 9 : P atterns of I nheritance When two genes are located on the same chromosome, they are considered linked, and their alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid cross involving flower color and plant height in which the genes are next to each other on the chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the other chromosome has genes for short plants and yellow flowers, then when the gametes are formed, the tall and red alleles will tend to go together into a gamete and the short and yellow alleles will go into other gametes. These are called the parental genotypes because they have been inherited intact from the parents of the individual producing gametes. But unlike if the genes were on different chromosomes, there will be no gametes with tall and yellow alleles and no gametes with short and red alleles. If you create a Punnett square with these gametes, you will see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes. Characters With More Complicated Inheritance Patterns In some cases, a gene may influence multiple characters. This phenomena is called pleiotro- py. A frequently used example of pleiotropy is sickle-cell disease. A single point mutation in the hemoglobin gene makes the protein product more susceptible to crystalizing which can lead to malformed red blood cells which don’t deliver oxygen as efficiently. This leads to many different observed symptoms such as pain, anemia, increased infections, and spleen damage to name a few. Alternately, characters can be influenced by many genes which is called polygenic inheri- tance. Examples of polygenic inheritance include skin color and height. Since there are many genes with many alleles, there are a huge number of possible allele combinations. This yields phenotypes which are far more variable than characters controlled by a single gene. Additionally, environmental factors have a strong influence in many characters. An example of this is the human disease diabetes which has a genetic component, but environmental fac- tors such as diet and fitness levels can have a large influence on the disease. The Test Cross Distinguishes the Dominant Phenotype Many human diseases are genetically inherited. A healthy person in a family in which some members suffer from a recessive genetic disorder may want to know if he or she has the disease-causing gene and what risk exists of passing the disorder on to his or her offspring. Of course, doing a test cross in humans is unethical and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases (Figure 9-17). A pedigree is a chart of the genetic history of a particular trait in a family going back several generations. A horizontal line between two elements indicates genetic parents and offshoots of that line indicate offspring. A shaded element shows that particular trait. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 233 VISUAL CONNECTION What are the genotypes of the individuals labeled 1, 2, and 3? Figure 9-17 | Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A? ” designation. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 234 C H A P T E R 9 : P atterns of I nheritance KEY TERMS allele – one of two or more variants of a gene that determines a particular trait for a charac- teristic carrier – heterozygous individual that does not express a trait but is able to pass it on to a future generation codominance – in a heterozygote, complete and simultaneous expression of both alleles for the same characteristic dihybrid – the result of a cross between two true-breeding parents that express different traits for two characteristics dominant – describes a trait that masks the expression of another trait when both versions of the gene are present in an individual F1 – the first filial generation in a cross; the offspring of the parental generation F2 – the second filial generation produced when F1 individuals are self-crossed or fertilized with each other genotype – the underlying genetic makeup, consisting of both physically visible and non-ex- pressed alleles, of an organism hemizygous – the presence of only one allele for a characteristic, as in X-linkage; hemizygos- ity makes descriptions of dominance and recessiveness irrelevant heterozygous – having two different alleles for a given gene on the homologous chromo- somes homozygous – having two identical alleles for a given gene on the homologous chromosomes hybridization – the process of mating two individuals that differ, with the goal of achieving a certain characteristic in their offspring incomplete dominance – in a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype law of dominance – in a heterozygote, one trait will conceal the presence of another trait for the same characteristic law of independent assortment – genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur law of segregation – paired unit factors (i.e., genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors linkage – a phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together model system – a species or biological system used to study a specific biological phenome- non to gain understanding that will be applied to other species monohybrid – the result of a cross between two true-breeding parents that express different traits for only one characteristic P – the parental generation in a cross D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 235 pedigree analysis – studying the genetic history of a particular trait going back several gen- erations phenotype – the observable traits expressed by an organism pleiotropy – a gene that influences multiple characters polygenic inheritance – a character that is influenced by many genes Punnett square – a visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid recessive – describes a trait whose expression is masked by another trait when the alleles for both traits are present in an individual recombination – the process during meiosis in which homologous chromosomes exchange linear segments of genetic material, thereby dramatically increasing genetic variation in the offspring and separating linked genes test cross – a cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the un- known parent is heterozygous or homozygous for the dominant trait trait – a variation in an inherited characteristic wild type – the most commonly occurring genotype or phenotype for a given characteristic found in a population X-linked – a gene present on the X chromosome, but not the Y chromosome D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8 236 C H A P T E R 9 : P atterns of I nheritance CHAPTER SUMMARY 9.1 | Mendel’s Experiments Working with garden pea plants, Mendel found that crosses between parents that differed for one trait produced F1 offspring that all expressed one parent’s traits. The traits that were visible in the F1 generation are referred to as dominant, and traits that disappear in the F1 generation are described as recessive. When the F1 plants in Mendel’s experiment were self- crossed, the F2 offspring exhibited the dominant trait or the recessive trait in a 3:1 ratio, con- firming that the recessive trait had been transmitted faithfully from the original P parent. Reciprocal crosses generated identical F1 and F2 offspring ratios. By examining sample sizes, Mendel showed that traits were inherited as independent events. 9.2 | Laws of Inheritance When true-breeding, or homozygous, individuals that differ for a certain trait are crossed, all of the offspring will be heterozygous for that trait. If the traits are inherited as dominant and recessive, the F1 offspring will all exhibit the same phenotype as the parent homozygous for the dominant trait. If these heterozygous offspring are self-crossed, the resulting F2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive. Because homozygous dominant and heterozygous indi- viduals are phenotypically identical, the observed traits in the F2 offspring will exhibit a ratio of three dominant to one recessive. Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Alleles segregate into gametes such that each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, in general, alleles are not more likely to segregate into a gamete with a particular allele of another gene. 9.3 | Extensions of the Laws of Inheritance Alleles do not always behave in dominant and recessive patterns. Incomplete dominance de- scribes situations in which the heterozygote exhibits a phenotype that is intermediate be- tween the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles for a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes, and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromo- some are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two. According to Mendel’s law of independent assortment, genes sort independently of each other into gametes during meiosis. This occurs because chromosomes, on which the genes reside, assort independently during meiosis and crossovers cause most genes on the same chromo- somes to also behave independently. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel’s law of independent assortment. However, recombination serves to exchange D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8  C H A P T E R 9 : P atterns of I nheritance 237 genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together. Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chro- mosomal space. Whether or not they are sorting independently, genes may interact at the level of gene prod- ucts, such that the expression of an allele for one gene masks or modifies the expression of an allele for a different gene. This is called epistasis. D o w n l o a d O p e n S t a x B i o l o gy 2 n d E d i t i o n f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / 8 d 5 0 a 0 a f- 9 4 8 b - 4 2 0 4 - a 7 1d - 4 8 2 6 c b a 7 6 5 b 8 @ 15. 4 3 D o w n l o a d O p e n S t a x C o n c e p t s o f B i o l o gy f o r f r e e a t h t t p : //c n x. o r g /c o n t e n t s / b 3 c1e1d 2 - 8 3 9 c - 4 2 b 0 - a 3 14 - e119 a 8 a a f b d d @ 15. 8