Mendelian Inheritance Basics PDF

Introduction to genetics CH 2 Mendalian & Nonmendalian inheritance Dr. Radhika G Bhardwaj Learning objectives Principles of heredity How genes are passed from generation to generation and How factors such as dominance influence...

Introduction to genetics CH 2 Mendalian & Nonmendalian inheritance Dr. Radhika G Bhardwaj Learning objectives Principles of heredity How genes are passed from generation to generation and How factors such as dominance influence their inheritance Mendel’s principles of segregation and independent assortment Probability and the behavior of chromosomes 1. Gregor Mendel Discovered the Basic Principles of Heredity Gregor Mendel put forth the basic principles of inheritance, publishing his findings in 1866. Much of Mendel’s success can be attributed to the seven characteristics of pea plants that he studied and his experimental approach. Gregor Johann Mendel discovered the principles of heredity by experimenting with peas. [James King-Holmes/Science Source.  Mendel Studied 7 Characters Variable characters of pea plants: ◦ Height ◦ Flower color ◦ Flower position ◦ Seed color ◦ Seed shape ◦ Pod color ◦ Pod shape Characters existed in two distinct variants in pea plants 5  The Garden Pea Mendel used the pea plant Pisum sativum in his studies of heredity. He examined seven characteristics that appeared in the seeds and in plants grown from the seeds. [Photograph by Charles Stirling/Alamy.] 2. Genetic Terminology At each locus, a diploid organism possesses two alleles located on different homologous chromosomes. The alleles identified here refer to traits studied by Mendel. Each phenotype results from a genotype developing within a specific environment. The alleles of the genotype, not the phenotype, are inherited. Phenotype can refer to any type of characteristic— physical, physiological, biochemical, or behavioral.  Thus, the condition of having round seeds is a phenotype, a body weight of 50 kilograms (50 kg) is a phenotype, and having sickle-cell anemia is a phenotype. The term characteristic or character refers to a general feature such as eye color; The term trait or phenotype refers to specific manifestations of that feature, such as blue or brown eyes. For many characteristics, both genes and environment are important in determining phenotypic differences. In Mendel’s peas, for example, the genotype, not the environment, largely determined the shape of the seeds. For other characteristics, environmental differences are more important. The height reached by an oak tree at maturity is a phenotype that is strongly influenced by environmental factors, such as the availability of water, sunlight, and nutrients. Nevertheless, the tree’s genotype imposes some limits on its height: an oak tree will never grow to be 300 meters (almost 1000 feet) tall, no matter how much sunlight, water, and fertilizer are provided. Thus, even the height of an oak tree is determined to some degree by genes. 3. Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance Monohybrid cross : Cross between two individuals that differ in a single characteristic— more specifically, a cross between individuals that are homozygous for different alleles at the same locus (AA × aa); also refers to a cross between two individuals that are heterozygous for two alleles at a single locus (Aa × Aa). P (parental) generation : First set of parents in a genetic cross. The conclusions that Mendel drew about inheritance from his monohybrid crosses further developed and formalized into the principle of segregation and the concept of dominance.  The principle of segregation (Mendel’s first law) - states that each individual diploid organism possesses two alleles for any particular characteristic, one inherited from the maternal parent and one from the paternal parent. - These two alleles segregate (separate) when gametes are formed, and one allele goes into each gamete. - Furthermore, the two alleles segregate into gametes in equal proportions.  Concept of dominance The concept of dominance states that when two different alleles are present in a genotype, only the trait encoded by one of them—the dominant allele—is observed/expressed in the phenotype. *The dominant allele is the allele that is expressed, and the recessive allele is the allele that is not expressed. Types of crosses Mendel carried out two types of crosses ◦ Self-fertilization ◦ Pollen and egg are derived from the same plant ◦ Cross-fertilization ◦ Pollen and egg are derived from different plants ◦ When plants with different traits are crossed, this is hybridization – progeny are called hybrids ◦ To cross-fertilize, Mendel transferred pollen into the flower of another plant 18  Mendel conducted monohybrid crosses. Mendel started with 34 varieties of peas and spent 2 years selecting those varieties that he would use in his experiments. He verified that each variety was pure-breeding (homozygous for each of the traits that he chose to study) by growing the plants for two generations and confirming that all offspring were the same as their parents. He then carried out a number of crosses between the different varieties Access the text alternative for slide images. 20 True-Breeding Lines A note on terminology: ◦ Character – The type of characteristic that can vary, such as “height” ◦ Trait, or variant – The version of the character, such as “tall” or “dwarf” Mendel started with plants that “bred true” for different characters ◦ True-breeding lines – Plants that always produce progeny with the same traits when self-fertilized (or bred to the same strain) 21 Access the text alternative for slide images. 22  Mendel’s Law of Segregation Mendel’s experiments with single-factor crosses The law of segregation and how it is related to gamete formation and fertilization Predicting outcomes of single-factor crosses using a Punnett square 23  Mendel’s Approach Mendel did not start with a hypothesis to explain the formation of hybrids ◦ But he believed that a quantitative analysis of crosses may reveal a mathematical relationship ◦ This is called an empirical approach ◦ General findings from such an approach are called empirical laws  Mendel’s Crosses Mendel mated true-breeding plants with one trait to plants with a different trait to create hybrids ◦ Mating looking at one character – single-factor cross ◦ When two parents are different variants for a character the cross produces single-character hybrids called monohybrids Mendel’s Single-Factor Cross Experiments Mendel studied seven characters, each with two variants ◦ e.g., Plant height variants were tall and dwarf His first experiments crossed only two variants of one character at a time He followed the characters for two subsequent crosses ◦ P generation – Parental generation F1 generation – 1st filial generation F2 generation – 2nd filial generation 26 27 DATA FROM MONOHYBRID CROSSES P Cross F1 generation F2 generation Ratio Tall X dwarf All tall 787 tall 2.84:1 stem 277 dwarf Purple X All purple 705 purple 3.15:1 white flowers 224 white Axial X All axial 651 axial 3.14:1 terminal flowers 207 terminal Yellow X All yellow 6,022 yellow 3.01:1 Green seeds 2,001 green Round X All round 5,474 round 2.96:1 wrinkled seeds 1,850 wrinkled Green X All green 428 green 2.82:1 yellow pods 152 yellow Smooth X All smooth 882 smooth 2.95:1 constricted pods 229 constricted TOTAL All dominant 14,949 dominant 2.98:1 5010 recessive 28 Interpreting the Data – Monohybrid Crosses 1 For all seven characters studied ◦ The F generation showed only one of the two parental traits 1 ◦ The F2 generation showed an ~ 3:1 ratio of the two parental traits These results refuted a blending mechanism of heredity ◦ The recessive trait “disappeared” entirely in the F1 But reappeared unchanged in the F2 The data suggested a particulate theory of inheritance Interpreting the Data – Monohybrid Crosses 2 Dominant and recessive traits: ◦ The trait that is exhibited in the F1 is called dominant ◦ The trait that is masked in the F1is called recessive In the F1, only the dominant trait appeared In the F2, the dominant trait plants outnumbered recessive trait plants with a 3:1 ratio Mendel’s Law of Segregation Mendel postulated: 1. When the two factors of a single character are different ◦ One is dominant and its effect can be seen ◦ The other is recessive and is not expressed 2. Genetic determinants of traits are passed down from generation to generation 3. Genes segregate from each other during the process that gives rise to gametes ◦ This is Mendel’s Law of Segregation 33 Punnett Squares A Punnett square is a grid that enables one to predict the outcome of simple genetic crosses ◦ Proposed by the English geneticist, Reginald Punnett Must know the genotype of the parents 34 The Punnett square can be used to determine the results of a genetic cross.  We will illustrate the Punnett square approach using the cross of heterozygous tall plants as an example Using a Punnett Square 1 1. Write down the genotypes of both parents Male parent = Tt Female parent = Tt 2. Write down the possible gametes each parent can make Male gametes: T or t Female gametes: T or t 3. Create an empty Punnett square 4. Fill in the possible genotypes of the offspring 36 Punnett square of a cross between two heterozygotes for one character Access the text alternative for slide images. 37 Using a Punnett Square 2 5. Determine proportions of genotypes and phenotypes ◦ Genotypic ratio ◦ TT : Tt : tt ◦ 1:2 :1 ◦ Phenotypic ratio ◦ Tall : dwarf ◦ 3 : 1 38 4. Dihybrid Crosses Reveal the Principle of Independent Assortment  Law of Independent Assortment Mendel’s experiments involving two-factor crosses The law of independent assortment Predicting the outcome of two-factor crosses using a Punnett square Using the multiplication or forked-line methods to predict the outcome of crosses involving 3+ genes 40 Mendel’s Two-Factor Cross Experiments Mendel also performed two-factor crosses ◦ Crossing individual plants that differ in two characters Example: ◦ Character 1 = Seed shape (round vs. wrinkled) ◦ Character 2 = Seed color (yellow vs. green) There are two possible patterns of inheritance for these characters – either linked or independent assortment 41  Dihybrid Crosses In addition to his work on monohybrid crosses, Mendel crossed varieties of peas that differed in two characteristics—that is, he performed dihybrid crosses. For example, he crossed one homozygous variety that had seeds that were round and yellow with another homozygous variety that had seeds that were wrinkled and green. The seeds of all the F1 progeny were round and yellow. He then allowed the F1 to self-fertilize and obtained the following progeny in the F2 : 315 round, yellow seeds; 101 wrinkled, yellow seeds; 108 round, green seeds; and 32 wrinkled, green seeds. Mendel recognized that these traits appeared in a ratio of approximately 9 : 3 : 3 : 1; that is, 916 of the progeny were round and yellow, 316 were wrinkled and yellow, 316 were round and green, and 116 were wrinkled and green. Principle of independent assortment (Mendel’s second law) States that genes encoding different characteristics (genes at different loci) separate independently; applies only to genes located on different chromosomes A common mistake is to think that the principle of segregation and the principle of independent assortment refer to two different processes ❑The principle of independent assortment is really an extension of the principle of segregation. The principle of segregation states that the two alleles at a locus separate when gametes are formed; The principle of independent assortment states that, when these two alleles separate, their separation is independent of the separation of alleles at other loci. Mendel’s dihybrid crosses revealed the principle of independent assortment. 46 47 DATA FROM DIHYBRID CROSSES P Cross F1 Generation F2 generation Round, yellow seeds X All round, 315 round, yellow seeds wrinkled, green seeds yellow 101 wrinkled, yellow seeds 108 round, green seeds 32 green, wrinkled seeds 48 Interpreting the Data - Dihybrid Crosses 1 The F2 generation contains seeds with novel combinations not found in the parental generation ◦ Round and green ◦ Wrinkled and yellow These non parental are predicted if the genes are segregating independently of each other Interpreting the Data - Dihybrid Crosses 2 Predicted phenotypic ratio in the F2 generation would be 9:3:3:1 if genes act independently of each other P Cross F1 generation F2 generation Ratio Round, yellow seeds X All round, yellow 315 round, yellow seeds 101 wrinkled, 9.8 wrinkled, green seeds yellow seeds 108 round, green seeds 32 3.2 green, wrinkled seeds 3.4 1.0 Mendel’s Law of Independent Assortment Two different genes randomly assort their alleles during the process that gives rise to gametes. The allele for one gene is found within a resulting gamete independently of whether the allele for a different gene is found in the same gamete. Two-Factor Crosses F1 self-fertilization – any two gametes can combine randomly during fertilization – 16 possible offspring Punnett squares can be used to predict the outcome of crosses involving two independently assorting genes 52 53 54 Three-factor crosses In crosses involving three or more independently assorting genes, a single Punnett square becomes cumbersome ◦ Would need 64 squares for three genes! ◦ Can use three Punnett Squares plus the multiplication method An alternative is the forked-line method 55 56 57  Mendelian Inheritance Basics Mendelian inheritance refers to the patterns of inheritance first described by Gregor Mendel. It is characterized by dominant and recessive alleles. Traits are inherited independently, following the Law of Segregation and the Law of Independent Assortment.  Non-Mendelian Patterns Non-Mendelian inheritance includes incomplete dominance, codominance, and multiple alleles. These patterns reveal that traits can be expressed in more complex ways than simple dominance. Understanding these patterns expands our knowledge of genetic variation.