General Biology 2 Lesson 1 - Genetics PDF

General Biology 2 Lesson 1 Genetics Genetics Branch of science which deals with the study heredity and variations. Heredity – transmission of characteristics Variation – differences among the traits that occur in member of the same species. Terminologies Pedigree This mak...

General Biology 2 Lesson 1 Genetics Genetics Branch of science which deals with the study heredity and variations. Heredity – transmission of characteristics Variation – differences among the traits that occur in member of the same species. Terminologies Pedigree This makes use of diagrams showing the ancestral relationships and transmission of genetic traits over several generations in a family. Proband The first family member that seeks medical attention. The individual in the pedigree that led to the construction of the pedigree. Autosomal Trait A trait whose alleles that control it are found in the autosomes (body chromosomes/non-sex chromosomes). Autosomal refers to the 22 numbered chromosomes as opposed to the sex chromosomes Genotype The gene pair an individual carries for a particular trait symbolized with a pair of letters. By convention, uppercase letter (e.g., A) for a dominant allele and lowercase letter (e.g., a) for the recessive allele. Any letter in the alphabet may be used. For a diploid organism with two alleles in a given gene pair, genotypes may be written as: Homozygous dominant, i.e. with two dominant alleles (DD) Heterozygous, i.e. with a dominant and recessive allele (Dd). The individual will show the dominant phenotype. Homozygous recessive, i.e. with two recessive alleles (dd) Phenotype The observable trait of an individual based on its genotype. Example: red flower, curly hair, blood types (i.e., the blood type is the phenotype) For a typical Mendelian trait, phenotypes may either be: Dominant. A trait that requires at least one dominant allele for the trait to be expressed (e.g., Dd) Recessive. A trait that requires two recessive alleles for the trait to be expressed (e.g., dd) Phenocopy A trait that is expressed due to specific environmental conditions (i.e., having hair that is dyed of a different color) and is not due to the genotype. Twins Identical twins. Also known as monozygotic twins, are derived from a single fertilization event. After the first cleavage or cell division of the zygote, the cells or blastomeres separate and become independent blastocysts implanted in the mother’s uterus. Fraternal twins. Also known as dizygotic twins, are derived from separate fertilization events (two eggs fertilized by two sperms) within the fallopian tube, resulting in two separate zygotes. Brief History Gregor Johann Mendel He laid the foundation for the formal discipline of genetics in 1866. When Mendel began his studies of inheritance using the Pea plant (Pisum sativum), there was no knowledge of chromosomes or the role and mechanism of meiosis. His work remained unnoticed until about 1900, following the rediscovery of his work, the concept of gene as a hereditary unit was established. Even today, they serve as the cornerstone of the study of Genetics. Research Method: Crossing Pea Plants Advantages of pea plants for genetic study There are many varieties with distinct heritable features or characters (such as flower color); character variants (such as purple or white flowers); called traits Mating can be controlled Each flower has sperm-producing organs (stamens) and an egg-producing organ (carpel) Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another Mendel’s Experimental Approach Mendel chose to track only those characters that occurred in two distinct alternative forms (such as purple or white flower color). He made sure that he started his experiments with varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) Mendel’s Experimental Approach In a typical experiment, Mendel mated two contrasting, true- breeding varieties, a process called hybridization. The true-breeding parents are the P generation (parental generation). The hybrid offspring of the P generation are called the F1 generation When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced Laws of Inheritance Law of Segregation (First Mendelian Law) Law of Segregation (First Mendelian Law) For every trait governed by a pair of alleles, these alleles segregate or separate during gamete formation in meiosis. Mendel’s Experiment Mendel derived the law of segregation by following a single character. The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character. A cross between such heterozygotes is called a monohybrid cross. Mendel’s Experiment When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple. When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white. Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation. EXPERIMENT P Generation (true-breeding parents) Purple White flowers flowers F1 Generation (hybrids) All plants had purple flowers Self- or cross-pollination F2 Generation 705 purple- 224 white flowered flowered plants plants Mendel’s Experiment Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids. Mendel called the purple flower color a dominant trait and the white flower color a recessive trait. The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation.  Mendel observed the same pattern of inheritance in six other pea plant characters, Mendel’s each represented Experiment by two traits. What Mendel called a “heritable factor” is what we now call a gene. Mendel’s Model Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring. Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes Concept 1: Alternative versions of genes account for variations in inherited characters For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers. These alternative versions of a gene are called alleles. Allele for purple flowers Allele - a variant of the sequence of nucleotides at Pair of Locus for flower-color gene homologous a particular chromosomes location (locus) on a DNA molecule Allele for white flowers Concept 2: For each character, an organism inherits two alleles, one from each parent Mendel made this deduction without knowing about the role of chromosomes. The two alleles at a particular locus may be identical, as in the true- breeding plants of Mendel’s P generation. Alternatively, the two alleles at a locus may differ, as in the F1 hybrids. Concept 3: If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance. In the flower-color example, the F1 plants had purple flowers because the allele for that trait is dominant. Concept 4 (now known as the LAW OF SEGREGATION): The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the organism. This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis. Useful Genetic Vocabulary An organism that has a pair of identical alleles for a gene encoding a character is called a homozygote and is said to be homozygous for that gene. In the parental generation, the purple-flowered pea plant is homozygous for the dominant allele (PP), while the white plant is homozygous for the recessive allele (pp). Homozygous plants “breed true” because all of their gametes contain the same allele—either P or p in this example. If we cross dominant homozygotes with recessive homozygotes, every offspring will have two different alleles—Pp in the case of the F1 hybrids of our flower color experiment.  Phenotype Genotype Purple PP 1 (homozygous) 3 Purple Pp (heterozygous) 2 Purple Pp (heterozygous) White pp 1 1 (homozygous) Ratio 3:1 Ratio 1:2:1  Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition. Therefore, we distinguish between an organism’s appearance or observable traits, called its phenotype, and its genetic makeup, its genotype. For the case of flower color in pea plants, PP and Pp plants have the same phenotype (purple flowers) but different genotypes. Note that the term phenotype refers to physiological traits as well as traits that relate directly to appearance. The Testcross How can we tell the genotype of an individual with the dominant phenotype? Such an individual could be either homozygous dominant or heterozygous. The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual. If any offspring display the recessive phenotype, the mystery parent must be heterozygous. TECHNIQUE Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If purple-flowered or If purple-flowered parent is PP parent is Pp Sperm Sperm p p p p P P Pp Pp Pp Pp Eggs Eggs P p Pp Pp pp pp RESULTS or All offspring purple 1/ offspring purple and 2 1/ offspring white 2 Law of Independent Assortment (Second Mendelian Law) Law of Independent Assortment (Second Mendelian Law) A pair of alleles for one trait will segregate or separate independently of another pair of alleles for another trait during meiosis. This law applies only to genes on different, non-homologous chromosomes or those far apart on the same chromosome. Genes located near each other on the same chromosome tend to be inherited together. Mendel’s Experiment Mendel identified his second law of inheritance by following two characters at the same time. Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters. A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently.  Mendel’s Experiment Imagine crossing two true-breeding pea varieties that differ in both of these characters—a cross between a plant with yellow-round seeds (YYRR) and a plant with green-wrinkled seeds (yyrr). The F1 plants will be dihybrids, individuals heterozygous for the two characters being followed in the cross (YyRr). But are these two characters transmitted from parents to offspring as a package? That is, will the Y and R alleles always stay together, generation after generation? Or are seed color and seed shape inherited independently? The next figure shows how a dihybrid cross, a cross between F1 dihybrids, can determine which of these two hypotheses is correct. EXPERIMENT P Generation YYRR yyrr Gametes YR yr F1 Generation YyRr Predictions Hypothesis of Hypothesis of dependent assortment independent assortment Sperm Predicted or offspring of 1/ 4 YR 1/ 4 Yr 1/ 4 yR 1/ 4 yr Sperm F2 generation 1/ 2 YR 1/ 2 yr 1/ 4 YR YYRR YYRr YyRR YyRr 1/ 2 YR YYRR YyRr 1/ Yr Eggs 4 YYRr YYrr YyRr Yyrr 1/ Eggs 2 yr YyRr yyrr 1/ yR 4 YyRR YyRr yyRR yyRr 3/ 1/ 4 4 1/ 4 yr Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 9/ 3/ 3/ 1/ 16 16 16 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Mendel’s Experiment The F1 plants, of genotype YyRr, exhibit both dominant phenotypes, yellow seeds with round shapes, no matter which hypothesis is correct. The key step in the experiment is to see what happens when F1 plants self-pollinate and produce F2 offspring. If the hybrids must transmit their alleles in the same combinations in which the alleles were inherited from the P generation, then the F1 hybrids will produce only two classes of gametes: YR and yr. As shown on the left side of Figure 14.8, this “dependent assortment” hypothesis predicts that the phenotypic ratio of the F2 generation will be 3:1, just as in a monohybrid cross:  Mendel’s Experiment The alternative hypothesis is that the two pairs of alleles segregate independently of each other. In other words, genes are packaged into gametes in all possible allelic combinations, as long as each gamete has one allele for each gene. In our example, an F1 plant will produce four classes of gametes in equal quantities: YR, Yr, yR, and yr. If sperm of the four classes fertilize eggs of the four classes, there will be 16 (4*4) equally probable ways in which the alleles can combine in the F2 generation. These combinations result in four phenotypic categories with a ratio of 9:3:3:1 (nine yellow round to three green round to three yellow wrinkled to one green wrinkled): Mendelian Modes of Inheritance Monohybrid Cross – one factor cross In pea plants, having axial position of flowers on stem (T) is dominant over the terminal position (t). A heterozygous axial flower position in a pea plant is allowed to pollinate by itself. 2. Dihybrid Cross – Two – factor cross In pea plants, let us use the same example as in monohybrid cross (Flower position). Then, let us combine the traits with yellow and green seed color. Cross heterozygous axial and yellow with another of the same kind. Find the phenotypic ratio of the offsprings. The Laws of Probability Govern Mendelian Inheritance Mendel’s laws of segregation and independent assortment reflect the rules of probability. When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss. In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles. Multiplication Rules Applied to Monohybrid Crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities. Probability in an F1 monohybrid cross can be determined using the multiplication rule. Segregation in a heterozygous plant is like flipping a coin: Each gamete has a 12 chance of carrying the dominant allele and a 12 chance of carrying the recessive allele.  We can apply the same reasoning to an F1 monohybrid cross. With seed shape in pea plants as the heritable character, the genotype of F1 plants is Rr. Segregation in a heterozygous plant is like flipping a coin in terms of calculating the probability of each outcome: Each egg produced has a 1⁄2 chance of carrying the dominant allele (R) and a 1⁄2 chance of carrying the recessive allele (r). The same odds apply to each sperm cell produced. For a particular F2 plant to have wrinkled seeds, the recessive trait, both the egg and the sperm that come together must carry the r allele. The probability that an r allele will be present in both gametes at fertilization is found by multiplying 1⁄2 (the probability that the egg will have an r) * 1⁄2 (the probability that the sperm will have an r). Thus, the multiplication rule tells us that the probability of an F2 plant having wrinkled seeds (rr) is 1⁄4. Likewise, the probability of an F2 plant carrying both dominant alleles for seed shape (RR) is 1⁄4.  Rr  Rr Segregation of Segregation of alleles into eggs alleles into sperm Sperm 1/ 2 R 1/ 2 r R R 1/ 2 R R r 1/ 1/ 4 4 Eggs r r 1/ r R r 2 1/ 1/ 4 4 Addition Rule The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities. The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous. Solving Complex Genetics Problems with the Rules of Probability We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters. A dihybrid or other multi-character cross is equivalent to two or more independent monohybrid crosses occurring simultaneously. In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied. To give two examples, the calculations for finding the probabilities of two of the possible F2 genotypes (YYRR and YyRR) are shown below: Probability of YYRR = 1/4 (probability of YY)  1/4 (RR) = 1/16 Probability of YyRR = 1/2 (Yy)  1/4 (RR) = 1/8 Seatwork In tomatoes, two pairs of gene affect the color of the ripe fruit as follows R, red flesh; r, yellow flesh; Y, yellow skin; y colorless skin Dominance is complete for red flesh and yellow skin. If the genes are independently segregating, calculate the expected phenotype and genotype ratios from the following crosses: a. Rryy x rrYy b. RrYy x rrYy c. RrYY x Rryy d. RrYy x RrYy Solution for Rryy x rrYy Possible combinations for Rrryy = Ry and ry Possible combinations for rrYy = rY and ry Ry Ry ry ry rY RrYy RrYy rrYy rrYy rY RrYy RrYy rrYy rrYy ry Rryy Rryy rryy rryy ry Rryy Rryy rryy rryy Probability Probability Probability Probability RrYy 4/16 = 1/4 Rryy 4/16 = 1/4 rrYy 4/16 = 1/4 rryy 4/16 = 1/4  Another Solution for Rryy x rrYy R r y y r Rr rr Y Yy Yy Combi Probability r Rr rr y yy yy RrYy 1/2 (Rr) × 1/2 (Yy) = 1/4 Probability Probability Rryy 1/2 (Rr) × 1/2 (yy) = 1/4 rrYy 1/2 (rr) × 1/2 (Yy) = 1/4 RR 0 YY 0 rryy 1/2 (rr) × 1/2 (yy) = 1/4 Rr 2/4 = 1/2 Yy 2/4 = 1/2 rr 2/4 = 1/2 yy 2/4 = 1/2 Phenotypic and Genotypic Ratios for Rryy x rrYy Combi Prob Phenotype Genotype RrYy 1/4 Red flesh, yellow skin heterozygous red flesh, heterozygous yellow skin Rryy 1/4 Red flesh, colorless skin heterozygous red flesh, homozygous colorless skin rrYy 1/4 Yellow flesh, yellow skin homozygous yellow flesh, heterozygous yellow skin rryy 1/4 Yellow flesh, colorless skin homozygous yellow flesh, homozygous colorless skin Phenotypic ratio = 1:1:1:1 Genotypic ratio = 1:1:1:1 Dominant and Recessive Traits in Humans Dominant Recessive Free earlobe Attached ear lobe Cleft chin No cleft chin Widow’s peak No widow’s peak Ability to roll the tongue Inability to roll the tongue Straight thumb Hitchhiker’s thumb Arm folding right on top Arm folding left on top With dimples Without dimples Dominant and Recessive Traits in Humans Dominant Recessive Huntington Disease Alkaptonuria Marfan Syndrome Color blindness Congenital night blindness Cystic fibrosis Neurofibromatosis Muscular dystrophy Porphyria Hemophilia Ehler-Danlos Syndrome Sickle-cell anemia Hypercholesterolemia Tay-Sach’s disease Achondroplasia Phenylketonuria