Patterns of Inheritance

1. Patterns of Inheritance Chapter 14: Mendel and the Gene Idea

2. Patterns of Inheritance • Parents and offspring often share observable traits. • Grandparents and grandchildren may share traits not seen in parents. • Why do traits disappear in one generation and reappear in another?

3. Possible Hypotheses • The “blending” hypothesis states that genetic material from the two parents blends together • Example: Blue and yellow paint blend to make green • The “particulate” hypothesis states that parents pass on discrete heritable units • These heritable units are genes that can be passed on to the next generation • Gregor Mendel documented a particulate mechanism through his experiments with garden peas

4. Gregor Mendel • Austrian monk • Analyzed patterns of inheritance • Asked why • Tested his theories making predictions based on math • Worked with garden peas

5. Pea plantsPisum sativum • Advantages of pea plants for genetic study: • Many varieties with distinct heritable features, or characters(such as flower color) • Character variants (such as purple or white flowers) are called traits • Mating of plants can be controlled • Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) • Cross-pollination (fertilization between different plants) easily done

6. Traits Mendel used

7. True breeding plants • Began with true-breeding varieties (pure-bred) • Self-pollinate • Always produce same offspring • Crossed with other true-breeding variety • Offspring called hybrid

8. Generations • Mendel mated two contrasting, true-breeding varieties • Process called hybridization • The true-breeding parents are the P generation • The hybrid offspring of the P generation are called the F1 (first filial) generation • Referred to as hybrids • When F1 individuals self-pollinate, the F2 (second filial) generationis produced

9. Crosses • Had lots of varieties • 7 traits with two forms of each trait • Crossing a tall plant with a short plant is a monohybrid cross • A monohybrid cross is involving only one trait. • Trait - plant height • Two variations - tall and short • Generations • P - parental (true-breeding): tall & short • F1 - first filial or son (children): all tall • F2 - next generation (grandkids): tall & short • What happened? How did the short plants reappear?

11. Mendel’s Conclusions 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: purple flowers and white flowers • These alternative versions of a gene are now called alleles • Each gene resides at a specific locus (location on a specific chromosome) • Therefore, we distinguish between an organism’s • phenotype or physical appearance • genotype or genetic makeup

12. Alleles

13. Genotype Terminology • If alleles are identical = homozygous • If both alleles are recessive = homozygous recessive • Genotype is aa • If both alleles are dominant = homozygous dominant • Genotype is AA • If both alleles are different = heterozygous • Dominant phenotype expressed • One dominant allele and one recessive allele (Aa) • An organism’s traits do not always reveal its genetic composition

15. Mendel’s Conclusions • For each character, an organism inherits two alleles, one from each parent • Factors (genes) that determine traits can be hidden or unexpressed. • Alleles may be identical (true-breeding plants) or different (F1 hybrids) • Dominant traits expressed in the F1 generation • Recessive traits not expressed in the F1 generation • Mendel observed the same pattern of inheritance in 7 pea plant characters, each represented by two traits • What Mendel called a “heritable factor” is what we now call a gene

16. Mendel’s Conclusions 3. If two alleles at a locus differ, then one (dominant allele) determines the organism’s appearance, and the other (recessive allele) has no noticeable effect on appearance • 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

18. Mendel’s Law of Segregation 4. The law of segregation states that the two alleles for a heritable character segregate (separate) 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 somatic cells of an organism • This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

19. Principle of Segregation • Two copies of each trait (gene) • Fully expressed gene - dominant • Other gene - recessive 2. Gametes only have one copy (haploid) 3. Fertilization restores pairs or two copies (diploid)

21. Monohybrid & Dihybrid Crosses • Mendel derived the law of segregation by following a single character • Mendel identified his second law of inheritance by following two characters at the same time • Crosses involving two traits are called dihybrid crosses • A dihybrid cross can determine whether two characters are transmitted to offspring as a package or independently • Using a dihybrid cross, Mendel developed the law of independent assortment

22. Law of Independent Assortment • The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation • This law applies only to genes on different, nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together

23. Probability Rules • Mendel’s laws 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 separate into gametes independently of another gene’s alleles • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities • Example: probability of 2 coins landing heads up is 1/4 (1/2 x 1/2 = 1/4)

24. Probability Rules • Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele • Similar to heads and tails • Another rule is needed to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous • The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities • Example: probability of one heads & one tails is 1/2 (1/4 + 1/4 = 1/2)

25. Probability Rules • These rules can be used to predict the outcome of crosses involving multiple characters • A dihybrid or other multicharacter crosses are 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 together

26. Punnett squares • Probability can be depicted through the use of a Punnett square • A diagram for predicting the results of a genetic cross between individuals of known genetic makeup • Predicts all possible offspring for all possible gametes with random fertilization • Same letter used for trait • Capital letter = dominant allele (A) • Lower case letter = recessive allele (a)

27. Setting up a Punnett Square • Step 1. Designate letters which will represent the genes/traits. • T = tall t = short • Step 2. Write down the genotypes (genes) of each parent. These are often given to you or are possible to determine. • TT (tall) X tt (short) - both homozygous or purebred • Step 3. List the genes that each parent can contribute. Parent 1 Parent 2

28. Setting up a Punnett Square • Step 4. Draw a Punnett square and write the possible gene(s) of one parent across the top and of the other parent along the side.

 • Step 5. Fill in each box of the Punnett square by transferring the letter above and in front of each box into each appropriate box. As a general rule, the capital letter goes first and a lowercase letter follows. • Step 6. List the possible genotypes and phenotypes of the offspring for this cross. • Genotypic Ratio: 4/4 or 100% Tt genotype • Phenotypic Ratio: 4/4 or 100% tall

29. Practice! • 1. Cross a homozygous tall plant with a short plant. What are the genotypic and phenotypic ratios? • 2. Cross a heterozygous tall plant with a homozygous tall plant. What are the genotypic and phenotypic ratios? • 3. Cross a heterozygous tall plant with a short plant. What are the genotypic and phenotypic ratios? 100% Tt; 100% Tall 50% Tt & 50% TT; 100% Tall 50% Tt & 50% tt; 50% Tall & 50 % Short

30. Testcross • Used to determine exact genotype • Individual expressing dominant phenotype • Could be heterozygous or homozygous dominant • Cross with homozygous recessive • Make prediction with Punnett square • If homozygous dominant: • TT x tt • All offspring Tt or tall plants • If heterozygous: • Tt x tt • Offspring - half Tt (tall), half tt (short)

32. More Practice!!! 1. In rabbits, the allele for black fur (B) is dominant over the allele for brown fur (b). If a heterozygous male mates with a heterozygous female, what are the chances that the offspring will have black fur? 2. In humans, dimples are dominant to no dimples. If a homozygous dominant man reproduces with a heterozygous female, what are the chances of having a child with no dimples? 3. In humans, freckles are dominant over no freckles. A man with freckles reproduces with a woman with freckles, but the children have no freckles. What chance did each child have for freckles? 4. If a man is homozygous for widow’s peak (dominant) reproduces with a woman homozygous for straight hairline (recessive), what are the chances of their children having a widow’s peak? A straight hairline? 5. In humans, pointed eyebrows (B) are dominant over smooth eyebrows (b). Mary’s father has pointed eyebrows, but she and her mother have smooth. What is the genotype of the father? 75% Black fur 0% No dimples 75% chance for freckles 100% Widow’s peak; 0% straight hairline Bb (heterozygous)

33. Review Questions Differentiate between the blending and particulate hypotheses of inheritance. Explain the importance of Gregor Mendel’s work with garden peas. Also, explain why he used garden peas. Define the following vocabulary associated with basic genetics: character, trait, hybrid, gene, allele, locus, genotype, phenotype, dominant, recessive, homozygous, & heterozygous. Differentiate between the P, F1, and F2 generations. Differentiate between monohybrid and dihybrid crosses. Explain Mendel’s four basic conclusions regarding inheritance patterns. Explain the three parts to the law of segregation. Explain the law of independent assortment. Properly construct a Punnett square for use in solving a genetics problem involving probability. Explain the idea of a testcross.