Chapter 4

1. Chapter 4 Mendelian Inheritance

2. 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. Gregor Mendel father of modern genetics Combined • plant breeding • Statistics • Careful recordkeeping Described hypothesis of transmission of traits now considered laws of inheritance

4. Mendel studied pea traits with two distinct forms

5. True breeding plants Plants which consistently have offspring with same trait as parent are true breeding plants.

6. Monohybrid cross • What happens when true breeding plants with two distinct forms of a trait are crossed? Progeny show only one form of the trait. The observed trait is called dominant. The masked trait is called recessive.

7. Test cross Test by crossing with a plant showing the recessive trait. Is a plant showing the dominant trait true-breeding or not? All tall offspring indicate parent is true-breeding Mixed offspring indicate parent is hybrid

8. Crossing hybrids to each other Hybrid parents show the dominant trait (tall). Offspring: • Dominant trait (tall) and • true breeding (1/4 total) • Dominant trait (tall) and • NOT true breeding (1/2 total) • Recessive trait (short) and • always true breeding • (1/4 total) Mendel concluded that among the hybrid parents the short trait (recessive) was hidden but not absent

9. Mendel’s data Crossed true-breeding plants differing at one of seven traits. Crossed hybrid offspring to each other (all show dominant trait). Counted offspring of hybrid crosses.

10. Law of segregation Why do traits “disappear” in one generation only to reappear in a subsequent generation? • Each plant possesses two distinct separable • units (alleles) for each trait inherited from • each parent. • Gametes contain ONE allele for each trait. • Only one version is observed in an individual. The unit (allele) does not disappear. It may be present but hidden.

11. Law of segregation

12. Alleles Mendel’s units (or “elementen”) are called alleles. • versions of the same gene or DNA sequence. • differ in DNA sequence at one or more sites.

13. Genotype indicates the combination of alleles present • Homozygous alleles are the same • Heterozygous alleles differ Phenotype indicates the trait observed. Terms distinguish the observed form and the underlying alleles present.

14. Phenotype Tall plant Short plant Genotype Homozygous dominant “tall-associated” alleles Heterozygous Homozygous recessive “short-associated” alleles Genotype and phenotype Abbreviation of genotype TT Tt tt

15. Modern terms for Mendel’s crosses • Mendel’s true-breeding plants were homozygous for the alleles of a trait. • Mendel’s hybrid were heterozygous for the alleles of a trait.

16. Wildtype most common version in the general population • wildtype phenotype • mutant phenotype • wildtype allele • mutant allele • most common phenotype • phenotype different from the • wildtype • most frequent allele • associated with the • common phenotype homozygous. • allele associated with the • mutant phenotype.

17. Law of segregation: the monohybrid cross Two heterozygous parents produce gametes with T or t allele equally frequently. Offspring genotypes 1/4 TT : 1/2 Tt :1/4 tt Offspring phenotypes 3/4 tall : 1/4 short

18. Most common Mode of inheritance indicates the patterns with which the mutant phenotype is associated. Autosomal recessive Autosomal dominant X-linked recessive X-linked dominant Y-linked mitochondrial

19. Autosomal dominant inheritance • Heterozygotes exhibit the affected phenotype. • Males and females are equally affected and may transmit the trait. • Affected phenotype does not skip generation.

20. Autosomal recessive inheritance • Heterozygotes carry the recessive allele but exhibit the wildtype phenotype. • Males and females are equally affected and may transmit the trait. • May skip generations.

21. Autosomal dominant Autosomal recessive Comparison of autosomal dominant and autosomal recessive inheritance Males and females affected? Yes Yes Males and females transmit the trait? Yes Yes Trait skips generations? No Yes At least one parent of affected child must be affected? Yes No

22. Law of independent assortment • Two genes on different chromosomes segregate their alleles independently. • The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.

23. Law of independent assortment

24. Independent assortment of two traits • In a dihybrid cross, parents with two differing traits are crossed. • Which allele is dominant? Heterozygous peas are round and yellow. Therefore round is dominant to wrinkled yellow is dominant to green

25. Two traits segregating independently

26. 315 round yellow peas 108 round green peas 101 wrinkled yellow peas 32 wrinkled green peas 416 423 140 133 : 1 3.18 Two traits segregating independently 2.97 :1

27. The probability of an event = # of chance of event total possible events Probability • No chance of event probability = 0 • (e.g. chance of rolling 8 on a six-sided die) • Event always occurs probability = 1 • (chance of rolling 1,2,3,4,5,or 6 on a six-sided die) The likelihood that an event will occur. The probabilities of all the possible events add up to 1.

28. Independent events The probability of independent events is calculated by multiplying the probability of each event. In two rolls of a die, the chance of rolling the number 3 twice: Probability of rolling 3 with the first die = 1/6 Probability of rolling 3 with the second die =1/6 Probability of rolling 3 twice = 1/6 x 1/6or1/36

29. Dependent events The probability of dependent events is calculated by adding the probability of each event. In one roll of a die, what is the probability of rolling either the number 5 or an even number? Probability of rolling the number 5 = 1/6 Probability of rolling an even number = 3/6 Probability of rolling 5 or an even number = 1/6 + 3/6 or 4/6

30. Independent events What is the chance of an offspring having the homozygous recessive genotype when both parents are doubly heterozygous?

31. Dependent events Parents are heterozygous for a trait, R. What is the chance that their child is carries at least one dominant R allele? Probability of child carrying RR = 1/4 Probability of child carrying Rr = 1/2 Probability of child carrying R_ = 1/4 + 1/2 = 3/4

32. Pedigreessymbolic representations of family relationships and inheritance of a trait

33. Autosomal dominant inheritance of brachydactyly Heterozygotes exhibit the phenotype.

34. Autosomal recessive inheritance of albinismHeterozygotes carry the recessive allele but exhibit the wildtype phenotype

35. A a A a AA Aa aa Aa Ellen is not affected and cannot carry aa genotype Ellen Michael ? Ellen and Michael’s parents must be carriers. Genetic predictions Ellen’s brother Michael has sickle cell anemia, an autosomal recessive disease. What is the chance that Ellen’s child has a sickle cell anemia allele (a)? chance Ellen is a carrier = 2/3 chance child inherits sickle cell allele = 1/2 Overall chance child carries sickle cell allele from Ellen = 2/3 x 1/2 = 1/3