Mendelian Genetics

1. Mendelian Genetics Chapter 9 Patterns of Inheritance

2. Gregor Mendel • 1850s, Mendel studied inheritance in pea plants by breeding them • Peas are good because they have distinctive traits, male and female parts, and are easy to manipulate • He crossed true breeding with other true breeding as well as with hybrids

3. Gregor Mendel • P = parental generation • F1 = First filial generation (P X P) • F2 = Second filial (F1 X F1) • True breeding (homozygous) = plants that only breed to produce one phenotype • Hybrid (heterozygous) = results of crosses between two plants that breed true for different phenotypes for the same trait

4. Gregor Mendel • Genotype = genetic makeup of an organism • Sequence of the gene • Phenotype = expressed genotype • Visible or measurable traits – based upon genotype

5. Gregor Mendel • He crossed a true breeding purple-flowered plant with a true breeding white-flowered plant • All of the F1 generation was purple

6. Gregor Mendel • He then crossed two F1’s with each other • The results (F2’s) were roughly 3 purples to 1 whites • White trait was absent in F1’s but reappeared in the F2’s

7. Gregor Mendel • The reappearance of the white trait meant that it was not altered in the F1, just “hidden” • Mendel did the same crosses, and got similar results, with other traits of the pea plant

9. Gregor Mendel

10. Gregor Mendel • Four ideas spawn from Mendel’s work: • Alleles account for different traits • Organisms inherit two alleles, one from each parent (may be same or different) • If alleles are different, then one is dominant over the other • The two alleles segregate (law of segregation) during gamete formation

11. Punnett Square • Predicts the statistical results of a cross

12. Punnett Square • A test cross, breeding a homozygous recessive with a dominant phenotype, but unknown genotype, can determine the identity of the unknown allele

13. Punnett Square • One Factor Crosses = crosses with only one trait • Monohybrid crosses = crossing one heterozygote with another • Two Factor Crosses = crosses with two traits • Dihybrid crosses = crossing one organism that is heterozygous for both traits with another that is heterozygous for both traits

14. Punnett Square • In monohybrid crosses, he noticed that genotypic and phenotypic ratios could be predicted • Genotypic ratio = 1:2:1 • Phenotypic ratio = 3:1

15. Punnett Square • Example of a dihybrid cross: • Crossing one plant that is heterozygous for seed shape and yellow seed coat (RrYy) with another plant that is also heterozygous for both traits (RrYy) • RrYy X RrYy • Dihybrid results point to the law of independent assortment

16. Independent Assortment • In our example (RrYy X RrYy), both parents can produce four possible gametes • RY • Ry • rY • Ry • Since both parents have four possible gametes, there are 16 possible combinations for fertilization

17. Independent Assortment

18. Independent Assortment • These possible combinations produce a genotypic ratio of 1:2:1:2:4:2:1:2:1 • It also produces a phenotypic ratio of 9:3:3:1 • The law of independent assortment simply means that during gamete formation, allelic pairs that code for different traits assort independent of each other

19. Dominance • Mendel was very lucky to have studied peas • They follow simple dominance laws • Not all cells follow the simple laws of dominance vs. recessiveness

20. Variations • Incomplete dominance = neither allele is shown in its dominant form; an intermediate phenotype is shown • Genotypic and phenotypic ratios are the same (1:2:1)

21. Variations • Codominance = heterozygote expresses a phenotype that is distinct from and not intermediate between those of the two homozygotes Ex. Human AB blood type

22. Variations • Codominance

23. Variations • Multiple alleles – gene exists as more than two alleles in the population • Rabbit coat color gene has 4 alleles: C, c, cch & ch • 5 phenotypes • 10 genotypes

24. Variations • Pleiotropy – one gene affects more than one phenotypic characteristic • Ex: sickle-cell disease affects much more than just the overall conformation of the hemoglobin protein

25. Variations • Epistasis – one gene at one locus affects the phenotype of another gene at another locus • Ex: mice coat color depends on two genes • One, the epistatic gene, determines whether or not pigment will be deposited in hair • Presence (C) is dominant to absence (c) • The second determines whether the pigment to be deposited is black (B) or brown (b) • An individual that is cc has a white (albino) coat regardless of the genotype of the second gene

26. Variations – epistasis • A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment • However, unlike the 9:3:3:1 offspring ratio of an normal Mendelian experiment, the ratio is 9 black, 3 brown, and 4 white

27. Variations • Polygenic – one phenotype determined by the combined effect of more than one gene • Ex: human skin color, eye color, height

28. Variations

29. Variations • Multifactorial Traits – determined by the combined effect of one or more genes plus the environment • Ex: heart disease, body weight, intelligence

30. Pedigrees • Autosomal Recessive Traits: • located on non-sex chromosomes • affects males and females • parents must be carriers or affected • affected individuals are homozygous recessive Ex: Albinism, Cystic fibrosis, Phenylketonuria, Sickle cell disease

31. Autosomal Recessive

32. Pedigrees • Autosomal Dominant Traits: • located on non-sex chromosomes • affects males and females • at least one parent is affected • does not skip generations • affected individuals are homozygous dominant or heterozygous Ex. Achondroplasia, Huntington disease, Lactose intolerance, Polydactyly

33. Autosomal Dominant