Introduction to Mendelian (classical”)Genetics

1. Introduction to Mendelian (classical”)Genetics People have understood for centuries that certain traits are inherited (transmitted from generation to generation) but did not understand how it happened.

2. Mendel’s experiments answered many of these questions, because his experiments were so carefully planned, executed, and quantified.

3. Mendelworked with peas

4. 7 Traits “either/or”

5. Peas produced many offspring Selected peas that “bred true” C. Controlled the breeding process Also, he could self-fertilize the plants (“self-cross”) Mendel’s Peas

6. D. Counted the offspring and analyzed the results E. He bred plants so that they differed in only one characteristic, or only two… F. For each trait there were only two possible outcomes Mendel’s Peas

7. Monohybrid cross (plants are identical except for one characteristic, or trait) Smooth vs. Wrinkled seed shape Result: all of the plants had smooth seeds. Next, he planted the seeds from this cross and self-fertilized them. What happened?

8. He collected over 7000 seeds and counted them. 5474 were smooth 1850 were wrinkled ¾ were smooth, and ¼ were wrinkled

9. Mendel’s Terminology P1 (parents) Smooth X wrinkled F1 (first generation) all smooth F2 (second generation) 5474 smooth 1850 wrinkled

10. Mendel examined seven traits and always got the same results. Interpretation: The F1 always showed only one of the two parental traits, and always the same trait. It didn’t matter which plant donated the pollen The trait that “disappeared” in the F1 generation reappeared in about 25% of the F2’s So traits did not blend, but remained unchanged from one generation to another

11. Mendel’s conclusions: Traits (what we now call genes) are not always expressed. Genes that are always expressed are called dominant genes Genes that are not expressed if a dominant gene is present are called recessive genes

12. The P1 plants and F1 plants have the same appearance (smooth seeds) but have different genes: (SS or Ss) The P1 plants produce only smooth seeds, but the F1 plants produce smooth and wrinkled seeds

13. These plants have the same PHENOTYPE But have different GENOTYPES Each parent contributes the same amount of genetic information!!!

14. Symbols for dominant and recessive traits Upper case letter-dominant version of the gene Lower case letter- recessive version Smooth seeds are dominant, so S stands for smooth seeds Wrinkled seeds are recessive, so s stands for wrinkled seeds

15. Homozygous Dominant A plant that always produces smooth seeds has two S alleles. The smooth seed allele is dominant to the wrinkled seed allele SS=homozygous dominant homozygous: both alleles are the same dominant: both alleles are dominant

16. Homozygous Recessive A plant that always produces wrinkled seeds has two s alleles: ss = homozygous recessive.

17. Heterozygous The F1 plants are Ss: heterozygous (one allele from each parent)

18. Mendel’s Law of Segregation Each parent has two genes for a trait (allele: variant version of a gene) Each gamete receives one of the two genes Parent SS ss Gametes S S s s Offspring Ss

19. Principle of independent assortment Crosses involving two traits: Yellow (Y) is dominant to green (y) Smooth (S) is dominant to wrinkled (s) P1 cross: Smooth yellow (SSYY) with wrinkled green, (ssyy) F1s are all smooth and yellow (SsYy) He crossed these with each other:

20. X = all SSYY ssyy SsYy SY Sy sY sy SSYY SSYy SsYY SsYy SY Sy sY sy SSYy SSyy SsYy Ssyy SsYY SsYy ssYY ssYy SsYy Ssyy ssYy ssyy

21. Out of 16 possible combinations; 9 have at least one copy of BOTH dominant alleles (S and Y) 3 have at least one copy of S and are homozygous recessive for yy 3 have at least one copy of Y and are ss 1 has recessive for ss and yy So the s and y alleles were distributed randomly into gametes during meiosis (independent assortment)

22. We now know (and Mendel did not) that this happens because genes are on chromo- somes. Genes Chromosomes Occur in pairs (alleles) occur in pairs (homologues) Members of a gene pair Homologues separate separate during meiosis during meiosis Members of one gene pair Members of one pair assort independently of of chromosomes other gene pairs assort independently during meiosis of others during meiosis

23. These laws apply to many examples of genetic inheritance. Variations have also been observed.

24. Other effects on phenotype Incomplete dominance Multiple alleles (continuous variation) Pleiotropic effects: one gene affects other genes cystic fibrosis, sickle cell anemia, PKU Environmental effects temperature sensitivity “risk factors” Penetrance: % of population express trait Expressivity: % that express variation in trait

25. Incomplete dominance

26. Multiple alleles

27. Multiple Alleles • More than two alleles in the population • (although any organism has two) • Both alleles may be expressed: codominant • Example : ABO blood groups • Three alleles, IA, IB and I • Type O is recessive (ii) • Type A person could be IAIA or Iai • What is genotype for type B? type AB? Type O?

28. Blood types are inherited in Mendelian fashion. You can use the format AO for a type A heterozygote, AA for a type A homozygote, etc. If two type A parents have a type O child, what must their genotypes be? AO and AO Can a type O man father a type B child? If so, what is the genotype of the child? Yes; type BO Can a type AB man and a type B woman have a type A child? Yes, if the woman’s genotype is BO

29. Epistasis

30. Environmental effects on gene expression

31. Polygenic inheritance

32. Sex linkage-Morgan’s experiment White-eyed male fly was crossed with a red-eyed female fly All of the F1 (offspring had red eyes). F1 flies were crossed with each other. A 3:1 red:white ratio was observed- but all of the white-eyed flies were male. The F1 females were test-crossed with the white-eyed males What is a testcross?

33. What is a testcross? Cross the organism with a homozygous recessive organism: A- X aa What will be the result if the test organism is AA? All of the offspring will have the dominant phenotype. What will be the result if the test organism is Aa? Half of the offspring will have the dominant phenotype And half will have the recessive phenotype.

34. Back to the testcross of F1 females with white-eyed males What happened? Phenotypic ratio was 1:1:1:1 red-eyed females white-eyed females red-eyed males white-eyed males As expected Why did recessive trait “disappear” in F1 females? The eye-color trait is on the X chromosome.

35. Except for the sex chromosomes, the other (autosomal) chromosomes are homologous pairs. Genetic Information on those chromosomes is inherited as pairs of alleles (homozygous or heterozygous). Sex chromosomes: in flies and humans, females have two X chromosomes and males have one X and one Y. Implications: when two gametes fuse, if both contain X chromosomes, the offspring is female. If one gamete contains X and the other Y, the offspring is male. (independent assortment of chromosomes)

36. Y X XX XY X XX XY X A female offspring must inherit which chromosome from her father? A male offspring must inherit which chromosome from his father? So if a recessive allele is on the X chromosome, a female needs two copies to have the recessive phenotype, but a male needs only one.

37. The Parental fly cross: let W=red eyes and w=white eyes (male) white-eyed, XwY (female) red-eyed, XWXW Xw Y XWXw XW XWY XWXw XWY XW All of the F1 flies have red eyes. The females are heterozygous (“carriers”). The males have inherited the red-eye gene from their mothers.

38. The F1 cross: Males: XwY Females: XWXw XW Y XWXW XWY XW XWXw XwY Xw All of the females have red eyes. Half of the males have red eyes and half have white eyes.

39. Testcross of the F1 females (XWXw) XWXw XwY Xw Y XWXw XWY XW XwXw XwY Xw Red-eyed female:white-eyed female:red-eyed male: White-eyed male 1:1:1:1

40. In flies, white eye color is sex-linked recessive Recessive, because red eyes are dominant to white Sex-linked, because the gene is on the X chromosome

41. How can you tell if a characteristic is inherited in a X (sex)-linked recessive manner? Males with the affected X chromosome, and homozygous females, are affected. Phenotype is seen more often in males. Affected males inherit the allele from their mothers and pass it on to their daughters. Daughters of affected males are usually heterozygous and thus unaffected. Sons of heterozygous mothers have a 50% chance of inheriting the gene.

42. Some X-linked recessive traits in humans • Color blindness (red or green) • Hemophilia • Duchenne muscular dystrophy • SCID (severe combined immune deficiency syndrome

43. X-linked dominant traits • Affected males produce all affected daughters and no affected sons • A heterozygous affected female will transmit the gene to half of her children (male or female) • About twice as many females as males are affected • Few of these traits are known in humans

44. Genes on Y chromosome are passed only from father to son Mitochondrial DNA is passed from mother to all offspring. Only daughters can pass on the same DNA to their offspring. In humans, patterns of inheritance are studied with pedigree analysis

45. Pedigree analysis Used in human genetic analysis humans don’t produce enough offspring for counting analysis Pedigree chart: a diagram that shows the membership and ancestral relationships in a family Pedigree analysis: use of family history to determine how a trait is inherited; used in the study of human heredity

46. Constructing a pedigree = male = female = mating = parents and children. Parents are the upper group, children the lower. From left to right, children are shown in birth order (so the son is the youngest child). or = unaffected by the trait in question or = affected by the trait in question

47. A pedigree of three generations I. grandparents II: parents III: most recent generation you Your mother Your father

48. Inheritance of an autosomal dominant trait Each affected child has at least one affected parent. Two affected parents can have an unaffected child.

49. Inheritance of an autosomal recessive trait Two unaffected parents can have an affected child.

50. Inheritance of sex-linked recessive traits Trait is seen much more often in males. Unaffected females may be “carriers” who pass the affected X chromosome to their sons. Affected males pass the affected chromosome to daughters but not sons.