Classical Genetics

1. Classical Genetics

2. Gregor Mendel: Father of Genetics

6. Law of Segregation: alleles separate during the formation of sex cells. Sex cells contain only one member of the pair of alleles

7. If genes are located on separate chromosomes, they are inherited independently of each other

8. Law of Dominance: when an organism carries alleles for two contrasting traits, only the dominant trait is shown

14. How could one determine whether an individual is homozygous dominant AA or heterozygous dominant Aa? Answer: Perform a Test Cross This is done by crossing it with an individual that is homozygous recessive for the same trait, then looking at the offspring

15. MendAlien Genetics • http://www.phschool.com/science/biology_place/biocoach/inheritance/genxfile.html • Try some crosses and see each concept we covered explained.

17. Incomplete Dominance

18. Incomplete Dominance When two alleles are equally dominant, they interact to produce a new phenotype that is an intermediate between the two alleles. Example: RED SNAPDRAGON X WHITE SNAPDRAGON → PINK SNAPDRAGON

19. Incomplete Dominance • A capital letter represents one incomplete dominance allele • A different capital letter represents the other incomplete dominant allele

20. Incomplete Dominance

21. Incomplete Dominance

22. Note: Nelson Textbook uses: CR CR= red CW CW =white CR CW=pink See Figure 1 on page 144

23. Incomplete Dominance • In humans, curly hair (HH) is incompletely dominant to straight hair (H’H’).  The heterozygous individual has wavy hair (HH’).

24. Codominance • In codominance, both alleles are expressed independently and are uniquely recognizable. • Example: • Red flower X white flower→ red and white spotted flowers

25. Codominance

26. Codominance • A capital letter represents one of the codominant alleles • A different letter represents the other codominant allele Note: Nelson Textbook uses: • CR CR= red • CW CW =white • CR =red and white spots • See Figure 2 on page 145 ·   

27. Codominance

28. This roan horse is heterozygous for coat color. Its fur appears grey because it has a mixture of black and white hairs, not because the individual hair colors are grey.

29. Roan Cow

30. In horses, gray horses (GG) are codominant to white horses (WW).  The heterozygous horses(GW) is an appaloosa horse (a white horse with gray spots on the rump and loins).

31. Codominance: Tabby Pattern

33. Dihybrid Crosses • Mendel also studied the inheritance of two separate traits in crossbreeding following the same procedures he had used for studying single traits. • Dihybrid Cross: a type of cross that involves two genes, each consisting of non-identical alleles.

34. Dihybrid Cross • Mendel proposed that alleles of different genes are assorted independently of one another during the formation of gametes. • This proved to be true. • This is called the Law of Independent Assortment

35. Dihybrid Cross Punnett Square • in a dihybrid cross question, all 4 possible gametes for one parent are placed along the top of the Punnett Square and all 4 gametes for the other parent are placed along the side. • Example: Heterozygous round, yellow pea plants RrYy: • 4 gametes for RrYy are: • RY, Ry, rY, ry

36. RrYy x RrYy

38. P Generation = RRYY x rryy ↓ F 1 Generation =RrYy ↓ F 2 Generation = 9/16 yellow, round 3/16 yellow wrinkled 3/16 green, round 1/16 green wrinkled

39. 9:3:3:1 Phenotype Ratio • A phenotype ratio of 9:3:3:1 occurs in the offspring of a mating of two organisms heterozygous for two traits.

43. Eye Colour: Dihybrid Cross BbGg x BbGg

44. F.Y.I. Eye Colour Details • At one locus (site=gene) there are two different alleles segregating: the B allele confers brown eye color and the recessive b allele gives rise to blue eye color. At the other locus (gene) there are also two alleles: G for green or hazel eyes and g for lighter colored eyes.The B allele will always make brown eyes regardless of what allele is present at the other locus. In other words, B is dominant over G. In order to have true blue eyes your genotype must be bbgg. If you are homozygous for the B alleles, your eyes will be darker than if you are heterozygous and if you are homozygous for the G allele, in the absence of B, then your eyes will be darker (more hazel) that if you have one one G allele.

45. Sex-Linked Traits

46. Sex-Linked Traits • Sex-linked genes are located on one of the sex chromosomes (X or Y) but not the other. • Since, typically the X chromosome is longer, it bears a lot of genes not found on the Y chromosome, thus most sex-linked genes are X-linked genes.

47. Morgan’s Experiments • In 1910, Thomas Hunt Morgan discovered that eye colour in fruit flies was sex-linked. • Morgan's results is due to the fact that the gene for eye colour in Drosophila is located on the X chromosome. • White eye colour is predominantly found in male flies.

48. Sex-Linked Traits • Morgan’s work on sex-linkage in fruit flies has been called one of the most important events in genetics. • His work with Drosophila proved true the theory that Mendel's "traits" are found on chromosomes.

49. Why are recessive sex-linked traits found more often in males? • Since females have two copies of the X chromosome, a female must inherit two defective genes. • Since a male has only one X chromosome, if it is defective, he will have the disorder. XcY • A female who appears normal but has one defective chromosome is called a carrier. XCXc

50. Carriers • X-linked traits are maternally inherited from carrier mothers. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele. • There are a few Y-linked traits; these are inherited from the father.