Genes and Heredity

1. Genes and Heredity Classical Genetics

2. Heredity • Genetics is the study of heredity. • Heredity – the passing of traits from parents to offspring • Heredity ensures that you have characteristics similar to your parents (but not exact copy) • Genes – units of instruction (located on chromosomes) that produce or influence a specific trait in the offspring (ie. Eye color) • Genome – a cell’s total hereditary endowment of DNA

3. Biological traits are controlled by genes of both parents but are unique to individual • ~8,000,000 combinations are possible between 23 chromosomes from each parent • Alleles – two or more alternate forms of a gene like brown hair vs. blonde hair • Gene locus – location of gene on a chromosome

4. Alleles Gene Locus

5. History Scientists noticed traits were passed on, but how? • Aristotle – suggested through blood (bloodlines) • Early Naturalists – believed in hybrids where species result from breeding between other species • Buffon (1700’s) – suggested head and limbs came from father, the rest from the mother • 1800’s – a blending of traits from both parents • Very late 1800’s – microscopes allowed for meiosis to be observed, there was speculation about chromosomal involvement

6. Gregor Mendel • Austrian Monk • 1822 – 1884 • Worked with garden peas to explain gene inheritance for plants and heredity • “Father of Genetics”

7. Why Peas??? • Have number of visible traits that are explainable in one of two ways • Ex) green vs. yellow peas • Ex) tall vs. short plants • Ex) wrinkled vs. smooth peas

8. 2) Garden peas are self-fertilizing and cross-fertilizing • Self-fertilization (Pollination) – pollen produced from male stamens attaches to female pistil inside same flower. • Cross-fertilization – pollen from male stamen attaches to female pistil inside another flower. • Mendel used cross-fertilization (removed stamen and transferred pollen from one plant to another plants pistil) • He combined male and female sex cells of different plants

9. Mendel’s experiments involved the crossing of pea plants with different traits to see what traits the offspring would have

11. Mendel’s Thoughts… • Why did the F1 generation produce all round seeds? • Why did the F2 generation produce more round that wrinkled seeds? • Does it make a difference if the characteristic came from the male or female plant? • In order to answer these questions, Mendel repeated the procedure with other characteristics

12. Mendel’s Discoveries • Discovered that one trait dominates another (no matter what characteristic came from the male or female sex cell) • Mendel established the general principles of genetics (later to become his laws of Inheritance)

13. Mendel’s Laws of Inheritance • Law of Unit Characteristics: traits are controlled by factors called genes which occur in pairs (one from each parent). Ex) RR, Rr, rr • Law of Dominance: the dominant form of a trait prevents or masks the expression of the recessive form. The dominant form is represented by capital letters (R) and the recessive form by lower case letters (r)

14. Law of Segregation: during gamete formation (meiosis), homologous pairs separate and gametes have one of each homologue • Law of Independent Assortment: genes which are located on separate chromosomes assort (separate) independently during meiosis. One gene does not effect where any other gene will move. In other words, all dominant genes do not end up in the same gamete.

15. NOTE: Just because a trait is dominant, it does not necessarily mean the majority of a population will express that phenotype • Most Dutch people are blue-eyed and blonde which are both recessive traits. • Dominant genes – determine the expression of the genetic trait in offspring (capital letter) • Recessive genes – overruled by dominant genes (lower case letter) *Normally the letters of genes follow the dominant gene name (capital) and will stay the same for recessive genes (lower case)

16. Genetic Terminology • Phenotype: the observed or displayed form of the genes carried by an individual (what you see); results from interaction between genes and the environment. • Ex. Round seeds or wrinkled seeds • Genotype – the combination of the genes that are carried on the chromosomes for a given trait • Ex. Round seeds = RR, Rr • Wrinkled Seeds = rr

18. Heterozygous – refers to the genotype in which the gene pair are different. Often called a hybrid or carrier. They would express the dominant phenotype • Ex. Rr = Round • Homozygous – refers to the genotype in which both genes are identical. Often called pure bred. • Ex. Homozygous dominant = RR • Ex. Homozygous recessive = rr

19. RATIOS • In terms of possible offspring phenotypes and genotypes, you may find it easier to write them as a ratio • Genotypic ratio: homo dominant : heterozygous : homo recessive • Ex. 9 RR, 3Rr, 1 rr  9:3:1 • Phenotypic ratio: Dominant: Recessive • Ex. 3 Round, 1 wrinkled  3:1

20. Monohybrid Crosses • Genetics problems are sometimes complex, so we simplify them using punnet squares • In a punnet square, the gametes from one parent are arranged along one side of the square and the gametes of the other parent are arranged along a second side • With this, you can derive all possible combinations of gametes • Monohybrid crosses (cross of a single trait) are the easiest to study

21. Ex) a plant heterozygous for seed shape is crossed with another plant which is also heterozygous Parents: Rr x Rr Possible gametes: R r R r Punnet Square

22. Genotypic Ratio: RR: Rr: rr = 1:2:1 • Phenotypic Ratio: Round:wrinkled = 3:1 • Thus, there is a 3 in 4 or 75% chance of producing a round seed plant when two heterozygous plants are crossed (or a 1 in 4 chance of producing a wrinkled seed plant)

23. Practice Examples • Using the trait for seed shape (round dominant to wrinkled), determine the phenotypic and genotypic ratios for the following crosses: • Homozygous dominant x homozygous dominant • Homozygous dominant x heterozygous

24. Practice Examples • Homozygous dominant x homozygous recessive • Heterozygous x homozygous recessive • Homozygous recessive x homozygous recessive

25. 2) Determine the phenotypic and genotypic ratios for a cross between a homozygous dominant red flower and a recessive white flower

26. 3) Determine the phenotypic and genotypic ratios for a cross between a man who is heterozygous for a skin pigment and an albino woman (no skin pigment is a recessive trait)

27. 4) Determine the phenotypic and genotypic ratios for a cross between two plants which are heterozygous for height (tall is dominant).

28. 5) Determine the phenotypic and genotypic ratios for both the F1 and F2 generations for a cross between a homozygous tall plant and a short plant

29. 6) Determine the phenotypic and genotypic ratios for a cross between a heterozygous hornless cow and a horned bull. What are the chances of producing a horned offspring?

30. Test Cross • Performed to determine the genotype of a dominant phenotype (by looking at them we cannot tell if they are homozygous or heterozygous) • Test cross is always performed between the unknown genotype and a homozygous recessive genotype

31. If the unknown is heterozygous, half of the offspring will be dominant and half will be recessive • If the unknown is homozygous dominant, all the offspring will be dominant

32. Ex. A farmer wants to know if his white sheep is pure bred or a hybrid so he crosses it with a black sheep White sheep = W ___ Black sheep = ww

33. Multiple Alleles • In Mendel’s experiments there were only two possible alleles and the dominant allele controlled the trait. • However, it is common for there to be more than two alleles for a particular trait • This is called multiple alleles

34. Ex) Fruit Flies (drosophila) can have eyes colored red, apricot, honey, and white (but it is only possible to have two of these different genes at one time) • There is a dominance hierarchy • Red is dominant to apricot, is dominant to honey, is dominant to white • We don’t use capital and lower-case letters. Instead we use capital letters with superscript numbers to express different genes and their combinations

35. E1 – Red • E2 – Apricot • E3 – Honey • E4 – White • Determine the phenotypic ratios from a cross between a fly with wild-type eye color (E1E4) to one with apricot eyes (E2E3) • Note: Wild-type refers to the ‘normal’ allele (the one found in majority of wild populations (not necessarily the most dominant though)

36. Incomplete Dominance • Sometimes hybrids have a blending of traits (not found by Mendel) • When two traits are equally dominant, they interact to produce a new phenotype – this is known as incomplete dominance • Two types of incomplete dominance: • Intermediate inheritance or non-dominance • Codominance

37. Intermediate inheritance • Results when neither allele is dominant • Hybrid phenotype is a blend or mixture of two phenotypes • Ex) Snapdragons – Red x White = Pink • This is also notated differently (with superscripts) FRFR x FWFW = FRFW

38. Determine the F1 phenotypic ratio from the crossing of two pink snapdragons

39. Codominance • Results when both alleles are dominant • Hybrid expresses both phenotypes • Ex) Shorthorn cattle = Red bull x White Cow = Roan Calf • Roan calf has intermingled white and red hair • Both genes expressed at same time

40. Also expressed in notation with superscript • HRHR x HWHW = HRHW • If a roan shorthorn cow is crossed with a white shorthorn bull, what is the probability that the offspring will be roan?

41. Blood Types • Most familiar example of multiple alleles and codominance in humans is blood type • Most common typing system divides individuals into four classes: A, B, AB, and O • There are three alleles for this: IA IB i

42. IA and IB are codominant • i is recessive to both of these

43. Phenotype A B AB O Genotype IAIA, IAi IBIB, IBi IAIB ii

44. If one parent has blood type A and the other has blood type B, determine the parental genotypes if the offspring had the following blood types: A) ½ AB, ½ B B) all AB

45. If one parent has blood type A and the other has blood type B, determine the parental genotypes if the offspring had the following blood types: C) ½ AB, ½ A D) ¼ A, ¼ B, ¼ AB, ¼ O

46. Lethal Alleles • When the expression or phenotype of a gene causes death, it is called a lethal allele • Lethal genes are usually recessive or mutant genes with a low frequency of occurrences in the population • Occasionally a fully dominant lethal allele can form

47. Dihybrid Crosses • What’s the probability of producing a yellow if two heterozygous plants (Yy) are crossed? • What’s the probability of producing a round seed if two heterozygous plants (Rr) are crossed? • So what’s the probability of producing a yellow, round seeded pea plant if two plants heterozygous for both traits are crossed?

48. A cross that deals with two different traits is called a dihybrid cross. • Dihybrid crosses can be solved with a larger 16 square punnet square • Mendel’s law of Independent Assortment was created from his study of dihybrid crosses – this law states that genes assort independently (one gene does not influence the inheritance of another when located on different chromosomes)

49. Mendel cross-pollinated pure-breeding plants that produced yellow, round seed coats with pure-breeding plants that produced green, wrinkled seed coats • Pure breeding round coat = RR • Pure breeding wrinkled coat = rr • Pure yellow = YY • Pure green = yy

50. Genotypes: • Pure yellow, round parent = YYRR • Pure green, wrinkled parent = yyrr • The F1 offspring produced from this cross are heterozygous for both yellow and round genotypes (YyRr) • In a dihybrid cross, there are 4 different possible phenotypes