Chapter 17 Mendelian (Classical) Genetics Biology 30

1. Chapter 17Mendelian (Classical) GeneticsBiology 30

2. Gregor Mendel, an Austrian monk tended the monastery garden and planted pea plants to study inheritance.

3. Mendel studied seven easily identifiable traits.- stem length (T = tall, t = short)- pod shape (I = inflated, i = pinched)- seed shape (R = round, r = wrinkled)- seed colour (Y = yellow, y = green)- flower position (A = axial, a = top)- flower colour (P = purple, p = white)- pod colour (G = green, g = yellow)

4. In one of Mendel’s experiments, he crossed parental plants with different seed shapes. The ratio of plants with round seeds to plants with wrinkled seeds in the F2 generation is 5474:1850 or 2.96:1. This is very close to a 3:1 ratio.

5. Mendel’s First Law: The Law of Segregation All individuals have two copies of each factor. These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every factor.

6. Monohybrid Cross (one trait) Representation of a monohybrid cross between two true breeding parent plants. One parent plant is homozygous for round seeds (RR), and the other parent plant is homozygous for wrinkled seeds (rr). The F1 generation is heterozygous for round seeds (Rr).

7. Punnett Square

8. For each trait that he tested, Mendel observed the same types of results and inferred the same pattern. This illustration shows a cross between true breeding purple-flowered plants and true breeding white-flowered plants. The ratio of phenotypes in the F2 generation is 3:1.

9. Test Cross A test cross is performed when the genotype of an organism displaying the dominant phenotype is unknown (ie – the genotype could either be homozygous dominant OR heterozygous dominant – same phenotype. By performing a test cross and examining the offspring, it is possible to tell the phenotype of that particular offspring.

10. Lets Practice!

11. Example: • Mendel Crossed true breeding plants that had yellow pods with true breeding plants that had green pods. • Predict the F1 and F2 generations using a punnettsquare and the genotypic and phenotypic ratios

12. Example: • In one of his experiments, Mendel counted 6022 yellow seeds and 2001 green seeds. What can you tell about this data? What can you infer about the parent plants?

13. Example: • The skin of tomatoes can be smooth (S) or hairy (s). The dominant allele S, cause smooth texture and the recessive allele s, causes peachy (hairy) texture. • What would the phenotype of the offspring be in the following crosses? Show your work • Ss x SS • Ss x ss

14. Example: • White sheep are dominant over black sheep, which is recessive. A farmer has a white sheep and wants to mate it but does not want any black sheep in his offspring. He does not know the genotype of his White sheep, how will he figure this out? Show all work

15. Mendel’s Second Law: The Law of Independent Assortment The two alleles for one gene segregate (assort) independently of the alleles for other genes during gamete formation. Mendel’s Law of Independent Assortment refers to dihybrid crosses involving 2 traits – the alleles for EACH trait will separate independent of each other.

16. DIHYBRID CROSSES For any dihybrid cross, individuals in the largest group (9) have at least one dominant allele for each gene (T_G_), where the underscore represents any one of the four alleles. In the intermediate groups (3), the individuals have at least one dominant allele for one gene but two recessive alleles for the other gene (T_ggor ttG_). The smallest group (1) is homozygous recessive for both traits (ttgg).

17. Lets Practice!

18. Example: • In poultry, ‘W’ white is dominant over ‘w’ red and feathered legs, ‘F’, is dominant over bare legs ‘f’. If a Wwff individual is crossed with a wwFf individual what are the genotypes and phenotypes of the offspring. Show all of your work

19. Example • In garden peas tall (T) plants are dominant over the dwarfed, (t) variety and the purple colour for flower petals is dominant to the white petal colour. In the example below, state the genotypic and phenotypic ratios of all of the offspring in each cross. Justify your answer. • TTPp x Ttpp

20. Example • If a certain ration for hair colour and texture in rabbits shows in the F1 generation; what are the possible genotypes of the parents? Justify your answer. • 19 black rough : 6 black smooth : 7 white rough : 2 white smooth

21. INCOMPLETE DOMINANCE – both alleles come Through as a blend of the original parent alleles The allele for red flowers in the four o’clock plant directs the synthesis of red pigment. When only one allele is present, the flower cannot make enough pigment to make the flowers red, resulting in incomplete dominance (pink flowers).

22. CODOMINANCE?! • Explain the difference between incomplete dominance and codominance:

23. Example: • In some flowers like carnations, the gene for red flowers (CR) and white flowers (CW) show incomplete dominance. The heterozygous results in a pink colour. Determine the phenotypic and genotypic ratios of the following crosses: Show your work. • Red x Pink

24. Example: • One day you went into a pet store and bought two mice, male and female, and both were grey in colour. When the mice had offspring there were 3 white, 7 grey and 3 black • What are the genotypes of the offspring?

25. Example: • In some chickens, the gene for feather colour is controlled by codominance. The allele for black is FB and the allele for white is FW. The heterozygous phenotype is known as erminette. • What is the genotype for black chickens? ____ • What is the genotype for white chickens? ____ • What is the genotype for erminette chickens? ____ • If two erminette chickens were crossed, what is the probability that: (Show your work) • They would have a black chick? ____% • They would have a white chick? ____%

26. Example: • In some cattle the genes for brown hair (HB) and for white hair (HW) are co-dominant. Cattle with alleles for both brown and white hair have both brown and white hairs. This condition gives the cattle a reddish colour, and is referred to as Roan (HBHW). For each of the following construct a punnett square and give phenotypic and genotype ratios of the offspring. • a roan cow and a white bull

27. When a man and a woman are both heterozygous for the sickle cell gene, there is a one in four chance that they will have a child with sickle cell disease. The sickle cell trait also provides some protection from malaria.

28. Alleles and chromosomes both segregate during meiosis. During anaphase I, the homologous chromosomes segregate (separate) and migrate to opposite ends of the cell. After telophase I, the homologous chromosomes are in separate cells. The resulting gametes are equally likely to contain each possible combination of alleles.

29. Use the following information to answer the next question. The coat colour of Labrador retrievers is determined by two alleles. The black allele, B, is dominant to the brown allele, b. A second pair of alleles, E and e, affects the expression of the coat colour: the homozygous recessive condition, ee, prevents the expression of black or brown and produces a pup with a yellow coat. GenotypePhenotype B _ E _ Black bbE _ Brown _ _ ee Yellow. If two Labrador retrievers with the genotype BbEe were to be crossed, what phenotypic ratio would be expected in their offspring? Ratio: __________ : __________ : __________ Phenotype: Black Brown Yellow (Record all three digits of your answer in the numerical-response section on the answer sheet.)

30. Mendel’s dihybrid crosses involved traits that were on separate chromosomes, and therefore separated independently from one another (eg – green or yellow pods and tall or short stems) • Morgan studied Drosophila (fruit flies) and found different offspring ratios than what Mendel documented – therefore certain traits were “linked” – or traveled together on the same chromosome • The closer the genes are on the chromosome, the greater the chance they will stay together during meiosis – and thus NOT assort independently

31. Crossing Over Thomas Morgan – fruit flies! Won a nobel prize proving that genes on chromosomes exist at specific sites and are arranged in a linear manner Showed that the further from the centromere a gene was the more likely it would cross over. Genes near the centromere were least likely to recombine Chromosomes were mapped for their genes and their frequencies of cross over were recorded His work

32. Making a Chromosome Map Relative position of genes are tracked and marked using chromosome maps. One map unit is defined as the distance between two areas on a chromosome where cross over is likely to occur. The map distance is the distance between the genes on a chromosome

33. Recombinant Types Page 600 Recombinant types are resulting offspring that look nothing like the P generation Meaning: The genes of the P generation must have crossed over or recombine to produce the F1 phenotypes

34. Recombination Frequency • The number of recombinant offspring (ones that do not • resemble either of the parents) is directly proportional to thedistanceof the genes on the chromosome • The greater the number of recombinants, the greater the distance between genes (translated into map units on the chromosome)

35. Mapping chromosomes: Genes are arranged linearly on the chromosome at specific gene loci (positions)

36. CHROMOSOME MAPPING Use the following information to answer the next two questions. The cross-over frequencies of four genes found on human chromosome 6 are shown below. Legend Gene Pairs Cross-over Frequencies 1 Diabetes mellitus 1 and 2 21% 2 Ovarian cancer 1 and 3 12% 3 Rh blood group 2 and 3 9% 4 Ragweed sensitivity 2 and 4 19.5% 1. The order in which the four genes listed in the legend above are located on chromosome 6 is _____, _____, _____, and _____. (Record your four-digit answer in the numerical-response section on the answer sheet.) 2. What is the approximate cross-over frequency between the diabetes mellitus gene and the ragweed sensitivity gene?

37. Cross of Fruit Flies Some traits are carried on the X chromosome

38. An allele for sex-linked colour blindness is passed to the next generation. Males can pass the X-linked recessive trait only to their daughters (A). Females who are heterozygous for the condition have a 50% chance of passing the recessive allele to a child (B).

39. A single gene is responsible for each of the 22 patterns that can be expressed in clover leaves.

40. Multiple Alleles and Blood Types

41. Distribution of ear length in corn, a polygenic trait. The Punnett square (A) shows a cross between corn with medium-length ears (AaBb). The resulting phenotype ratio of 1:4:6:4:1 is graphed in (B). In graph (C), three genes control ear length, resulting in a more gradual-length distribution curve.

42. Pedigree Symbols

43. Hemophilia (X-linked Recessive) in the European Families

44. Y-Linked Inheritance • We will now look at how various kinds of traits are inherited from a pedigree point of view. • Traits on the Y chromosome are only found in males, never in females. • The father’s traits are passed to all sons. • Dominance is irrelevant: there is only 1 copy of each Y-linked gene (hemizygous).

45. Sex-Linked Dominant • Mothers pass their X’s to both sons and daughters • Fathers pass their X to daughters only. • Normal outsider rule for dominant pedigrees for females, but for sex-linked traits remember that males are heterozygous and express whichever gene is on their X. • XD = dominant mutant allele • Xd = recessive normal allele

46. Sex-Linked Recessive • males get their X from their mother • fathers pass their X to daughters only • females express it only if they get a copy from both parents. • expressed in males if present • recessive in females • Outsider rule for recessives (only affects females in sex-linked situations): normal outsiders are assumed to be homozygous.

47. Autosomal Dominant • Assume affected outsiders are assumed to be heterozygotes. • All unaffected individuals are homozygous for the normal recessive allele.

48. Autosomal Recessive • All affected are homozygotes. • Unaffected outsiders are assumed to be homozygous normal • Consanguineous matings are often (but not always) involved.