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Posted: December 28th, 2022

Question 1. A cross between white-eyed males and pink-eyed females

Assignment 3

Question 1. A cross between white-eyed males and pink-eyed females (pure breeding) in a particular insect species produces an F1 generation with all wild-type red eyes. In genetics, the term “pure breeding” refers to individuals that are homozygous for a particular trait. When two pure breeding individuals are crossed, the resulting offspring are called the F1 generation.

In your example, if the white-eyed males and pink-eyed females are pure breeding for their respective eye color traits, then the F1 generation will have a combination of the genes inherited from each parent. This is known as a hybrid.

If the white eye color trait is dominant and the pink eye color trait is recessive, then all of the F1 individuals will have red eyes, which is the wild-type trait. This is because the dominant trait will mask the expression of the recessive trait.

However, if the F1 individuals are then allowed to breed with each other, the resulting F2 generation will have a mix of individuals with red eyes (the wild-type), white eyes, and pink eyes. This is because the F2 generation will have a combination of both dominant and recessive traits, and the expression of these traits will depend on the specific combination of genes inherited from the parents. When these F1 flies are interbred, the F2 results obtained are shown in the Table below. Deduce how eye colour is inherited in this particular insect species and explain how you came to this conclusion. (How many genes are involved? Are they X-linked or autosomal? Are there any gene interactions?) (Hint: Examine the overall phenotypic ratio for males and females combined.) (6 marks)
Sex Eye Colour Number of Offspring
Female red 911
Male white 604
Male red 445
Female pink 292
Male pink 148
Total # offspring 2400

Question 2. Two true breeding lines of a particular insect species were isolated: black shells, long thorax and red shells, short thorax. When these two lines are crossed, the F1 offspring all have black shells and long thorax. When these F1 are crossed with red shelled, short thorax insects, the following results are obtained:
Black shell, long thorax 82
Red shell, short thorax 78
Black shell, short thorax 37
Red shell, long thorax 43
A. Deduce the map distance between the genes for shell colour and thorax length. Show your calculation. (4 marks)
B. Which assumption would you make prior to determining map distance? (1 mark)
Question 3. A scientist determines, after many years of studying fruit flies, that violet eyes (v) and green wings (g) are recessive. She generates three pure-breeding strains: one that is homozygous recessive for violet eyes only; one that is homozygous recessive for green wings only; and a third that is doubly homozygous recessive for violet eyes and green wings. The true-breeding parental strains in the first cross have violet eyes (and wild type wings) and wild type eyes with green wings. She interbreeds the first two strains and all of the F1 progeny are phenotypically wildtype so the F1 offspring would be wild type for both traits. Next, she mates wildtype F1 females to doubly homozygous recessive males. The F2 offspring produced from this testcross are shown below:
Violet eyes only 207
Violet eyes, green wings 41
Wild-type phenotype 42
Green wings only 210

a. Denoting v+ and g+ as the wildtype alleles, state the genotypes of the flies that were used in the testcross. (3 marks)
b. Are the genes for eye colour and wing colour linked? Why do you think so? (2 marks)
c. Determine the distance in map units between the genes for eye colour and wing colour. Show your calculation. (2 marks)
Question 4 A geneticist obtains two female salamanders (Mary and Jane), both of which are heterozygous for two genes of interest, P/p and Q/q. Each of these female salamanders are separately crossed with a male (Jon) which is homozygous recessive for both genes (p/p and q/q) and the results are shown below. Using appropriate diagrams, explain the difference in the results of these two crosses. (4 marks)

Mary Jane
Offspring phenotype Number Number
PQ 36 5
pq 35 6
Pq 5 35
pQ 4 36

Question 5. A set of two-point crosses among fruit flies is conducted between genes for black eyes (be), brown wings (bw), vestigial wings (vg), ebony body colour (e), and curly wings (cw). The results of these crosses are shown below. Deduce a genetic map for these 5 genes. (6 marks)
Genes in Cross Non-Recombinant Offspring Recombinant Offspring
bw, be 2224 117
bw, cw 2609 823
bw, e 3200 3200
bw, vg 5172 2379
be, cw 4614 1706
be, e 4150 4150
be, vg 2796 1434
cw, e 3116 3116
cw, vg 2102 305
vg, e 4559 4559

Question 6 (12 marks)
Provide short answers to the following:
a. Discuss the differences and at least one similarity between recombination and independent assortment. (4 marks)
b. Geneticists often assume that map distances less than 7 to 8 map units (mu) are quite accurate. Map distances that exceed this threshold significantly are assumed to be less accurate and the level of accuracy declines as map distances increase. Briefly explain this observation. (2 marks)
c. Compare and Contrast gene maps, physical maps and cytogenetic maps. (6 marks)

Question 7 (12 marks)
In fruit flies, ebony body colour (a mutation) is autosomal recessive and white eyes (another mutation) is X-linked recessive. Deduce the phenotypes and phenotypic ratios of both the F1 and F2 generations of the following:
a. A cross between a pure breeding wild-type male and a white-eyed, ebony body female. (6 marks)
b. The reciprocal cross, i.e., a pure breeding wild-type female and a white-eyed, ebony body male. (6 marks)
Question 8 (4 marks)
In testcross experiments, the frequency of recombination between genes a and b is 0.2; between b and c, 0.3; between c and d, 0.4; and between a and c, 0.5. Deduce the various possibilities for the order of these four genes on this chromosome. (4 marks)
Question 9 (4 marks)
In a short essay, no more than 500 words, discuss the ethical considerations surrounding the Human Genome Project. In your essay identify at least two ethical issues and concerns that are identified, and explain some of the proposed ways to mitigate some of the concerns. (4 marks)
Question 10 (10 marks)
An experiment is performed with Drosophila. In the parental generation, two true-breeding flies are crossed. The female parent is brown and wingless, and the male parent is black with normal wings. All of the flies in the F1 generation are brown and have normal wings. An F1 female is then crossed with a true-breeding black, wingless male, and the results of this cross are as follows: 85 brown winged flies; 728 black winged flies; 712 brown wingless flies; 75 black wingless flies.
A. State the genotypes of the flies in the P generation (assign your own allelic symbols). (2 marks)
B. State the genotype of the flies in the F1 generation. (2 marks)
C. Deduce the genetic distance between the genes for body colour and wings. (4 marks)
D. In another series of experiments, it was calculated that the recombination frequency between the gene for wing size and the gene for antenna length is 5% (i.e., the genetic distance between them is 5 m.u). List all possible recombination frequencies between the gene for body colour and the gene for antenna length. (2 marks)

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