Question

1. Imagine that you as a fruit fly researcher cross true breeding, wild-type flies to true breeding, eyeless flies in the P generation. The resulting F1 generation is made up of 100% wild-type flies with normal eyes. Based on Mendelian genetics, what phenotype ratio would you expect to see for a F1 monohybrid cross? How would you go about determining the genotype ratio in the F2 generation?
2. In cactus, the relationship between Gene S and Gene N is known to be epistatic. Gene S controls the sharpness of spines in a type of cactus. Cacti with the dominant allele, S, have sharp spines, whereas homozygous recessive ss cacti have dull spines. At the same time, the second gene, N, determines whether or not cacti have spines. Homozygous recessive nn cacti have no spines at all. Briefly explain why this genetic interaction is considered epistasis. Which gene is considered epistatic to the other?
3. In Drosophila melanogaster, vestigial wings are determined by a recessive allele of a gene that is linked to a gene with a recessive allele that determines black body color. T. H. Morgan crossed black-bodied, normal-winged females and gray-bodied, vestigial-winged males. The F1 were all gray bodied, normal winged. The F1 females were crossed to homozygous recessive males to produce testcross progeny. Morgan calculated the map distance to be 17 map units. Which of the phenotypes used in the experiment would be considered parental? Which phenotypes would be recombinants? What would be the percentages of the resulting phenotypes of the testcross progeny?
4. The most commonly occurring mutation in people with cystic fibrosis is a deletion of a single codon. What is the result of this type of mutation at the protein level? Which class of mutation would have an even more severe effect on the protein?

Please explain!

Answer 1

1) A monohybrid cross for is performed between a Normal eyed Wild type (AA) and true breeding eyeless flies (aa).

Parental cross : AA (Wild Type) x aa (Eyeless)

Gametes: A A     a a

F1 Progeny

Male/Female	A	A
a	Aa	Aa
a	Aa	Aa

Genotype: All are heterozygotes

Phenotype: All all Normal Eyed

2) In a cross between two F1 generations.

F2 cross : F1 x F1

Gametes: A a A a

F2 progeny:

Male/Female	A	a
A	AA	Aa
a	Aa	aa

Genotype: 1AA : 2Aa : 1aa

Phenotype: 3 Normal eyed : 1 eyeless

3) The Following conclusion can be made from the question

S_ - Sharp Spines

ss - Dull spines

N_ - Spines

nn- No spines

The Phenomenon observed here is know and Recessive epistasis (9:3:4). Recessive epistasis occurs when the recessive allele at one loci or gene masks or modifies the effect alleles at other loci or gene. Here the function of the gene products of both the alleles occur in series, that is a precursor molecule X is converted to an intermediate precursor A by S_ genotype and this intermediate A precursor is converted to final phenotype by N_ genotype.

4) The recessive allele ss is epistatic over the expression of N allele.