Question

You are studying development in a new species offruit fly, Drosophila mitensis, and you find a fly that has curly wings as seen here:

                                                           

                                                                          

                                                                                                        

                                                                                                         Curly Wings                   Normal Wings

The Curly wings phenotype is due to a mutation at a single locus. Assume that you have flies that are true- breeding for the curly wing phenotype (cur^-cur^-) and flies that are true-breeding for normal wings (cur⁺cur⁺).

·Propose a genetic cross to determine whether the mutant phenotype is dominant or recessive, and list the genotype(s) of the F1 offspring you would obtain from this cross. Use cur^- for the allele associated with curly wings and cur⁺ for the allele associated with normal wings.

·What phenotype(s) would you expect to see in the F1 generation if the mutant phenotype is dominant? In what proportion would you see these phenotypes?

·What phenotype(s) would you expect to see in the F1 generation if the mutant phenotype is recessive? In what proportion would you see these phenotypes?

You perform the experiment and find that the curly wing phenotype is recessive. A friend of yours has been studying a mutation that causes short antenna. The short antenna phenotype is recessive to the wild-type long antenna phenotype. You designate that mutant allele (ant^-) and the wild-type allele (ant⁺)

You set up the following crosses:

Curly wings, long antenna                                 Normal wings, short antenna

Cross 1:          cur^-cur^-ant⁺ant⁺Xcur⁺cur⁺ant^-ant^-

↓

F1:                              Normal wings,Long antenna

↓

Cross 2:       Normal wings, long antenna        X                     Curly wings, short antenna

                        (F1 from cross 1)

What type of cross is cross 2?

If the two genes are unlinked…

What phenotypic classes do you expect to see from cross 2 and in what ratios?

You perform cross 2, examine 1000 progeny and see that the genes are linked. In the table below, give the genotypes and phenotypes for the parental and recombinant progeny.

Please note that you must use the following nomenclature. Use cur^- and cur⁺ for the alleles of the wing gene. Use ant^- and ant⁺ for the alleles of the antennae gene.

	Number of progeny	Genotype	Phenotype
Nonrecombinant	431 + 429

Recombinant	69 + 71

What is the recombination frequency between the wing and the antennae genes?

Answer 1

In order to find out whether the mutant phenotype is dominant or recessive, we need to cross the homozygous mutant genotype with the homozygous wild-type genotype.

If the mutant phenotype is dominant then in F1. We will get all the progenies will have the mutant phenotype.

If the mutant phenotype is recessive then in F1. We will get all the progenies will have the wild-type phenotype.

cur-cur-ant+ant+Xcur+cur+ant-ant-

Cross 2 - It is the type of test cross where we need to find out the genotype of an F1 parent. It helps us to establish whether the F1 parent is homozygous dominant or heterozygous dominant.

If the genes are not linked then we will get following progeny

ant+cur+cur-ant-

cur-ant+cur-ant-

cur+ant- cur-ant-

cur-ant- cur-ant-

All the progeny will be in 1:1:1:1 ratio.

​

	Number of progeny	Genotype	Phenotype
Nonrecombinant	431 + 429	ant+cur+cur-ant-	Wild-type for wing wild-type for antenna
		cur-ant- cur-ant-	Curly wing and mutant antenna

Recombinant	69 + 71	cur-ant+cur-ant-	wild-type for antenna Curly wing
		cur+ant- cur-ant-	Wild-type for wing mutant antenna

Recombination frequency is given by number of recombinant/ total number of progeny

number of recombinants = 140

total number of progeny = 1000

140/1000

0.14

recombination frequency between the wing and the antennae genes is 0.14