Female Drosophila flies from a true-breeding stock with purple eyes ( pr ), norm

Question

Female Drosophila flies from a true-breeding stock with purple eyes (pr), normal wings (+dp), and short feet (sf) are crossed with male flies from a true-breeding stock with red eyes (+pr), dumpy wings (dp), and normal feet (+sf). Both sexes of the F1 offspring have phenotypes +pr +dp +sf. F1 females are then crossed with males from a true-breeding stock phenotype pr dp sf and the following eight classe of progeny are counted: (+ means wild type) (I am having a hard type with figuring out the genotypes since the phenotypes are given)

pr dp sf = 36

Which allele is recessive? Are the 3 genes autosomal or sex linked and how do we know? What are the genotypes of the orignal parents? What are the genotypes of the F1 females and their male mates? What is the order of these genes on the chromosome arm & how do we know? Calculate the recombination percentages and Draw and Label the map of these 3 genes.

pr dp sf = 36

Explanation / Answer

prdp+sf / pr+dpsf (female)

Purple, normal wing, short feet

pr+dpsf+ / pr+dpsf+ (male)

Red eyes, dumpy wings, normal feet

prpr+dpdp+sfsf+ (F1 progeny)

Red eyes, normal wing, normal feet

Recessive alleles are: pr (purple eyes), dp (dumpy wings), sf (short feet)

The three genes are autosomal linked, because if it would have been sex linked then the phenotype of males would have been purple eyes, normal wing and short feet.

Genotype of original parents: pr+dp+sf+ and prdpsf

Genotypes of F1 males and females: prpr+dpdp+sfsf+

Order of Genes:

pr +dp sf =154 (parental)

+pr dp +sf = 159 (parental)

pr +dp +sf = 9 (double recombinants)

+pr dp sf  = 12 (double recombinants)

In double recombinants, gene in the middle shuffles. On comparision of parental and double recombinants, we can see sf shuffles.

So, the gene order is pr - sf - dp

Recombination frequency and map distance

Recombination frequency = number of recombinants / total number of progeny

Frequency of recombinants with single cross over between pr and sf = 51 + 46 / 500 = 0.194 = 19.4 %

Frequency of recombinants with single cross over between sf and dp = 36 + 33 / 500 = 0.138 = 14.0 %

Frequency of recombinants with double cross over = 12 + 9 / 500 = 0.042 = 4.2 %

Actual frequency of recombination between pr and sf = 19.4 + 4.2 = 23.6 %

Actual frequency of recombination between sf and dp = 14.0 + 4.2 = 18.2 %

Frequency of recombination is nothing but map distance (expressed as mU), so

pr-----sf = 23.6 mU

sf-----dp = 18.2 mU

pr-------dp = 41.8 mU = 42.0 mU

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