In Drosophila melanogaster, a gene \"ebony\" produces a dark body colour when ho
ID: 18475 • Letter: I
Question
In Drosophila melanogaster, a gene "ebony" produces a dark body colour when homozygous, and an independently assorting gene "black" has a similar effect (that is the black and ebony body colours are indistinguishable).
a. What is the colour of the F1 progeny from a cross between homozygous ebony and homozygous black?
b. What phenotypic proportions are expected in the progeny of an F1 x F1 cross?
c. What phenotypic ratios are expected among progeny of a cross between F1 and the ebony parental type?
Explanation / Answer
a: Monohybrid Cross is a method of determining the inheritance pattern of a trait between two single organisms.[1] A monohybrid cross is a cross between parents who are true-breeding for a trait; i.e., both are homozygous for one allele of the gene, for example AA x aa, in which A is the dominant allele for a trait and a is the recessive allele for that same trait. The cross between their offspring is referred to as a dihybrid cross, in which parents are both heterozygous at one locus.
b: Since the F1 generation all had gray eyes and long tails, then gray is dominant to pink eyes and long is dominant to short tails. The statement "pure-breeding" means homozygous. So, if the allele for gray is P and pink is p and the allele for long is S and short is s, then the gray eyed mouse with a long tail is PPSS and the pink-eyed mouse with the short tail is ppss, so the Parental (P) cross is:
gray eyes, long tails PPSS x ppss pink eyes, short tails
The F1 is:
PpSs, all gray eyes and long tails
Cross the F1s to get an F2 generation:
PpSs x PpSs
Note that this is a dihybrid cross (both organisms are heterozygotes at the same two genes), so the expected phenotype ratio in the F2 is:
9 P-S-, gray eyes, long tail
3 ppS-, pink eyes, long tail
3 P-ss, gray eyes, short tail
1 ppss, pink eyes, short tail
and this is approximately the ratios you see (88:27:31:10).
The complete genotypes from the dihybrid cross are:
PpSs x PpSs
PPSS, gray eyes, long tail
PPSs, gray eyes, long tail
PPSs, gray eyes, long tail
PPss, gray eyes, short tail
PpSS, gray eyes, long tail
PpSs, gray eyes, long tail
PpSs, gray eyes, long tail
Ppss, gray eyes, short tail
PpSS, gray eyes, long tail
PpSs, gray eyes, long tail
PpSs, gray eyes, long tail
Ppss, gray eyes, short tail
ppSS, pink eyes, long tail
ppSs, pink eyes, long tail
ppSs, pink eyes, long tail
ppss, pink eyes, short tail
As for question a), a monohybrid cross is a cross between two organisms, both of which are heterozygous at the gene of interest:
Aa x Aa
gives a 3:1 phenotypic ratio, assuming simple dominance and recessiveness:
AA, dominant phenotype
Aa, dominant phenotype
Aa, dominant phenotype
aa, recessive phenotype
c: Based on these findings, these scientists hypothesized that certain alleles of one gene were somehow coupled with certain alleles of another gene; however, they were not sure how this could occur. This phenomenon is now known as genetic linkage, and it generally describes an inheritance pattern in which two genes located in close proximity to each other on the same chromosome have a biased association between their alleles. This, in turn, causes these alleles to be inherited together instead of assorting independently. Genetic linkage is a violation of the Mendelian principle of independent assortment
i hope it helps you dear :)))