Test cross
What was the phenotypic ratio for the offspring resulting from this testcross?
Results for the F1 generation for monohybrid cross between a female fly with brown eye (BW) color with a male fly having ebony body (E) color are as shown below.
Fly lab Lab Notes for Steven F
Parents
(Female: BW) x (Male: E)
Offspring
Phenotype Number Proportion Ratio
Female: + 494 0.5067 1.027
Male: + 481 0.4933 1.000
Total 975
All the offspring had a wild-type phenotype for the brown eyes and ebony body. This shows that, both brown eye and ebony body are recessive genes. The phenotypic ratio for the offspring resulting from the test cross was brown eye: Ebony color: wild eye color: wild ebony body of 0:0:1:1.
Based on this phenotypic ratio, determine whether the F1 wild-type female male was double homozygous or double heterozygous for the eye color and body color alleles. Explain your answer.
The offspring consisted of flies with double heterozygous for the eye and body color. This is because one parent had a recessive gene for ebony color while the other one had a recessive gene for brown eye. This enabled each of the offspring to receive an allele for the two genes from the parents. With the BB being the allele for wild eye color and bb for the recessive one and EE and ee, the alleles for wild and ebony body color respectively, the F1 generation can be illustrated as in Table 1 below.
If your answer was double heterozygous, describe an expected phenotypic ratio for the offspring produced from a testcross with a homozygous fly.
A test cross between homozygous fly would result to a F1 generation with all wild phenotypic characteristics. There are no recessive alleles in the parent to result into a heterogeneous offspring.
Lethal Mutations
What phenotypic ratio did you observe in the F1 generation?
Crossing two flies with mutations for antennae shape gave a phenotypic ratio of 1:1 for wild against aristapedia.
What were the phenotypes?
The offspring had both the wild as well as aristapedia mutation phenotypes.
What results did you obtain with this cross?
However, a cross of a wild-type fly with a fly that had the aristapedia mutation gave a phenotypic ratio of 1:1 for wild against aristapedia.
Perform an F1 cross between two flies with the aristapedia phenotype. What phenotypic ratio did you observe in the F2 generation?
A cross between two flies having the aristapedia phenotype gave an F2 generation with a phenotypic ratio of wild: aristapedia of 1: 2.
Develop a hypothesis to predict the phenotypic ratio for the F1 generation. Mate these flies. What phenotypic ratio did you observe in the F1 generation?
Designing a cross between two flies with curly wing shape and stubble bristles gave a phenotypic ratio in F2 generation of wild: stubble bristles: curly wing: stubble bristles and curly wing of 1:2:2:4. A hypothesis of a phenotypic ratio of wild: stubble bristles: curly wing: stubble bristles and curly wing of 1:1:1:2 was tested and the hypothesis rejected.
Repeat this procedure for an F1 cross between two flies that express the curly wing and stubble bristle phenotypes. Are the phenotypic ratios that you observed in the F2 generation consistent with what you would expect for a lethal mutation? Why or why not? Explain your answers.
A cross of F1 generation between two flies expressing the stubble bristle and curly wing phenotypes gave the same phenotypic ratio of wild: stubble bristles: curly wing: stubble bristles and curly wing of 1:2:2:4. This is because the two set of families had the same genomic makeup for the two traits.
Sex Linkage
What phenotypes and ratios did you observe in the F1 generation?
Performing a cross between a female fly with a tan body and a wild-type male gave phenotypic ratio of 1: 1 for the wild: Tan body in the F1 generation. All the flies with tan body were males.
Based on what you know about Mendelian genetics, did the F2 generation demonstrate the phenotypic ratio that you expected? If not, what phenotypic ratio was obtained with this cross?
When the F1 flies were crossed, the F2 generation comprised of a phenotypic ratio of 1:1 for the wild: Tan color flies. These results were as expected.
Crossing a female fly with the vestigial wing size mutation and a white-eyed male gave wild flies. A cross between the F1 generation gave a ratio of 9:3:3:1 of wild: white-eye: vestigial wings: vestigial and white eyes. Examining the phenotypes and sexes of each fly, the ratio was as shown below
Is there a sex and phenotype combination that is absent or underrepresented? If so, which one?
What does this result tell you about the sex chromosome location of the white eye allele?
The sex chromosome of the white-eye allele is located in the Y chromosome.
