Hi,
I'm just trying to learn gene mapping and I'm a bit confused as to why the answer this question is D. I just don't get it, and I'd really appreciate it if someone could help work it through so I'd understand how they got that answer to be wrong and the other one's to be right.
Esmé has her Bio199 project planned. She will determine the map distance between the
tubby and vestigial genes in Drosophila.
tb+ normal, long body
tb tubby, short body
vg+ big wings
vg small wings
She began by crossing true-breeding, homozygous parents and picking an F1 female from
this cross to testcross.
Esmé generated the following table of the progeny from the testcross:
Observed phenotype number
normal, long body and big wings 283
normal, long body and small wings 1294
tubby, short body and big wings 1418
tubby, short body and small wings 241
Which of the following statements is not true about this experiment?
A. The recessive alleles of the F1 heterozygous female were in trans.
B. The F1 heterozygous female genotype was tb+ vg/tb vg+.
C. The frequency of recombination was 16%.
D. vg is 19 map units from tb.
E. All of the statements are true.
Thanks in advance.
To determine the answer to this question, we need to understand the concept of gene mapping and how it is applied in this experiment.
In this experiment, Esmé is studying the map distance between the tubby (tb) and vestigial (vg) genes in Drosophila (fruit flies). She begins by crossing true-breeding, homozygous parents and selects an F1 female from this cross for a testcross. The F1 female is heterozygous for both genes.
The progeny from the testcross are observed and the following phenotypes and numbers are recorded:
- normal, long body and big wings: 283
- normal, long body and small wings: 1294
- tubby, short body and big wings: 1418
- tubby, short body and small wings: 241
Now, let's analyze each statement to determine which one is not true:
A. The recessive alleles of the F1 heterozygous female were in trans.
This statement refers to the arrangement of the alleles on the homologous chromosomes. If the recessive alleles of tb and vg were in trans, it means one chromosome carries both recessive alleles while the other chromosome carries both dominant alleles. This statement is true because the F1 female genotype is tb+ vg/tb vg+.
B. The F1 heterozygous female genotype was tb+ vg/tb vg+.
This statement describes the genotype of the F1 female. It indicates that the F1 female carries one dominant allele (tb+) and one recessive allele (vg) for each gene. This statement is true.
C. The frequency of recombination was 16%.
The frequency of recombination is determined by the number of recombinant offspring divided by the total number of offspring. In this case, the recombinant offspring can be either normal, long body and small wings or tubby, short body and big wings. Adding up the numbers, we have 1294 + 1418 = 2712 recombinant offspring. Dividing this by the total number of offspring (283 + 1294 + 1418 + 241 = 3236) gives us a value of approximately 83.8%, which is not 16%. Therefore, this statement is not true.
D. vg is 19 map units from tb.
Map units (also known as centimorgans) represent the distance between genes on a chromosome. To calculate map units, we use the formula: % recombination = map distance. Since the frequency of recombination in this experiment is approximately 83.8%, the map distance between vg and tb is around 83.8 map units, not 19. Therefore, this statement is not true.
E. All of the statements are true.
Since we have already determined that statements C and D are not true, it means that not all of the statements are true. Therefore, the correct answer is D. vg is 19 map units from tb.
To summarize, the statement that is not true about this experiment is that vg is 19 map units from tb. The correct map distance is approximately 83.8 map units based on the observed recombinant frequencies.