Let’s suppose that you have made a karyotype of a female fruit fly
with red eyes and found that it has three X chromosomes instead
of the normal two. Although you do not know its parents, you do
know that this fly came from a mixed culture of flies in which
some had red eyes, some had white eyes, and some had eosin eyes.
Eosin is an allele of the same gene that has white and red alleles.
Eosin is a pale orange color. The red allele is dominant and the
white allele is recessive. The expression of the eosin allele, however, depends on the number of copies of the allele. When females
have two copies of this allele, they have eosin eyes. When females
are heterozygous for the eosin allele and the white allele, they have
light-eosin eyes. When females are heterozygous for the red allele
and the eosin allele, they have red eyes. Males that have a single
copy of the eosin allele have eosin eyes.
You cross the XXX red-eyed female with a white-eyed male and
count the numbers of offspring. You may assume that this unusual female makes half of its gametes with one X chromosome and half
of its gametes with two X chroExplain the 3:1 ratio between female and male offspring. What is
the genotype of the original mother, which had red eyes and three
X chromosomes? Construct a Punnett square that is consistent
with these datamosomes. The following results for
the offspring were obtained:
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