Video Transcript
Red–green color blindness is an
X-linked recessive condition in humans. The allele that allows humans to
see these colors correctly, uppercase B, is dominant to the allele that causes
red–green color blindness, lowercase b. A male that does not have red–green
color blindness reproduces with a heterozygous female. Which of the following Punnett
squares correctly predicts the genotypes of their offspring?
This question asks us about an
X-linked recessive condition. So let’s review how X-linked
conditions differ from normal autosomal inheritance. But first let’s remove the answer
choices so we have more room to work with.
Females have two copies of the X
chromosome: one from the biological mother and one from the biological father. On the other hand, males have one
copy of the X chromosome and one copy of the Y chromosome. When constructing a Punnett square
for X-linked conditions, we separate our sex chromosomes as we would for alleles for
a gene. So females can produce two types of
gametes. One will contain the first copy of
the X chromosome, while the second gamete will contain the other copy. Males can also produce two types of
gametes: one containing the X chromosome and the other containing the Y
chromosome.
Now let’s take another look at our
question. We are told that red–green color
blindness is an X-linked recessive condition. So the male can inherit one copy of
the allele and have this trait, while the female would need to inherit two copies of
the allele. We’re also told that those with the
dominant allele won’t be color-blind. So males with this genotype won’t
be color-blind, and females with these genotypes won’t be color-blind.
Next, we’re told that a male who is
not color-blind reproduces with a heterozygous female. You might remember that
heterozygous means the female will contain both a dominant and recessive allele for
the particular gene. So now we can list the male gametes
here and the female gametes here.
Now let’s look at the possible
combinations that these gametes can produce in the offspring. The first genotype is a female with
two dominant alleles that will not be color-blind. The second genotype is a female
that is heterozygous and not color-blind. The third genotype is a male with a
dominant allele that will not be color-blind. And the fourth genotype is a male
with a recessive allele that will be color-blind.
Therefore, the only table that
correctly indicates the genotype is given in answer choice (D).
