Video Transcript
The pedigree chart provided shows
the inheritance of hemophilia within a family. What is the most likely genotype of
individual A? (A) X uppercase H X uppercase
H. (B) X uppercase H Y lowercase
h. (C) X uppercase H Y. (D) X uppercase H X lowercase
h.
To answer this question, we need to
understand how hemophilia is passed from parent to child. We can see from the key provided
that females are indicated with circles and males with squares. An affected male or female is
indicated with a filled black square or circle, respectively.
With a first overview of the
pedigree chart, we notice that only males are affected by hemophilia, while females
are carriers. This suggests that the disorder
could be inherited by a sex chromosome. But which one, the X or the Y
chromosome? We can see that the three sons of
the original father who has hemophilia are not affected by the disease. As these sons must have inherited a
Y chromosome from their father, this demonstrates that the disorder cannot be linked
to the Y chromosome.
However, we can see that the two
daughters are both carriers. These daughters would have
inherited one X chromosome from their mother and one from their father. This confirms that hemophilia is a
sex-linked recessive disorder. This means that the abnormal gene
responsible for hemophilia is carried on the X chromosome. And a male needs only one copy of
the allele to be affected, whereas a female needs two copies of the allele to be
affected.
Now we have a better idea of the
mode of inheritance of this disorder, so let’s come back to the question. We want to know what the genotype
of individual A will be the most likely. Individual A is a female with two X
chromosomes we don’t know is affected or not. Her partner is a male with an X and
Y chromosome who is unaffected, and together they have three children. Since the father is unaffected,
we’ll indicate his genotype with an uppercase H on the X chromosome.
In order to determine the genotype
of individual A, we need to infer it from the phenotypes of the children. The first child is male and not
affected by the disease. So he doesn’t have the recessive
allele on X and must have had the dominant allele. The second child is male and is
affected by hemophilia, which means he has the recessive allele on the X chromosome,
indicated with the lowercase h. The third child is a female and is
a carrier. This means that she must have one
dominant allele and one recessive allele.
There’s two possible outcomes for
the mother’s genotype, either X lowercase h X lowercase h or X uppercase H X
lowercase h. So she’s either affected or a
carrier, since we know that the father isn’t contributing the recessive allele.
Let’s look at the first possibility
by drawing out a Punnett square. Here are the possible outcomes
along with the circle and square illustrations to more easily visualize this. As we can see, if the mother’s
genotype was X lowercase h X lowercase h, then all of the male offspring would be
affected. Since we have an unaffected male
offspring, we know that this can’t be possible.
Now, let’s look at the second
possibility where the mother’s genotype is X uppercase H X lowercase h. Here are the possible outcomes for
this second cross. Each of the phenotypes in the
children are represented in the Punnett square, so this mother’s genotype is the
most likely. Therefore, the option that
represents the most likely genotype of individual A is given by answer choice (D), X
uppercase H X lowercase h.
