Video Transcript
The gene that causes beards to grow
on the faces of humans is located on an autosome but is usually only expressed in
males and not females. What is this an example of? (A) A sex-limited trait, (B) a
sex-influenced trait, (C) a sex-linked trait.
To answer this question, let’s look
at a few key terms. An autosome is any chromosome that
isn’t a sex chromosome. In humans, our DNA is organized as
chromosomes, which you can see here represented in pink. The chromosomes numbered one to 22
are called autosomes. And two chromosomes, called X and
Y, are called the sex chromosomes because they determine our biological sex. We get one set of these chromosomes
from each of our biological parents. So, from our biological mother, we
get 22 autosomes and a single X chromosome. And from our biological father, we
get a second set of 22 autosomes and either an X or a Y chromosome. If two X chromosomes are inherited,
then the child is a biological female. And if an X and Y are inherited,
then the child is a biological male.
Each of these chromosomes have a
different set of genes, which you can see here represented as these blue lines. Because we get one set of
chromosomes from each of our biological parents, we get two sets of genes in total
that can be slightly different from each other that we call alleles. These genes and their alleles can
express certain traits, just like the gene that causes beards to grow in this
question. Since this question is asking us
about how traits like beard growth differ between sexes, let’s look at the three
different ways this can happen. It will also be useful to write out
some additional information to answer this question, like the type of chromosomes
that are affected, as well as any patterns for how this trait is expressed.
Sex-limited traits are due to genes
on autosomes, and the expression of these traits are limited so that normally only
one sex is affected. An example of this is with
lactation. In females, the mammary glands
mature during puberty due to hormones. Then, during pregnancy, different
genes involved in lactation become activated. In males, the mammary glands
normally don’t mature because males don’t produce the right hormones. So, even though they contain the
right genes for lactation, males don’t normally lactate.
Sex-influenced traits are due to
genes on autosomes, and their expression is variable between sexes. This is different from sex-limited
traits, where generally only one sex is affected. An example of this is with some
forms of baldness, which can affect both males and females, though it’s more common
in males. In males, the trait is dominant,
while in females, it’s recessive. So, in females, two copies of the
recessive allele are needed for baldness. This may be influenced by
testosterone levels, which are normally higher in males than in females, and is what
leads to the variability in expression that we see in this example.
Sex-linked traits are due to genes
on sex chromosomes and are generally more common in males than in females. For example, color blindness is due
to genes on the X chromosome and can occur in both males and females. But it’s more common in males. Color blindness is recessive,
meaning you need two copies of the allele in females to express this trait. If only one copy of the recessive
allele is present, then this trait is not expressed. In males, it’s a bit different
because they only have one copy of the X chromosome. And in this case, it only requires
one copy of the allele to cause color blindness. Because only one copy is needed,
this makes color blindness more common in males.
Now let’s look at this question
again and try to answer it. The gene that causes beards to grow
is located on an autosome and is only expressed in males and not females. Looking at the different options,
these features are best represented by a sex-limited trait. Therefore, the correct option is
given by answer choice (A), a sex-limited trait.
