Hello, Bea here, and welcome back to Biology! Do you remember the last time you fell and scraped
your knee? Ouch! Or maybe you got a paper cut? But the bleeding probably stopped within a few
minutes. That's because your body knows how to make proteins that cause clots to form,
which stops the bleeding. Way back in Unit 1, we briefly talked about a disease,
hemophilia, which prevents your body from forming these blood clots. We said that
we would talk more about it during our genetics unit, and here we are!
We also said in Unit 1 that hemophilia was sometimes called the "royal disease"
because it was famously passed down from England's Queen Victoria in the 1800s to
many of her descendants, who married into royal families across Europe,
spreading the condition into Russia, Germany, and Spain.
Curiously, it was only known to affect males in the family. Nine of Queen Victoria's direct male
descendants were known to have hemophilia, and at least five died at a young age because of it.
Mild falls and accidents that would have been harmless for most ended in tragedy
for them instead. Why was it only males who were affected? We posed the question back in Unit 1,
and today we'll answer it. It has to do with two special chromosomes we'll be learning about.
And speaking of chromosomes, hopefully you also remember what a karyotype is:
the picture of all the chromosomes in an organism. The human karyotype shows that
we have 23 pairs of chromosomes, but the 23rd pair is often labeled as X or Y. And it turns
out that these chromosomes determine whether you are born male or female,
as well as other traits, like whether or not you have hemophilia.
Before we get started, let's look at our goals for this lesson. By the end,
you'll be able to explain how chromosomal inheritance determines biological sex,
describe possible effects of nondisjunction in the X and Y
chromosomes, and predict the genotype and phenotype ratios of sex-linked traits.
For expecting parents, one of the most common questions they get asked is "Are you having a
boy or a girl?" With today's technology, we can usually tell long before the baby
is born which anatomy it will have. But what actually determines this at a genetic level?
It has to do with one of those 23 pairs of chromosomes. The first 22 pairs are known
as autosomes, and they contain genes that function exactly the same for everyone.
But the 23rd pair is known as the sex chromosomes,
or allosomes, and comes in two different versions. They are often called X and Y,
simply because when they were discovered in the early 1900s by Edmund Beecher Wilson,
their shape under the microscope during replication loosely resembled those two letters.
While the X and Y chromosomes are considered to be the 23rd pair,
it is important to note that they are not homologous chromosomes like the other 22
pairs because they contain different genes. The X chromosome contains around
1,000 genes, many of which are critical for everyone. The Y chromosome is smaller,
having around 200 genes, most of which relate directly to male development. This may give you
a clue about how the combination of chromosomes will determine the biological sex of the baby.
If the baby inherited an X chromosome from both its parents, it will be born a biological female.
If it inherited an X from one parent but a Y from the other, it will be a biological male. So,
it turns out that everyone has at least one X chromosome, but biological males also have a Y
chromosome. It's important to understand that there are exceptions to this. These exceptions
include rare cases where individuals with XX chromosomes develop male anatomy. Further,
as individuals grow up, they may not identify with the sex assigned to them at birth.
Because females only have two X chromosomes, the gamete that comes from the female parent
is guaranteed to contain an X chromosome. But the chromosome in the male parent's gamete could be
either his X or his Y. We can set up a Punnett square to show how this inheritance works.
To fill in our Punnett square, we can put the parents on the top
and left. We list their genotypes as X and X/Y.
Remember that in this case, the genotypes represent entire chromosomes instead of individual
genes, but it still works the same way in the Punnett square. We fill in the boxes by combining
what we see above and to the left. Boxes with two X's represent a baby that is born female, and
boxes with X/Y represent a baby that is born male. So, what was the big reveal for our mystery baby?
This baby was born a girl! She must have gotten an X chromosome from both parents.
They're so sweet when they're sleeping.
In the last unit, we looked at possible mutations involving chromosomes. One of
the most common of these was nondisjunction, where two chromosomes fail to separate properly
in meiosis. You may remember that this can cause Down syndrome when it occurs on chromosome 21,
also known as Trisomy 21. But the allosomes, our X and Y chromosomes,
are no exception. They can sometimes experience nondisjunction as well.
Let's take the example that the two X chromosomes in a female gamete fail to
separate during anaphase of Meiosis I. The gametes formed here will either have both
X chromosomes or neither. If one of the gametes with two X's combines with a male
gamete containing a Y, the offspring will have two X chromosomes and one Y, or XXY.
The chromosome combination XXY is known as Klinefelter syndrome, and individuals
with this variation are biologically male due to the presence of the Y chromosome,
but they will most likely have a much higher risk of health problems, delayed puberty,
unusual body proportions, and infertility. Of course, other combinations can occur
due to nondisjunction as well. Non-disjunction could lead to an
offspring with three X chromosomes or only one or only a Y chromosome.
Many of these combinations could also result from nondisjunction in the gamete from the
male parent. Remember that it doesn't matter which parent the X chromosome comes from,
only the final tally of X and Y chromosomes. Any number other than 2 can cause significant
problems for our bodies. You'll learn more about these chromosomal mutations in the lesson PDF.
As part of the PDF in the previous lesson, you should have read about linked genes. Anytime
two genes are on the same chromosome, we say they are linked, as they are often inherited together.
Genes that are on the allosomes, or the X and Y chromosomes,
are said to be sex-linked traits. Many of these traits have nothing to do with
biological sex but rather control critical processes for our bodies.
For example, it just so happens that the gene responsible for making the protein that allows
our blood to clot properly when we get a cut is on the X chromosome. The dominant form of the
gene will produce the necessary protein, but genes with only the recessive mutation will not. Having
one working copy of the gene is enough. So the genotypes Big H Big H, as well as Big H little H,
will form the clotting factor and have no problem forming blood clots to stop minor bleeding.
Because females have two copies of the X chromosome, they have two chances to get
a working allele for the clotting factor. Even if one chromosome has the recessive mutation,
chances are, the other chromosome will have a working copy, and they will be just fine. However,
it will be important in a few minutes to note that this female is known as a carrier,
meaning she has a recessive allele for a condition, but she doesn't show the trait.
What trait do you think we're talking about here? Remember our story from the
beginning about Queen Victoria passing hemophilia throughout the royal family?
When a person can't produce the protein necessary to make blood clots,
it is known as hemophilia, and it is inherited as a recessive trait on the X chromosome.
We saw that females have two chances to get a working copy of the necessary gene.
But what about males? They only have one X chromosome, which they inherit from
the female parent. If this X chromosome has the recessive mutation, that's it;
they have hemophilia. No second chances because they didn't get an X chromosome from the male
parent. And remember, in our previous example, that the female ended up being a carrier? This
means that she could pass hemophilia to any of her male children, even though she
didn't have hemophilia herself. This was the case with Queen Victoria; she was a carrier.
Let's look at the inheritance of hemophilia in a Punnett square. We'll use our example
of the female who is a carrier. We'll assume that Dad has a working copy of the gene. Both
parents are able to make blood clots, as shown in the top and left of our square.
To indicate that the gene is on the X chromosome, we'll use the letter X with
a superscript of either Big H or little H for the dominant or recessive allele.
Mom is a carrier, so she is X Big H X little H. That is, X Big H and Y. He doesn't have an
allele for this gene on the Y chromosome, so we'll just put a Y. We'll fill in our boxes like always,
and let's first determine which genotypes are male and female using symbols. The XX's are female,
and the XY's are male. Do any of these boxes represent an offspring with hemophilia?
As long as they have at least one Big H, no problem. Both females have at least one Big H,
and the first male does but not the second. So, if they have a boy, there is a 50% chance
that he will have hemophilia. If they have a girl, there is a 50% chance that she will be a carrier.
For a female to have hemophilia, she would have needed to get the recessive allele from
both Mom and Dad, which means her dad must have had hemophilia, and Mom either had it as well
or was a carrier. It does happen sometimes, but it's far less common in females than in males.
You'll learn about some other X-linked traits in the lesson PDF.
During our lesson today, we looked at how the combination of X and Y chromosomes determines
if an offspring is born male or female. It's important to note that these biological terms
don't assume the vast array of identities that people can hold. We saw cases in which the usual
combination of chromosomes is disrupted due to nondisjunction, and we looked at an example of a
recessive trait, which is on the X chromosome, and why this makes it so much more likely in males.
Next time, we'll dig deeper into another topic that we broached in Unit One:
pedigrees! Remember our family trees of genetics? We'll build a big one next time. Until then,
remember that biology isn't just science; it's the way of life!