Captions are on! Click CC at bottom right to turn off. Would you like to know one of the most underappreciated
pieces of cytoplasm out there? Platelets. We take for granted the function of our platelets,
which are fragments of cytoplasm that help stop us from bleeding. They help our blood to clot when we get hurt. But there is a disorder called hemophilia
that can affect those platelets and even a basic cut could be dangerous for them because
they could bleed continuously. We have many treatments for the symptoms of
Hemophilia now that have greatly improved outcomes with this disorder. Although it wasn’t always that way. Hemophilia is a sex-linked, recessive trait
which means it is different from basic Mendelian genetic problems. We still use the terms dominant and recessive
for alleles—but this time—those alleles are on sex chromosomes. This is the case with sex-linked traits. What is a sex chromosome? Recall that humans have 46 chromosomes. Chromosomes are made up of DNA and protein. They contain your genes. Well two of your 46 chromosomes -they are
called the sex chromosomes. In a karyotype, it is usually the last two
chromosomes that are the sex chromosomes. The sex chromosomes are called X and Y chromosomes
but it has nothing to do with the shape of the chromosome. That’s kind of confusing but please don’t
think that Y chromosomes are shaped like a Y and X chromosomes are shaped like a X. That used to always confuse me—that has
nothing to do with their name. The reason they got those names is actually
kind of interesting so to the Google for that. Everyone has a X chromosome. But if you have another X chromosome—meaning
you have two X chromosomes—you are female. And if you have a Y chromosome—meaning you
have a X and Y chromosome—you are male. There are also genetic disorders where you
can have extra copies of sex chromosomes but we are not going into that for this clip. Sex-linked traits are traits that are specifically
on the sex chromosomes. Most sex-linked traits tend to be on the X
chromosome, because it is larger than the Y chromosome and contains more genes than
the Y chromosome. The disorder hemophilia is like this. We will use the letter “H” to represent
an allele for not having hemophilia and a letter “h” to represent an allele for
having hemophilia. Hemophilia is a recessive disorder, which
is why it is being represented by a lowercase letter h. Only, it must be placed on the sex chromosomes
as a superscript. Like an exponent. Let me explain what I mean by that. A woman that does not have hemophilia could
have the genotype XHXH or XHXh. Because as long as she’s got at least one
dominant allele—that dominating allele—will be what shows. So no hemophilia, since again, hemophilia
is a recessive disorder. The only way for her to have hemophilia would
be the genotype XhXh. Because only when there is no dominant present
will that recessive show up, at least in this type of trait. For a male to not have hemophilia, his genotype
would have to be XHY. Notice how I didn’t put anything on the
Y chromosome—again, most sex linked traits are on the X chromosome. If he has the genotype XhY, then he has hemophilia. He doesn’t have two X chromosomes, so in
this disorder, he either has it or he doesn’t. There is no heterozygous genotype for the
male so he cannot be a carrier. So let’s say two people that do not have
hemophilia have children. However, let’s say the woman is a carrier. That means she is heterozygous. How do you do a sex-linked Punnett square
cross for this kind of trait? Step 1) Identify the genotypes of the parents. So the mother is XHXh. She doesn’t have hemophilia because of the
dominant allele present but she is a carrier. The male, if he does not have hemophilia,
must be XHY. There’s no other option for him. Step 2) Place one parent on the top, outside
of the square like this. Place the other parent on the left, outside
of the square, like this. Step 3) Cross them! For formatting purposes, place X chromosomes
before Y. You also write any sex chromosomes with dominant
letters first. The results you get in the squares would be
the offspring—the babies. The genotype ratio could be written out like
this. The phenotype ratio—remember that these
are the traits—can be written out that there is a 75% chance that a child will be born
without hemophilia and a 25% chance that a child would have hemophilia, for this boy
here. Notice that in this type of example of a sex
linked recessive disorder—boys are more likely to inherit this disorder—because
they only have one X chromosome. This is true for many other sex linked recessive
disorders, such as color blindness. Well that’s it for the amoeba sisters and
we remind you to stay curious!

Articles Tags:

Leave a Reply

Your email address will not be published. Required fields are marked *