Download Sex linked genetic disorders are associated with problems with the

Sex linked genetic disorders are associated with problems with the sex
chromosomes.
X-linked disorders
The Y chromosome is shorter than the X chromosome and therefore does not
contain the same genes as the X. Altered genes on certain parts of the X chromosome
of a male (XY) where there is no corresponding gene on the Y chromosome are said to
be X-linked. This means only one dysfunctional protein is expressed because there is
no other X chromosome to compensate for defect
Females with 1 altered gene are carriers but females with 2 altered genes will show
symptoms of disorder.
Colorblindness, hemophilia, male pattern baldness are examples
Hemophilia is a rare blood clotting disorder. People born with hemophilia have little or no clotting
factor. Clotting factor is a protein needed for normal blood clotting. There are several types of clotting
factors. These proteins work with platelets to help the blood clot. Hemophilia is caused by a gene
found on the X chromosome. It is recessive
Color blindness occurs when there is a problem with the color-sensing granules (pigments) in certain
nerve cells of the eye. If just one pigment is missing, you may have trouble telling the difference
between red and green. This is the most common type of color blindness. If a different pigment is
missing, you may have trouble seeing blue-yellow colors. People with blue-yellow color blindness
usually have problems identifying reds and greens, too.
The most severe form of color blindness is achromatopsia. A person with this rare condition cannot see
any color, so they see everything in shades of gray. Achromatopsia is often associated with lazy
eye, nystagmus (small, jerky eye movements), severe light sensitivity, and extremely poor vision.
A man who has normal vision marries a woman who is color-blind. What are the possible
phenotypes and genotypes?
A color blind male marries a woman who is a carrier for colorblindness. What are the possible
phenotype and genotype percentages of their children?
Male sex determination: The main Y gene is called the SRY gene, which is the master gene
that specifies maleness and male features. It is the single gene that sets off the initial cascade
of hormone changes that make a person male. It is not the entire Y chromosome, but just this
gene that is necessary for maleness. There is evidence of this in rare diseases where the SRY
gene is missing. People who are genetically male with XY chromosomes, but with a mutation
or deletion of this SRY gene on the Y chromosome, will be female despite having most of the
Y chromosome. And people who are genetically female with XX but also have a tiny piece of
the Y chromosome with this gene, will become male despite their female-like XX
chromosomes.
NEWS: In the May 4, 2008 online issue of Nature, Ryohei Sekido and Robin Lowell-Badge
reported that the SRY gene affects gender by activating and regulating a critical autosomal
gene known as Sox9. If the Sox9 gene becomes active in an embryo with two X
chromosomes, it causes male gonads to form instead of ovaries, and the individual develops
into an anatomical male. If the Sox9 gene does not turn on in an embryo with XY sex
chromosomes, the gonads develop into ovaries, and the individual becomes a female
anatomically. The authors suggest that this could happen in up to 1 in 20,000 genetically male
embryos (XY).
http://www.pbs.org/wgbh/nova/miracle/program_adv.html
Expansion
Sex-linked genes
A boy, whose parents and grandparents had normal vision, is color-blind. What are the
genotypes for his mother and his maternal grandparents. Use XB for the dominant normal
condition and Xb for the recessive, color-blind phenotype.
The bison herd on Konza Prairie has begun to show a genetic
defect. Some of the males have a condition known as "rabbit
hock" in which the knee of the back leg is malformed slightly.
We do not yet know the genes controlling this trait but for the
sake of our question, we shall assume it is a sex-linked gene
and that it is recessive. Now, suppose that the herd bull (the
dominant one which does most of the breeding) who is
normal (XN) mates with a cow that is a carrier for rabbit hock.
What are his chances of producing a normal son?
If he mates with this cow every year, what percentage of their daughters have normal knees?
What percentage of their daughters will be carriers of rabbit hock?
A woman with red-green color-blindness has a mother with normal vision. Knowing that colorblindness is a sex-linked recessive gene, can you determine what her father's phenotype is?
If so, what is it?
The woman marries a man with normal vision. What is the probability they will have sons who
are red-green color-blind?
What is the probability they will have daughters who are red-green color-blind?
A rancher owns a bull with many desirable characteristics. Unfortunately, he also has a sex-linked trait
that in the recessive form leads to no pigment formation in the iris of the eye. This makes the bull very
sensitive to sunlight and could lead to blindness. The rancher wishes to breed him to a cow that will
minimize the chances of any offspring showing this trait. She would especially like to produce another
bull with most of his sire's desirable qualities but without the nonpigmented eye. Two cows with the
dominant normal colored eyes (XN) are available that have been genetically typed for this particular
trait. Cow 1 has a genotype of XN XN and cow 2 is XNXn. Which of these two cows should the rancher
choose as a mate to her bull if she wishes to minimize the occurrence of the nonpigmented eye in his
offspring?
What percentage of the male offspring from the preferred cross will have nonpigmented eyes?
Will crossing the bull with this cow eliminate the trait from the herd?
If not, why not?
Clouded leopards are a medium sized, endangered species of cat,
living in the very wet cloud forests of Central America. Assume that
the normal spots (XN, pictured here) are a dominant, sex-linked trait
and that dark spots are the recessive counterpart. Suppose as a
Conservation Biologist, you are involved in a clouded leopard
breeding program. One year you cross a male with dark spots and a
female with normal spots. She has four cubs and, conveniently, two
are male and two female. One each of the male and female cubs
have normal spots and one each have dark spots.
What is the genotype of the mother?
Suppose a few years later, you cross the female cub that has normal spots with a male that also has
normal spots. How many of each genotype will be found in the cubs (assume 4)?
Will any of the cubs from this latest cross have dark spots?
If so, how many and of what sex will they be?
We have previously mentioned that it’s very important to have exactly two copies of each chromosome
(one from the mother and one from the father), and more or less chromosomes would be an abnormal
number that can cause problems. How is it, then, that we can get by with females being XX and having
two copies of all of the genes on the X chromosome, while males, being XY, only have one copy of
most of those genes because there are no corresponding places on the Y chromosome?
Dr. Barr noticed a dark spot in the nucleus of each cell in the body of female
mammals. Mary Lyon figured out what this was and what was going on here. In
a female mammal, during embryonic development, one X at random is turned
off in each of her cells and condenses to form the dark spot. Mary Lyon called
these inactivated X-chromosomes “Barr bodies” in honor of Dr. Barr. She also
figured out that as those embryonic cells divide, all daughter cells of each of
those cells will have the same X turned off.
This is illustrated by calico cats. Coat color in cats is an X-linked gene, with
alleles for black and orange-brown, so XBXB and XBY cats will have a black coat,
while XOXO and XOY will have an orange-brown coat. Another possible
combination for female cats would be XBXO. Both of the color alleles would be expressed, so the cat
would end up being partially brown and partially black.
Origin of Calico Coats mentioned, during embryonic development, one X, at random, turns off in each
cell in a female’s body. For a cat who is XBXB or XOXO, since both Xs are the same, this won’t be
noticed, but if a female is XBXO, in some of her cells the XB will be turned off while in others, the XO will
be turned off. As these cells multiply by mitosis, this will lead to patches of skin where black hair will be
produced, while other patches will produce orange-brown fur. She will end up with the patchy coat color
typical of calico or tortoiseshell cats.
There is a similar, X-linked gene in human females for the presence of sweat glands in the skin. A
woman who is heterozygous for this gene will have patchy skin containing some areas with and some
without sweat glands. This discussion will hopefully lead you to think of several “what-if” questions:
Q: Can a male cat ever be calico?
A: Yes, if he’s XXY, which would be abnormal. It turns out that abnormal numbers of X chromosomes
aren’t as serious as other chromosome number abnormalities, because all but one X turn off, so an
XXX individual would have two Barr bodies, an XXY individual would have one, etc. (By the way, that’s
an “easy” thing to test: as in the photo, above, all that’s needed is a sample of some of the “loose”
cheek cells from inside of someone’s mouth. Those are stained with a special dye, and if Barr bodies
are present, they will show up as a “black spot.” )
Adenosine Deaminase (ADA) deficiency
Huntingtons disease,
Neurofibromatosis 1,
Marfan Syndrome,
Hereditary nonpolyposis colorectal cancer
Hereditary multiple exostosesis
Cystic fibrosis
Alpha-1 antitrypsin deficiency
Sickle cell anemia,
Tay-Sachs disease,
Achondroplasia (Dwarfism)
Spinal muscular atrophy
Hypophosphatemia,
Aicardi Syndrome,
galactosemia
Chokenflok Syndrome
Hemophilia A,
Duchenne muscular dystrophy,
Color blindness,
Androgenetic alopecia
Leber's Hereditary Optic Neuropathy