Download Sex Linked / "X" Linked Genetics Recall

Sex Linked / "X" Linked Genetics
Recall:
Autosomes = the chromosomes other than the sex chromosomes.
Human cells contain 23 pairs of chromosomes.
There are 22 pairs of autosomes and 2 sex chromosomes ("X" and "Y") in the nuclei.
human karyotype
Sex/Gender chromosomes = chromosomes that determine the gender of an organism.
In humans, females have two X chromosomes (genotype = XX).
In humans, males have one X and one Y chromosome (genotype = XY).
The "X" chromosomes is the larger chromosome.
The "Y" chromosome is the smaller chromosome.
The X chromosome carries hundreds of genes but only a few of these have anything
to do directly with gender.
Chromosomal Determination of Sex
In other organisms there are different methods of determining the
gender of an individual.
In some reptiles, the temperature of the eggs as they are forming will
trigger the gender of the individuals.
In humans the father's gamete (Sperm) determines the sex of
offspring!
X is found in the ovum
X or Y is found in the sperm
The offspring is
either XX or XY
Sex Linked / "X" Linked Genetics
Genes that are carried by sex chromosomes are said to be sex linked or
X-linked traits.
In the case of X-linked genetics, the gene is found on chromosome X so we show the X
or Y chromosome with a superscript for the allele.
The "a" recessive allele
will be expressed in his
phenotype.
We use the symbol
Xa.
The "a" recessive allele
will not be expressed in
her phenotype.
A
We use the symbol X .
Are you colour-blind?
Normal 8
Red-green deficiencies 3
Red-green deficiencies read 17
Total colour blindness should not read
any numeral
Ex 1:
A man who has normal vision is married to a woman who is a carrier for red-green
colour blindness (her father was red-green colour blind).
a) What is the probability that their children are red-green colour blind?
b) What is the probability that their sons are red-green colour blind?
c) What is the probability that their daughters are red-green colour blind?
= Normal Vision,
= Red-Green colour blindness
Genotypes of parents: Father =
, Mother =
_____ of their children would be red-green colour blind.
_____ of their sons would be red-green colour blind.
_____ of their daughters would be red-green colour blind.
XN
Y
Xn
Facts about X-Linked Traits:
- Almost all the genes on the X have no counterpart on the Y chromosome
- Fathers can not pass X-linked alleles to their sons.
- Father WILL pass X-linked alleles to their daughters.
- Mothers can pass sex-linked alleles to both sons and daughters.
X-linked recessive traits are primarily expressed in the phenotype of males.
(men only have one X chromosome and there are no corresponding gene on the Y
chromosome in most cases)
X-linked recessive traits in females are often masked in their phenotype by a dominant
normal allele on the other chromosome. This explains why women are frequently
carriers of X-linked traits but rarely have them expressed in their own phenotypes.
Sex Linked Disorders
There are about 1100 human X-linked genes.
Most of them code for something other than female anatomical traits.
Many of the non-sex determining X-linked genes are responsible for abnormal conditions
such as ...
* hemophilia
* red-green colour blindness
* congenital night blindness,
* duchene muscular dystrophy
Other Sex-Linked Genetic Disorders:
Sex-chromosome abnormalities may also be caused by nondisjunction of one or more
sex chromosomes. Any combination (up to XXXXY) produces maleness. Males with
more than one X are usually underdeveloped and sterile. XXX and X women are known,
although in most cases they are sterile.
Baldness in humans is a dominant, sex-influenced trait.
This gene is on the autosomes, not the sex chromosomes, but
how it is expressed is influenced by the person’s sex (due to
hormones present, etc.).
A man who is BB or Bb will be bald and will be non-bald only if
he is bb. A woman will only be bald if she is BB and non-bald if
she is Bb or bb (it’s almost like baldness is dominant in males
and recessive in females).
Actually, because of the influence of other sex-related factors,
most women who are BB never become totally bald like men do,
but rather, their hair becomes “thin” or sparse.
Y-linked traits
Y-linked traits are controlled by alleles on the Y chromosome.
Y-linked traits are known as holandric traits as they are “wholly male”.
Do the words homozygous or heterozygous apply to Y-linked traits?
NO! Because there is only ever one Y chromosome present at a time!
Are females affected by Y-linked traits?
NO! Because females don’t carry Y chromosomes!
An example of a Y-linked trait is HAIRY EARS!
All of the sons of an affected male will display this Y-linked
trait.
Ex 2:
A woman who has X- linked hypophosphatemia, which
affects bone development. She marries a man with normal
bone structure. What is the probability that the woman and
her husband will have a child with the disorder?