Download Sex Linked / "X" Linked Genetics

Sex Linked / "X" Linked Genetics
Autosomes = the chromosomes other than the sex chromosomes. Each member of an autosome pair (in diploid organisms) is of similar length and in the genes it carries. Human cells contain 23 pairs of chromosomes.
There are 22 pairs of autosomes and 2 sex chromosomes ("X" and "Y") in the nuclei.
karyotype = picture of the set of chromosomes for an individual
Autosomal Dominant Inheritance Autosomal Recessive Inheritance 1
Sex Linked / "X" Linked Genetics
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 organism there are different methods of determining the geneder of an individual.
In some reptiles, the temperature of the eggs as they are forming will trigger the gender of the individuals.
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Sex Linked / "X" Linked Genetics
Only men inherit Y chromosomes so they are the only ones to inherit Y­linked traits. Both men and women can get the X­linked traits since both inherit X chromosomes.
It is the father's gamete (sperm) that determines the gender of the offspring.
The sperm donates either an X allele or a Y allele while the egg donates an X allele.
Gametes are the sex cells (egg or sperm) that carry only one allele.
A gamete is a "haploid" (alleles in singles) set of genes for an egg or a sperm.
The offspring is either XX or XY X is found in the ovum
X or Y is found in the sperm
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Sex Linked / "X" Linked Genetics
Genes that are carried by either sex chromosome are said to be sex linked or X­linked traits.
In the case of X­linked 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.
The "a" recessive allele will not be expressed in her phenotype.
Xa.
XA.
We use the symbol We use the symbol Sex Linked / "X" Linked Genetics
The X allele in the normal form is symbolized by ...
XN" when the normal form of the gene is dominant n
an "X " when the normal form of the gene is recessive
an "
Genotypes of parents: Father = XNY
Father = XnY
Mother = XNXN
Mother = XNXn
Mother = XnXn
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Sex Linked / "X" Linked Genetics
There are about 1100 human X­linked genes. Most of them code for something other than female anatomical traits. Many of the non­sex determining X­linked genes are responsible for abnormal conditions such as ...
* hemophilia * red­green colour blindness
* congenital night blindness,
* duchene muscular dystrophy Are you colour­blind?
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Facts about X­Linked Traits:
­ Almost all the genes on the X have no counterpart on the Y chromosome
­ Fathers can not pass X­linked alleles to their sons.
­ Father can pass X­linked alleles to their daughters.
­ Mothers can pass sex­linked alleles to both sons and daughters.
­ If a sex linked trait is due to a recessive allele:
* A female will express the phenotype only if she is homozygous recessive.
* A male with one recessive allele from his mother will express the phenotype.
The inheritance of these genes on the X chromosome follows special rules because males have only a single X chromosome.
X­linked recessive traits are primarily expressed in the phenotype of males. This is due to the fact that men only have one X chromosome. and there are no corresponding gene on the Y chromosome in most cases. X­linked 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 X­linked traits but rarely have them expressed in their own phenotypes.
Sex Linked / "X" Linked Genetics
Colour Vision:
Colour vision is due to a gene found on the X chromosome (not on the Y chromosome).
Having one bad copy of this gene in a male (XY) guarantees a colour blind phenotype.
Should a heterozygous females for colour­blindness be able to see colour?
Suppose: XN= colour vision and Xn= red­green colour blind vision
The retina of a heterozygous (XNXn) female will have some cells with the "XN" allele inactivated and other cells with the "Xn" allele inactivated.
A heterozygous carrier of red­green colour blindness has some color blind cells in her retina. The non­colorblind cells enable her to see colour.
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Sex Linked / "X" Linked Genetics
Sample Genetics Problem:
A man who has normal vision is married to a woman who is a carrier for red­green color blindness (her father was red­green color blind). a) What is the probability that their children are red­green colour blind?
b) What is the probability that their sons are red­green colour blind?
c) What is the probability that their daughters are red­green colour blind?
XN= Normal Vision, Xn= Red­Green colour blindness
Phenotypes of parents: Father = XNY, Mother = XNXn
The Retina of a heterozygous (XNXn) female will have some cells with the "XN" allele inactivated and other cells with the "Xn" allele inactivated.
A heterozygous carrier of red­green colour blindness has some colour blind cells in her retina. The non­color blind cells enable her to see colour.
Sample Genetics Problem:
A man who has normal vision is married to a woman who is a carrier for red­green color blindness (her father was red­green color blind). a) What is the probability that their children are red­green colour blind?
b) What is the probability that their sons are red­green colour blind?
c) What is the probability that their daughters are red­green colour blind?
XN= Normal Vision, Xn= Red­Green colour blindness
Phenotypes of parents: Father = XNY, Mother = XNXn
XNXN XNY
N
n
n
X X XY
25% of their children would be red­green colour blind.
50% of their sons would be red­green colour blind.
0% of their daughters would be red­green colour blind.
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Sex Linked / "X" Linked Genetics
Sex­Linked Genetic Disorders:
Sex­chromosome 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. Testicular Feminization
The body cells of people with testicular feminization are insensitive to testosterone and therefore develop the female phenotype even though they have a Y chromosome.
It has an X­linked recessive mode of inheritance.
Guevodoces
Guevodoces refers to a condition in which the male phenotype develops after puberty.
The affected individual is usually an XY male, but he appears female from birth as a result of delayed testosterone production. At puberty, however, his testes descend and his penis finally develops.
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Baldness in humans is a dominant, sex­influenced 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 non­bald only if he is bb. A woman will only be bald if she is BB and non­bald if she is Bb or bb (it’s almost like B is dominant in males and b is dominant in females). Actually, because of the influence of other sex­related factors, most women who are BB never become totally bald like men do, but rather, their hair becomes “thin” or sparse. If two parents are heterozygous for baldness, what are the chances of their children being bald? Use a Punnett square to illustrate this. Note: because the sex of a person does make a difference in how the gene is expressed, you need to set this up as a dihybrid cross to account for the sex of the children.
X­LINKED INHERITANCE
1. Hemophilia is a sex­linked recessive trait in which the blood does not clot properly. a) Which parent passes on this trait to which children? Explain.
b) What is the chance that a man with hemophilia will pass this trait onto his children? Show all possibilities.
2. A man with hemophilia marries a female carrier of the disease.
a) What is the likelihood that their daughters will carry the disease? b) What is the likelihood that their sons will carry the disease?
c) What is the likelihood that their daughters will have the disease?
3. Colour­blindness is a sex­linked recessive trait. A woman with normal vision whose father was colour­blind marries a colour­blind man. Give the genotypes and phenotypes of their children. What ratio of their children can be expected to be colour­blind?
4. A man with normal colour vision whose father was colour­blind marries a woman carrier of the colour­blind allele. What is the likelihood that their children will be colour­blind?
What is the likelihood that their children will be carriers of the trait?
5. Male pattern baldness is recessive and inherited on the X chromosome. Neither of your parents has the condition, but your mother's brother does. What is the chance that you will inherit the gene? Explain.
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