Download Sex-Linked Genes - Mr. Kleiman`s Wiki

Sex-Linked Genes
Below is a human karyotype. This is a picture of all of the chromosomes in one human cell. All cells in
the body of one individual (except in the gametes) will have this same set of chromosomes.
Unsorted Chromosomes
Male Karyotype
Sex Chromosomes:
 The X and the Y chromosomes are called sex chromosomes because they determine the gender
of an individual

In mammals, if you have two X chromosomes, you will be a female
if you have one X and one Y chromosome you will be a male


The X Chromosome is larger than the Y chromosome, therefore it codes for more traits.
Sex Chromosomes do not only code for sexual characteristics!
Let us create an irregular Punnett square using the entire sex chromosomes, rather than just one allele,
to show the probability of offspring being male or female.
P generation: Mother = XX Father = XY
X
Y
X
X
XX
XX
XY
XY
“Genotype” =
2 XX :
50%
2 XY
50%
Phenotype = 2 female : 2 male
50%
50%
In reality the human world population is approximately 51% male and 49% female.
Why are the X and Y chromosomes NOT considered to be homologous chromosomes even though they
pair up and separate during meiosis?
_____________________________________________________________________________________
_____________________________________________________________________________________




This means that alleles on the sex chromosomes are a special case for inheritance.
We call genes on the X or Y chromosomes sex linked traits.
Females have no chance of receiving Y-linked genes and males only receive ONE X chromosome
This means that males only have one allele for all X-linked traits. They are more likely to show
recessive X-linked traits because they cannot be masked by a dominant allele
Sex Linked Diseases / Disorders
 Since the Y chromosome is small and doesn’t code for many traits, Y-linked diseases are rare
 Most sex-linked disorders (faulty genes on sex chromosomes) are X-linked.
 We need special Punnett squares to show the inheritance of X-linked Characteristics which show
whole sex chromosomes
Eg. Hemophilia – a rare recessive X-linked disease that prevents blood from clotting.
What are the chances of offspring getting hemophilia if only the mother is a carrier of the hemophilia
allele, but does not have hemophilia herself?
Let A = allele for normal blood clotting
a = allele for Hemophilia
XA
Xa
XA
XA XA
XA Xa
Y
A
a
X Y
X Y
“Genotype” =
Phenotype =
P Generation:
Mother = XAXa
Female
1 XA XA : 1 XA Xa
25%
25%
No Hemophilia :
75%
Father = XAY
Male
1 XA Y : 1 Xa Y
25%
25%
Hemophilia
25%
If Male :
If Female:
50% chance
0% chance
Why do males have more of a chance of having Hemophilia than females?
_____________________________________________________________________________________
_____________________________________________________________________________________
On a separate page, answer the following question:
In humans, colorblindness (b) is an example of a sex-linked recessive trait. If a colourblind male
reproduces with a female who is not colourblind, but carries the colourblindness (b) allele, what are the
chances that their children will be colourblind? Compare the chances if it is a son vs. a daughter.
Sex-Linked Genes
Below is a human ___________________. This is a picture of all of the chromosomes in one human cell.
All cells in the body of one individual (except in the gametes) will have this same set of chromosomes.
Unsorted Chromosomes
Male Karyotype
Sex Chromosomes:
 The X and the Y chromosomes are called sex chromosomes because they determine the
________________________ of an individual

In mammals, if you have two X chromosomes, you will be a ___________________
if you have one X and one Y chromosome you will be a ___________________


The X Chromosome is ____________ than the Y chromosome, therefore it codes for more traits.
Sex Chromosomes do not only code for sexual characteristics!
Let us create an irregular Punnett square using the entire sex chromosomes, rather than just one allele,
to show the probability of offspring being male or female.
P generation: Mother = XX Father = XY
X
Y
X
X
XX
XX
XY
XY
“Genotype” =
2 XX :
50%
2 XY
50%
Phenotype = 2 female : 2 male
50%
50%
In reality the human world population is approximately 51% male and 49% female.
Why are the X and Y chromosomes NOT considered to be homologous chromosomes even though they
pair up and separate during meiosis?
_____________________________________________________________________________________
_____________________________________________________________________________________




This means that alleles on the sex chromosomes are a special case for inheritance.
We call genes on the X or Y chromosomes ____________________________________________
Females have no chance of receiving Y-linked genes and males only receive ONE X chromosome
This means that males only have one allele for all X-linked traits. They are more likely to show
recessive X-linked traits because they cannot be masked by a dominant allele
Sex Linked Diseases / Disorders
 Since the Y chromosome is small and doesn’t code for many traits, Y-linked diseases are rare
 Most sex-linked disorders (faulty genes on sex chromosomes) are X-linked.
 We need special Punnett squares to show the inheritance of X-linked Characteristics which show
whole sex chromosomes
Eg. Hemophilia – a rare recessive X-linked disease that prevents blood from clotting.
What are the chances of offspring getting hemophilia if only the mother is a carrier of the hemophilia
allele, but does not have hemophilia herself?
Let A = allele for normal blood clotting
a = allele for Hemophilia
XA
Xa
XA
XA XA
XA Xa
Y
A
a
X Y
X Y
“Genotype” =
Phenotype =
P Generation:
Mother = XAXa
Female
1 XA XA : 1 XA Xa
25%
25%
No Hemophilia :
75%
Father = XAY
Male
1 XA Y : 1 Xa Y
25%
25%
Hemophilia
25%
If Male :
If Female:
50% chance
0% chance
Why do males have more of a chance of having Hemophilia than females?
_____________________________________________________________________________________
_____________________________________________________________________________________
On a separate page, answer the following question:
In humans, colorblindness (b) is an example of a sex-linked recessive trait. If a colourblind male
reproduces with a female who is not colourblind, but carries the colourblindness (b) allele, what are the
chances that their children will be colourblind? Compare the chances if it is a son vs. a daughter.