Download Sex-Linked Traits

Sex-Linked Traits
Karyotype: ______________________________________________
Autosomes: ______________________________________________.

Autosomes are the same for both males and females.
Sex chromosomes: ______________________________________________.

The sex chromosomes are the 23rd pair of chromosomes.
______________________________________________
______________________________________________
Which parent determines the sex of an offspring – _________
Why?

All moms have the genotype XX. When egg cells are made, _________________
____________________________

All dads have the genotype XY. When sperm cells are made, ________________
_____________________________

Therefore, males and females are born in roughly a 50:50 ratio.
Diagnosing Disorders from Karyotypes



Some disorders can be diagnosed by looking at a person’s karyotype
Most are caused by _____________________ during meiosis
o Nondisjunction – _____________________________________________
Disorders:
o _________________________ (_____________________) – individual
has 3 of the 21st chromosome instead of 2
o _________________________ – female has only one X for her sex
chromosome
o _________________________ – male has or more extra X chromosome
SEX-LINKED TRAITS: ______________________________________________

NOTE: The Y chromosome is much smaller than the X chromosome and only
contains a few genes. Most sex-linked traits are on the X chromosome.

In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (Xh)
to being normal (XH). The heterozygous female is called a carrier. Cross a
carrier female with a normal male.
XHXh x XHY
XH
Xh
XH
Y
Genotypic ratio:

Phenotypic ratio:
Cross a carrier female with a male with hemophilia.
XHXh x XhY
XH
Xh
Xh
Y
Genotypic ratio:
Phenotypic ratio:

In humans, red-green colorblindness is a sex-linked trait. People with red-green
colorblindness can not tell the difference between red and green. Colorblindness
is the result of a recessive allele. Cross a female with colorblindness with a male
with normal vision.
XnXn x XNY
Xn
Xn
XN
Y
Genotypic ratio:

Phenotypic ratio:
Why are sex-linked traits more common in males than in females?
o ____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
____________________________________________________________
o ______________________________________________
 If female gets a “bad” from one parent, she could still get a “good”
X, and become a carrier (has the trait but isn’t expressed). The
only way for the female to express the trait is to inherit two “bad”
X’s.
 If a male gets a “bad” X, the only other sex chromosome is a Y.
The “bad” X is dominant to the Y, therefore, the male will express
the trait.
Inheritance Of Color Blindness In Men & Women
Sex-Linked Genes Located On X Chromosome:
+
o
= Normal Vision (Dominant)
= Color Blindness (Recessive)
Sex
Color-blind
Normal Vision
Male
XoY
X+Y
Female
XoXo
X+X+ X+Xo
Cross Between A Color-blind Man (XoY) and
Heterozygous Normal Vision Woman (X+Xo)
Gametes
Xo
Yo
X+
X+Xo
X+Y
Xo
XoXo
XoY
In the above cross, four different possible offspring are produced:
1. XoY: Color-blind Boy (1/4 or 25%)
2. X+Y Normal Vision Boy (1/4 or 25%)
3. XoXo Color-blind Girl (1/4 or 25%)
4. X+Xo Heterozygous Normal Vision Girl (1/4 or 25%)
Note: The heterozygous normal vision girl carries the recessive gene for
color blindness. On the average, she will pass this gene on to half of her
sons and half of her daughters.