CHAPTER 12
SEX-LINKED TRAITS
Karyotype: a picture of chromosomes.
Body Cells vs. Sex Cells
 Autosomes:
the first 22 homologous
pairs of chromosomes.
 Autosomes are the same for both
males and females.
 Sex cells: Determine sex of offspring
Is it Male or Female? Explain

Sex chromosomes: determines the sex
of the individual.
The
sex chromosomes are the 23rd pair
of chromosomes.
XX =female
XY=male
Which parent determines the sex of
an offspring?
 DAD
 Why?
All moms have the genotype XX.
When egg cells are made, they will
all carry a single X chromosome.
All dads have the genotype XY. When
sperm cells are made, 50% will have
an X chromosome and 50% will have a
Y chromosome.
 Therefore, males and females are
born in roughly a 50:50 ratio.

SEX-LINKED TRAITS


Those traits that are controlled by
genes on the X or Y chromosomes.
NOTE: The Y chromosome is much
smaller than the X chromosome and
only contains a few genes. Most sexlinked 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 ___
male
XH
Y
XH
Xh
XH XH
XH Xh
XH
Y
Xh Y
Results:
Genotypic ratio:
1 XHXH :1 XHXh : 1 XHY :1 Xh Y
Phenotypic ratio:
2 normal females:
1normal male:
XH
1 male with
hemophilia
Y
XH
Xh
XH XH
XH Xh
Normal Female
Normal Female
XH
Y
Normal Male
Xh Y
Male w/
hemophilia
Now You Try

Cross a carrier female with a male
with hemophilia.
__ XHXh __ X __ XhY ___
Results:
Genotypic ratio:
1 XHXh :1 XhXh : 1 XHY :1 Xh Y
XH
Phenotypic ratio:
Xh
1 normal female:
1 female w/ Hemopholia:
Xh
1normal male:
1 male with hemophilia
Y
XH
Xh
Normal Female
XH
Y
Normal Male
Xh Xh
Female w/
hemophilia
Xh Y
Male w/
hemophilia
Genetic Sex-Linked Disorders
Color Blindness



Cause: x-linked
recessive
1/10 males have,
1/100 females have.
Why the difference?
Individuals are unable
to distinguish shades
of red-green.
Genetic Sex-Linked Disorders
2- Hemophilia A and B
Recessive disorders that affect 1 of
5,000 males.
These interfere with
normal blood clotting and occur
on
the X chromosome.
Using Fruit Flies to Study Genetics





Thomas Hunt Morgan, 1908
Among normal red-eyed Drosophila he found a
mutated white-eyed fly (male).
Since it was a recessive gene that was expressed
only in male flies, Morgan suggested that the
chromosome was located on the X chromosome.
Miniature winged mutation was transmitted on the
same gene. These two characteristics did not
always travel together.
Morgan suggested that the chromosomes could
swap parts and received the Nobel prize in 1934.
Drosophila
melanogaster
X-linked Inheritance


A situation where the genes that cause a
disorder are located on the X chromosome.
More male victims than female
–
–
–
Hemophilia
Color-blindness
Duchenne muscular dystrophy
Polygenic Inheritance
Traits that are influenced by several
Genes.
 Show many degrees of variation.
Examples:
 Height
 Hair color
 Skin Color
 Eye Color
