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.
Polygenic Inheritance
Traits that are
influenced by several
Genes.
 Show many degrees of
variation.
Examples:
 Height
 Hair color
 Skin Color
 Eye Color
