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?

DADWhy? 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 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.

mal

e__ XHXh __  X  __ XHY ___

XH XH

XH Y Xh Y

XH Xh

XH

Y

XH Xh

Results:Genotypic ratio: 1 XHXH :1 XHXh : 1 XHY :1 Xh Y

XH XH

Normal Female

XH Y

Normal Male

Xh YMale w/

hemophilia

XH Xh

Normal FemaleXH

Y

XH XhPhenotypic ratio:2 normal females: 1normal male: 1 male with 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 XH

Normal Female

XH Y

Normal Male

Xh YMale w/

hemophilia

Xh Xh

Female w/ hemophilia

XH

Y

XH XhPhenotypic ratio:1 normal female: 1 female w/ Hemopholia:1normal male: 1 male with 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