CHAPTER 14 NOTESSEX-LINKED TRAITS

Karyotype: a picture of chromosomes.

Autosomes:

the first 22 homologous pairs of chromosomes.

• Autosomes are the same for both males and females.

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 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 _____

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

XH Xh

XH XHXH

Y

XH Xh

XH XHXH XHXh

Y

XH Xh

XH XHXH XHXh

Y XHY

XH Xh

XH XHXH XHXh

Y XHY XhY

Genotypic ratio:

1 XHXH :1XHXh :1XHY :1XhY

Phenotypic ratio:

2 normal females: 1normal male: 1 male with hemophilia

Cross a carrier female with a male with hemophilia.

XHXh X _____

Cross a carrier female with a male with hemophilia.

XHXh X XhY

XH Xh

Xh

Y

XH Xh

Xh XHXh

Y

XH Xh

Xh XHXh XhXh

Y

XH Xh

Xh XHXh XhXh

Y XHY

XH Xh

Xh XHXh XhXh

Y XHY XhY

Genotypic ratio:

1 XHXh :1XhXh :1XHY :1XhY

Phenotypic ratio:

1 normal female: 1 female with hemophilia:1 normal male: 1 male with hemophilia

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 _____

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

Xn Xn

XN XNXn

Y

Xn Xn

XN XNXn XNXn

Y

Xn Xn

XN XNXn XNXn

Y XnY

Xn Xn

XN XNXn XNXn

Y XnY XnY

Genotypic ratio:

2 XNXn : 2 XnY

Phenotypic ratio:

2 normal females: 2 males with colorblindness

Why are sex-linked traits more common in males than in females?

• Because a male only has to inherit ONE recessive allele in order to get a sex-linked trait and a female has to inherit TWO recessive alleles in order to acquire the sex-linked trait.

It is easier to inherit one recessive allele than two.

If the female only inherits one recessive allele, then they are a carrier but have the normal phenotype.