SEX-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 (X h ) to being normal (X H ). The heterozygous female is called a carrier. Cross a carrier female with a normal male. X H X h X _____

In humans, hemophilia is a sex-linked trait. Having hemophilia is recessive (X h ) to being normal (X H ). The heterozygous female is called a carrier. Cross a carrier female with a normal male. X H X h X X H Y

XHXH XhXh XHXH Y

XHXH XhXh XHXH XHXHXHXH Y

XHXH XhXh XHXH XHXHXHXH XHXhXHXh Y

XHXH XhXh XHXH XHXHXHXH XHXhXHXh YXHYXHY

XHXH XhXh XHXH XHXHXHXH XHXhXHXh YXHYXHYXhYXhY

Genotypic ratio: 1 X H X H :1X H X h :1X H Y :1X h Y Phenotypic ratio: 2 normal females: 1normal male: 1 male with hemophilia

Cross a carrier female with a male with hemophilia. X H X h X _____

Cross a carrier female with a male with hemophilia. X H X h X X h Y

XHXH XhXh XhXh Y

XHXH XhXh XhXh XHXhXHXh Y

XHXH XhXh XhXh XHXhXHXh XhXhXhXh Y

XHXH XhXh XhXh XHXhXHXh XhXhXhXh YXHYXHY

XHXH XhXh XhXh XHXhXHXh XhXhXhXh YXHYXHYXhYXhY

Genotypic ratio: 1 X H X h :1X h X h :1X H Y :1X h Y 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. X n X n 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. X n X n X X N Y

XnXn XnXn XNXN Y

XnXn XnXn XNXN XNXnXNXn Y

XnXn XnXn XNXN XNXnXNXn XNXnXNXn Y

XnXn XnXn XNXN XNXnXNXn XNXnXNXn YXnYXnY

XnXn XnXn XNXN XNXnXNXn XNXnXNXn YXnYXnYXnYXnY

Genotypic ratio: 2 X N X n : 2 X n Y 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.