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 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.

Examples of X-linked traits: 1. Hemophilia 2. Color Blindness 3. Muscular Dystrophy 4. Icthyosis simplex (scaly skin)

Hemophilia Hemophilia- Lacking in the ability to clot blood There is a gene on the “X” chromosome that control blood clotting People who have hemophilia are missing the protein to clot blood They can bleed to death by minor cut.

In humans, hemophilia is a sex-linked trait 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 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 Y

XH Xh XHXH Y

XH Xh XHXH XHXh Y

XH Xh XHXH XHXh Y XHY

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 Y

XH Xh XHXh Y

XH Xh XHXh XhXh Y

XH Xh XHXh XhXh Y XHY

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

Colorblindness A person with normal color vision sees a number seven in the circle above. Those who are color blind usually do not see any number at all.

Colorblindness RED-GREEN COLORBLINDNESS: People with red-green color blindness see either a three or nothing at all. Those with normal color vision see an 8.

In humans, red-green colorblindness is a sex-linked trait 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 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 Y

Xn XN XNXn Y

Xn XN XNXn Y

Xn XN XNXn Y XnY

Xn XN XNXn Y XnY

Genotypic ratio: 2 XNXn : 2 XnY Phenotypic ratio: 2 normal females: 2 males with colorblindness

Muscular Dystrophy - Results in weakening/loss of muscles - Caused by defective version of gene that codes for muscle

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.