Sex-Linked Punnett

KaryotypeA photograph of a cell’s chromosomes arranged in pairs according to size

Normal Karyotype: 23 pairs of chromosomes

•pairs 1-22 are autosomes• pair 23 are sex chromosomes

24-5

Sex Chromosomes

X

Y

X chromosome• has over 1,000 genes• most genes on the X chromosome do not have corresponding alleles on the Y chromosome

Y chromosome• has only a few dozen genes• some genes are unique only to the Y chromosome

Sex-Linked TraitsThose traits whose genes

are found on the X chromosome but NOT on the Y chromosome

Examples of Sex-linked Traits:

Red-green colorblindnessMale Pattern BaldnessHemophilia (Queen Victoria was a carrier)

Duchenne Muscular Dystrophy (fatal)

Color Blindness “tests”

The test to the left is simpler.

The individual with normal color vision will see a 5 revealed in the dot pattern.

An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.

The test below is simpler. The individual with normal color vision will see a 5 revealed in the dot pattern. An individual with Red/Green (the most common) color blindness will see a 2 revealed in the dots.

Females = XX –carry 2 alleles for a sex-linked trait

Males = XY – only carry 1 allele for a sex-linked trait, so it is easier for them to get sex-linked diseases

Sex-Linked Traits, cont.

Males receive sex-linked traits from their mother because their mother gives them the X chromosome and their father gives them the Y chromosome

Example of Sex-Linked Trait

H = Normal blood clotting h = Hemophilia

Genotypes Phenotypes HH Normal Hh Normal/Carrier hh Hemophilia

Example of Sex-Linked TraitNormal male:Male with Hemophilia:Normal female (not a carrier for

hemophilia – homozygous): Normal female (carrier for

hemophilia – heterozygous): Female with Hemophilia:

XHY

XhY

XHXH

XhXh

XHXh

Sample Problems

1. XHXh x XHYChances of:a. hemophiliac

child? b. hemophiliac

son? c. hemophiliac

daughter? d. daughter

who is carrier?

XHXH

XH

YXhY

XHXh

XH

Y

XH Xh¼ or 25%

½ or 50%

0%

½ or 50%

2. A woman who is a carrier for hemophilia marries a hemophiliac man.

Chances of:a. hemophiliac

child? b. hemophiliac

son? c. hemophiliac

daughter?

XHXh

XH

YXhY

XhXh

Xh

Y

XH Xh

2/4 or 50%

½ or 50%

½ or 50%

XHXh x XhY

3. A normal man marries a Homozygous normal woman.

Chances of: a. Will any of

their kids be hemophiliacs?

XHXH

XH

YXHY

XHXHXH

Y

XH XH

0/4 or 0%

XHXH x XHY

A normal woman (whose father is a hemophiliac) marries a hemophiliac man.

What are their genotypes?

XHX? XhY

Her dad - XhY

XHXh and XhY