Sex-linked TraitsUsing punnett squares

INHERITANCE OF SEX CHROMOSOMES

Humans have 23 pairs of chromosomes

Autosomes: 22 pairs are non-sex determining chromosomes

the 23rd pair determines the sex of the offspring

female sex chromosomes - XX

male sex chromosomes - XY

( y is smaller than x and has different genes)

Who determines the child's sex?

the male

SEX- LINKED TRAITS

an inherited trait that has a gene on the x chromosome but no corresponding gene on the y chromosome (genes that only exist only on the sex chromosomes)

Examples of human sex-linked traits

Hemophilia Red-green color blindness Muscular dystrophy Male pattern baldness

How to work sex-linked trait problems

Hemophilia: is a rare, inherited bleeding disorder in which your blood doesn’t clot normally. If you have hemophilia, you may bleed for a longer time than others after an injury. You also may bleed internally, especially in your knees, ankles, and elbows. This bleeding can damage your organs or tissues and, sometimes, be fatal.

All possible genotypes & phenotypes:

genotypes phenotypes

XHXH

XHXh

XhXh

XHYXhY

Normal femaleFemale carrierFemale hemophiliacNormal maleMale hemophiliac

What is a carrier?

carrier - expresses the normal characteristic but has both dominant and recessive genes (have the gene for a trait but do not express the trait themselves)

for sex-linked traits, males cannot be carriers

XHXh

How to work sex-linked trait problems:start with this type of punnett square

X X X X

X Y X YY

X

X X

How to work sex-linked trait problems:put the genes on the X chromosome

X X X X

X Y X YY

X

X X

Cross a normal husband with a carrier wife

H

H

h

H H

H

H h

h