Sex-linked Inheritance

Objectives

• Know the difference between autosomal chromosomes and sex chromosomes.

• Understand sex-linked inheritance.

• Be able to create a family pedigree.

• Solve sex-linked problems using a Punnett square

Gender Determinationin Humans

• Humans have 23 pairs of chromosomes.• 22 of them are called autosomal.• The 23rd pair are the sex chromosomes.• The sex chromosomes determine

whether an offspring will be male or female.

• XX = Female• XY = Male

Pedigree

XY

XY XY

XX

XX

Father Mother

Daughter

Son Son

Marriage

Children

Oldest Youngest

Red/Green Colorblindness• The gene which allows us to

distinguish between red and green is on the X chromosome.

= Female with normal visionXCXCXCXc

= Female carrier

XcXc= Female who is colorblind

Red/Green Colorblindness• For males, it appears to be easier

to inherit sex-linked traits.

= Male with normal vision

= Male who is colorblind

XCYXcY

Pedigree

?? ????

Father Mother

Daughter

Son Son

XYc XXC C

Pedigree

XY XX

Father Mother

Daughter

Son Son

c C C

XXcC XYC XYC

Using a Punnett Square

XXC cXYc

Y

Xc

Xc

XCXc

XC

Xc

Xc

Xc

XC YY

Genotype & Phenotype

XXC cXYc

Y

Xc

Xc

XCXc

XC

Xc

Xc

Xc

XC YY

Female Carrier Female Colorblind

Male Normal Male Colorblind

Hemophilia

XX

XX

XX

XY

XY

H

HH

H

h

hh

h

= Female normal

= Female carrier

= Female with hemophilia

= Male normal

= Male with hemophilia

A sex-linked bleeding disorder (dangerous)

Review

• What is the general format for a pedigree?

• Why are Punnett Squares better to use sometimes?

• Why do sex-linked traits affect

males more frequently than

females?

Important Point

• Some sex-linked traits are dominant. This means that even if the female is only a carrier of the illness, she will be affected with the disease.

• The dominant gene dominates the gene pair.

Practice Problems

1. Muscular dystrophy is a sex-linked illness. Draw a pedigree showing the sons and daughter of an affected father and a normal mother.

2. Draw a Punnett Square to show the same illness for an affected father and a carrier mother.