Download Sex-Linked Genetics Problems File

Name _____________________________
Sex-Linked & Dihybrid Genetics Problems
-- Draw Punnett squares to support your answers -1. Hemophilia is a recessive sex-linked (X-linked) trait.
Show how the following will be represented:
Genotype
Phenotype
Homozygous dominant alleles
Female
Heterozygous alleles
Homozygous recessive alleles
Dominant allele
Male
Recessive alleles
a) If a woman who carries the hemophilia allele has children with a normal male, what
proportion of the male children will be hemophiliacs? The female children?
b) If a normal woman has children with a hemophiliac male, what proportion of the male
children will be hemophiliacs? The female children?
2. A man with color-blindness (sex-linked recessive) and a woman who is a carrier for
color-blindness are planning to have children. As their genetic counselor, find out the
probability of their having a color-blind child.
3. Coat color in cats is an X-linked gene, with alleles for black and orange, so XBXB and XBY
cats will have a black coat, while XOXO and XOY will have an orange coat. Another possible
combination for female cats would be XBXO. Both of the color alleles would be expressed,
so the cat would end up being partially orange and partially black (calico).
a) What type of dominance is this? ________________________________
b) A calico cat is bred with an orange cat.
What are genotypic and phenotypic ratios for each gender of offspring.
Extention Questions: Dihybrid Crosses (Not required at the SL level, but still fun!)
4. Free ear lobes (F) are dominant in humans to the recessive attached ear lobe allele (f).
Show the cross between a carrier for hemophilia that is homozygous for free ear lobes and a
man with hemophilia and attached ear lobes.
5. Straight thumbs (T) are dominant in humans to the recessive curved thumb allele (t). Show
the cross between a woman homozygous for normal vision with a curved thumb and a man
with color-blindness and a straight thumb - but his mother has a curved thumb.