Genetics Worksheet 5: Sex LinkageName:

Sex Linkage is a non- Mendelian trait because the genes are located on the sex chromosomes and therefore do not independently assort. Males only have one X-chromosome so whatever the allele they possess on that chromosome is their phenotype. Females have two X-chromosomes and therefore can be heterozygous for a trait.

  1. Why is it more likely for a male to have an X-linked trait than a female?
  1. Can men be carriers of a recessive X-linked disease? Explain.
  1. In humans, the gene for hemophilia is located on the X chromosome with no corresponding gene on the Y. Hemophilia (inability to clot your blood) is caused by a recessive allele.

Male / Female
Without disease / XAY / XAXA
Carrier of the disease / XAXa
With disease / XaY / XaXa
  1. If a man and a woman, both with normal clotting factors, have a hemophiliac son, what are their genotypes? and
  2. Complete the following Punnett square to illustrates how two non diseased people could have a son with the disease.Place an asterisk next to the son with the disease.
  1. If this woman, with normal clotting factors, were widowed and later had children with a hemophiliac man, complete a Punnett square to show the type(s) of children that could be expected.

What is the phenotypic ratio?

hemophiliac male: normal clotting male:

hemophiliac female: normal clotting female

  1. Color blindnessis the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. It is a recessive trait. Color blindness is usually sex-linked because the genes that produce photo-pigments in humans are carried on the X chromosome.
  1. A color-blind man has children with a woman with normal vision. Their son had normal vision but their daughter was color-blind. Complete the following Punnett square to show how that is possible.
  1. What is the probability that they will have a colorblind son?
  2. What is the probability that they will have a daughter with normal vision?
  1. The exact genes carried on the X chromosome varies among species.In cats, one of several genes controlling fur color is located on the X chromosome. The gene has two alleles: orange fur (XO) and black fur (XB). A female cat heterozygous to that gene (XBXO) will not be orange, instead, her coat will be apatchworkof orange and black, a condition known astortoiseshell.See the chart below.

Phenotype / Black / Orange / Tortoiseshell
Genotype / XBY or XBXB / XOY or XOXO / XBXO
  1. Why can’t male cats have a tortoiseshell phenotype?
  1. In the Punnett square below show a mating of a black male cat and an orange female cat.
  1. What is the phenotypic ratio?

orange male: black male: orange female: black female: calico female

  1. Besides sex linkage does this gene show simple dominance, incomplete dominance, or codominance? How do you know?
  1. Thomas Hunt Morgan, born in Lexington, KY, is considered the father of modern genetics because he discovered sex linked traits in fruit flies. Fruit fly eye color is linked to the X chromosome and has two alleles, red (XR) and white (Xr) .Red-eyes are dominant over white-eyes.
  1. A homozygous red-eyed female is mated to a white-eyed male. What are the genotypesof the parents?and
  1. Use the Punnett Square below to show the F1 generation.
  1. What is the genotype and phenotype of the male flies? and
  1. What is the genotype and phenotype of the female flies?and
  1. Now use the Punnett Square below to show a cross of the F1 to get the F2 generation.
  1. What is the phenotypic ratio for the F2?

red-eyed male: white-eyed male: red-eyed female: white-eyed female

  1. One gene for hypertensionor high blood pressure appears to be Y-linked in rats.
  1. Is it possible for female rats to get this form of hypertension? Why?
  1. Can a male rat with hypertension have any sons without hypertension? Why?
  2. Use the Punnett Square below to show a male rat with hypertension having offspring with a female rat without hypertension.
  1. What is the phenotypic ratio?

hypertensive male: non-hypertensive male:

hypertensive female: non-hypertensive female

  1. Scientists use pedigrees to help show how sex linked traits can be inherited in families. Determine the genotypes for each person in the family in this pedigree. Remember circles represent females and squares represent males. If the shape is shaded then the person has the recessive disorder. You may use any letter you choose to show this X-linked recessive disease.

Person 1:

Person 2:

Person 3:

Person 4:

Person 5:

Person 6:

Person 7:

Person 8:

Person 9:

Person 10:

Person 11:

Person 12:

Person 13:

Person 14:

Person 15: