11.3 Study Guide

Question 1

Explain the difference between sex-linked and autosomal genes. What is the primary difference between the genes of offspring resulting from these different modes of inheritance?

Answer 1

Sex-linked genes are found on the sex chromosomes, while autosomal genes are found on any of the non-sex chromosomes. While autosomal genes produce uniform phenotypic ratios between all offspring, sex-linked genes can produce differing phenotypic ratios based on the sex of the offspring.

Question 2

Define a carrier. How can X-inactivation affect how a carrier is affected or unaffected by a sex-linked recessive gene?

Answer 2

Because carrier females have one copy of a sex-linked dominant gene and one copy of a disease-causing sex-linked recessive gene, X-inactivation of the chromosome with the dominant gene may cause the recessive gene to be partially expressed instead of only the dominant gene being expressed.

Question 3

What is the purpose of a pedigree? Give three examples of a symbol from a pedigree and its meaning.

Answer 3

A pedigree is used to determine things like a gene’s mode of inheritance by using a chart to track how the gene is passed through several generations of a family.
Possible Examples: Circle = Female; Square = Male; Shaded = Affected; Half-Shaded = Carrier; Cut in Half = Deceased; Double Offspring Line = Twins; Brackets = Adopted; Triangle = Miscarriage.

Question 4

Explain why males are more likely than females to be affected by sex-linked recessive genes.

Answer 4

Males are more likely than females to be affected by sex-linked recessive genes because they only require one copy of the gene to be affected, as their two sex chromosomes are different from each other and code for different things.

Question 5

Using a Punnett square or a forked diagram, find the phenotypic ratios of the offspring of a cross between an X(A)X(a) parent and an X(a)Y parent, given that the gene in question is an X sex-linked recessive gene.

Answer 5

1/4 Carrier Female : 1/4 Affected Female : 1/4 Unaffected Male : 1/4 Affected Male

Question 6

Find a gene’s mode of inheritance given a pedigree with two affected parents, one affected son, and one unaffected daughter.

Answer 6

Based on the pedigree, the gene could only have an autosomal dominant mode of inheritance.

Question 7

Find a gene’s mode of inheritance given a pedigree with a carrier mother, an affected father, an affected daughter, and an unaffected son.

Answer 7

Based on the pedigree, the gene could actually follow any mode of inheritance except for autosomal dominant.