• You need to know how to use Mendel's laws of segregation and independent assortment to analyze patterns of inheritance.

• You need to know how dominant, recessive, codominant, sex-linked, polygenic, and multiple allele modes of inheritance cause observed inheritance patterns. 

Hemophilia is a sex-linked, recessive trait. Which of the following describes the probability of hemophilia in the offspring of a man who does not have hemophilia and a woman whose father is a hemophiliac?

A. Each of their sons will have hemophilia. 

B. None of their daughters will have hemophilia. 

C. Their sons have a 25% chance of having hemophilia. 

D. There is a 50% chance that their daughters will have hemophilia.

Alleles for the A and B blood cell antigens are codominant. The condition where no antigens are present on the blood cells (type O blood) is a recessive trait. Which set of parents can most likely produce a child with type O blood? 

A. one parent with type AB blood, and the other parent with type A blood

B. one parent with type AB blood and the other parent with type O blood

C. one parent with heterozygous type A blood, and the other parent with type O blood

D. one parent with homozygous type A blood, and the other parent with homozygous type B blood

One of the parents of a child has phenylketonuria (PKU), which is caused by recessive alleles. The other parent does not have the PKU alleles. What is the chance that the couple will have a child with phenylketonuria? 

A. 0%

B. 50%

C. 75%

D. 100%

Hitchhiker's thumb (H) is dominant to no hitchhiker's thumb (h). A woman who does not have hitchhiker's thumb marries a man who is heterozygous for hitchhiker's thumb. What is the probable genotypic ratio of their children? 

A. 0% Hh: 100% hh

B. 50% Hh: 50% hh

C. 75% Hh: 25% hh

D. 100% Hh: 0% hh

This diagram shows a pedigree for a recessive genetic disorder.

What is the genotype of individual 6?

A. XHXH

B. XHXh

C. XHY

D. XhY

This diagram shows a diploid cell with two pairs of homologous chromosomes. 

Due to independent assortment, what is the possible genetic make-up of gametes produced by this organism?

A. SsTt

B. Ss, Tt

C. S, s, T, t

D. ST, St, sT, st

The table lists the trials for fruit color where allele R exhibits incomplete dominance over allele R'. 

Heterozygous fruit have orange phenotypes. What percent of offspring are expected to have an orange phenotype if the parent plants are orange (RR') and yellow (R'R')?

A. 25%

B. 50%

C. 75%

D. 100%

Mendel hypothesized that reproductive cells have only one factor for each inherited trait. This hypothesis is supported by which observation? 

A. Haploid cells are produced by mitosis.

B. Diploid cells are produced by mitosis. 

C. Haploid cells are produced by meiosis.

D. Diploid cells are produced by meiosis.

A human baby boy inherits a recessive allele from his mother. In which circumstance would he most likely show the trait coded for by the recessive allele?

A. The baby inherits the dominant allele from his father. 

B. The allele is on an autosomal chromosome and the baby is a twin. 

C. The allele is on the X chromosome.

D. The allele is on the Y chromosome. 

1. Correct answer: B. None of their daughters will have hemophilia.

Sex-linked traits are only present on sex chromosomes. Hemophilia is an X-linked chromosome so a father who does not have hemophilia must donate a non-hemophilia gene on the X chromosome he contributes to his daughter. For a girl to have a sex-linked condition, both of her X chromosomes have to have the allele for that condition.

2. Correct answer: C. one parent with heterozygous type A blood, and the other parent with type O blood

In order for a recessive condition to be expressed, the individual must have two recessive alleles, one from the mother and one from the father. C is the only condition in which both parents possess an O allele to pass on to the offspring.

3. Correct answer: A. 0%

For a recessive condition to be passed on, the offspring must inherit one allele for that condition from each parent. In this case, the father has no PKU alleles to pass on to the child, so even if the mother passes one on, it will not be expressed.

4. Correct answer: B: 50% Hh: 50% hh 

See Punnett square below:

5. Correct answer: D. XhY

Individual 6 is the offspring of individuals 1 and 2. Males are always represented with squares. If the disorder is recessive, the Father’s genotype must be XhY for the disorder to be expressed. Because the condition is expressed in the son (individual 6), we can infer that the mother’s genotype is heterozygous (XHXh) because she must have contributed an X chromosome with the allele for the gene to her son, but did not have the condition herself. Individual 6 has the genotype XhY.

6. Correct answer: D. ST, St, sT, st

Gametes are haploid and contain only half of the number of chromosomes of the parent cells (one of each chromosome type).

7. Correct answer: B. 50% of the offspring would have an orange phenotype.

8. Correct answer: C. Haploid cells are produced by meiosis.

Although each cell in the human body has two copies of each chromosome with one allele for each trait on them, the gametes (egg and sperm) have only one copy of each chromosome with one allele for each trait.

9. Correct answer: C. The allele is on the X chromosome.

Traits coded for on the X chromosome will always be expressed in males because there is no counterpart allele to mask the expression. Boys have XY genotype and girls have XX genotype.