Unit 6: Genetics

Color blindness is a sex-linked recessive trait. A mother with normal color vision and a color blind father have a color blind daughter. Which statement is true about the offspring?

• All of their daughters must be color blind.

• Some of their sons can have normal color vision.

• All of their sons will carry a gene for color blindness.

• None of their daughters will carry a gene for color blindness.

Answer explanation

Some of their sons can have normal color vision. The normal vision mother must be a carrier and have one allele for colorblindness. The daughters would definitely receive the colorblind allele from their father, so there is no "might" about it. The daughters have a 50% chance of receiving the colorblind allele from their mother. The sons also have a 50% chance of receiving the colorblind allele from their mother. Since this allele is sex-linked and X-linked, the sons cannot receive the allele from their father.

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Snapdragons, a flowering plant popular with gardeners, can have red, white, and pink colored petals. When snapdragons with red petals (RR) are crossed with snapdragons having white (WW) petals, the offspring have pink petals. The pink coloration (RW) is a phenotype in between the red and white parental phenotypes. What is this kind of inheritance pattern called?

• bipolar inheritance

• recessive inheritance

• classic dominance

• incomplete dominance

Answer explanation

When the phenotype of the offspring is intermediate or between the phenotypes of the parents, the situation is referred to as incomplete dominance.

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A man who is normal for color vision marries a woman who is colorblind. What is the probability their offspring will be colorblind if all of their children are male?

• 75%

• 100%

• 50%

• 25%

Answer explanation

100% of their offspring will be colorblind because the recessive allele will be expressed in male offspring that inherit it. Colorblindness is sex-linked on the X chromosome and recessive. Since males have only one X-chromosome, if this chromosome has the red-green color blind allele, the males will be colorblind.

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A pedigree chart like this one is characteristic of _________________ disorders.

• dominant allelic

• sex-linked dominant

• sex-linked recessive

• autosomal recessive

Answer explanation

A pedigree chart like this one is characteristic of sex-linked recessive disorders such as hemophilia.

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Color blindness is a sex-linked trait. If we could see the pedigree chart for several more generations of the family illustrated here, we would expect

• no females to ever be color blind.

• more females to be color blind.

• an equal number of males and females that are color blind.

• more males to be color blind.

Answer explanation

If we could see the pedigree chart for several more generations of the family illustrated here, we would expect more males to be color blind. Because the sex-linked trait is X-linked and recessive, the males will inherit the condition with one allele on the inherited X chromosome. The females must inherit two alleles to have the condition, but this is not impossible to do.

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The production of pink flowered plants from a cross between a red flowered plant and a white flowered plant is an example of

• codominant inheritance.

• complete dominance.

• incomplete dominance.

• multiple alleles.

Answer explanation

The production of pink flowers results from incomplete dominance in a genetic cross between a red flowering plant and a white flowering plant. The expression of the recessive allele is not completed blocked by the expression of the dominant allele.

Mr. Jones has blood type B and Mrs. Jones has blood type AB. What is the probability that they will have a child with blood type A if both of Mr. Jones’s parents were AB?

• 100%.

• 25%.

• 50%.

• 0%.

Answer explanation

0%.Mr Jones must be BB if both parents were AB. Otherwise he would be AB or AA. He does not have a recessive O gene that would allow any of his kids to have type A blood. They would all be B or AB.

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The ABO blood type system in humans demonstrates

• incomplete dominance.

• multiple allelism.

• complete integration.

• sex-linkage.

Answer explanation

Multiple allelism. More than two possible alleles exist for expression of blood type in humans.