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Answers to Concept Checks

1. b
2. With complete dominance, the heterozygote expresses the same phenotype as that of one of the homozygotes. With incomplete dominance, the heterozygote has a phenotype that is intermediate between the two homozygotes. With codominance, the heterozygote has a phenotype that simultaneously expresses the phenotypes of both homozygotes.
3. d
4. b
5. c
6. Gene interaction is interaction between genes at different loci. Dominance is interaction between alleles at a single locus.
7. d
8. Cross a bulldog homozygous for brindle with a Chihuahua homozygous for brindle. If the two brindle genes are allelic, all the offspring will be brindle: bb × bb → all bb (brindle). If, on the other hand, brindle in the two breeds is due to recessive genes at different loci, then none of the offspring will be brindle: a⁺a⁺ bb × aa b⁺b⁺ → a⁺a b⁺b.
9. Both sex-influenced and sex-limited traits are encoded by autosomal genes whose expression is affected by the sex of the individual organism possessing the gene. Sex-linked traits are encoded by genes on the sex chromosomes.
10. Cytoplasmically inherited traits are encoded by genes in the cytoplasm, which is usually inherited only from the female parent. Therefore, a trait due to cytoplasmic inheritance will always be passed through females. Traits due to genetic maternal effect are encoded by autosomal genes and can therefore be passed through males, although any individual organism’s trait is determined by the genotype of the maternal parent.
11. Methylation of DNA
12. Cross two eyeless flies and cross an eyeless fly with a wild-type fly. Raise the offspring of both crosses in the same environment. If the trait is due to a recessive mutation, all the offspring of the two eyeless flies should be eyeless, whereas at least some of the offspring of the eyeless and wild-type flies should be wild type. If the trait is due to a phenocopy, there should be no differences in the progeny of the two crosses.
13. Polygeny refers to the influence of multiple genes on the expression of a single characteristic. Pleiotropy refers to the effect of a single gene on the expression of multiple characteristics.

WORKED PROBLEMS

Problem 1

A geneticist crosses two yellow mice with straight hair and obtains the following progeny:

a. Provide a genetic explanation for the results and assign genotypes to the parents and progeny of this cross.
b. What additional crosses might be carried out to determine if your explanation is correct?

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Solution Strategy

What information is required in your answer to the problem?

a. A genetic explanation for the inheritance of color and hair type in the mice. Genotypes of the parents.
b. Examples of other crosses that might be carried out to determine if the explanation given in (a) is correct.

What information is provided to solve the problem?

Phenotypes of the parents.
Phenotypes and proportions of different types of progeny.

For help with this problem, review:

Lethal Alleles in Section 5.1 and ratios for simple genetic crosses (Table 3.5).

Solution Steps

a. This cross concerns two separate characteristics—color and type of hair. First, let’s look at the inheritance of color. Two yellow mice are crossed, producing yellow mice and gray mice.

Hint: Examine the progeny ratios for each trait separately.

We learned in this chapter that a 2 : 1 ratio is often produced when a recessive lethal gene is present:

Recall: A 2 : 1 ratio is usually produced by a lethal gene.

Now, let’s examine the inheritance of the hair type. Two mice with straight hair are crossed, producing mice with straight hair and mice with fuzzy hair. We learned in Chapter 3 that a 3 : 1 ratio is usually produced by a cross between two individuals heterozygous for a simple dominant allele:

Hint: Table 3.3 gives phenotypic ratios for simple genetic crosses.

We can now combine both loci and assign genotypes to all the individual mice in the cross:
b. We could carry out a number of different crosses to test our hypothesis that yellow is a recessive lethal and straight is dominant over fuzzy. For example, a cross between any two yellow mice should always produce yellow and gray offspring, and a cross between two gray mice should produce only gray offspring. A cross between two fuzzy mice should produce only fuzzy offspring.

Problem 2

In some sheep, the presence of horns is produced by an autosomal allele that is dominant in males and recessive in females. A horned female is crossed with a hornless male. One of the resulting F₁ females is crossed with a hornless male. What proportion of the male and female progeny from this cross will have horns?

Solution Strategy

What information is required in your answer to the problem?

Proportion of male and female progeny that have horns.

What information is provided to solve the problem?

Symbols for horned and hornless alleles.
The presence of horns is due to an autosomal gene that is dominant in male and recessive in females.
A horned female is crossed with a hornless male. A resulting F₁ female is crossed with a hornless male to produce progeny.

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For help with this problem, review:

Sex-Influenced and Sex-Limited Characteristics in Section 5.3.

Solution Steps

The presence of horns in these sheep is an example of a sex-influenced characteristic. Because the phenotypes associated with the genotypes differ for the two sexes, let’s begin this problem by writing out the genotypes and phenotypes for each sex. We will let H represent the allele that encodes horns and H⁺ represent the allele that encodes hornless. In males, the allele for horns is dominant over the allele for hornless, which means that males homozygous (HH) and heterozygous (H⁺H) for this gene are horned. Only males homozygous for the recessive hornless allele (H⁺H⁺) are hornless. In females, the allele for horns is recessive, which means that only females homozygous for this allele (HH) are horned; females heterozygous (H⁺H) and homozygous (H⁺H⁺) for the hornless allele are hornless. The following table summarizes genotypes and associated phenotypes:

Hint: Write out the genotypes and the associated phenotype for each sex.

Recall: When a trait is dominant, both the homozygote and the heterozygote express the trait in their phenotypes.

Genotype	Male phenotype	Female phenotype
HH	horned	horned
HH⁺	horned	hornless
H⁺H⁺	hornless	hornless

In the problem, a horned female is crossed with a hornless male. From the preceding table, we see that a horned female must be homozygous for the allele for horns (HH) and a hornless male must be homozygous for the allele for hornless (H⁺H⁺); so all the F₁ will be heterozygous; the F₁ males will be horned and the F₁ females will be hornless, as shown in the following diagram:

A heterozygous hornless F₁ female (H⁺H) is then crossed with a hornless male (H⁺H⁺):

Therefore, of the male progeny will be horned, but none of the female progeny will be horned.