Ch. 14 - Mendel and the Gene

Freeman - Biological Science 8th Edition

Freeman8th EditionBiological ScienceISBN: 9780138276263Not the one you use?Change textbook

Freeman 8th Edition

Ch. 14 - Mendel and the Gene

Problem 2

Chapter 14, Problem 2

Why is the pea wrinkle-seed allele a recessive allele?
a. It 'recedes' in the F2 generation when homozygous parents are crossed.
b. The trait associated with the allele is not exhibited in heterozygotes.
c. Individuals with the allele have lower fitness than that of individuals with the dominant allele.
d. The allele is less common than the dominant allele. (The wrinkled allele is a rare mutant.)

Verified step by step guidance

Understand the concept of dominant and recessive alleles: In genetics, a dominant allele is one that expresses its trait even when only one copy is present (heterozygous condition), while a recessive allele only expresses its trait when two copies are present (homozygous condition).

Analyze the options given: The question provides four options, each suggesting a reason why the pea wrinkle-seed allele is recessive.

Evaluate option b: 'The trait associated with the allele is not exhibited in heterozygotes.' This option aligns with the definition of a recessive allele, as the trait (wrinkled seeds) would not be visible in heterozygous individuals (those with one dominant and one recessive allele).

Consider the other options: Option a refers to the behavior of alleles in the F2 generation, which is more about Mendelian inheritance patterns rather than the definition of recessiveness. Option c discusses fitness, which is not directly related to whether an allele is recessive. Option d mentions allele frequency, which does not determine dominance or recessiveness.

Conclude that option b is the most accurate explanation for why the pea wrinkle-seed allele is considered recessive, as it directly relates to the expression of the trait in heterozygotes.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Recessive Allele

A recessive allele is one that does not produce a phenotype when paired with a dominant allele in a heterozygous organism. It requires two copies (homozygous) to express the trait. In the context of pea plants, the wrinkle-seed allele is recessive because its trait is not visible in heterozygotes, who display the dominant round-seed phenotype.

Heterozygote

A heterozygote is an organism that has two different alleles for a particular gene, one dominant and one recessive. In pea plants, heterozygotes for the seed shape gene have one allele for round seeds and one for wrinkled seeds, but only the round seed phenotype is expressed due to the dominance of the round-seed allele.

Dominant Allele

A dominant allele is one that expresses its phenotype even when only one copy is present in a heterozygous organism. In the case of pea plants, the round-seed allele is dominant over the wrinkle-seed allele, meaning that peas with at least one round-seed allele will exhibit the round-seed phenotype, masking the presence of the recessive wrinkle-seed allele.

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Dominant vs. Recessive Alleles

Related Practice

Textbook Question

The genes for the traits that Mendel worked with are either all located on different chromosomes or behave as if they were. How did this help Mendel recognize the principle of independent assortment?

a. Otherwise, his dihybrid crosses would not have produced a 9 : 3 : 3 : 1 ratio of F2 phenotypes.

b. The occurrence of individuals with unexpected phenotypes led him to the discovery of recombination.

c. It led him to the realization that the behavior of chromosomes during meiosis explained his results.

d. It meant that the alleles involved were either dominant or recessive, which gave 3 : 1 ratios in the F1 generation.

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Textbook Question

The alleles found in haploid organisms cannot be dominant or recessive. Why?

a. Dominance and recessiveness describe which of two possible phenotypes are exhibited when two different alleles occur in the same individual.

b. Because only one allele is present, alleles in haploid organisms are always dominant.

c. Alleles in haploid individuals are transmitted like mitochondrial DNA or chloroplast DNA.

d. Most haploid individuals are bacteria, and bacterial genetics is completely different from eukaryotic genetics.

Textbook Question

Two black female mice are crossed with a brown male. In several litters, female I produced 9 black offspring and 7 brown; female II produced 57 black offspring. What deductions can you make about the inheritance of black and brown coat color in mice? What are the genotypes of the parents?

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Textbook Question

A plant with orange, spotted flowers was grown in the greenhouse from a seed collected in the wild. The plant was self-pollinated and gave rise to the following progeny: 88 orange with spots, 34 yellow with spots, 32 orange with no spots, and 8 yellow with no spots. What can you conclude about the dominance relationships of the alleles responsible for the spotted and unspotted phenotypes? What can you conclude about the genotype of the original plant that had orange, spotted flowers?