Ch. 14 - Mendel and the Gene Idea

Campbell - Campbell Biology 11th Edition

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Urry 11th Edition

Ch. 14 - Mendel and the Gene Idea

Problem 10

Chapter 14, Problem 10

In 1981, a stray black cat with unusual rounded, curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the curl cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true-breeding variety. How would you determine whether the curl allele is dominant or recessive? How would you obtain true-breeding curl cats? How could you be sure they are true-breeding?
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Verified step by step guidance

Step 1: Begin by understanding the concept of dominant and recessive alleles. A dominant allele is one that expresses its trait even if only one copy is present, while a recessive allele requires two copies to express the trait.

Step 2: To determine if the curl allele is dominant or recessive, perform a test cross. Mate the curl cat with a cat that has normal ears (homozygous recessive for the ear trait). Observe the offspring: if all offspring have curled ears, the curl allele is likely dominant. If some offspring have normal ears, the curl allele is likely recessive.

Step 3: To obtain true-breeding curl cats, continue breeding cats with curled ears. If the curl allele is dominant, breed cats that are homozygous dominant (both alleles are for curled ears). If the curl allele is recessive, breed cats that are homozygous recessive (both alleles are for curled ears).

Step 4: To ensure the cats are true-breeding, perform several generations of breeding and observe the offspring. True-breeding cats will consistently produce offspring with the same curled ear trait.

Step 5: Confirm true-breeding status by genetic testing, if available, to verify the homozygosity of the alleles responsible for the curled ears. This ensures that the trait will be passed on reliably to future generations.

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Key Concepts

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

Dominant and Recessive Alleles

Dominant alleles are those that express their trait even when only one copy is present, while recessive alleles require two copies to express the trait. To determine if the curl allele is dominant or recessive, you would perform a test cross with a homozygous recessive individual. If the curl trait appears in all offspring, the allele is likely dominant; if not, it is recessive.

True-Breeding

True-breeding organisms consistently pass down specific phenotypic traits to their offspring. This occurs when the organism is homozygous for the trait in question. To develop true-breeding curl cats, you would need to breed individuals that consistently produce offspring with the curl trait, ensuring they are homozygous for the curl allele.

Test Cross

A test cross involves breeding an individual with a dominant phenotype but unknown genotype with a homozygous recessive individual. This helps determine the genotype of the dominant individual. If any offspring display the recessive phenotype, the dominant individual is heterozygous. This method is crucial for confirming true-breeding status in curl cats.

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Test Crosses

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Textbook Question

The genotype of F₁individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four genes, what are the probabilities that F₂ offspring will have the following genotypes?

a. aabbccdd

b. AaBbCcDd

c. AABBCCDD

d. AaBBccDd

e. AaBBCCdd

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Textbook Question

What is the probability that each of the following pairs of parents will produce the indicated offspring? (Assume independent assortment of all gene pairs.)

a. AABBCC×aabbcc→AaBbCc

b. AABbCc×AaBbCc→AAbbCC

c. AaBbCc×AaBbCc→AaBbCc

d. aaBbCC×AABbcc→AaBbCc

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Textbook Question

Karen and Steve each have a sibling with sickle-cell disease. Neither Karen nor Steve nor any of their parents have the disease, and none of them have been tested to see if they carry the sickle-cell allele. Based on this incomplete information, calculate the probability that if this couple has a child, the child will have sickle-cell disease.

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Textbook Question

In tigers, a recessive allele of a particular gene causes both an absence of fur pigmentation (a white tiger) and a cross-eyed condition. If two phenotypically normal tigers that are heterozygous at this locus are mated, what percentage of their offspring will be cross-eyed? What percentage of cross-eyed tigers will be white?

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Textbook Question

In maize (corn) plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when homozygous. At a different locus, the dominant allele P causes purple kernel color, while the homozygous recessive genotype pp causes red kernels. If plants heterozygous at both loci are crossed, what will be the phenotypic ratio of the offspring?

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Textbook Question

The pedigree below traces the inheritance of alkaptonuria, a biochemical disorder. Affected individuals, indicated here by the colored circles and squares, are unable to metabolize a substance called alkapton, which colors the urine and stains body tissues. Does alkaptonuria appear to be caused by a dominant allele or by a recessive allele? Fill in the genotypes of the individuals whose genotypes can be deduced. What genotypes are possible for each of the other individuals?

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