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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First, determine if the curl allele is dominant or recessive by breeding the original curl cat with a normal-eared cat. If any of the offspring have curled ears, the curl allele is likely dominant. If none of the offspring have curled ears, the curl allele is likely recessive.

To develop a true-breeding line, if the curl trait is dominant, continue to breed the curled ear cats with each other. Selectively breed those that consistently produce curled ear offspring.

If the curl trait is recessive, breed two cats that have curled ears together, as they would be homozygous recessive (cc). Their offspring should all have curled ears if they are true-breeding.

To ensure the cats are true-breeding, perform test crosses with normal-eared cats. If the offspring from these crosses all exhibit the expected phenotype (curled or normal ears based on the dominant or recessive nature of the allele), then the line can be considered true-breeding.

Maintain detailed breeding records including phenotypes of all offspring to monitor the inheritance pattern over several generations. This will help confirm the genetic consistency of the curled ear trait in the population.

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Key Concepts

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

Dominance in Genetics

In genetics, dominance refers to the relationship between alleles, where one allele can mask the expression of another. A dominant allele will express its trait even in the presence of a recessive allele, while a recessive allele only expresses its trait when paired with another recessive allele. Understanding whether the curl allele is dominant or recessive is crucial for predicting the traits of offspring in breeding experiments.

True-Breeding

True-breeding organisms are those that consistently produce offspring with the same phenotype when mated with one another. This occurs when the organism is homozygous for a particular trait, meaning it has two identical alleles. To develop true-breeding curl cats, one would need to breed individuals that exhibit the curl trait and analyze the offspring to ensure they consistently display the same trait.

Punnett Squares

Punnett squares are a tool used in genetics to predict the genotypes and phenotypes of offspring from a cross between two parents. By setting up a Punnett square with the alleles of the curl cat and its mate, one can visualize the potential genetic combinations and determine the likelihood of producing offspring with the curl trait. This method is essential for understanding inheritance patterns and confirming whether the curl allele is dominant or recessive.

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Textbook Question

The genotype of F1 individuals 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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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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