Ch. 14 - Mendel and the Gene Idea

Problem 11

Chapter 14, Problem 11

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?

Verified step by step guidance

Step 1: Understand the problem. In this case, we are dealing with a recessive allele that causes two traits: absence of fur pigmentation (white tigers) and a cross-eyed condition. The tigers in question are heterozygous, meaning they carry one dominant and one recessive allele. The dominant allele is for normal pigmentation and eye condition, while the recessive allele is for white fur and cross-eyed condition.

Step 2: Use the Punnett square to predict the genotypes of the offspring. Since both parents are heterozygous (let's denote the dominant allele as 'A' and the recessive allele as 'a'), the Punnett square will look like this: AA, Aa, aA, aa. This means that 25% of the offspring will be homozygous dominant (AA), 50% will be heterozygous (Aa or aA), and 25% will be homozygous recessive (aa).

Step 3: Determine the percentage of offspring that will be cross-eyed. Since the cross-eyed condition is caused by the recessive allele, only the homozygous recessive offspring (aa) will be cross-eyed. According to the Punnett square, this is 25% of the offspring.

Step 4: Determine the percentage of cross-eyed tigers that will be white. Since the white fur and cross-eyed condition are caused by the same recessive allele, all cross-eyed tigers will also be white. Therefore, 100% of cross-eyed tigers will be white.

Step 5: Summarize the results. 25% of the offspring will be cross-eyed and white, and 100% of cross-eyed tigers will be white.

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Key Concepts

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

Mendelian Genetics

Mendelian genetics is the study of how traits are inherited through generations based on the principles established by Gregor Mendel. It involves understanding dominant and recessive alleles, where a recessive allele only expresses its trait when two copies are present. In this case, the recessive allele causes both the white fur and cross-eyed condition in tigers.

Punnett Square

A Punnett square is a diagram used to predict the genotypes of offspring from a genetic cross. By organizing the alleles of the parents, it allows for the visualization of possible combinations and the probabilities of each genotype. In this scenario, it will help determine the percentage of offspring that will be cross-eyed when two heterozygous tigers are mated.

Phenotype and Genotype

Phenotype refers to the observable traits of an organism, while genotype refers to the genetic makeup that determines those traits. In this question, the phenotypically normal tigers have a heterozygous genotype, meaning they carry one dominant and one recessive allele. Understanding the relationship between genotype and phenotype is crucial for predicting the traits of the offspring.

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