Problem 11 - Campbell Biology

Question

In tigers, a recessive allele of a particular gene causes both an absence of fur pigmentation (a white tiger) and a crosseyed 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 tigers with crossed eyes will also be white?

Step-by-Step Solution

Answer

25% of offspring will be cross-eyed. 100% of cross-eyed tigers will also be white.

Step 1: Define the alleles

Let the dominant allele be denoted as A (normal fur pigmentation) and the recessive allele as a (white fur and cross-eyed condition).

Step 2: Determine the genotype of the parents

Since the tigers are heterozygous at this locus, their genotype is Aa.

Step 3: Set up the Punnett Square

Create a Punnett Square to determine the possible genotypes of the offspring. List the alleles from each parent on the top and side of the square. The combinations will be AA, Aa, Aa, and aa.

Step 4: Analyze the Punnett Square results

The Punnett Square shows these genotype frequencies: 25% AA, 50% Aa, and 25% aa. Only the aa genotype results in the cross-eyed and white fur condition.

Step 5: Calculate the percentage of cross-eyed offspring

Since only the homozygous recessive (aa) results in cross-eyed tigers and this represents 25% of the offspring, 25% of the offspring will be cross-eyed.

Step 6: Calculate the percentage of white tigers among the cross-eyed ones

All cross-eyed tigers (genotype aa) will also be white because the allele responsible for being cross-eyed also causes the white fur condition. Hence, 100% of the cross-eyed tigers will be white.

Key Concepts

Punnett Square

A Punnett Square is a simple diagram used in genetics to predict the genotype and phenotype of offspring from two parents. Imagine you are playing a genetic lottery where the combination of alleles (either dominant or recessive) determines traits in the offspring. The Punnett Square method helps you visualize this.

In our tiger example, we have two heterozygous tigers (genotype Aa). To set up the Punnett Square:

• List one parent's alleles (A and a) on the top.
• List the other parent's alleles (A and a) on the side.

By filling in the squares, you get the possible combinations: AA, Aa, Aa, and aa. This square provides easy-to-see genotype frequencies, showing the likelihood of traits being passed to the offspring.

Recessive Allele

Alleles are versions of a gene that dictate specific traits. A recessive allele is an allele that will only express its characteristic if an organism has two copies (homozygous recessive). If an organism has one dominant and one recessive allele, the dominant one will mask the effect of the recessive one.

In our exercise, the recessive allele (a) causes both white fur pigmentation and a cross-eyed condition in tigers. This recessive trait only appears when the tiger has a genotype of aa. If a tiger has either AA or Aa, the dominant A will mask the recessive trait.

Understanding the recessive allele's behavior is critical in predicting the traits of the offspring, especially when using the Punnett Square.

Genotype

Genotype refers to the genetic makeup of an organism, particularly its set of alleles. This is the basis for determining an organism's phenotype—the physical expression of a trait.

In the case of our tiger example:

• AA results in normal pigmentation.
• Aa results in normal pigmentation (due to dominant A).
• aa results in the recessive traits appearing (white fur and cross-eyed condition).

Using these genotypes within the Punnett Square allows us to predict and calculate the probabilities of each possible phenotype in offspring. It shows how likely it is for tigers to be cross-eyed or white based on their genetic makeup and the inherent behavior of the recessive allele.