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 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?

Step-by-Step Solution

Answer

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

Step 1: Understand the Question

We need to find the percentages of two characteristics (cross-eyed and white) in the offspring of two phenotypically normal, heterozygous tigers.

Step 2: Identify Genotypes and Phenotypes

Let the allele for normal phenotype be denoted as 'A' (dominant) and the allele for recessive trait as 'a' (recessive). Phenotypically normal tigers have genotype 'Aa'.

Step 3: Set up the Punnett Square

A Punnett square is used to determine the genotypes of the offspring from two heterozygous tigers (Aa x Aa).

Step 4: Complete the Punnett Square

| | A | a ||---|---|---|| A | AA | Aa || a | Aa | aa |

Step 5: Analyze the Genotypes of Offspring

The genotypic ratio is: 1 AA (normal), 2 Aa (carriers, phenotypically normal), and 1 aa (recessive, showing white and cross-eyed traits).

Step 6: Determine the Percentage of Cross-eyed Offspring

Only the 'aa' genotype will lead to cross-eyed condition, which represents 1 out of 4 or 25% of the offspring.

Step 7: Determine the Percentage of White Cross-eyed Tigers

All cross-eyed tigers will be white because the recessive allele affects both traits. Thus, 100% of cross-eyed tigers will be white.

Key Concepts

Punnett square

A Punnett square is a visual tool used in genetics to predict the genotypes of offspring from two parents. It's essentially a grid that shows all possible combinations of alleles passed from each parent. This helps demonstrate how traits are inherited.

For example, when two heterozygous tigers (both ‘Aa’) mate, the alleles are combined in the Punnett square as follows:

• Each parent contributes one allele per offspring.
• The combinations can be AA, Aa, Aa, and aa.

This makes it easier to see how dominant and recessive traits could be passed down.

recessive allele

A recessive allele is one that must be inherited from both parents for a trait to be expressed. Unlike dominant alleles which can show their effect even if only one is present, recessive alleles need a pair to exhibit their traits.

In tigers, the recessive allele ‘a’ causes both the absence of fur pigmentation (white coloration) and a cross-eyed condition. If a tiger inherits two recessive alleles (aa), it will exhibit these traits.

When looking at heterozygous tigers (Aa), each only carries one recessive allele, which is why they appear phenotypically normal.

phenotypes and genotypes

Phenotype refers to the visible traits of an organism, while genotype refers to the genetic makeup. In our tiger example:

• Genotype AA: Normal phenotype, showing neither white coloration nor cross-eyed condition.
• Genotype Aa: Carries one recessive allele, but appears phenotypically normal.
• Genotype aa: Displays the recessive traits (white and cross-eyed).

Understanding the genotype can help predict the phenotype of offspring using the Punnett square.

heredity

Heredity is the process of passing genetic information from parents to offspring. This determines traits and characteristics visible in the next generation.

When two phenotypically normal, heterozygous tigers reproduce, the traits of their offspring are determined through the combination of their alleles.

In our exercise, this means understanding how dominant (A) and recessive (a) alleles interact will show how frequently traits like being cross-eyed and having white fur appear in their cubs. Using the Punnett square, we can predict:

• 25% chance of being cross-eyed (aa).
• 100% of cross-eyed tigers will also be white.