Problem 1 - Biology of Humans: Concepts, Applications, and Issues

Question

Which of the following crosses could produce offspring with the recessive phenotype? a. \(A A \times a a\) b. \(A a \times A a\) c. \(A A \times A a\) d. \(A A \times A A\)

Step-by-Step Solution

Answer

Cross b (Aa x Aa) can produce offspring with the recessive phenotype.

Step 1 – Understand the Phenotype

A recessive phenotype appears only when the individual has two copies of the recessive allele (aa).

Step 2 – Determine Genotypes of Parents

For each cross, determine the genotypes of the parents and how they combine. The crosses provided are AA x aa, Aa x Aa, AA x Aa, and AA x AA.

Step 3 – Analyze Cross a. AA x aa

Parents are homozygous dominant (AA) and homozygous recessive (aa). All offspring will have the genotype Aa (heterozygous), so no recessive phenotype.

Step 4 – Analyze Cross b. Aa x Aa

Parents are both heterozygous (Aa). Using a Punnett square: - Offspring genotypes can be: AA, Aa, aa. - There is a 25% chance of offspring being aa (recessive phenotype).

Step 5 – Analyze Cross c. AA x Aa

Parents are homozygous dominant (AA) and heterozygous (Aa). All offspring will have the genotype AA or Aa, so no recessive phenotype.

Step 6 – Analyze Cross d. AA x AA

Parents are both homozygous dominant (AA). All offspring will have the genotype AA, so no recessive phenotype.

Step 7 – Identify the Correct Cross

Only the cross Aa x Aa (b) can produce offspring with the recessive phenotype (aa).

Key Concepts

Recessive Phenotype

A recessive phenotype only appears when an individual has two copies of the recessive allele. Alleles are different forms of a gene, and they come in pairs: one from each parent. In this context, we use lowercase letters (like 'a') for recessive alleles. For the phenotype to be recessive, the genotype of the organism must be 'aa'. When both alleles are recessive, the traits they control will express this, leading to the recessive phenotype.

Punnett Square

A Punnett square is a tool we use to predict the genotypes of offspring from a genetic cross. It’s a simple grid that helps visualize how alleles from each parent combine. Each parent’s potential alleles are written along the top (father) and side (mother) of the grid. By filling in the squares, you can see all possible combinations of alleles:

For example, if both parents are heterozygous (Aa), we place 'A' and 'a' along the top and the side. The squares will show:

• AA
• Aa
• Aa
• aa

This visual representation shows that there’s a 25% chance for 'aa' (recessive phenotype).

Genotype Analysis

Genotype refers to the genetic makeup of an organism, specifically the alleles present. When analyzing genotypes in genetic crosses, it is essential to understand how different combinations can impact the phenotype. For instance, two organisms with the genotypes 'Aa' and 'Aa' might produce:

• 25% AA (homozygous dominant)
• 50% Aa (heterozygous)
• 25% aa (homozygous recessive)

By examining these potential genotypes, you can predict the likelihood of different phenotypes appearing in offspring. This is what helps in identifying which crosses will produce specific traits.

Heterozygous and Homozygous

In genetics, heterozygous means having two different alleles for a trait (e.g., 'Aa'), while homozygous means having two identical alleles (e.g., 'AA' or 'aa').

• Homozygous dominant (AA): Both alleles are dominant.
• Homozygous recessive (aa): Both alleles are recessive.
• Heterozygous (Aa): One allele is dominant, and one is recessive.

This distinction is crucial because it helps explain how certain traits are inherited and appear in offspring. In our exercise, the key to finding offspring with a recessive phenotype is identifying when 'aa' can occur, which is only possible when both parents carry a recessive allele.

Dominant and Recessive Alleles

Alleles can be dominant or recessive. A dominant allele is represented by a capital letter (A) and will mask the presence of a recessive allele (a) when both are present in a heterozygous individual. In other words, in an 'Aa' genotype, the dominant trait will be visible because it overshadows the recessive trait.

For a recessive allele to express its trait, it must be in a homozygous recessive condition (aa). This means the individual received a recessive copy of the gene from both parents. Understanding the interaction between dominant and recessive alleles is fundamental in predicting the outcomes of genetic crosses. In our example, only the cross between two heterozygous individuals (Aa x Aa) can produce offspring with the 'aa' genotype, which exhibits the recessive phenotype.