Problem 3 - Biology Today and Tomorrow Without Physiology

Question

The offspring of the cross \(A A \times a a\) are _____ a. all \(A A\) b. all aa c. all \(A a\) d. \(1 / 2 A A\) and \(1 / 2\) aa

Step-by-Step Solution

Answer

All offspring will be \(Aa\) (heterozygous).

Step 1: Understanding the Parent Alleles

In the given genetic cross, we have two parent genotypes: one is homozygous dominant, denoted as \(AA\), and the other is homozygous recessive, denoted as \(aa\). This means that one parent contributes two dominant alleles (\(A\)), and the other contributes two recessive alleles (\(a\)).

Step 2: Setting Up the Punnett Square

A Punnett square is used to predict the genotypes of the offspring from this cross. We place the alleles from each parent on the top and side of the square. From parent 1 (\(AA\)), we have two \(A\) alleles. From parent 2 (\(aa\)), we have two \(a\) alleles.

Step 3: Filling Out the Punnett Square

Each cell in the Punnett square represents a possible genotype of the offspring. Every pairing results in one allele from each parent. Thus, the cells will show the genotype \(Aa\) by combining one \(A\) allele and one \(a\) allele.

Step 4: Analyzing the Offspring Genotypes

All the squares in the Punnett square show the genotype \(Aa\). Therefore, every offspring of this cross will have the genotype \(Aa\), which is heterozygous.

Key Concepts

Homozygous

In genetics, the term "homozygous" refers to an organism that has two identical alleles for a particular gene. Alleles are various forms of a gene, and they can be dominant or recessive.
A homozygous genotype can either be homozygous dominant, with two dominant alleles (like AA), or homozygous recessive, with two recessive alleles (like aa).
This concept is significant in inheritance as it determines purity of the genetic trait. Parents passing homozygous alleles consistently pass only one type of allele to their offspring. As we saw in the genetic cross question, one parent was homozygous dominant (AA), and the other was homozygous recessive (aa).

• Homozygous dominant means both alleles are the same and dominant (e.g., AA).
• Homozygous recessive means both alleles are the same and recessive (e.g., aa).

When dealing with homozygous pairs, the parent will contribute one type of allele (either both dominant or both recessive) during reproduction.

Heterozygous

The term "heterozygous" applies to an organism that has two different alleles for a specific gene – one from each parent.
This often results in the expression of the dominant trait, because in many cases, the dominant allele will mask the presence of the recessive allele.
In our example with the cross of two parents, all offspring were heterozygous (3Aa).

• Heterozygous organisms have one dominant and one recessive allele (e.g., Aa).
• Despite having a recessive allele, a heterozygous individual's traits are usually dominated by the dominant allele.

Heterozygous genotypes reflect genetic diversity and can lead to varied expression of traits, potentially offering selective advantages in different environments.
They are particularly important because they are carriers of recessive alleles, which can appear in future generations if passed along.

Punnett Square

The Punnett square is a helpful tool in genetics that is used to predict the possible genotypes of offspring resulting from a genetic cross.
It was named after Reginald C. Punnett, who devised this method. It's essentially a grid that allows you to visualize and determine the probabilities of the outcomes of genetic crosses.

• The alleles from one parent go on the top and from the other parent on the side.
• Each square in the grid represents a possible combination of alleles that an offspring can inherit.

For the particular cross of homozygous dominant (3AA) and homozygous recessive (3aa), every possible outcome recorded in the Punnett square was heterozygous (3Aa).
This illustrates how the Punnett square can be used not only to deduce the genetic makeup of offspring but also to understand the likelihood of different traits being passed on.