Problem 6 - Biology of Humans: Concepts, Applications, and Issues

Question

How many different gametes could a person with the genotype Aabbcc form? (The genes are on different chromosomes.) a. 2 b. 4 c. 16 d. 64

Step-by-Step Solution

Answer

2 different gametes

Step 1: Identify Genotype Components

The genotype given is Aabbcc. Here, 'A', 'a', 'b', and 'c' represent different alleles of genes on different chromosomes. The possibility of forming different gametes depends on which alleles can combine in new ways.

Step 2: Determine Heterozygous and Homozygous Genes

For the genotype Aabbcc, 'A' is heterozygous (A/a), while 'bb' and 'cc' are homozygous (only one form each: 'b' and 'c').

Step 3: Calculate Possible Gamete Combinations

Since 'A' can be either 'A' or 'a' and both 'b' and 'c' are fixed (since they are homozygous), only the heterozygous gene can contribute to different combinations. Each heterozygous locus can produce two different gametes. Here, only one gene (A/a) is heterozygous.

Step 4: Determine Number of Combinations

The number of different gametes is determined by the formula: \[2^n\]Where \(n\) is the number of heterozygous gene pairs. In this case, \(n = 1\), so the number of different gametes is \[2^1 = 2\].

Step 5: Conclude the Answer

Since there are 2 possible different gametes, the correct answer is (a) 2.

Key Concepts

Genotype

Genotype refers to the genetic makeup of an organism. It describes the set of genes an organism carries and can be represented by letters. For example, in the genotype Aabbcc, the letters A, a, b, b, c, c represent different alleles of genes. Each pair of letters represents the genes inherited from each parent.

• 'A' and 'a' are alleles of one gene.
• 'b' and 'b' are alleles of another gene.
• 'c' and 'c' are alleles of yet another gene.

Each of these genes is located on different chromosomes, meaning they can assort independently during gamete formation. In essence, the genotype is a description of the genetic instructions that will guide an organism's traits.

Heterozygous

An organism is heterozygous for a particular gene if it has two different alleles for that gene. In the context of our exercise with the genotype Aabbcc, the gene represented by 'A' is heterozygous because the two alleles are different ('A' and 'a'). This mixture of alleles can lead to variations in traits depending on which allele is dominant.

For the genotype Aabbcc, only the 'A' gene is heterozygous.

• This means this gene can produce different combinations (A or a) during gamete formation.

This heterozygosity creates the potential for genetic diversity in offspring.

Homozygous

When an organism is homozygous for a gene, it means it has two identical alleles for that gene. In our given genotype Aabbcc, both the 'b' and 'c' genes are homozygous because each allele is the same ('bb' and 'cc').

• This indicates there is a single form of the gene being passed on.
• No variation can occur from homozygous gene pairs during gamete formation.

This is why, in this genotype, only the heterozygous gene (A) contributes to different combinations, while homozygous genes (bb and cc) do not.

Alleles

Alleles are different versions of the same gene. Each individual inherits one allele from each parent for every gene. For instance, in the genotype Aabbcc, the letters 'A' and 'a' are alleles for the same gene because they occur at the same location (locus) on homologous chromosomes.

• 'A' and 'a' may lead to different traits being expressed.
• 'b' and 'b' are identical alleles, leading to a single trait being expressed.
• 'c' and 'c' are also identical alleles, leading to a single trait being expressed.

Alleles play a crucial role in determining the diversity of traits an organism can pass on to its offspring.

Chromosomes

Chromosomes are long, thread-like structures made of DNA and proteins. They carry genes in a linear sequence. Humans have 23 pairs of chromosomes, totaling 46. In the exercise, the genes A, a, b, b, c, and c are located on different chromosomes.

• This separation (independent assortment) during meiosis allows for genetic diversity.
• One chromosome of each pair comes from each parent.

Chromosomes ensure genes are accurately passed down to gametes and then to offspring. Each parent's pair of chromosomes recombines to produce unique gamete combinations, facilitating genetic variation in successive generations.