Problem 1 - Biology Today and Tomorrow with Physiology

Question

A heterozygous individual has ________ a. the same allele on both homologous chromosomes b. two different alleles of a gene c. a haploid condition, in genetic terms

Step-by-Step Solution

Answer

A heterozygous individual has two different alleles of a gene (Option B).

Step 1: Understand the Concept of Alleles

First, we need to understand what alleles are. Alleles are different forms of a gene that are found at the same place on a chromosome. Homozygous means having two of the same alleles, while heterozygous means having different alleles.

Step 2: Clarify Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes containing genes for the same characteristics. Each individual receives one chromosome of each pair from each parent. When talking about heterozygous conditions, we are referring to having different alleles on homologous chromosomes.

Step 3: Evaluate Option A

Option A states that a heterozygous individual has the same allele on both homologous chromosomes. This describes a homozygous condition, not a heterozygous one.

Step 4: Evaluate Option B

Option B refers to having two different alleles of a gene, which is exactly the definition of a heterozygous individual. Thus, this option correctly describes a heterozygous individual.

Step 5: Evaluate Option C

Option C suggests a haploid condition, which refers to having a single set of unpaired chromosomes, typical of gametes. Heterozygosity, however, is a condition involving pairs of alleles and is relevant to diploid organisms, which have paired chromosomes.

Step 6: Conclusion

After evaluating all options, we can conclude that option B is correct, describing a heterozygous individual as having two different alleles of a gene.

Key Concepts

Alleles

In the world of genetics, alleles serve as the foundational building blocks. They are different versions of a gene that occur at a specific locus on a chromosome.
Think of alleles as different flavors of ice cream, where each flavor represents a variation of a gene. For example, a gene determining eye color can have alleles for blue, brown, or green eyes.

• Every individual inherits two alleles for each gene— one from each parent.
• Alleles can be dominant, meaning they overpower the presence of other alleles, or recessive, which only show their effect if two copies exist.

Understanding alleles is key to grasping broader genetic principles such as homozygosity and heterozygosity, which describe how alleles are paired on homologous chromosomes.

Homozygous vs Heterozygous

These terms describe how alleles pair up on homologous chromosomes. Being homozygous means having two identical alleles for a particular gene.
For example, inheriting two alleles for blue eyes, one from each parent, is a true homozygous trait.

• Homozygous can be further specified as homozygous dominant (e.g., BB) or homozygous recessive (e.g., bb).
• Heterozygous means having two different alleles for a gene, like one allele for brown eyes (B) and one for blue eyes (b).

The distinction between homozygous and heterozygous is crucial for determining genetic traits. While homozygous traits carry certainty about what trait will be expressed, heterozygous traits may lead to a dominant allele overshadowing a recessive one.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes that almost look identical and have the same gene sequence. Each individual inherits one chromosome of each pair from each parent, making them part of our diploid cell structure.

• While these chromosomes carry the same types of genes, they may have different alleles.
• This means that while two chromosomes may carry genes for eye color, one might have an allele for brown eyes and the other for blue.

Understanding homologous chromosomes helps explain how genetic inheritance happens. They are involved in meiosis, the process of creating haploid cells from diploid cells, and assure genetic diversity.
These chromosomes are a fundamental part of how traits are passed from generation to generation.