Problem 11 - Biology of Humans: Concepts, Applications, and Issues

Question

The genotype of a person with two copies of the same allele is _______________ and the genotype of a person with two different alleles for a trait is ______________.

Step-by-Step Solution

Answer

Homozygous; Heterozygous

Step 1 - Understand the Concepts of Alleles

Alleles are different versions of a gene. In genetics, organisms usually have two alleles for each trait, one inherited from each parent.

Step 2 - Define Homozygous

When a person has two identical alleles for a trait, the genotype is called homozygous. This can be either homozygous dominant (e.g., AA) or homozygous recessive (e.g., aa).

Step 3 - Define Heterozygous

When a person has two different alleles for a trait, the genotype is called heterozygous (e.g., Aa).

Step 4 - Apply the Definitions

Using the definitions from the previous steps, the genotype of a person with two copies of the same allele is homozygous, and the genotype of a person with two different alleles for a trait is heterozygous.

Key Concepts

alleles

Alleles are essential to understanding genetics. Think of alleles as different versions of the same gene. For example, imagine a gene that determines eye color. The gene can have alleles that code for blue or brown eyes. When you inherit your genes from your parents, you get one allele from each parent for every trait. This combination of alleles determines many of your physical traits.

Alleles can be either dominant or recessive. Dominant alleles are the versions of a gene that will be expressed if at least one copy is present (e.g., B for brown eyes). Recessive alleles will only show their effect if both copies are the same (e.g., b for blue eyes). Understanding alleles is crucial for determining how traits are inherited and expressed.

homozygous

When both alleles for a given trait are the same, the genotype is called homozygous. This could mean either two dominant alleles (homozygous dominant) or two recessive alleles (homozygous recessive). For example, if a person has brown eyes and their alleles are BB, they are homozygous dominant for the eye color trait. If they have blue eyes with alleles bb, they are homozygous recessive for the same trait.

Homozygous genotypes are simple to understand because the trait depends on only one type of allele. There’s no mix; it’s either all dominant or all recessive. This makes predicting the inheritance of certain traits straightforward, provided you know the parents' genotypes.

heterozygous

A heterozygous genotype arises when an individual has two different alleles for the same trait (e.g., Bb for eye color). In this example, the person inherited one allele for brown eyes from one parent and one allele for blue eyes from the other. Usually, the dominant allele (B for brown) will mask the effect of the recessive allele (b for blue), and the individual's eyes will be brown.

Heterozygous genotypes are interesting because they involve a combination of different alleles. This can result in dominant traits being expressed, with recessive traits remaining hidden. Understanding heterozygosity is key in genetics, as it explains why children can have a mix of traits from both parents, and why some traits may seem to 'skip' a generation.