Overview of Human Genetics: Videos & Practice Problems

Understanding the difference between genotype and phenotype is crucial in genetics. The genotype represents the genetic combination of alleles, written as pairs of letters, while the phenotype is the physical expression of these traits, such as pea color. Dominant alleles, symbolized by capital letters, mask the effects of recessive alleles, represented by lowercase letters. For example, a homozygous dominant genotype (YY) and a heterozygous genotype (Yy) both result in a yellow phenotype, demonstrating how dominant traits influence observable characteristics.

Dominant vs. Recessive Alleles

Understanding the concepts of genotype and phenotype begins with the distinction between dominant and recessive alleles. Alleles are different versions of specific genes, and they can be classified as either dominant or recessive. A dominant allele exerts its effects whenever it is present, symbolized by a capital letter (e.g., Y for yellow peas). In contrast, a recessive allele has no effect if a dominant allele is present and is represented by a lowercase letter (e.g., y for green peas).

Gregor Mendel's experiments with pea plants revealed that the allele for yellow peas (Y) is dominant over the allele for green peas (y). This means that if an organism has at least one dominant allele (Y), the dominant trait (yellow peas) will be expressed, while the recessive trait (green peas) will only be expressed when both alleles are recessive (yy).

In summary, the dominant allele masks the effect of the recessive allele whenever it is present. This foundational understanding of alleles sets the stage for exploring the differences between genotype, which refers to the genetic makeup of an organism (e.g., YY, Yy, or yy), and phenotype, which is the observable expression of that genotype (e.g., yellow or green peas). As we continue, we will delve deeper into these concepts and their applications in genetics.

An allele that exerts its effects whenever it is present is:

• A. Recessive.
• B. Heterozygous.
• C. Dominant.
• D. Homozygous.
• E. Homologous.

Answer: (C)

Genotype & Phenotype

In genetics, understanding the distinction between genotype and phenotype is crucial. The genotype refers to the genetic makeup of an organism, specifically the combination of alleles it possesses. In diploid organisms, which have two copies of each gene, the genotype is represented as a pair of letters. There are two main types of genotypes: homozygous and heterozygous. Homozygous genotypes consist of two identical alleles for a gene, which can be either dominant (e.g., two capital letters, such as YY) or recessive (e.g., two lowercase letters, such as yy). In contrast, heterozygous genotypes contain two different alleles, typically one dominant and one recessive (e.g., Yy).

The phenotype, on the other hand, is the observable physical expression of the genotype. It reflects the traits that result from the genetic combinations. For instance, in pea plants, a homozygous dominant genotype (YY) and a heterozygous genotype (Yy) both produce a yellow phenotype, while a homozygous recessive genotype (yy) results in a green phenotype. This occurs because the dominant allele masks the effect of the recessive allele when both are present. Therefore, while the phenotype can indicate certain traits, it does not provide definitive information about the underlying genotype, especially when dominant alleles are involved.

In summary, the genotype is the genetic combination of alleles represented as a pair of letters, while the phenotype is the physical trait expressed as a result of that genotype. Understanding these concepts is essential for further exploration of genetic inheritance and traits.

If the two alleles for a particular gene are identical the gene pair is:

• A. Homozygous.
• B. Heterozygous.
• C. Recessive.
• D. Homologous.
• E. Dominant.
• F. Dissimilar.

Answer: (A)

If an individual is homozygous for a particular trait:

• A. Each parent contributed a different allele for that trait.
• B. One parent contributed two different alleles for that trait.
• C. Each parent contributed the same allele for that trait.
• D. One parent contributed two copies of the same allele for that trait.

Answer: (C)

Here’s what students ask on this topic:

What is the difference between genotype and phenotype?

The genotype refers to the genetic makeup of an organism, specifically the combination of alleles it possesses for a particular gene. It is typically represented as a pair of letters, such as YY, Yy, or yy. On the other hand, the phenotype is the observable physical or physiological trait that results from the genotype, such as the color of a pea (yellow or green). Dominant alleles in the genotype mask the effects of recessive alleles, which means that both homozygous dominant (YY) and heterozygous (Yy) genotypes can produce the same phenotype (e.g., yellow peas). Understanding this distinction is essential for studying inheritance patterns and predicting traits in organisms.

What are dominant and recessive alleles?

Dominant alleles are versions of a gene that exert their effects whenever they are present, even if only one copy is inherited. They are typically represented by a capital letter (e.g., Y for yellow pea color). Recessive alleles, on the other hand, only express their effects when two copies are present, as they are masked by dominant alleles. Recessive alleles are represented by lowercase letters (e.g., y for green pea color). For example, in pea plants, the allele for yellow peas (Y) is dominant over the allele for green peas (y). This means that both YY and Yy genotypes result in yellow peas, while only the yy genotype produces green peas.

What is a homozygous genotype, and how does it differ from a heterozygous genotype?

A homozygous genotype occurs when an organism has two identical alleles for a specific gene. This can be either homozygous dominant (e.g., YY) or homozygous recessive (e.g., yy). In contrast, a heterozygous genotype occurs when an organism has two different alleles for the same gene (e.g., Yy). The key difference is that homozygous genotypes have identical alleles, while heterozygous genotypes have one dominant and one recessive allele. In terms of phenotype, a homozygous dominant and a heterozygous genotype often produce the same trait due to the dominance of one allele.

How do dominant alleles mask the effects of recessive alleles?

Dominant alleles mask the effects of recessive alleles because they are expressed in the phenotype whenever they are present. This occurs because the dominant allele produces a functional protein or trait that overrides the effect of the recessive allele, which may produce a non-functional or less effective protein. For example, in pea plants, the dominant allele for yellow peas (Y) results in yellow coloration, even if the recessive allele for green peas (y) is also present. Thus, both YY and Yy genotypes result in yellow peas, while only the yy genotype expresses the green phenotype.

Why can’t we visually distinguish between homozygous dominant and heterozygous phenotypes?

We cannot visually distinguish between homozygous dominant (e.g., YY) and heterozygous (e.g., Yy) phenotypes because both genotypes produce the same observable trait. This is due to the dominant allele masking the effect of the recessive allele in the heterozygous genotype. For example, in pea plants, both YY and Yy genotypes result in yellow peas, as the dominant allele (Y) determines the phenotype. To differentiate between these genotypes, genetic testing or breeding experiments are required.