Overview of Human Genetics: Videos & Practice Problems

Alleles, the different versions of genes, can be dominant or recessive, with dominant alleles (capital letters) masking recessive ones (lowercase letters) in phenotype expression. The genotype represents the genetic makeup with homozygous (identical alleles) or heterozygous (different alleles) pairs, while the phenotype is the observable trait, such as pea color. Dominant-recessive inheritance explains why heterozygous genotypes express dominant phenotypes. Understanding genotype and phenotype relationships is essential for grasping genetic inheritance and predicting traits in diploid organisms.

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 in human genetics?

Genotype refers to the genetic makeup of an organism, specifically the combination of alleles it carries for a particular gene. It is represented by pairs of letters, such as $Yy$, where each letter represents an allele. Phenotype, on the other hand, is the observable physical or biochemical characteristic resulting from the genotype, such as pea color or eye color in humans. For example, a genotype with one dominant allele ($Y$) and one recessive allele ($y$) will produce a phenotype that expresses the dominant trait. Understanding this distinction is crucial for studying inheritance patterns and predicting traits in organisms.

How do dominant and recessive alleles affect the phenotype?

Dominant alleles mask the effect of recessive alleles when both are present in an organism's genotype. This means that if an individual has at least one dominant allele, the phenotype will express the dominant trait. Recessive alleles only influence the phenotype when two copies are present (homozygous recessive). For example, in pea plants, the allele for yellow peas ($Y$) is dominant over the allele for green peas ($y$). Therefore, both homozygous dominant ($\mathrm{YY}$) and heterozygous ($\mathrm{Yy}$) genotypes result in yellow peas, while only homozygous recessive ($\mathrm{yy}$) results in green peas.

What are homozygous and heterozygous genotypes?

Homozygous genotypes consist of two identical alleles for a specific gene. This can be either homozygous dominant, with two dominant alleles (e.g., $\mathrm{YY}$), or homozygous recessive, with two recessive alleles (e.g., $\mathrm{yy}$). Heterozygous genotypes have two different alleles for the same gene, one dominant and one recessive (e.g., $\mathrm{Yy}$). The phenotype of heterozygous individuals typically reflects the dominant allele because it masks the recessive allele's effect.

Why can't we visually distinguish between homozygous dominant and heterozygous individuals based on phenotype?

Because the dominant allele masks the recessive allele in heterozygous individuals, both homozygous dominant ($\mathrm{YY}$) and heterozygous ($\mathrm{Yy}$) genotypes produce the same dominant phenotype. For example, both genotypes result in yellow peas. This means that visually, the phenotype does not reveal whether an organism carries two dominant alleles or one dominant and one recessive allele. Genetic testing or breeding experiments are required to determine the exact genotype.

How are alleles represented in genetic notation?

Alleles are represented by letters, with dominant alleles shown as uppercase letters and recessive alleles as lowercase letters. For example, the dominant allele for yellow pea color is represented as $Y$, while the recessive allele for green pea color is represented as $y$. This notation helps easily distinguish between dominant and recessive alleles when writing genotypes.