Genetics and Allele Frequencies: Videos & Practice Problems

The modern synthesis combines Darwinian evolution and Mendelian genetics, focusing on how alleles are inherited in populations. Key concepts include phenotype, genotype, alleles, and the gene pool, which represents all alleles in a population. Allele frequency, calculated as the proportion of a specific allele to the total alleles, is crucial for understanding evolution. For two alleles, p and q, the relationship p + q = 1 holds. Changes in allele frequency indicate evolutionary processes, emphasizing the importance of genetic variation in populations.

Mendelian and Population Genetics

The modern synthesis of evolutionary biology integrates Darwinian evolution with Mendelian genetics, emphasizing the inheritance of alleles in diploid organisms, which possess two sets of chromosomes. A fundamental tool in this study is the Punnett square, which allows predictions about the offspring's traits based on parental genotypes.

Key vocabulary includes:

• Phenotype: The observable traits of an organism, influenced by genetics and the environment. For example, the coat color of rabbits, such as brown or white, represents their phenotypes.
• Gene: A segment of DNA that encodes for a specific trait or protein. In the rabbit example, the gene responsible for coat color is of particular interest.
• Allele: Different versions of a gene. For the coat color gene, there are two alleles: dominant (represented as A) and recessive (represented as a).
• Genotype: The specific combination of alleles an organism possesses. For instance, a brown rabbit with the genotype aa is homozygous recessive, while a white rabbit could be either homozygous dominant (AA) or heterozygous (Aa).

While Mendelian genetics focuses on single matings, population genetics expands this view to consider allele inheritance across entire populations. This approach introduces the concept of the gene pool, which encompasses all alleles present in a population, regardless of the organisms they belong to. For example, if we analyze a population of rabbits, the gene pool would include all alleles from both brown and white rabbits.

From the gene pool, we can derive allele frequency, which is the proportion of a specific allele within the gene pool. For instance, if 60% of the alleles are dominant (A) and 40% are recessive (a), these percentages represent the allele frequencies. Changes in allele frequency over time are indicative of evolution, highlighting the importance of understanding the gene pool and its dynamics in the context of modern biology.

Genetics and Allele Frequencies Example 1

In genetics, a gene pool represents the total collection of alleles within a population. For instance, consider two gene pools: Gene Pool 1 contains 400 dominant alleles (A) and 600 recessive alleles (a), while Gene Pool 2 has 5,005,000 of each allele. To determine the frequency of the little a allele in each pool, we calculate the total number of alleles. In Gene Pool 1, the total is 1,000 alleles, leading to a frequency of the little a allele calculated as:

Frequency of little a in Gene Pool 1 = \(\frac{600}{1000} = 0.6\) or 60%.

In Gene Pool 2, the total number of alleles is 10,000, resulting in a frequency of:

Frequency of little a in Gene Pool 2 = \(\frac{5,005,000}{10,000} = 0.5\) or 50%.

Thus, Gene Pool 1 has a higher frequency of the little a allele compared to Gene Pool 2.

When considering the frequency of heterozygotes (individuals with one dominant and one recessive allele), it is important to note that allele frequencies alone do not provide enough information to make predictions about genotype distributions. Therefore, without additional data on how these alleles are combined in individuals, we cannot determine which population has a higher frequency of heterozygotes. This highlights the need for further analysis of genotype distributions to understand the dynamics of allele frequencies and their implications for evolution.

When all the alleles in a population are considered together, independently of the individuals they come from, we call that the ___________.

• A. Genotype.
• B. Gene pool.
• C. Phenotype.
• D. Mendelian genetics.

Answer: (B)

Which of the following terms is correctly matched to its definition?

• A. Allele: an organism’s traits.
• B. Phenotype: an organism’s specific set of alleles.
• C. Heterozygote: having two copies of the same allele.
• D. Genotype: an organism’s specific set of alleles.

Answer: (D)

Allele Frequencies

Evolution is defined as a change in allele frequency within a population. To calculate allele frequency, you divide the number of a specific allele by the total number of alleles present. This process is straightforward and can be broken down into manageable steps.

In a typical scenario involving one gene with two alleles, we denote the frequencies of these alleles as p and q. Here, p represents the frequency of the first allele (often denoted as A), while q represents the frequency of the second allele (denoted as a). It is important to note that the sum of these frequencies will always equal 1, or 100% of the alleles in the gene pool: p + q = 1.

To calculate the allele frequencies, consider a sample population consisting of 100 homozygous dominant individuals (AA), 250 heterozygotes (Aa), and 150 homozygous recessive individuals (aa). The first step is to count the total number of each allele:

For the dominant allele (A), the calculation is as follows:

Number of A alleles = 2 × (Number of AA homozygotes) + (Number of Aa heterozygotes)

Substituting the values: Number of A alleles = 2 × 100 + 250 = 450.

For the recessive allele (a), the calculation is:

Number of a alleles = (Number of Aa heterozygotes) + 2 × (Number of aa homozygotes)

Substituting the values: Number of a alleles = 250 + 2 × 150 = 550.

Now, to find the total number of alleles in the population, you can either add the number of each allele or use the formula:

Total alleles = 2 × (Total number of individuals)

In this case, the total number of individuals is 100 + 250 + 150 = 500, so:

Total alleles = 2 × 500 = 1000.

With the total number of alleles calculated, you can now find the allele frequencies:

p = (Number of A alleles) / (Total alleles) = 450 / 1000 = 0.45

q = (Number of a alleles) / (Total alleles) = 550 / 1000 = 0.55

This means that 45% of the alleles in the population are the dominant allele (A), while 55% are the recessive allele (a).

In cases where only one allele exists in the population, the frequency of that allele will equal 1, indicating that it is fixed. This fixation means that there is no genetic variation for that gene, which is essential for evolution to occur.

The number of individuals in a population with each of the following genotypes is listed below. What is the frequency of the small t allele?


TT: 640
Tt: 200
tt: 160

• A. 0.32
• B. 0.16
• C. 0.26
• D. 0.52

Answer: (C)

Gia and Kojo collect a sample of beetles to calculate the frequency for the W allele. After genotyping the individuals in their sample, they count the following genotypes:
WW: 50
Ww: 170
ww: 120


Gia calculates the frequency by taking: $W=\frac{50+50+170}{680}$


Kojo calculates the frequency by taking: $W=\frac{2\left(50\right)+170}{50+170+120}$


Who calculated the frequency incorrectly, and what was their mistake?

• A. Kojo because he divided by the number of individuals instead of total alleles.
• B. Kojo because he multiplied the number of homozygotes by 2 but not the number of heterozygotes.
• C. Gia because she added instead of multiplying the number of W alleles found in the homozygote.
• D. Gia because she did not multiply the total number of individuals by 2.

Answer: (A)

Here’s what students ask on this topic:

What is the modern synthesis in genetics?

The modern synthesis is the integration of Darwinian evolution and Mendelian genetics. It combines the principles of natural selection, as proposed by Charles Darwin, with the genetic inheritance patterns discovered by Gregor Mendel. This synthesis explains how evolutionary processes, such as natural selection, act on genetic variation within populations. Key concepts include phenotype (observable traits), genotype (genetic makeup), alleles (different versions of a gene), and the gene pool (all alleles in a population). The modern synthesis emphasizes the importance of genetic variation and changes in allele frequencies as the basis for evolution.

How do you calculate allele frequency in a population?

To calculate allele frequency, divide the number of a specific allele by the total number of alleles in the population. For a gene with two alleles, represented as p and q, the formula is:

number of specific alleletotal number of alleles

For example, if there are 450 big A alleles and 550 little a alleles in a population, the total number of alleles is 1000. The frequency of big A (p) is 450/1000 = 0.45, and the frequency of little a (q) is 550/1000 = 0.55. The sum of p and q always equals 1.

What is the difference between phenotype and genotype?

Phenotype refers to the observable traits or characteristics of an organism, such as eye color, height, or fur color. These traits can be influenced by both genetic and environmental factors. Genotype, on the other hand, is the genetic makeup of an organism, specifically the set of alleles it possesses. For example, in rabbits, the genotype could be AA, Aa, or aa, where A and a are different alleles of a gene. The genotype determines the potential phenotypes an organism can express.

What is a gene pool and why is it important in population genetics?

A gene pool is the complete set of all alleles present in a population. It represents the genetic diversity available for natural selection to act upon. The gene pool is important in population genetics because it determines the potential for evolutionary change. By studying the gene pool, scientists can track how allele frequencies change over time, which is a measure of evolution. A diverse gene pool provides a population with a greater ability to adapt to changing environments and survive selective pressures.

What does it mean when an allele is fixed in a population?

An allele is considered fixed in a population when its frequency reaches 1, meaning it is the only allele present for a particular gene. This occurs when all individuals in the population are homozygous for that allele. Fixation indicates a lack of genetic variation for that gene, which can limit the population's ability to adapt to environmental changes. Evolution requires genetic variation, so a fixed allele means that no further evolutionary change can occur for that gene unless new mutations arise.