Traits and Variance: Videos & Practice Problems

Inherited traits can be classified into continuous traits, like human height, which can take on an infinite number of values, and categorical traits, which fall into discrete categories, such as flower color. Categorical traits include threshold traits, requiring a specific combination of genetic and environmental factors, and meristic traits, which are countable. Complex inheritance involves multiple genes affecting a trait, while simple inheritance follows Mendelian ratios. Polygenic inheritance results in varied phenotypes, calculable using formulas like $(m^{7}G)$ and $(V_A)$.

Traits and Variance

Inherited traits can be categorized into two main types: continuous traits and categorical traits. Continuous traits, such as human height, can take on an infinite number of values within a range. For example, a person's height can be measured with great precision, allowing for values like 5 feet 7.123 inches. In contrast, categorical traits are divided into distinct categories, such as the color of a flower (purple or white) or the number of spots on a Dalmatian, which can only be whole numbers (e.g., 1, 2, or 30 spots, but not 30.67).

Within categorical traits, there are two classifications: threshold traits and meristic traits. Threshold traits, like type 2 diabetes, require a combination of genetic and environmental factors to be expressed. Not everyone with genetic markers for type 2 diabetes will develop the condition, as they may not reach the necessary threshold of contributing factors. Meristic traits, on the other hand, involve counting discrete values, such as the number of eggs a bird lays, which can only be whole numbers (e.g., 1, 2, or 3 eggs).

When classifying traits, it is essential to distinguish between continuous and categorical. For instance, human weight and foot size are continuous traits, as they can be measured with decimal precision. In contrast, the number of kittens in a cat litter is categorical, as it can only be whole numbers.

Traits are inherited through two primary mechanisms: complex inheritance and simple inheritance. Complex inheritance involves multiple genes and environmental factors, as seen in human height, which is influenced by various genetic contributions. Simple inheritance, however, typically involves a single trait and can be analyzed using Mendelian ratios. For example, a 3:1 or 9:3:3:1 ratio indicates simple inheritance.

Polygenic inheritance, a form of complex inheritance, involves multiple genes contributing to a single phenotype. Each gene behaves according to Mendelian principles, but the overall phenotype results from the cumulative effect of all contributing alleles. In this context, alleles can be classified as additive, which contribute to the phenotype, or non-additive, which do not. This distinction is crucial for understanding how traits manifest in the phenotype.

For example, in a cross between white corn and purple corn, the presence of additive alleles determines the resulting kernel color. The F2 generation may exhibit a range of phenotypes, including purple, dark red, light red, and white, depending on the number of additive alleles present. The formula for determining the number of genes involved in a trait is given by:

$$1:4^n = \text{ratio of parental phenotypes}$$

Where \( n \) represents the number of polygenes. If the ratio of parental phenotypes is 1:16, solving for \( n \) reveals that there are 2 genes involved.

Additionally, to calculate the number of phenotypic categories observed for a trait controlled by multiple genes, the formula is:

$$2n + 1$$

For example, if a trait is controlled by 4 genes, the calculation would be \( 2(4) + 1 = 9 \) phenotypic categories.

Understanding these concepts and formulas is essential for analyzing inheritance patterns and predicting phenotypic outcomes in various organisms.

A trait controlled through polygenic inheritance was observed in a series of experiments. A brown eyed rabbit was mated with a blue eyed rabbit. 130 F2 offspring were produced. 2 offspring had brown eyes and 2 offspring had blue eyes. How many polygenes control eye color in rabbits?

• A. 1
• B. 2
• C. 3
• D. 4

Answer: (C)

If a trait is controlled by 5 polygenes, how many phenotypic categories will be observed in the F2 generation?

• A. 2
• B. 5
• C. 10
• D. 11

Answer: (D)

Polygenic inheritance is what type of inheritance?

• A. Simple
• B. Complex
• C. Additive
• D. Non-additive

Answer: (B)

Here’s what students ask on this topic:

What is the difference between continuous and categorical traits?

Continuous traits can take on an infinite number of values within a range. Examples include human height and weight, where measurements can be highly precise, such as 5.678 feet or 130.456 pounds. Categorical traits, on the other hand, fall into discrete categories. These include traits like flower color (e.g., purple or white) or the number of spots on a Dalmatian, which can be counted but not fractioned. Categorical traits can be further divided into threshold traits, which require a combination of genetic and environmental factors to be expressed, and meristic traits, which are countable, like the number of eggs a bird lays.

How do you determine if a trait follows complex inheritance?

Complex inheritance involves multiple genes and possibly environmental factors affecting a trait. To determine if a trait follows complex inheritance, you can look at the phenotypic ratios in the F2 generation. Unlike simple Mendelian inheritance, which follows predictable ratios like 3:1 or 9:3:3:1, complex inheritance often results in a variety of phenotypes. For example, human height is controlled by multiple genes, making it a complex trait. Additionally, the presence of varied phenotypes and the use of formulas like $1^4ⁿ=\frac{1}{16}$ can help identify polygenic inheritance.

What are threshold traits and how are they different from other categorical traits?

Threshold traits are a type of categorical trait that require a specific combination of genetic and environmental factors to be expressed. For example, type 2 diabetes is a threshold trait because it requires a certain number of genetic predispositions and environmental triggers to manifest. Unlike other categorical traits, which are straightforward and countable (e.g., number of spots on a Dalmatian), threshold traits involve a more complex interplay of factors. You either have the trait or you don't, but the expression depends on reaching a certain threshold of contributing factors.

How can you calculate the number of genes involved in a polygenic trait?

To calculate the number of genes involved in a polygenic trait, you can use the formula $1^4ⁿ=\frac{1}{16}$, where $n$ represents the number of polygenes. For example, if the ratio of parental phenotypes in the F2 generation is 1:16, you can solve for $n$ to find that $n$ equals 2, indicating two genes are involved. This formula helps in identifying the genetic complexity of the trait.

What is the formula to calculate the number of phenotypic categories in polygenic inheritance?

The formula to calculate the number of phenotypic categories in polygenic inheritance is $2n+1$, where $n$ is the number of polygenes. For example, if a trait is controlled by 4 genes, the number of phenotypic categories would be $2\times 4+1=9$. This formula helps in predicting the diversity of phenotypes that can be observed in the F2 generation.