Problem 6 - Biological Science

Question

Which of the following is the strongest evidence that a trait might be influenced by polygenic inheritance? a. \(\mathrm{F}_{1}\) offspring of parents with different phenotypes have an intermediate phenotype. b. \(\mathrm{F}_{1}\) offspring of parents with different phenotypes have the dominant phenotype. c. The trait shows qualitative (discrete) variation. d. 'The trait shows quantitative variation.

Step-by-Step Solution

Answer

d. The trait shows quantitative variation.

Step 1: Option a: Intermediate phenotype

This option suggests that F1 offspring between parents with different phenotypes display a phenotype that is intermediate between the parent phenotypes. Though this can sometimes be an indication of incomplete dominance, it does not necessarily provide strong evidence for polygenic inheritance. So, this option is not the strongest evidence.

Step 2: Option b: Dominant phenotype

This option indicates that F1 offspring of parents with different phenotypes show the dominant phenotype. This is more characteristic of simple Mendelian inheritance, where one gene controls the trait and there is a clear dominant and recessive allele. Therefore, this option also does not provide strong evidence for polygenic inheritance.

Step 3: Option c: Qualitative variation

This option suggests that the trait shows qualitative or discrete variation. As mentioned in the analysis, qualitative variation is more characteristic of traits controlled by single genes, and polygenic inheritance typically shows quantitative variation. So, this option is not the strongest evidence either.

Step 4: Option d: Quantitative variation

The final option states that the trait shows quantitative or continuous variation. This is characteristic of polygenic inheritance, where multiple genes control a single trait, resulting in a wider range of possible phenotypes. Thus, this option provides the strongest evidence that the trait may be influenced by polygenic inheritance. To summarize, the strongest evidence that a trait might be influenced by polygenic inheritance is:

Step 5: Answer

d. The trait shows quantitative variation.

Key Concepts

Quantitative Variation

Quantitative variation is a key feature of polygenic inheritance, where a single trait is influenced by multiple genes. This results in a continuous range of phenotypes, like varying heights in humans or different kernel colors in corn. Instead of distinct categories, quantitative traits exhibit a spectrum, like shades of a color.
These traits are measured rather than categorized, which is why you might find them described by numbers (e.g., height in centimeters) instead of divisions like tall or short. Each gene involved in a polygenic trait contributes a small amount to the phenotype. Therefore, the more genes involved, the greater the range of phenotypes observed.

• Examples of quantitative traits: height, skin color, intelligence, and weight.
• Evaluated using statistical methods, as these traits typically follow a bell-shaped distribution known as the normal distribution.

Intermediate Phenotype

An intermediate phenotype occurs when the offspring exhibit a trait that is a mix between their parents' phenotypes. This is typical in cases of incomplete dominance, a different genetic phenomenon that can sometimes be confused with polygenic inheritance.
In incomplete dominance, neither allele is completely dominant over the other, resulting in a blending of traits. However, intermediate phenotypes are not definitive proof of polygenic inheritance.

• Intermediate phenotypes demonstrate a partial expression of alleles.
• Example: A red flower crossed with a white flower may produce pink offspring.

Understanding the difference between incomplete dominance and polygenic inheritance is crucial, as multiple genes influence polygenic traits rather than just two alleles.

Mendelian Inheritance

Mendelian inheritance describes how traits are passed from parents to offspring through distinct alleles, either dominant or recessive. Named after Gregor Mendel, who discovered these patterns of inheritance, it is a simpler form of genetic crossing compared to polygenic inheritance.
In Mendelian inheritance:

• Traits are determined by single genes.
• There is a clear ratio in offspring phenotypes, often seen in a 3:1 pattern for dominant to recessive traits.

Traits that follow Mendelian inheritance usually show qualitative variation, where there are distinct phenotypic categories such as color or shape. This contrasts with the continuous variation seen in polygenic inheritance.

Qualitative Variation

Qualitative variation represents traits that appear in distinct, separate categories without continuous variation. This is typical for traits governed by Mendelian inheritance, where a single gene determines the trait. These traits are often easy to categorize, like having a specific blood type or a particular flower color.
Qualitative traits do not form a gradient; they are either present or absent. For instance, a plant is either tall or short according to traditional Mendelian concepts.

• Examples include flower color (red or white) and earlobe attachment (attached or free).
• Unlike quantitative traits, these don't follow a normal distribution and are more predictable and straightforward.

Understanding the distinction between qualitative and quantitative variations is key in genetics, helping differentiate between single-gene and multi-gene influences on traits.