Question

An inbred strain of plants has a mean height of \(24 \mathrm{cm} .\) A second strain of the same species from a different geographical region also has a mean height of \(24 \mathrm{cm}\). When plants from the two strains are crossed together, the \(\mathrm{F}_{1}\) plants are the same height as the parent plants. However, the \(\mathrm{F}_{2}\) generation shows a wide range of heights; the majority are like the \(P_{1}\) and \(F_{1}\) plants, but approximately 4 of 1000 are only \(12 \mathrm{cm}\) high, and about 4 of 1000 are \(36 \mathrm{cm}\) high. (a) What mode of inheritance is occurring here? (b) How many gene pairs are involved? (c) How much does each gene contribute to plant height? (d) Indicate one possible set of genotypes for the original \(P_{1}\) parents and the \(F_{1}\) plants that could account for these results. (e) Indicate three possible genotypes that could account for \(\mathrm{F}_{2}\) plants that are \(18 \mathrm{cm}\) high and three that account for \(\mathrm{F}_{2}\) plants that are \(33 \mathrm{cm}\) high.

Short answer

Answer: The mode of inheritance is Polygenic Inheritance, and each gene contributes 12 cm to the plant height.

Step-by-step solution

(a) Determine the mode of inheritance

Since the F1 plants have the same height as the P1 plants, it suggests a type of dominance inheritance is occurring here. Additionally, the presence of discrete height values in the F2 generation is indicative of a quantitative trait with multiple genes involved. Therefore, the mode of inheritance is Polygenic Inheritance.

(b) Determine the number of gene pairs involved

In the F2 generation, we see a 1000:6:4 ratio of the 24 cm (majority height): intermediate height (12 and 36 cm) plants. Since there are two extremes (12 and 36 cm), this suggests that 2 gene pairs are involved, each showing incomplete dominance.

(c) Calculate the gene contribution to plant height

Since there are two gene pairs involved and each gene pair is showing incomplete dominance, we can determine the contribution of each gene to plant height by finding the difference between the extremes and the average height. \( \frac{36 cm - 12 cm}{2} = 12 cm \), so each gene contributes 12 cm to the plant height.

(d) Propose a set of genotypes for P1 parents and F1 plants

Let's use A and B as the gene pairs, and use the 12 cm height increment as the dominance effect. Possible genotypes for original P1 parents: - First P1 parent: AABB (24 cm) - Second P1 parent: AABB (24 cm) Possible genotype for F1 plants (after crossing the P1 parents): AaBb (also 24 cm)

(e) Propose possible genotypes for F2 plants that are 18 cm and 33 cm high

For 18 cm F2 plants, we need to find combinations that produce one less increment (remember that we determined each gene contributes 12 cm to the plant height): - AaBB (18 cm) - AAbb (18 cm) - aaBB (18 cm) For 33 cm F2 plants, we need to find combinations that produce one extra increment: - AaBb (33 cm) - AABb (33 cm) - AABB (33 cm)

Key concepts

Genetics of Plant Height

The genetics underlying plant height is a fascinating area of study, involving complex interactions between multiple genes and their environment. In our textbook exercise, we see an example of how these genetic interactions can influence the height of a plant species. The data from the F2 generation of crossed inbred strains suggest a polygenic inheritance pattern. This pattern occurs when multiple genes, or gene pairs, contribute to a single phenotypic trait – in this case, plant height.

Each gene pair may have a certain incremental effect on the height, and together they produce a range of plant heights in the offspring. It's much like a mixing board where each slider (gene) adjusts the volume (trait expression) slightly, combining to create the overall sound (the phenotype). The distribution of heights seen in the F2 generation is indicative of a quantitative trait controlled by multiple genes rather than a single gene determining height.

Gene Pairs and Plant Trait Expression

The interplay of gene pairs significantly influences the expression of plant traits, such as height. In the example provided, we discovered that the trait for plant height is regulated by at least two gene pairs. We interpret the wide range of heights in the F2 generation to mean that different combinations of alleles from these genes lead to varying expressions of height. This polygenic inheritance results in a continuous variation of the trait, rather than distinct categories that you'd see with a single gene.

Moreover, each gene pair contributes to the plant height in a unique way, and by breeding plants with certain genotypes, we can predict the range of possible heights of their offspring. Understanding how these genes work together to influence plant traits is crucial in plant breeding and agricultural practices, where desired characteristics need to be enhanced or minimized.

Incomplete Dominance

In the case of incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes, unlike complete dominance where one allele completely masks the effect of another. Observing the F2 generation of the inbred plant strains, we see heights that do not match exactly to either parent strain but fall in between. This outcome is indicative of incomplete dominance at work.

The plants with intermediate heights, such as those 18 cm and 33 cm tall, are illustrative of this genetic phenomenon. For instance, a plant with a genotype of AaBB would have one allele coding for the average height and another contributing to additional height, resulting in an intermediate stature. This gene interaction showcases how the effects of alleles can be mixed to produce a trait within a range of expression rather than at distinct heights, creating the variation we observe in the plant population.