Polygenic Inheritance

Definition and Introduction:

Polygenic inheritance refers to those traits that are controlled by more than one gene that are often at different loci-positions on chromosomes. Such genes are termed as non-allele genes. A polygenic trait is complex in expression because the phenotype-the observable characteristics-is the composite result of multiples genes. Due to this complexity, polygenic traits manifest as continuous variation rather than abrupt categories. This renders the relationship between genotype and phenotype far more complicated than what is described under simple Mendelian inheritance.

Important characteristics of polygenic inheritance include:

1. Continuous Variation:

In polygenic traits, phenotypic categories are not sharply defined. Unlike that of a single gene traits (as is observed in Mendelian characters - for e.g., pea plant flower color), it presents a phenomenon of continuous variation. This simply means that the traits fall within a spectrum, thereby range from one extreme to another; such as height, skin color, and kernel color in wheat. Unlike Mendelian traits, which sometimes present abrupt gradations, the polygenic traits present smooth gradations.

2. Multiple Non-allelic Genes:

Under polygenic inheritance, multiple genes together serve as functional units that contribute to the expression of a single trait. Once more, each of the non-allelic genes has two alleles: one additive and one non-additive. The additive alleles join in the expression of the phenotype, while the non-additive ones contribute nothing in this regard. Collectively, the effects add up to produce the ultimate expression of the trait

3. Additive Effect of Alleles:

In polygenic traits, the alleles at various loci are additive that means each contributing allele has a small and nearly equal effect to the phenotype. The allelic pairs do not show any dominance. For instance, in the case of the color of wheat kernel, as discussed later, more additive alleles contribute towards darker kernel color and conversely, fewer additive alleles dictate lighter colors.

4. No Dominance or Non-allelic Gene Interaction:

Unlike the Mendelian inheritance, in which dominant and recessive alleles exist, the case of polygenic inheritance shows no dominance. This means that no one allele suppresses the effect of another. Moreover, there is no interaction among the non-allelic genes; every gene acts individually to contribute to the overall phenotype.

Example: Nilsson-Ehle's Experiment with Wheat Kernel Color:

Probably the best known of all experiments on polygenic inheritance is that made by Swedish geneticist Herman Nilsson-Ehle, showing that several genes are involved in the control of wheat kernel color. He did this experiment by:

P (Parent) Generation:

Nilsson-Ehle crossed one plant having dark red kernels with another one having white kernels.

F1 Generation:

All the offspring from this cross (F1 generation) produced intermediate pink kernels (RW), with none displaying either of the dark red or white phenotypes that the parents displayed.

F2 Generation:

If F1 plants were allowed to self-cross, the F2 presented with five different phenotypic classes of kernel color: dark red, red, pink, light red, and white. These phenotypes also occurred in a ratio of 1:4:6:4:1.

Nilsson-Ehle interpreted these results to suggest that two loci of genes control kernel color. From each locus, two alleles exist-one red pigment and nonadditive. For example, the two alleles are additive that express effects in combination. In other words, each gene contributes equally to the overall color so the more additive alleles mean a darker kernel color. This experiment had permitted such continuous variation yet to show Mendelian rules despite the polygenic nature of the trait.

Genetic Explanation and Additive Model:

The phenotypes in polygenic traits are additive sums of contributions from many genes. Each of these contributing genes has two alleles, one additive and one non-additive. For illustration purposes, consider three pairs of genes (Aa Bb Cc) that control kernel color:

If all six contributing alleles were present, the phenotype would be dark red kernels (AA BB CC).

If no additive alleles were present, the phenotype would be white kernels (aa bb cc).

If three contributing alleles were present, then the phenotype would be intermediate red kernels (Aa Bb Cc).

If more gene pairs than this are added, then even more phenotypic classes appear, each with its different combination of additive alleles.

Polygenic Traits and Normal Distribution:

The distribution of polygenic traits often follows a normal distribution or is said to have a bell-shaped curve in large populations-this is also known as a Gaussian distribution. In such instances, most individuals of a population tend to show phenotypes close to the average value for that population, while very few individuals may show extreme ones. This kind of distribution is symmetric and is hence characterized by its mean-mean and variance-variance spread of values about the mean. The standard deviation-which is the square root of the variance-is, therefore, a measure of just how spread out the phenotypes are about the average.

Counting the Number of Gene Pairs:

The number of gene pairs responsible for a polygenic trait is obtained by determining the phenotypic ratio of the F2 generation. This requires the determination of the proportion of F2 progeny that exhibit the extreme most phenotypes. The formula applied is;

Number of extreme phenotypes =1/4^n

For example, if 1/64 of the offspring in the F2 generation express extreme phenotypes, then three gene pairs are involved because .

Number of Phenotypic Classes: With the rise in number of gene pairs, the number of potential phenotypic classes also rises. Making use of the formula, the number of distinct phenotypic classes in the F2 generation for gene pairs is calculated as

{Number of phenotypic classes} = 2n + 1

Quantitative vs. Qualitative Genetics: Polygenic inheritance differs from Mendelian, or qualitative, inheritance in the following aspects:

1. Nature of Traits:

Qualitative Genetics: Qualitative features are controlled by single genes and can be classified into distinct categories (tall or short plants).

Quantitative Genetics: Qualitative features have a continuous distribution of variation and are quantitatively regulated by many genes. They include height and skin color.

2. Variation:

Qualitative Features: These have discontinuous variation, where differences are distinct categories.

Quantitative Traits: Show continuous variation, or a range of phenotype.

3. Number of Genes:

Qualitative Traits: Inherited in simple Mendelian patterns, determined by a single gene or a few genes; the actions of these genes are directly recognizable.

Quantitative Traits: Inherited in complex patterns, determined by several genes whose individual contributions are very small and that can't be distinguished easily.

4. Statistical Analysis:

Qualitative Traits: Analysis is direct, based on simple counts and ratios.

Quantitative Traits: Population parameters, which include mean and variance, will have to be estimated by statistical methods.

Practice Problems:

1. Calculation of Height

Of course, a cross between a plant having genotype aa bb with a height of 40 cm and one plant having genotype AA BB with a height of 60 cm. If the dominant allele adds quantitatively to height, then the expected height of the F1 progeny would be 50 cm. The 20 cm difference between the two parent plants is attributed to the presence of dominant alleles, with each allele contributing 5 cm. The F1 progeny, having two contributing alleles, would be 10 cm taller than the base height of 40 cm (i.e., ).

2. Gene Pairs Calculation:

In a cross where 2/125 of the F2 offspring are as extreme as one of the parents, the number of gene pairs involved can be calculated with the formula. Since is approximately equal to , it means that three gene pairs must be controlling the trait since.

This treatise on the theory of polygenic inheritance spells out its very basic principles, the Nilsson-Ehle experiment, genetic mechanisms, statistical analysis, and the difference between qualitative and quantitative characters. In this report, the polygenic nature was pointed out to indicate how characteristics of this type followed Mendel's laws in principle at the gene level but complicated by complex phenotypic expressions.

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