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Chapter 3: Problem 40

In polygenic systems, how many phenotypic classes corresponding to number of polygene "doses" are expected in selfs a. of strains with four heterozygous polygenes? b. of strains with six heterozygous polygenes?

Short Answer

Expert verified
9 classes for four polygenes, 13 classes for six polygenes.

Step by step solution

01

Understand Polygenic Systems

Polygenic systems involve multiple genes (polygenes) contributing to a single trait. The number of phenotypic classes in polygenic inheritance corresponds to the number of 'doses' or combinations of alleles affecting the trait.
02

Formula for Phenotypic Classes

For 'n' heterozygous polygenes, the number of phenotypic classes can be calculated using the formula: \(2n + 1\). This considers all possible combinations of alleles that might contribute to phenotypic variation.
03

Calculate for Four Polygenes

Using the formula from Step 2, for four heterozygous polygenes: \(n = 4\). Therefore, the number of phenotypic classes is \(2(4) + 1 = 9\) classes.
04

Calculate for Six Polygenes

Similarly, for six heterozygous polygenes: \(n = 6\). Thus, the number of phenotypic classes is \(2(6) + 1 = 13\) classes.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Phenotypic Classes
In a polygenic inheritance system, phenotypic classes represent the varied outcomes observable in a given trait.
Such variations are the result of different combinations of alleles at multiple genetic loci contributing to a trait.
Each class corresponds to a unique phenotypic expression or 'dose' of these contributing alleles.
  • The number of phenotypic classes means the number of visible variations possible in the population.
  • To determine the number of these classes, the formula used is \(2n + 1\) where \(n\) is the number of heterozygous polygenes.
  • This accounts for all the possible combinations of alleles inherited from both parents.
For example, using this calculation, when there are four heterozygous polygenes, the variety in traits becomes more noticeable with 9 different phenotypic classes. Similarly, six heterozygous polygenes result in 13 phenotypic classes, showcasing the vast array of expressions possible.
Heterozygous Polygenes
Heterozygous polygenes refer to pairs of genes where different alleles are present at corresponding loci on a pair of chromosomes.
In the context of polygenic inheritance, these genes contribute variably to a single trait, with each gene having an additive effect.
This means that instead of one dominant gene determining a trait, multiple genes come into play.
  • This setup explains why traits such as height, skin color, and eye color show a range of variations in the population.
  • Each heterozygous polygene contributes its effects, leading to a melding of traits from both alleles.
  • These combinations provide a continuous distribution of trait outcomes.
Hence, as the number of heterozygous polygenes increases, the complexity and variety of potential trait outcomes also increase. This is why even slight variations in polygenetic configurations can lead to noticeable phenotypic diversity.
Alleles
Alleles are different versions of a gene that occur at a particular locus on a chromosome.
In the context of polygenic inheritance, alleles play a crucial role because their different combinations determine the overall expression of a trait.
Each parent typically contributes one allele to the offspring for any given gene.
  • This can result in combinations that are either homozygous, where the alleles are the same, or heterozygous, where they are different.
  • In cases of multiple alleles at several loci, particularly in polygenic traits, the interactions between these alleles bring out varied phenotypic expressions.
  • For a given polygene, both alleles contribute to the trait, with each exerting its influence on the outcome.
Understanding alleles and their interactions provides insight into the complex nature of hereditary traits and polygenic variations. This knowledge helps explain the vast differences we observe in inherited characteristics among individuals.

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Most popular questions from this chapter

Assume independent assortment and start with a plant that is dihybrid \(A / a ; B / b\) a. What phenotypic ratio is produced from selfing it? b. What genotypic ratio is produced from selfing it? c. What phenotypic ratio is produced from testcrossing it? d. What genotypic ratio is produced from testcrossing it?

In tomatoes, red fruit is dominant over yellow, twoloculed fruit is dominant over many-loculed fruit, and tall vine is dominant over dwarf. A breeder has two pure lines: (1) red, two-loculed, dwarf and (2) yellow, many-loculed, tall. From these two lines, he wants to produce a new pure line for trade that is yellow, twoloculed, and tall. How exactly should he go about doing so? Show not only which crosses to make, but also how many progeny should be sampled in each case.

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