Multiple Allelism and Polygenic Traits Flashcards
What are Modifying factors vs Non-Modifying factors?
Modifying Factors- phenotypic variability is affected only (intra/ inter allelic interactions and environment)
Non-Modifying Factors- genotypic variability and phenotypic variability are affected (multiple allelism, polygenic traits, linkage of genes)
What are examples of Modifying factors?
Intra-allelic Interactions
Inter-allelic Interactions
Environmental Influences
What are examples of Non- Modifying factors?
Multiple Allelism
Polygenic Traits
Linkage of Genes
What is Multiple Allelism?
The presence of more than two alleles per gene locus (most genes have more than two alleles)
How is Multiple Allelism different from Mendel’s assumed conception of genes.
Mendel assumed the existence of two alleles per gene locus versus multiple allelism where most genes have more than two alleles…
However, each gene locus can only carry two alleles of the multiple allelic series.
Mutations continue to create new…
gene variants
What are alleles/ allelomorphs?
Gene variants that result in novel (new) phenotypes
How does Multiple Allelism affect Genetic Variability?
It is directly proportional… the greater number of multiple alleles with respect to a gene, the greater is the genetic variability.
Number of alleles No. of genotypes
2 3
3 6
4 10
5 15
n n/2 (n+1)
What type of distribution does Polygenic traits generate in the F2 generation?
A normal distribution (larger at the bottom and smaller at the top)
*genotypic variation in the F2 generation increases with the number of genes
What is Additive Genetic Effects? When are they expressed?
Additive genetic effects is when each allele manifests a certain phenotypic value to the individual. The overall phenotype is the summation of the allelic effects the alleles that contribute to the character.
e.g. Two genes control plant height A = 6 cm, a = 3 cm, B = 10 cm and b = 4 cm. AaBb will have a height of 6+3+10+4 = 19 cm.
It is found in Polygenic traits, NOT Oligogenic traits
What does the distribution tell us about Phenotypic Variation in polygenic traits?
- Large number of genes ensure a wide distribution
- Additive genetic effects ensure a symmetric distribution
- Large environmental influence ensures that the phenotypic classes merge with one another to produce a smooth distribution
- Phenotypic distributions are quantitative in nature, hence normal distribution.
Does Polygenic Inheritance use segregation ratios like Oligogenic traits?
No! For polygenic traits, F2 variation doesn’t fall into discrete classes but is continuous – segregation ratios cannot be calculated.
- Inheritance has to be studied using quantitative genetic analysis, which involves the partitioning of the total phenotypic variation into its components.
Calculation Terms to Note
Total phenotypic variability = VPh (subscript)
Total genotypic variability = VG (subscript)
Variability due to dominance = VD (subscript)
Variability due to epistasis = VEp (subscript)
Variability due to additive genetic effects = VA (subscript)
Variability due to environment = VE (subscript)
Broad sense heritability = H^2
Narrow sense heritability = h^2
How do you calculate Total Phenotypic Variability (VPh)?
VPh = VG + VE
VPh = (VA + VD + VEp) + VE
What is Broad Sense Heritability?
Measures the magnitude of environmental influences on the character.
Broad sense heritability (H^2) = Variation in genotype/ Variation in phenotype
= VG / VPh
Environmental influence = [1 – H^2]
What is Narrow Sense Heritability?
Measures the strength of association between genotype and phenotype (breeding value).
Narrow sense heritability (h^2) = Variation due to additive genetic effects/ Variation in phenotype
= VA / VPh
Dominance and epistasis = [1 – h^2]
What are the consequences of Narrow Sense Heritability being high? (Breeding Value)
- the modifying influences are small
- phenotype is a good indicator of genotype since phenotype = genotype + modifying factors
- selection of parents for hybridization by breeders based on phenotype will yield superior progeny
What are the consequences of Narrow Sense Heritability being low? (Breeding Value)
- the modifying influences are high
- phenotype is not a good indicator of genotype
- progress in breeding will be slow.
