Genetic Variation
Genetic variation refers to the variation in the genome sequences between individual organisms of a species. Individual differences or population differences can both be referred to as genetic variations. It is primarily caused by mutation, but other factors such as genetic drift and sexual reproduction also play a major role.
Quantitative Genetics
Quantitative genetics is the part of genetics that deals with the continuous trait, where the expression of various genes influences the phenotypes. Thus genes are expressed together to produce a trait with continuous variability. This is unlike the classical traits or qualitative traits, where each trait is controlled by the expression of a single or very few genes to produce a discontinuous variation.
Explain Twin studies of quantitative complex traits?
Twin studies are usually done in human genetic studies to understand the inheritance of quantitative traits. The expression of a trait is affected by both environmental and genetic factors.
Twin studies permit the understanding of the estimation of the heritability of traits in humans. The average heritability of trait indicates that how much phenotypic expression variation is due to the genetic variation.
There are two kinds of twins- DZT(dizygotic (two different zygotes, different genotypes)) and MZT(monozygotic (when a single zygote splits into two, identical genotype)). Twin studies are helpful in understanding the inheritance pattern and expression of complex traits. Such studies have been useful in studying diseases like diabetes and cardiovascular diseases.
- Phenotypic variance is environmental variance in MZT
Studying twins is important because the phenotypic variations of traits in monozygotic twins (MZT) can only be due to environmental factors. The phenotypic variation in this case is treated as an environmental variance. Their genotype is identical. Dizygotic twin (DZT) are however like any two siblings having different genotypes (50 percent different).
- Concordant and discordant twins (both MZT and DZT)
If the phenotypic expression of a trait is the same in both the twins, the twins are called concordant, if it is different, they are discordant. The difference in the phenotypic expression, in that case, is attributed to the environmental factors in the case of MZT. The comparison between DZT and MZT using this concept can be helpful in assessing the complex trait inheritance. For example, if MZT show high concordance but DZT do not, this can be deduced that the variation in DZT is due to genetic variations.
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