LectureChapter 5 - Polygenic and multifactorial inheritanceChapter 6 - Cytogenetics

Today we discussed one of the most important concepts in quantitative genetics.

We defined heritability and talked about some of its implications. We discussed the use of twin studies as tools the estimate heritability of different traits, and the importance of using both, monozygotic (MZ) and dizygotic (DZ) twin studies.

We discussed a few examples of multifactorial traits in humans: Skin color, IQ, and obesity.

Then we started discussing the chapter on cytogenetics, a field that focuses on studying chromosomes, using karyotypes as the main tool.

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Lecture, chapter 5 - Polygenic and multifactorial inheritance

We started covering quantitative genetics, the subfield of genetics that studies polygenic and multifactorial inheritance based on observations on phenotypic variation.

We discussed the principle of regression to the mean and how statistics is used as an important tool in quantitative genetics. We focused on concepts that are specific to quantitative genetics, such as phenotypic distribution and distribution of environments.

We mentioned features of the interaction of genes and environment as preparation to discuss the concept of heritability.

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Lab 03 - Epistasis and hypothesis testing

Genetic corn

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Yesterday we used genetic corn to test a prediction based on Mendelian principles, about the inheritance of two genes.

The color of corn kernels, although just one trait, is controlled by two separate genes (R and C) that affect pigmentation in the aleurone, which may or may not be pigmented. If transparent the color of the kernel will be yellow or white, and when pigmented it will be purple or red. In our case we only had purple and yellow kernels in cobs that were obtained as the F2 generation from a cross from double homozygote parent plants (RRCC x rrcc).

By doing a count of kernels, students were able to predict the phenotypic proportions of purple and yellow kernels. The predictions were compared to the observations and tested using a chi-square test, with a significance level of 5% (0.05, numerically, but not conceptually equal to α).

When the hypothesis (observed counts = expected counts) was rejected (if it was rejected), results were explained as the consequence of an epistatic interaction that prevented the R and C genes of showing the phenotypic proportions predicted by Mendelian inheritance.

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Lecture, chapter 5 - Polygenic and multifactorial inheritance

After finishing chapter 4 on pedigree analysis (discussing age-related phenotypic expression, penetrance, and expressivity), we started chapter 5, on multifactorial and polygenic inheritance.

We discussed the differences between continuous and discontinuous traits, and how they are related to the number of genes that affect them. Discontinuous traits must be described in terms of measurements taken in a population rather than qualitatively describing individuals. We defined complex, multifactorial, and polygenic traits.

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