LectureChapter 5 - Polygenic and multifactorial inheritanceChapter 6 - Cytogenetics

We finished the chapter on polygenic and multifactorial inheritance with a discussion of the importance of twin studies in studying heritability and on some human complex traits whose relative importance of genetic and environmental factors is still not completely understood: skin color, IQ, and propensity to obesity

We started the chapter on cytogenetics, with an overview of chromosome structure and nomenclature, and the process of making a human karyotype. We followed by discussing abnormalities in chromosome number: polyploidy and aneuploidy (we described some specific cases).

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Lab 03 - Epistasis and hypothesis testing

Genetic corn

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In this lab 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 purple and yellow kernels, students were able to predict the phenotypic proportions of yellow and purple kernels. The predictions were compared to the observations and tested using a chi-square test, with a significance level of 5% (α=0.05).

When the hypothesis (observed values = expected values) 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

We continued the lecture we started yesterday, on polygenic and multifactorial inheritance, a field of genetics that departs from the Mendelian approach. Although individual genes behave according to Mendel's principles, traits that are controlled my more than one gene or are affected by the environment don't show Mendelian proportions in the phenotype of offspring generations.

We defined complex traits, including polygenic and multifactorial, and we introduced the concept of quantitative genetics. We discussed some concepts that are important in the field, like phenotypic distribution, distribution of environments, norm of reaction, and heritability.

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