20. Quantitative Genetics

Write a short essay that discusses the difference between the more traditional Mendelian and neo-Mendelian modes of inheritance (qualitative inheritance) and quantitative inheritance.

A trait controlled through polygenic inheritance was observed in a series of experiments. A brown eyed rabbit was mated with a blue eyed rabbit. 130 F2 offspring were produced. 2 offspring had brown eyes and 2 offspring had blue eyes. How many polygenes control eye color in rabbits?

If a trait is controlled by 5 polygenes, how many phenotypic categories will be observed in the F2 generation?

Polygenic inheritance is what type of inheritance?

Which of the following traits would you expect to be inherited as quantitative traits?

body weight in chickens

Which of the following traits would you expect to be inherited as quantitative traits?

growth rate in sheep

Which of the following traits would you expect to be inherited as quantitative traits?

milk production in cattle

Which of the following traits would you expect to be inherited as quantitative traits?

fruit weight in tomatoes

How can we ascertain the number of polygenes involved in the inheritance of a quantitative trait?

How do we know that threshold traits are actually polygenic even though they may have as few as two discrete phenotypic classes?

In this chapter, we focused on a mode of inheritance referred to as quantitative genetics, as well as many of the statistical parameters utilized to study quantitative traits. Along the way, we found opportunities to consider the methods and reasoning by which geneticists acquired much of their understanding of quantitative genetics. From the explanations given in the chapter, what answers would you propose to the following fundamental questions:

What findings led geneticists to postulate the multiple-factor hypothesis that invoked the idea of additive alleles to explain inheritance patterns?

A dark-red strain and a white strain of wheat are crossed and produce an intermediate, medium-red F₁. When the F₁ plants are interbred, an F₂ generation is produced in a ratio of 1 dark-red: 4 medium-dark-red: 6 medium-red: 4 light-red: 1 white. Further crosses reveal that the dark-red and white F₂ plants are true breeding

How many additive alleles are needed to produce each possible phenotype?

A dark-red strain and a white strain of wheat are crossed and produce an intermediate, medium-red F₁. When the F₁ plants are interbred, an F₂ generation is produced in a ratio of 1 dark-red: 4 medium-dark-red: 6 medium-red: 4 light-red: 1 white. Further crosses reveal that the dark-red and white F₂ plants are true breeding

Based on the ratios in the F₂ population, how many genes are involved in the production of color?

The use of nucleotide sequence data to measure genetic variability is complicated by the fact that the genes of many eukaryotes are complex in organization and contain 5' and 3' flanking regions as well as introns. Researchers have compared the nucleotide sequence of two cloned alleles of the γ-globin gene from a single individual and found a variation of 1 percent. Those differences include 13 substitutions of one nucleotide for another and three short DNA segments that have been inserted in one allele or deleted in the other. None of the changes takes place in the gene's exons (coding regions). Why do you think this is so, and should it change our concept of genetic variation?

List as many human traits as you can that are likely to be under the control of a polygenic mode of inheritance.