Key Question 1
For a particular character, how do we answer the question, Is the observed variation in the character influenced at all by genetic variation? Are there alleles segregating in the population that produce some differential effect on the character or is all the variation simply the result of environmental variation and developmental noise (see Chapter 1)?
Key Question 2
If there is genetic variation, what are the norms of reaction of the various genotypes
Key Question 3
For a particular character, how important is genetic variation as a source of total phenotypic variation?
Are the norms of reaction and the environments such that nearly all the variation is a consequence of environmental difference and developmental instabilities or does genetic variation predominate?
Key Question 4
Do many loci (or only a few) vary with respect to a particular character? How are they distributed throughout the genome?
OUTLINE
- Genes and quantitative traits
- Some basic statistical notions
- Genotypes and phenotypic distribution
- Norm of reaction and phenotypic distribution
- Determining norms of reaction
- The heritability of a quantitative character
- Quantifying heritability
- Locating genes
CHAPTER OVERVIEW
However, most actual variation between organisms is quantitative, not qualitative
Wheat plants in a cultivated field or wild asters at the side of the road are not neatly sorted into categories of “tall” and “short,” any more than humans are neatly sorted into categories of “black” and “white.”
Height, weight, shape, color, metabolic activity, reproductive rate, and behavior are characteristics that vary more or less continuously over a range Even when the character is intrinsically countable, the number of distinguishable classes may be so large that the variation is nearly continuous
e.g.
say, a corn plant 8 feet tall and another one 3 feet tall—a cross between them will not produce a Mendelian result. Such a corn cross will produce plants about 6 feet tall, with some clear variation among siblings. The F2 from selfing the F1 will not fall into two or three discrete height classes in ratios of 3 :1 or 1:2:1. Instead, the F2 will be continuously distributed in height from one parental extreme to the other.(高中生物练习题)
How do we study quantitative traits when they show such a complex relation between genotype and phenotype? The analysis of a continuously varying character can be carried out by an array of investigations
