Continuous phenotypic variation

1
Continuous phenotypic variation

• The most common variation acted on by natural
  selection.
• Phenotype of individuals determined by
  measurement.
• Changes and patterns of variation determined
  statistically.

2
A contrast to Mendelian genetics
Quantitative phenotypic characters common in
nature Range of variation Note shape
Distribution of the heights of 1,000 Harvard
College students aged 18 to 25
(Adapted from Castle.)
3
White spotting in Dutch rabbits a quantitative
character
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Possible genetic basis for some
continuous Characters Contributing and
noncontributing alleles No dominance Each
contributing allele makes a small contribution
to the phenotypic expression
Note shape
The results of crosses between two strains of
wheat differing in three gene pairs that
determine grain color
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The mathematical model of continuous character
inheritiance

• Two kinds of alleles
• contributing a, b, c, etc.
• non-contributing a, b, c, etc.
• Genes a, b, c, etc. in different chromosomes.
• Locations in chromosomes called Quantitative
  Trait Loci (QTLs)
• Binomial expansion emulates independent
  assortment of the alleles.

6
Percent survival of 16 different types of D.
melanogaster flies exposed to a uniform dose of
DDT
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BUT!