7 Continuous Variation

Variation without natural discontinuities, is called continuous variation, and character that exhibit it are called quantitative characters or metric characters, because their study depends on measurement instead of on counting.

The branch of genetics concerned with metric characters is called quantitative genetics or biometrical genetics.

How the intrinsically diiscontinous variation caused by genetic segregation is translated into the continuous variation of metric characters?

  • one is the simultaneous segregation of many genes affecting the character
  • the other is the superimposition of truly continuous variation arising from non-genetic causes.

A gene with an effect large enough to cause a recognizable discontinuity even in the presnece of segregation at other loci and of non-genetic variation can be studied by Mendelian methods, whereas a gene whose effect is not large enough to cause a discontinuity cannot be studied individually. The distinction is reflected in the terms major gene and minor gene.

Variation caused by the simultaneous segregation of many genes may be called polygenic variantion, and the minor genes concerned are sometimes referred to as polygenes.

7.1 Metric characters

The frequency distributions of most metric characters approximate more or less closely to normal curves. Sometimes, however, the scale of measurement (such as in logarithmic units) must be modified if a distributiong approximating to the normal is to be obtained.

There are two kinds of evidence supporting that metric characters really are controlled by genes at many loci:

  • The first is indirect: observations in general agree well with what is expected frome the theory of polygenic inheritance
  • The second is direct experimental evidence: loci affecting quantitative traits can be mapped on chromosomes by methods. And the loci are called Quantitative Trait Loci, or QTLs.

7.2 Properties of metric characters

Two basic genetic phenomena concerned with metric characters:

  • the first is the resemblance between relatives
  • the second is inbreeding depression (近交衰退), with its converse hybrid vigour (杂种优势), or heterosis.

The properties of a population that we can observe in connection with a metric character are means, variances and covariances.