continuously varying traits | continuous and discontinuous variations
CONTINUOUSLY VARYING TRAITS
Genotype interacts with environment to produce phenotype. Phenotypic expression
of traits has two aspects:
(i) Qualitative
(ji) Quantitative
Qualitative differences are large and more obvious, but quantitiave variations are small
and less striking. Some traits, like pea seed shape, show discontinuous qualitative
variations with two sharply distinct phenotypes, round or wrinkled; others like 4 O’clock
flower colour can have three phenotypes, red, pink and white; still others like ABO
blood group system have four qualitatively different phenotypes A, B, AB and O. But
many traits like height, weight, intelligence and skin colour in humans, and grain colour
in wheat exibit continuous quantitative variation over a range of many phenotypes.
endel focused on traits that showed only two qualitatively different phenotypes which
could be determined by just two alternate alleles of a single gene. Darwin observed small
continuous variations within individuals of a population. Such a range of phenotypic
spectrum of a trait cannot be traced to a single gene with two alleles. Even a few multiple
alleles of a single gene cannot make such a wide range of phenotypes.
MA continuously varying trait is encoded by alleles of two or more different gene
pairs found at different loci, all influencing the same trait in an additive way. These
quantitative traits, are called polygenic traits, and their genes are polygenes. Each
polygene has a small positive or negative effect on the character. Polygenes supplement
each other and sum of positive and negative effects of all individual polygenes produce
quantitative phenotypes of a continuously Varying trait.
Wheat grains vary in colour from white to dark red. This trait shows a continuous
spectrum of colour variation. (Fig 22-10). Some grains are white, some are deep red but
most grains have shades in between from light pink to moderately dark red. Nilsson
Ehle studied the genetics of wheat grain colour. When he crossed a true breeding
dark red grain plant with a true breeding white grain plant, all Fi grains had light red
colour, intermediate between two parental shades. It seemed as if it was a case of
incomplete dominance. But when Fi.grains were grown to mature plants and crossed
with each other, F2 grains had exactly seven shades of colour in the ratio of 1 dark red
: 6 moderately dark red : 15 red : 20 light red : 15 pink : 6 light pink : 1 white (Fig. 22-11).
Three different gene pairs, i.e. Aa, Bb, Cc at three different loci contribute to the wheat
grain colour. Each individual would contain six alleles for the trait. Alleles A, B and C
code for an equal amount (dose) of red pigment, which is a positive effect. But none
of a, b and c encode red pigment, which is a no (zero) dose negative effect. If all the six
alleles code for red pigment (AABBCC), the grain is dark red; when none of the six alleles
encode red pigment (aabbcc), the grain is white. When a grain has one allele for red
pigment (Aabbcc or aaBbcc or aabbCc) its colour is light pink; if it has two alleles for the
pigment (AaBbcc or aaBbCc or AabbCc) it is pink, if it has three pigment alleles (AaBbCc
or AABbcc or AabbCC), it will be light red. Similarly four alleles colour dose (AABBcc or
aaBBCC or AAbbCC) will make red and five alleles colour dose (AABBCc or AABbCC or
AaBBCC) will produce moderately dark red grain. Thus the colour phenotype of the
grain is the sum of the individual effects of all the six alleles.Environmental factors like
light, water and nutrients also influence the amount of grain colour. Environmental
variations make the distribution of phenotypes more smooth and continuous.
Human skin colour is also a quantitative trait which is controlled by three to six gene
pairs. The greater the number of pigment specifying genes, the darker the skin. A child
can have darker or lighter skin than his parents.
Human height is a more complex polygenic trait. The perfectly continuous variation
in range of human heights produces a smooth bell - shaped curve (Fig 22-12). A few
people are very tall or very short, but most individuals fall in the average or mean
value. This trait is controlled by many pairs of genes at different loci. Even multiple
alleles may be possible at each locus. More the number of alleles for shortness, the
shorter the height will be. Similarly greater the number of alleles for tallness, the taller
the height will be. Environment also has a strong influence on height, intelligence and
skin colour in humans. Constant exposure to sun darkens skin. Poor nutrition prevents
achieving genetically determined height. Healthy and encouraging social environment
promotes intellegence
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