Lessons 58–59 · 2 lessons · A. Zikiryayev, A. To‘xtayev, I. Azimov, N. Sonin. Biology. Basics of Cytology and Genetics, Grade 9, revised and expanded 5th edition. “Yangiyul Poligraph Service”, Tashkent, 2019
59
Variation
Textbook: pp. 158–161
GoalExplain the kinds of variation, modification variation and the norm of reaction.
Variation is the differences between individuals of one species; it provides the material for natural and artificial selection. The non-hereditary kind is modification (phenotypic) variation; the hereditary kinds are combinative (new combinations of genes in sexual reproduction, including crossing over) and mutational variation; ontogenetic variation is the change of traits with age. In modification variation the genotype does not change but the phenotype does under the environment: Himalayan rabbit fur grows black in a cold place and white in a warm one; the underwater and surface leaves of a water lily differ; ultraviolet light increases skin pigment. The norm of reaction is the range within which a trait can change, set by the genotype: narrow for some traits (heart and brain size), wide for others (fat content of milk, leaf size). Modifications are not inherited, are group-wide (similar in many individuals under the same conditions), depend on the environment and their limits depend on the genotype.
Worked examples
Two plants grown from one clone: 40 cm in the sun and 25 cm in the shade. The genotype is the same, the difference is 40 − 25 = 15 cm — a modification; cuttings taken from them and planted elsewhere grow according to the new conditions.
If features inherited from two parents show up in a new combination, that is combinative variation; the whitening of a rabbit’s dark patch in warmth is a modification.
Class activity
“Modification or mutation?”. In your home or school garden (with permission) compare 5 leaves each of one species grown on a sunny and a shady side: measure their length and tabulate. Is the difference hereditary or environmental? Why?
Practice
1
What is modification variation?
A change of phenotype under the environment without a change of genotype; it is not inherited.
2
What is the norm of reaction?
The limits within which a trait can change, set by the genotype.
3
If a tree’s leaf in the sun is 9 cm and in the shade 14 cm, what is the difference in cm?
5
4
Why is the dark fur of a Himalayan rabbit, caused by cold, not passed to its offspring?
It is a phenotypic modification and the genotype has not changed; the offspring again develop according to the environment.