Heredity and Evolution class 9th
1a) Heredity :-
The transfer of characters or traits from the parents to their off springs is called heredity.
The differences between the characters or traits among the individuals of the same species are called variations.
Importance of Variation :
(1) Depending upon the nature of variations different individuals would
have different kinds of advantage. Example, Bacteria that can withstand heat will survive better in a heat
wave. (ii) Main advantage of variation to species is that it increases the chances of
its survival in a changing environment. Free ear lobes and attached ear lobes are two variants found in human populations.
Accumulation of variations during reproduction
When organisms reproduce, the off springs show minor variations due to inaccuracies in DNA copying. These variations are less in asexual reproduction and more in sexual reproduction.
Some variations are useful variations and they help the organism to adjust to the changes in the environment. Some variations do not help the organism to adjust to the changes in the environment and they may die and become extinct.
Mendel and His Work on Inheritance
- Gregor Johann Mendel (1822 & 1884) : Started his experiments on
plant breeding and hybridisation. He proposed the laws of inheritance in living organisms.
Mendel was known as Father of Genetics.
- Plant selected by Mendel : Pisum sativum (garden pea). Mendel used a
number of contrasting characters for garden pea.
Medel’s Experimental Material : He chose Garden Pea (Pisum sativum) as his experiment material because of:
(i) Availability of detectable contrasting traits of several characters. (ii) Short life span of the plant. (iii) Normally allows self-fertilisation but cross-fertilisation can also be
carried out. (iv) Large no. of seeds produced.
- Mendel’s Experiments : Mendel conducted a series of experiments in
which he crossed the pollinated plants to study one character (at a time).
Cross between two pea plants with one pair of contrasting characters is called a monohybrid cross.
Example : Cross between a tall and a dwarf plant (short).
Rules for inheritance of characters (traits) :-
Characters are transferred through genes present in the DNA molecules in the chromosomes present in the nucleus of the cell.
The inheritance of characters is due to the fact that both the father and mother contributes equal amount of genetic material to the child. So for each trait there are two factors one from the father and one from the mother.
Gregor Johann Mendel conducted experiments with garden pea plants and determined the rules for the inheritance of traits.
When plants having one pair of character (Eg:- tall and short plant) was crossed (Monohybrid cross) :-
Mendel selected pea plants having one pair of character – a tall pea plant and a short pea plant. He selected pure tall (TT) and pure short (tt) pea plants and cross pollinated them. He obtained all tall plants (Tt) in the first generation (F1 ). When the first generation plants were self pollinated, he obtained tall and dwarf plants in the ratio 3:1 in the second generation. (F2)
The ratio of pure tall (TT), hybrid tall (Tt) and pure dwarf (tt) was in the ratio 1:2:1
The trait that is expressed in the F1 generation is called the dominant trait and the trait that is supressed in the F1 is called the recessive trait.
When plants having two pairs of characters (Eg:- shape and colour of seeds) were crossed (Dihybrid cross) :-
Mendel selected pea plants having two pairs of characters – shape and colour of seed. He selected plants having round yellow seeds (RRYY) and wrinkled green seeds (rryy) and cross pollinated them. He obtained all plants with round yellow seeds (RrYy) in the F1 generation. When these plants were self pollinated in the F2 generation out of 16 plants, 9 had round yellow (RrYy), 3 had round green (Rryy), 3 had wrinkled yellow (rrYy) and 1 had wrinkled green (rryy) seed. In the ratio 9:3:3:1.