NCERT Class 10 The most Important Basic concepts of chapter 9 (Heredity and Evolution) part -1

The most Important points of Heredity and Evolution (part-1)

1. The progeny always has some variations from its parents: These variations may be subtle or clearly observable. Over generations these variations accumulate and result in a progeny that is very different from its ancestors.

2. In asexually reproducing organisms, there are subtle variations over generations. In sexually reproducing organisms the variation is large over generations. 

3. Variations result in organisms belonging to a species being different from each other. Some of the variations may be helpful to the organisms in the prevailing natural conditions and thus the organisms having such positive variations would survive. Whereas organisms with variations not suited to the prevailing natural conditions may not survive. This concept is known as the “survival of the fittest”.

4. Gregor Johann Mendel (1822- 1884) is known as the “Father of genetics”. He experimented with pea plant (pisum sativum) and brought out the three rules of inheritance:

• law of dominance

• law of segregation

• law of independent assortment. 

5. Homozygous plant: Mendel crossed homozygous plant (TT) with dwarf(tt) pea plant. The progeny (F1) was all tall (Tt). The genotype of the F1 generation plant (Tt) was however not as that of tall parent plant (TT). When the F1 generation plants were self pollinated the F2 progeny had a tall:dwarf ratio of 3:1 as depicted below in punnett square: 

	T	t
T	TT	Tt
t	Tt	tt

 

Genotypic ratio— TT : Tt : tt = 1 : 2 : 1

Phenotypic ratio— Tall : Dwarf = 3 : 1

This indicates that both tall and dwarf  trait were inherited in F1 generation but only tall trait was exhibited in all the progeny. Thus tall character (T) was dominant over dwarf (t) and so that dwarf character was not depicted even though it was present in F1 generation Plants (law of dominance). Traits like ‘T’  are called dominant traits while those like ‘t’ are called recessive traits.

6. If pea plants with two different pairs of characteristics (e.g., tall/dwarf and round seeds/wrinkled seeds) are bred with each other, the F1 progeny would have all tall plants with round seeds. This implies that round seed is a dominant character over wrinkled seeds. In F2 progeny there would be some tall Plants with round seeds and short plants with wrinkled seeds. However, there would be some plants with mixed characters—tall plants with wrinkled seeds and short Plants with round seeds. This depicts that tall/dwarf trait and round/wrinkled trait are inherited independent of each other (law of independent assortment). The following punnett square explains this:

Parents: TTRR

F1: TtRr

F2:

       

	TR	Tr	tR	tr
TR	TTRR	TTRr	TtRR	TtRr
Tr	TTRr	TTrr	TtRr	Ttrr
tR	TtRR	TtRr	ttRR	ttRr
tr	TtRr	Ttrr	ttRr	ttrr

Phenotypic ratio: Tall Round : Tall Wrinkled : Dwarf Round : Dwarf Wrinkled = 9:3:3:1.

7. How do the traits get expressed?

A trait in an organism results from the action of proteins (e.g., growth hormone in plants is a protein which is manufactured from a gene). Each protein is made by information from the gene/s for that protein. In the example of pea plant, there are two genes for plant height—‘T’ and ‘t’. ‘T’ is dominant over ‘t’. In a heterozygous plant (Tt), only the gene ‘T’ will be able to make protiens as it is dominant over the ‘t’ gene. This protein would be the growth hormone which will result in the plant being tall. 

8. Each parent contributes one set of genes to the offspring. These genes are located on DNA strands known as chromosomes which are present in pairs in an organism. Each cell in an organism has two copies of each chromosome, one each from each parent. The germ cells of the organism has only one member of the pair of chromosome. When two gametes fuse to form the progeny the pair of chromosomes is restored.

• 9. Sex determination: In humans the sex is genetically determined. Humans have 23 pairs of chromosomes out of which 22 pairs have similar members. The 23rd pair in men has one chromosome termed X and the other shorter one termed as Y. In women both the chromosomes are similar and are X and X. The gametes in men are of two types having either X and Y chromosome, while gametes in females are of one type, i.e., all having X chromosome.

The offspring can be of two types— with XX(girl) or XY(boy) chromosomes.