9. Heredity and Evolution | Class 10 CBSE | Web Notes | Part 1: Accumulation of Variation, Heredity

Heredity (inheritance) is the transmission of characters from parents to offspring.

Evolution is the change in the characteristics of a species over several generations.


Inheritance from previous generation provides common basic body design & minute changes for next generation. Second generation gets inherited differences and newly created differences.

In asexual reproduction, the resultant individuals get only very minor differences due to small inaccuracies in DNA copying. However, in sexual reproduction, greater diversity is generated.

The variations in a species have no equal chances of surviving in the environment. Based on the nature of variations, different individuals would have different kinds of advantages. E.g. Bacteria that can withstand heat will survive better in a heat wave.

Selection of variants by environmental factors is the basis of evolution.


Inherited Traits

A child bears all the basic features of a human being. However, it does not look exactly like its parents.

Human populations show a great variation. E.g. Variation in ear lobes. Most of the individuals have free ear lobes (dominant trait) and some have attached ear lobe (recessive trait).

Rules for the Inheritance of Traits – Mendel’s Contributions

Father and mother contribute equal amounts of genetic material to the child. i.e., each trait is influenced by paternal and maternal DNA. Thus, for each trait there will be two versions in each child.

Gregor Johann Mendel (1822–1884) worked out the main rules of such inheritance.

Mendel used several contrasting visible characters of garden peas. E.g. round/wrinkled seeds, tall/short plants, white/violet flowers etc.

He crossed a tall plant & a short plant. In first generation (F1 progeny), all plants were tall. There were no halfway characteristics (no ‘medium-height’ plants). This means that only one parental trait (tall) was expressed. The expressed trait is called Dominant. The suppressed trait is called Recessive.

Mendel allowed F1 tall plants to reproduce by self-pollination. The second-generation (F2) progeny was 75% tall and one quarter (25%) short. This indicates that both traits (tall and short) were inherited in the F1 plants, but only the tallness trait was expressed. Thus he proposed that two copies of factor (now called genes) control traits in sexually reproducing organism. They may be identical or different, based on the parentage.

Phenotypic ratio (Tall: Short) =

Genotypic ratio (TT: Tt: tt) = 1:2:1

To confirm genotypic ratio, each F2 plants are crossed with a pure recessive (tt) variety. It is called Test cross.

TT & Tt are tall plants, while tt is a short plant. It means a single copy of ‘T’ can make the plant tall (dominant trait), but two copies of ‘t’ (tt) is needed to get short plant (recessive trait).

Cross of coloured flowered & white flowered plants:

Here, F1 produced all coloured flower. So it is dominant trait and white flower is recessive. F2 produced coloured flowered plants and white flowered plants in 3:1 ratio.

Cross between pea plants showing two different characteristics

When a tall plant with round seeds and a short plant with wrinkled seeds is crossed, the F1 progenies will be tall and round seeded. i.e., tallness and round seeds are dominant traits.

If the F1 progeny undergo self-pollination, the following types of F2 progeny are produced:

Parental type combination

New combinations

o  Tall, round seeds.

o  Short, wrinkled seeds.

o  Tall, wrinkled seeds.

o  Short, round seeds.

New combinations are formed due to independent inheritance of tall/short trait and round seed/wrinkled seed trait.

Similarly, formation of new combinations of traits in F2 when factors controlling for seed shape & seed colour recombine to form zygote leading to form F2 offspring.

How do these Traits get Expressed?

DNA is the information source to make proteins in a cell.

A section of DNA that provides information for one protein is called the gene for that protein.

Genes control traits producing proteins. E.g.

Plant height depends on a growth hormone. It is synthesised due to an enzyme (protein). This enzyme is synthesised due to a gene.

o  Efficient enzyme → more hormone → tall plant.

o  Alteration of the gene → less efficient enzyme → less hormone → short plant.

According to Mendelian experiments, both parents contribute DNA equally (copies of same genes) to the progeny. Thus each pea plant inherits 2 sets of all genes. For this, each germ cell must have only one gene set.

In Mendel’s experiment, the characteristics ‘R’ and ‘y’ independently inherited because they are not linked. This indicates that each gene set is not in a single DNA thread (i.e., not in a whole gene set), but in separate independent pieces, each called a chromosome. Thus, each cell has two copies of each chromosome, one each from male (paternal) & female (maternal) parents. Every germ cell takes one chromosome (maternal or paternal).

When two germ cells combine, they restore the normal chromosome number in the progeny. It ensures the stability of DNA of the species. Such a mechanism of inheritance is used by all sexually reproducing organisms. Asexually reproducing organisms also follow similar rules of inheritance.

Sex Determination

There are different strategies for sex determination.

Some species rely entirely on environmental cues. E.g. In a few reptiles, the temperature at which fertilised eggs are kept determines whether they become male or female.

In animals such as snails, individuals can change sex. It indicates that their sex is not genetically determined.

In human beings, the sex is genetically determined. The genes inherited from parents decide whether an individual be boy or girl.

All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and have 22 such pairs. But one pair (sex chromosomes) is not always a perfect pair. Women have a perfect pair called XX. But men have a mismatched pair in which one is a normal-sized X while the other is short called Y (XY).

Half the children will be boys and half will be girls.

All children inherit an X chromosome from their mother.

Sex of the children is determined by father. A child who inherits X chromosome from father will be a girl, and one who inherits Y chromosome will be a boy.

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