Heredity and Evolution | Class 10 CBSE | Web Notes | Part 1

Heredity (inheritance) is the transmission of characters from parents to offspring.
Evolution is the change in the characteristics of a species over several generations.

ACCUMULATION OF VARIATION DURING REPRODUCTION

Inheritance from the previous generation provides a common basic body design and minute changes for the next generation. The second generation inherits these 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 unequal chances of surviving in the environment. Based on the nature of variations, different individuals have different kinds of advantages. For example, bacteria that can withstand heat will survive better in a heat wave.
Selection of variants by environmental factors is the basis of evolution.

A child bears all the basic features of a human being. However, it does not look exactly like its parents.
Human populations show great variation. For example, variation in ear lobes. Most individuals have free ear lobes (dominant trait), and some have attached ear lobes (recessive trait).

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 and a short plant. In the 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 a factor (now called genes) control traits in sexually reproducing organisms. They may be identical or different, based on the parentage.

Phenotypic ratio (Tall: Short) = 3:1

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

To confirm the genotypic ratio, each F2 plant is crossed with a pure recessive (tt) variety. This is called a test cross.
TT and Tt are tall plants, while tt is a short plant. This means a single copy of ‘T’ can make the plant tall (dominant trait), but two copies of ‘t’ (tt) are needed to get a short plant (recessive trait).
Cross of colored flowered and white flowered plants: Here, F1 produced all colored flowers. So, the colored flower is the dominant trait, and the white flower is recessive. F2 produced colored flowered plants and white flowered plants in a 3:1 ratio.

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
Tall, round seeds. Short, wrinkled seeds.	Tall, wrinkled seeds. Short, round seeds.

New combinations are formed due to the independent inheritance of the tall/short trait and the round seed/wrinkled seed trait.
Similarly, the formation of new combinations of traits in F2 occurs when factors controlling seed shape and seed color recombine to form a zygote, leading to F2 offspring.

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 by producing proteins. For example:
Plant height depends on a growth hormone, which is synthesized due to an enzyme (protein). This enzyme is synthesized due to a gene.

According to Mendelian experiments, both parents contribute DNA equally (copies of the same genes) to the progeny. Thus, each pea plant inherits two sets of all genes. For this, each germ cell must have only one gene set.
In Mendel’s experiment, the characteristics ‘R’ and ‘y’ were 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 the male (paternal) and 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. This ensures the stability of the 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.

There are different strategies for sex determination.
Some species rely entirely on environmental cues. For example, in a few reptiles, the temperature at which fertilized eggs are kept determines whether they become male or female.
In animals such as snails, individuals can change sex, indicating that their sex is not genetically determined.
In human beings, the sex is genetically determined. The genes inherited from parents decide whether an individual will be a boy or a 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.
The sex of the children is determined by the father. A child who inherits an X chromosome from the father will be a girl, and one who inherits a Y chromosome will be a boy.

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