CBSE Class 10 Science Heredity And Evolution Notes Set 09

These CBSE Class 10 Science notes for Chapter 8, Heredity, cover accumulation of variation during reproduction, inherited and acquired traits, Mendel's monohybrid and dihybrid crosses and his laws, how genes and DNA control traits, and how the sex of a child is determined in human beings. Each topic is written in short numbered points with Punnett squares and a table of important ratios for quick revision before exams.

Accumulation of Variation During Reproduction

Reproduction gives rise to new individuals that are similar to their parents, but not exactly the same. The differences between the individuals of a species, and between the offspring and their parents, are called variations.

How variations arise

1. Asexual reproduction: one parent is involved, and the offspring is almost an exact copy (a clone). Only minor variations arise, due to small inaccuracies in DNA copying.
2. Sexual reproduction: two parents are involved, and each offspring receives some characters from the father and some from the mother. Crossing over and the random separation of chromosomes during gamete formation add to the variation, so sexual reproduction generates much greater diversity.
3. The second generation will have the differences it inherits from the first generation as well as newly created differences.

Importance of variation

1. Different individuals have different advantages depending on the nature of their variations. For example, bacteria that can withstand heat will survive better in a heat wave.
2. The most important advantage of variation to a species is that it increases the chance of its survival in a changing environment.
3. All variations do not have equal chances of survival. Variation is useful for the species, though it is not necessary for the survival of every individual.
4. Inheritance from the previous generation provides both a common body design and subtle changes in it, which are passed to the next generation.

Heredity

Heredity is the transmission of characters (traits) from the parents to the offspring, that is, from one generation to the next. Characters such as height, facial features, body colour and eye colour that can be seen in an individual are called traits. Genetics is the branch of biology that studies heredity and variation.

Inherited traits and acquired traits

Only the changes in the DNA of the germ cells (gametes) are passed to the next generation. For example, cutting the tails of mice generation after generation does not produce tailless mice, since the genes of the germ cells are not changed.

Mendel and His Experiments

Gregor Johann Mendel (1822-1884), an Austrian monk, is called the Father of Genetics. He worked out the rules of inheritance by experimenting on the garden pea plant (Pisum sativum). He studied one character at a time and kept a count of the individuals showing each trait in every generation, which helped him to arrive at the laws of inheritance.

Why Mendel chose the garden pea

1. It is easy to grow and has a short life cycle, so many generations can be studied in a short time.
2. It has clear, easily visible contrasting traits.
3. Its flowers are bisexual and normally self-pollinating, and cross-pollination can also be done easily.
4. Each plant produces many seeds in one generation.

The seven pairs of contrasting characters

Terms used in heredity

Monohybrid cross (one character)

1. Mendel crossed a pure tall plant (TT) with a pure dwarf plant (tt). The seeds grew into the F1 generation, and all the plants were tall (Tt).
2. He then allowed the F1 plants to self-pollinate. In the F2 generation, three-fourths of the plants were tall and one-fourth were dwarf, a phenotypic ratio of 3 : 1 and a genotypic ratio of TT : Tt : tt = 1 : 2 : 1.
3. The dwarf trait was not lost in F1; it was hidden by the tall trait. So tallness is the dominant trait and dwarfness is the recessive trait.
4. A single copy of T is enough to make a plant tall, but both copies must be t for a plant to be dwarf.

Parents: TT × tt. Gametes: T and t. F1: all Tt (tall):

F2 (Tt × Tt):

Law of dominance: when two pure parents with contrasting traits are crossed, only one trait (the dominant trait) appears in the F1 generation. Law of segregation (purity of gametes): the two alleles of a pair separate when gametes are formed, so each gamete carries only one allele.

Dihybrid cross (two characters)

1. Mendel crossed a pure plant with round, yellow seeds (RRYY) with a pure plant with wrinkled, green seeds (rryy). All the F1 plants had round, yellow seeds (RrYy), so round and yellow are dominant.
2. He self-pollinated the F1 plants. The F2 plants were of four types: round yellow, round green, wrinkled yellow and wrinkled green.
3. Round yellow and wrinkled green are the parental combinations. Round green and wrinkled yellow are new combinations.
4. The number of plants of each type was in the ratio 9 : 3 : 3 : 1 (Mendel counted 315 round yellow, 108 round green, 101 wrinkled yellow and 32 wrinkled green seeds out of 556).
5. Each character taken alone still gives the 3 : 1 ratio. The shape of the seed and its colour are therefore inherited independently of each other.

The F1 plant RrYy forms four kinds of gametes, RY, Ry, rY and ry, in equal numbers. The F2 generation is:

Mendel's laws of inheritance

Reasons for the success of Mendel's experiments

1. He selected true-breeding (pure) lines of the pea plant.
2. He studied one trait at a time, and later two traits.
3. He used a large number of plants, so the numbers were reliable.
4. He recorded and counted the results of every generation and applied mathematics to them.

How Traits Get Expressed

1. Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides the information for one protein is called the gene for that protein.
2. The proteins made by genes may be enzymes or hormones that control the chemical reactions in the cell. A trait is the result of several such reactions.
3. Example: The height of a plant depends on a plant hormone. If the gene for the enzyme that makes the hormone works well, much hormone is made and the plant is tall. If the gene has an alteration, the enzyme is less efficient, less hormone is made and the plant is short.
4. Genes are located on chromosomes, which are thread-like structures in the nucleus made of DNA and proteins.

Mechanism of inheritance

1. Both parents contribute equal amounts of genetic material to the child, so each trait has two copies of its gene, one from the father and one from the mother.
2. Each gene has two or more alternative forms (alleles); one allele may be dominant over the other.
3. During gamete formation (meiosis), the number of chromosomes is halved and each gamete gets only one allele of each gene.
4. At fertilisation the male and female gametes fuse and the normal number of chromosomes is restored in the zygote: 23 + 23 = 46 in human beings.
5. This keeps the amount of DNA constant from one generation to the next, and the same mechanism works in all sexually reproducing organisms.

Sex Determination

Different species use different strategies to determine the sex of their offspring. In some reptiles (for example some turtles and lizards), the temperature at which the fertilised eggs are kept decides whether the young ones are males or females. In snails, individuals can change sex, so sex is not genetically determined. In human beings, the sex is genetically determined by the sex chromosomes.

1. Human cells have 23 pairs of chromosomes. 22 pairs are autosomes and one pair is the sex chromosomes.
2. A woman has a perfect pair of sex chromosomes, both called X (XX). A man has a mismatched pair: a normal-sized X and a short Y (XY).
3. All the eggs of a woman carry an X chromosome. A man makes two kinds of sperms in equal numbers, half with X and half with Y.
4. If a sperm carrying X fertilises the egg, the child is a girl (XX). If a sperm carrying Y fertilises the egg, the child is a boy (XY).
5. All children inherit an X chromosome from their mother, whether they are boys or girls. So the sex of the children is determined by what they inherit from their father, and not from their mother.
6. The chance of a boy and a girl is equal (50% each) in every birth; it is a matter of chance which sperm fertilises the egg.

Sex determination in human beings

Important Ratios at a Glance

Quick Revision

1. Variations arise from errors in DNA copying and from the mixing of genes in sexual reproduction; they help a species survive in a changing environment.
2. Inherited traits are controlled by genes and are passed on; acquired traits affect only body cells and are not inherited.
3. A gene is a section of DNA on a chromosome that codes for one protein; each trait is controlled by two copies of a gene.
4. In a monohybrid cross the F1 shows the dominant trait and the F2 ratio is 3 : 1 (genotypes 1 : 2 : 1).
5. In a dihybrid cross the F2 ratio is 9 : 3 : 3 : 1, and two new combinations appear, which shows independent assortment.
6. A recessive trait is expressed only when both alleles are recessive (homozygous).
7. Gametes have half the number of chromosomes; the zygote has the full number.
8. In human beings the father decides the sex of the child: X sperm gives a girl and Y sperm gives a boy.