1. Heredity – The transmission of characters (or traits) from the parents to their off-springs is called heredity.

2. Variation– The differences in the characteristics (or traits) among the individuals of a species is called variation.

3. Genotype and phenotype mean different things. The genotype is the set of genes in our DNA which is responsible for a particular trait. The phenotype is the physical expression, or characteristics, of that trait.

4. Gene is the unit of inheritance. Gene is the part of a chromosome which controls the appearance of a set of hereditary characteristics.

5. Chromosomes are long thread-like structures present in the nucleus of a cell.

6. Deoxyribonucleic Acid (DNA) is a chemical in the chromosome which carries the hereditary characters or traits in a coded form from one generation to the next in all the organisms. It is a section of the gene.

7. DNA —> Gene—> Chromosome

8. We study the mechanism by which variations are created and inherited.

9. The long-term consequences of the accumulation of variations are also an interesting and we shall be studying it under evolution.

10. Heredity is the transmission of characteristics, physical or mental from parents to offspring that is, from one generation to the successive generation.

11. The reproductive processes give rise to new individuals that are similar, but subtly different. These differences shown by the individuals of a species are termed as variations.

12. Some amount of variation is always seen and these appear more pronounced for species that reproduce sexually when compared to those having asexual reproduction. The second generation will have differences that they inherit from the first generation, as well as newly created differences.

13. Depending on the nature of variations, different individuals would have different kinds of advantages. Bacteria that can withstand heat will survive better in a heat wave. Selection of variants by environmental factors forms the basis for evolutionary processes.


14. The rules of heredity determine the process by which traits and characteristics are reliably inherited. Let us take a closer look at these rules.

15. Inherited Traits – It is the transmission of particular characteristics from parents to their offspring, generation to generation, which bears all basic features with a great deal of variation. An inherited trait is, therefore, a genetically determined characteristic that distinguishes a person.

16. Mendel’s Contributions

(i) Gregor Mendel, an Austrian monk was the first to carry out scientific studies on transmission of characteristics from the parent to the offspring.

(ii) He did this by using different varieties of pea plant (Pisum sativum) which he grew in his garden.

17. Mendel chose pea plants for studying inheritance because –

(i) Pea plants had a number of clear-cut differences/ contrasting character which was easy to identify.

(ii) Pea plants were self-pollinating which enabled them to produce next generation early.

(iii) Many generations of pea plants can be produced in a comparatively short time span.

18. Mendel used a number of contrasting characters of garden peas- shape of seeds (round and wrinkled), colour of seeds (yellow and green), height of pea plants (tall or dwarf), colour of flowers (white or violet) and so on.

19. Hybrid – A new form of plant resulting from a cross (or breeding) of different varieties of a plant is known as hybrid.

monohybrid cross

20. Monohybrid cross– When we breed two pea plants having one contrasting characteristic each (of one trait each) to obtain new plants, then it is called monohybrid cross.

(i) In the first experiment, Mendel considered the phenotype – an externally exhibited trait only one at a time.

(ii) He first ensured pure-bred tall and pure-bred short plants. He selected one tall and one short plant from the pure-bred. He called this the P or parental generation.

(iii) He cross-pollinated the tall and short plants.

(iv) In the first generation, he got all tall plants. This generation was called the first filial or F1 generation. The plants that are produced in the F1 generation are called hybrids as they have a mixture of traits of both the parents.

(v) Then the plants from F1 generation were self-pollinated and the next generation was called the second filial or the F2 generation. In this generation, the ratio of tall to short plants was found to be 3:1.

(vi) Of the tall plants, one-third was found to breed true and the other two-thirds on self-pollinating again produced plants in 3:1 ratio of tall and short plants.

(vii) Here tallness was dominant character and shortness was recessive character.

(viii) Since, in this case only one trait, i.e., height was considered; this cross is called the monohybrid cross.

21. Dihybrid cross– When we breed two pea plants having two contrasting characteristics each (of two traits each) to obtain new plants, then it is called dihybrid cross.

(i) Two pairs of contrasting character were taken (i) Round/Wrinkled and (ii) Yellow/Green.

(ii) Round and Yellow are dominant characters and Wrinkled and Green are recessive characters.

(iii) In F1 generation we only obtain Round and Yellow Seeds.

                                    R           r

 Y             RY         rY

y            Ry           ry

(iv) In F2 generation, the phenotype seed obtained were in the ration 9:3:3:1 [Round and Yellow seeds (9): Round and Green seeds (3): Wrinkled and Yellow seeds (3): Wrinkled and Green seed (1)]

(v) Thus, the tall/short trait and the round seed/wrinkled seed trait are independently inherited.

22. Mendel’s 3 Principles of Inheritance – Based on his experiments Mendel Laws inheritance as: –

(i) Law of Segregation – According to this law allele pairs separate or segregate during gamete formation, and randomly unite at fertilization.

(ii) Law of Independent Assortment – According to this law when two or more characteristics are inherited, individual hereditary factors assort independently during gamete production, giving different traits an equal opportunity of occurring together.

(iii) Law of Dominance – It states that states that one of the factors for a pair of inherited traits will be dominant and the other recessive unless both factors are recessive

23. Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene for that protein. If the gene for an enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics or traits.


24. In some animals, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female.

25. In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined.

26. In human beings, the sex of the individual is largely genetically determined.

27. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X. But men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY.

sex determination

28. Half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.

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