Science Class Ten Science Class ten Hindi Version Class 10 Subject List

Heredity And Evolution

Heredity:- The passing of traits from the parents to offspring is called heredity.This is heredity which is responsible for many commonly observable facts; likesiblings looking similar in overall appearance.

Genotype:- The complete set of genes in an organism’s genome is calledgenotype.

Phenotype:- The observable characters in an organism make the phenotype.Phenotype is a result of genotype’s interaction with the environment. Due to thisreason, many phenotypes are not inheritable.

Acquired Traits:- Traits; which are acquired due to interaction with theenvironment; are called acquired traits. Acquired traits are not inheritable. Forexample; if a boxer develops bulging biceps, it does not mean that his son wouldbe borne with bulging biceps.

Inheritable Traits:- Traits; which can be expressed in subsequent generations; arecalled inheritable traits. Such traits bring a change in the genotype of theorganism and hence become inheritable.

Accumulation of Variations During Reproduction:-

Asexual reproduction involves a single parent and is hence not ideal ideal forfacilitating variations. Some minor variations do occur due to inaccuracies in DNAreplication. But the quantum of variations would be too little and would take toomany years to show effect.

Sexual reproduction, on the other hand, is ideal for facilitating variations becausetwo parents are involved in it. The offspring’s genotype is contributed by twoparents and hence chances of variations are very high.

Rules of Inheritance

Gregor Johann Mendel conducted experiments on pea plants and proposed therules of inheritance; based on his observations. Mendel observed that charactersare often present in pairs. A pair of contrasting characters is called allele.

Possible Reasons of Pea Plants Used by Mendel:

Pea can be termed as biennial plant, i.e. two generations of a pea plantcan grow in a given year. This means that Mendel could get enough timeto observe a larger number of generations.Many easily identifiable and contrasting characters are present in peaplants.Cross pollination can be easily induced in pea plants.

Monohybrid Cross: The cross inwhich just two contrasting charactersare studied is called monohybridcross. Mendel did monohybrid crossfor his first experiment. He selected apair of contrasting characters for thatexperiment.

Let us take the example of crossbetween tall plants and short plants.The figure; given here shows theresults of this experiment.

TT represents the genotype of tallplant and tt represents the genotypeof short plants. In the F2 generation,all plants were tall but their genotypewas Tt; which means they were not pure tall plants. This could be established bythe appearance of the character of shortness in the F2 generation; in which mostof the plants were tall and some of the plants were short. This experiment showedthat the character of shortness of recessive in F1 generation and hence could notbe observed. The ratio of number of tall plants to that of short plants in F2

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be observed. The ratio of number of tall plants to that of short plants in F2generation was 3 : 1.

Mendel’s First Law: The Law of Segregation: Every individual possesses a pair ofalleles for a particular trait. During gamete formation, a gamete receives only onetrait from the alleles. A particular trait can be dominant or recessive in a particulargeneration.

Dihybrid Cross: Thecross in which twopairs of charactersare studied is calleddihybrid cross. Inhis secondexperiment, Mendelused dihybrid cross.

Let us takeexample betweenplants with roundand green seeds

and those with wrinkled and yellow seeds. The genotype of round and greenseeds is shown by RRyy and that of wrinkled and yellow seeds is shown by rrYY.In the F1 generation, all plants produced round and yellow seeds; which meansthat wrinkled texture was the recessive character and so was the green colour ofseeds. When plants of F1 generation were allowed to self pollinate; it wasobserved that most of the plants in F2 generation produced round and yellowseeds. Some plants produces round green seeds, some produced wrinkled yellowseeds and some produced wrinkled green seeds. The ratio was 9 : 3 : 3 : 1; asshown in the figure.

Mendel’s Second Law: Law of IndependentAssortment: Alleles of different characters separateindependent from each other during gameteformation.

In the above example; alleles of texture wereassorted independently from those of seed colour.

Sex Determination in Humans:

Somatic cells in human beings contain 23 pairs ofchromosomes. Out of them the 23rd pair iscomposed of different types of chromosomes which are named as X and Ychromosomes. The 23rd pair contains one X and one Y chromosome in a male.On the other hand, the 23rd pair in a female contains X chromosomes. Thismeans that all the eggs would have X chromosome as the 23rd chromosome,while a sperm may have either X or Y chromosome as the 23rd chromosome.When a sperm with X chromosome fertilizes the egg, the resulting zygote woulddevelop into a female child. When a sperm with Y chromosome fertilizes the egg,the resulting zygote would develop into a male child.

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