Class X – Heredity and Evolution (PYQs) – Solutions

Ans. DNA stands for deoxyribonucleic acid. It is self-replicative; molecule present in all living organisms as the main constituent of chromosomes. It passes on genetic characteristics to offsprings.

Ans. DNA is found in the nucleus of the eukaryotic cells and cytoplasm of prokaryotic cells. It is also present in plastids and mitochondria.

Ans. Heredity

Ans. Evolution

Ans. Gene is the unit of heredity present in chromosomes, DNA which directs specific protein synthesis, when it is passed from the parent to the offspring.

Ans. DNA

Ans. Heredity is a process in which traits are passed onto the offspring from parents

Ans. All variations will not be able to survive in the environmental changes, which is an essential requirement for survival.

Ans. Gene or DNA

Ans. No, mother does not play an important role because mother has only ‘X’ chromosomes whereas father has both X and Y chromosomes. When offspring takes ‘X’ from mother and ‘Y’ from father, then baby boy is born.

If offspring takes ‘X’ chromosomes from mother as well as father, girl child is born. The probability of getting boy or girl is 50 : 50.

Ans. The information is not sufficient because information is confined upto two generation. We need information about third generations to predict the dominant trait.

Ans. The traits which are visible to us are called phenotype, e.g. tall or dwarf (short) are phenotype traits of the plants.

Ans. Genotype shows the genetic constituent of an organism, i.e. it is the description of genes present in an organism. For example, TT, Tt or tt where T and t are the different forms of the same gene. The genotype of tall plant is TT and that of dwarf plant is ‘tt’.

Ans. Mendel’s first law, also known as the law of segregation states that ‘during gamete formation, the alleles of the character segregate in such a way that each gamete carries only one allele for each gene’.

Ans. Mendel’s second law, also known as the law of independent assortment states ‘that for different traits genes segregate independently during the formation of gametes’.

Ans. (i) Colour of eye, (ii) Height, (iii) Colour of skin

Ans. Monohybrid cross is the cross between two individuals with one pair of different traits.

Ans. Those chromosomes which decide whether the offspring will be male or female are called sex chromosomes. Many animals have two types of chromosomes. Human beings have ‘X’ and ‘Y’ sex chromosomes.

Ans. Chromosomes are long thread like structures which contain genetic information of an individual. These are located in the nucleus of a cell.

Ans. Phenotype.

Ans. In snail, sex is determined by temperature i.e., environmental factor.

Ans. Garden pea (Pisum Sativum)

Ans. Violet flowers

Ans. Individual chromosomes can be identified by their lengths, position of centromere and binding pattern of staining and shape.

Ans. Male

Ans. Offspring always takes 23 chromosomes from father and 23 chromosomes from mother and in total has 46 chromosomes. It means that total number of chromosomes remain constant.

Ans. We cannot predict because information is not sufficient. We can decide dominant or recessive trait if we have the data for at least three generations. More information is required.

Ans. (i) Law of dominance. (ii) Law of segregation (iii) Law of independent assortment

Ans. Yes, each trait is influenced by DNA of both parents.

Ans. The generation produced by the offsprings of F₁ generation is called F₂ or second generation.

Ans. yyrr – wrinkled, green seeds

yyRR – Round, green seeds.

Ans. According to law of dominance, dominant trait is expressed in F₁ generation although dominant as well as recessive traits are inherited. Single copy of dominant trait is sufficient for expression of tall dominant trait in F₁ generation.

Ans. A gene is present on a segment of DNA chromosome.

(i) It provides information of one protein to another and is responsible for its change.

(ii) It ensures stability of DNA.

It means change in gene segment of DNA will bring change in DNA.

Ans. Genes are located on chromosomes at fixed positions. They are made of nucleic acid (DNA) therefore acidic in nature.

Ans. (i) Mendel worked on pea plant.

(ii) He found that traits are controlled by genes which come in pairs.

(iii) Genes are inherited as separate units, one from each parent.

Ans. Rr and Rr

Ans. Red colour of flower is dominant trait while white colour is the recessive trait.

Ans. Monohybrid cross, phenotype ratio is 3 : 1.

Ans. Tall because it is a dominant trait whereas dwarf is a recessive trait.

Ans. 3 : 1 is ratio between tall to dwarf.

Ans. In F₂ generation, recessive genes are also expressed in homozygous condition.

Ans. Justification: Women produce only one type of ovum (carrying X chromosome) and males produce two types of sperms (carrying either X or Y chromosome) in equal proportions. So the sex of a child is a matter of chance depending upon the type of sperm fertilising the ovum.

Ans. Chromosomes are long thread like structures which contain heriditary information of individual and are carrier of genes.

The total number of chromosomes present in both human male and female is 46. Out of these, two chromosomes are the sex chromosomes. In human males, the two sex chromosomes present are X and Y, while in human female, both sex chromosomes are X.

During sexual reproduction, the new individual is formed by the fusion of gametes from both the parents. These gametes are haploid in nature, i.e. they contain only one set of the chromosomes. They are formed by the meiosis, a type of cell division which reduces chromosome number to half. When these haploid gametes fuse during fertilisation, the two nuclei of these gametes fuse and the chromosome number is then restored to normal. Hence, the progeny formed has the same number of chromosomes as that of the parents.

39. After self-pollination in pea plants with round, yellow seeds, following types of seeds were obtained by Mendel:

Analyse the result and describe the mechanism of inheritance which explains these results.

Ans. The ratio obtained is 9:3:3:1 in which parental as well as new combinations are observed. This indicates that progeny plants have not inherited a single whole gene set from each parent. Every germ cell takes one chromosome from the pair of maternal and paternal chromosomes. When two germ cells combine, segregation of one pair of characters is independent of other pair of characters.

Ans. Mendel carried out dihybrid crosses by crossing two pea plants differing in contrasting traits of two characters. For example, he crossed a pea plant having yellow colour and round seed characters with another pea plant bearing green colour and wrinkled seed characters. In the F2 generation, he obtained pea plants with two parental and two recombinant phenotypes as yellow round and green wrinkled (parental) and yellow wrinkled and green round (recombinant). This indicated that traits separated from their original parental combinations and got inherited independently.

Ans. Mendel demonstrated that traits can be either dominant or recessive through his monohybrid cross. He crossed true-breeding tall (TT) and dwarf (tt) pea plants. The seeds formed after fertilisation were grown and the plants that were formed represent the first filial or F1 generation. All the F1 plants obtained were tall.

Then, Mendel self-pollinated the F1 plants and observed that all plants obtained in the F2 generation were not tall. Instead, one-fourth of the F2 plants were short.

From this experiment, Mendel concluded that the F1 tall plants were not true breeding; they were carrying traits of both short height and tall height. They appeared tall only because the tallness trait was dominant over the dwarfness trait. This shows that traits may be dominant or recessive.

Ans. Mendel demonstrated that traits are inherited independently through his dihybrid cross. He considered two traits at a time, seed colour and seed shape in which yellow colour (YY) and round shape (RR) are dominant over green colour (yy) and wrinkled shape (rr), respectively.

Mendel cross bred the plants and observed that the F2 progeny of dihybrid cross had a phenotypic ratio of 9 : 3 : 3 : 1 and produced nine plants with round yellow seeds, three plants with round green seeds, three plants with wrinkled yellow seeds and one plant with wrinkled green seeds.

In this experiment, he found that round yellow and wrinkled green are parental combinations whereas round green and wrinkled yellow are the new combinations. In a dihybrid cross between two plants having round yellow (RRYY) and wrinkled green seeds (rryy), four types of gametes (RY, Ry, rY, ry) are produced. Each of these gametes segregate independently of each other and each has a frequency of 50% of the total gametes produced.

From this experiment, he concluded that when two pairs of traits are combined together in a hybrid, one pair of character segregates independently of the other in terms of character. This is known as the law of independent assortment.

Ans. Law of dominance: Some alleles are dominant while others are recessive. An organism with at least one dominant allele will display the effect of dominant trait e.g. If we carry out a cross between pure breed tall pea plant with pure dwarf pea plant then F1 progeny will be tall plant because it is dominant trait whereas dwarf is a recessive trait.

Ans. Acquired traits even if cause genetic changes in non-reproductive tissues, cannot be passed on to next generation. Genetic changes in reproductive tissues can only be passed on to next generation.