## NCERT Exemplar Class 10 Science Solutions

### Chapter 09-Heredity and Evolution-Long Answers

Q.37. Does geographical isolation of individuals of a species lead to formation of a new species? Provide a suitable explanation.
Answer. Yes, geographical isolation of individuals of a species leads to speciation. Splitting of existing species into two new species followed by their geographical isolation brings about reproductive isolation. Populations of a species get separated into two groups by accumulation of genetic variations in them. These newly formed groups are then isolated by any geographical barrier.
Reproductive isolation is absence of interbreeding between members of different species. Geographical barriers prevent interbreeding between two groups. This in turn causes isolation of gene pool of section of population from rest; inhibits gene flow (spread of genes between populations) and thereby leads to formation of new species.
Q.38. Bacteria have a simpler body plan when compared with human beings. Does it mean that human beings are more evolved than bacteria? Provide a suitable explanation.
Answer. Evolution is the process of accumulation of adaptive changes in population’s genetic makeup through successive generations. Mutation in gene pool of population leads to introduction of new / altered traits. Now, natural selection acts on these traits and favour only those traits which makes the organism more adapted to the prevailing climatic conditions. It can be said that only those mutations which give survival and reproductive advantage to organisms are favoured by natural selection. Increased number of organisms having these favoured traits leads to evolution of species with time.
Evolution is not related to complexity of body structure. It is not acceptable to say that all complex organisms are more evolved and all simpler organisms are less evolved. Dinosaurs were much more complex organisms which got disappear followed by appearance of birds on earth (organism with less complex body structure as compared to dinosaurs). Although bacteria are simpler organisms as compared to human but have evolved with time that enable them to withstand extremes of climatic conditions (thermophillic bacteria, halophiles etc) and to develop many adaptive traits to survive under harsh conditions (antibiotic resistance). Thus, evolutionary process does not prove the bacteria being less evolved organisms as compared to human.

Q.39. All the human races like Africans, Asians, Europeans, Americans and others might have evolved from a common ancestor. Provide a few evidences in support of this view.
Answer. Human races are anthropogenic concept; it has no roots in biology. Body plan, morphology, anatomy, physiology and metabolism is common to all human. They are share constant chromosome number (23 pairs) and same genetic as almost 99.9% DNA is same in all human races. And also all the human race can freely inter-breed to produce fertile offspring.
Evolutionary events also indicate origin of human from a common African ancestor. The earliest members of the human species, Homo sapiens can be traced there. Taking into account of all these evidences, all human races seem to have common ancestor.
Q.40. Differentiate between inherited and acquired characters. Give one example for each type.
Answer. Differences between inherited and acquired characters are:

Q.41. Give reasons why acquired characters are not inherited.
Answer. Characters that are determined by environmental factors alone and genetic makeup of organisms does not play any role are called as acquired characters. These characters are developed during lifetime of an organism. Since genetic constitution does not determine these traits, they remain confined to the individual only and are never passed from parents to offspring.
For example: Body size of an individual is determined by health status, nutrition, presence or absence of any disease, severity of disease, physical activity etc. Genes do not determine the size of body as it is developed by above mentioned environmental factors during lifetime of the individual.
Q.42. Evolution has exhibited a greater stability of molecular structure when compared with morphological structures. Comment on the statement and justify your opinion.
Answer. Evolution is the process of accumulation of adaptive changes in population’s genetic makeup through successive generations. Mutation in gene pool of population leads to introduction of new / altered morphological structures. Now, natural selection acts on modified structures and favor only those which makes the organism more adapted to the prevailing climatic conditions. Therefore, with passage of time, evolution leads to changes in morphology of organisms to impart them the survival and reproductive advantage. For example, evolution of homologous, analogous and vestigial organs.
On the other hand, biomolecules (lipids, proteins, DNA and RNA) have maintained a comparatively stable structure through evolution. The basic scaffold of these molecules has not been changed. DNA in all organisms is made up of pentose sugar, phosphate moiety and nitrogenous bases. Organisms differ only in the sequence of nitrogenous bases which in turn impart different forms of particular trait to them. Otherwise, biomolecules have maintained a stable structure.

Q.43. In the following crosses write the characteristics of the progeny Cross.

(a) A cross between two pure breeding dominant parents gives pure breeding dominant offspring. All progeny in this cross will have genotype RRYY and will exhibit round and yellow phenotype.

(b) A cross between two dihybrid dominant recessive parents gives phenotypic ratio of 9: 3: 3:1 in offspring.

Progeny – Round yellow : Round green : Wrinkled yellow : Wrinkled green = 9 : 3 : 3 : 1
(c) A cross between two pure breeding recessive parents gives pure breeding recessive offspring. All progeny in this cross will have genotype “rryy” and will exhibit wrinkled and green phenotype.

(d) A cross between two pure breeding dominant and recessive parents gives heterozygous dominant offspring. All progeny in this cross will have genotype RrYy and will exhibit round and yellow phenotype.

Q.45. Study the following cross showing self pollination in F1. Fill in the blank:

Answer. A dihybrid cross shows inheritance of two different traits simultaneously. A cross between two pure breeding dominant and recessive parents gives heterozygous dominant offspring. All F1 progeny in this cross will have genotype RrYy and will exhibit round and yellow phenotype. Self cross in $F_{1}$ progeny gives $F_{2}$ generation and follow law of independent assortment of characters.

Q.46. In Question 44, what are the combination of characters in the $F_{2}$ progeny? What are their ratios?
Answer. A dihybrid cross shows inheritance of two different traits simultaneously. A cross between two pure breeding dominant and recessive parents gives heterozygous dominant offspring. All $F_{1}$ progeny in this cross will have genotype RrYy and will exhibit round and yellow phenotype. Self cross in $F_{1}$ progeny gives $F_{2}$ generation and follow law of independent assortment of characters.

Combination of characters:
Phenotypic ratio= 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green. So, F2 generation exhibits both parental and recombinant offspring. Hence, the ratio is = 9 : 3 : 3 : 1
Q.47. Give the basic features of the mechanism of inheritance.
Answer. Transmission of genetic characteristic from one generation to the next is termed as inheritance. The basic features of mechanism of inheritance are following:
(i) All genetic characters are controlled by genes. There is atleast one gene for each trait.
(ii) Each gene has two or more alternating forms called as alleles. One of the allele is dominant while other is recessive.
(iii) Genes are linearly arranged on chromosomes.
(iv) Each parent possesses a pair of genes for each characteristic on a chromosome.
(v) Meiosis during gamete formation reduces the chromosome number to half in gametes and fusion of male and female gametes during fertilization restore the normal chromosome number. This fusion also creates new combination of genes in zygote as zygote has half chromosome set from father and half from mother.
(vi) In this way, offspring inherit a pair of genes for each trait from its parents.
Q.48. Give reasons for the appearance of new combination of characters in the $F_{2}$ progeny in question 45.
Answer. $F_{1}$ plants have round and yellow seeds. Cron between generations, they give rise to new combinations in $F_{2}$ generation with round-yellow, round green, wrinkled yellow and wrinkled green in the ratio of 9 : 3 : 3 : 1.
This indicates that the chances for the pea seed to be round or wrinkled do not depend on their chances to be yellow or green. Each pair of alleles is independent of the other pair. This is known as principal of independent assortment.