CBSE Class 10 Science Heredity And Evolution Notes Set A

Class-X

Chapter 9: Heredity and Evolution

Chapter Notes

Key learning:

1) Variations arise during the process of reproduction. They may be few in asexual reproduction, but many in case of sexual reproduction.

2) The minor variations arising during asexual reproduction are caused by slight inaccuracies in DNA copying. In sexual reproduction, variations are also caused by crossing over process of meiosis.

3) Beneficial variations help the species to survive better in the environment.

4) Nature selects the beneficial variations thereby leading to evolution.

5) Reproduction produces offsprings with similar body design of the parents. However the offspring are not identical, but show a great deal of variation from the parents.

6) Sexually reproducing organisms like humans have 2 (or more) versions of genes for each trait, called alleles.

7) Gregor Johann Mendel carried out several experiments on pea plants. He carried out large number of monohybrid and dihybrid crosses using many contrasting characteristics and put forward several important conclusions.

8) In case of monohybrid cross with pure variety of plants, the phenotypic ratio obtained in F2 generation is 3:1.

9) In case of dihybrid cross involving 2 pairs of contrasting characters, the phenotypic ratio obtained in F2 generation is 9:3:3:1.

10) Mendel concluded that out of any pair of contrasting characters, one is dominant and the other recessive.

11) The homozygous dominant trait is denoted by two capital letters whereas the homozygous recessive trait is denoted by two small letters.

12) The factors or genes controlling a particular trait separate from each other during gamete formation. Hence gamete is always pure as far as contrasting characters are considered. Each gamete will possess only one gene set.

13) In crossing if two or more traits are involved, their genes assort independently, irrespective of the combinations present in the parents.

14) Genes carry information for producing proteins, which in turn control the various body characteristics.

15) For a particular trait, the offspring receives one allele from the father and one allele from the mother.

16) The combination of the male and female germ cells gives a diploid zygote. Thus the normal diploid number of chromosomes in the offspring is restored.

17) Different mechanisms are used for sex determination in different species.

18) The sex of human offspring is genetically determined.

19) Humans have 22 pairs of autosomes and one pair of sex chromosomes.

20) Females have similar sex chromosomes XX, whereas males have an imperfect pair i.e. XY. All eggs carry X chromosom

21) The sex of the child depends on whether the egg fuses with the sperm carrying X chromosome (resulting in a girl) or with the sperm carrying Y chromosome (resulting in a boy).

22) Variations beneficial to a species have a greater chance of flourishing in the species than the harmful or neutral variations.

23) Genetic drift can alter gene frequencies in small population and provide diversity without any survival benefits.

24) Several factors like environment, mutations, reproduction etc can cause alterations in gene frequencies in a population over generations, leading to evolution.

25) Changes occurring in the DNA of germ cells are heritable whereas changes taking place in the non-reproductive tissues are not inherited.

26) Charles Darwin proposed that evolution of species occurred by natural selection, but he did not know the underlying mechanism.

27) Natural selection, genetic drift, variations and geographical isolation can lead to speciation in sexually reproducing organisms.

28) Gene flow between the members of a population prevents speciation.

29) The fundamental characteristics used to classify organisms are
- presence of prokaryotic or eukaryotic cells
- whether the organism is unicellular or multicellular - ability to perform photosynthesis - presence of endoskeleton or exoskeleton in heterotrophic organisms.

30) Classification of living organisms is closely related to their evolution.

31) As we go back in time to trace common ancestors, we find that all organisms must have arisen and radiated from a single species, which in turn originated from non-living material. Thus life arose from non-living matter.

32) Study of homologous organs, e.g. hand of man and wing of bird, helps in tracing the evolutionary relationship between different species.

33) Analogous organs, e.g. wing of insect and wing of bird, do not have common origins, but arose in different species to fulfill similar functions.

34) Fossils help in tracing evolutionary pathways.

35) The age of fossils can be determined by using the relative method or the isotope dating method.

36) Evolution is not a one-step process, but a continuous process occurring in several stages.

37) Complex organs are formed slowly over many generations, sometimes with intermediate forms playing an important role.

38) Sometimes the use of certain features gets modified with time. For example- Feathers may have provided insulation initially, but later became associated with flight.

39) Evolutionary studies have shown that birds are closely related to reptiles.

40) Humans have carried out artificial selection for various features of cabbage and produced different vegetables. 

41) Molecular phylogeny can also be used to trace evolutionary relationships. Here the DNA of different species is compared. Greater the differences in DNA, more distantly related are the species.

42) Disappearance of the existing species is not a requirement for formation of new species.

43) The new species formed are better adapted to the environment, but they need not be superior to the existing species.

44) The common ancestor of humans and chimpanzees evolved in different ways to produce the present forms.

45) Evolution produces more diverse and complex body forms over time, but the newly formed species are not more progressive than the already existing ones. So it is wrong to say that evolution produces progressive higher forms from lower ones.

46) All human beings, whether fair skinned or dark skinned, belong to the same species i.e. Homo sapiens that originated in Africa.

47) The human ancestors gradually migrated from Africa to various parts of the world like Asia, Europe, Australia and America. Thus they spread to different parts of the Earth and adapted as best as they could to their environmental conditions.

Top Definitions

1) F₁ generation- The generation resulting immediately from a cross of the first set of parents (parental generation).

2) F₂ generation – Offspring resulting from a cross of the members of F₁ generation.

3) Progeny - The offspring produced as a result of reproduction of the parents.

4) Dominant trait - A genetic trait is considered dominant if it is expressed in a person who has only one copy of that gene.

5) Recessive trait – A genetic trait that is expressed only when two copies of the gene are present.

6) Homozygous - having two identical alleles of the same gene

7) Heterozygous - having dissimilar alleles at corresponding chromosomal loci 8) Monohybrid cross – A type of crossing in which only one pair of contrasting characters are considered.

9) Dihybrid cross – A type of cross that involves two sets of characteristics.

10) Allele – Either of a pair (or series) of alternative forms of a gene that can occupy the same locus on a particular chromosome and that control the same character.

11) Somatic cells- All cells forming the body of an organism, except the reproductive cells.

12) Sex chromosomes – Either of a pair of chromosomes, usually designated X or Y, in the germ cells of most animals, that combine to determine the sex and sex-linked characteristics of an individual.

13) Gene – A segment of DNA that is involved in producing a polypeptide chain and forms the basic unit of heredity.

14) Trait –A trait is a distinct variant of a phenotypic character of an organism that may be inherited or environmentally determined.

15) Haploid cell – Cell having only one complete set of chromosomes

16) Diploid cell – Cell having two sets of chromosomes, one of paternal origin, the other maternal.

17) Micro-evolution – Evolution resulting from small specific genetic changes that can lead to a new subspecies.

18) Genetic drift - It refers to the random change in gene frequencies in a small population, presumably owing to chance rather than natural selection, thereby providing diversity without any adaptations.

19) Speciation - The process of formation of a new species.

20) Homologous organs – Organs of different organisms which may be dissimilar externally, but are similar in origin and in fundamental structural plan.

21) Analogous organs – Organs of different organisms which are similar in function and external appearance, but dissimilar in origin and structural plan.

22) Fossils – All preserved traces of living organisms.

23) Molecular phylogeny - The use of a gene's molecular characteristics to trace the evolutionary history of organisms.

Top diagrams / charts  (img)

So the ratio of plants in F₂ generation is 3(tall) : 1(short)
Inheritance of a trait (e.g. height) over two generations

…………. F₂ So the ratio of plants in F₂ generation is 9 (Round yellow seeds) : 3 (Round green seeds) : 3(Wrinkled yellow seeds): 1 (Wrinkled green seeds)

Independent inheritance of two separate traits, shape and colour of seeds 

Choose the correct answer from the given options.

Question. Sugarcane field has little variation because of
(a) sexual reproduction
(b) asexual reproduction
(c) Both (a) and (b)
(d) None of these
Answer : B

Question. A Mendelian experiment consisted of breeding tall pea plants bearing violet flowers with short pea plants bearing white flowers. In the progeny, all bore violet flowers, but almost half of them were short.
This suggests that the genetic makeup of tall plant can be depicted as
(a) TTWW
(b) TTww
(c) TtWW
(d) TtWw
Answer : C

Question. When two parents are crossed, the offsprings are referred to
(a) F1 generation
(b) F2 generation
(c) Either F1 or F2
(d) None of these
Answer : A

Question. A cross between two individuals result are in ratio of 9:3:3:1. The cross is
(a) Monohybrid cross
(b) Dihybrid cross
(c) Cross-pollination
(d) Self pollination
Answer : B

Question. Select the statements that describe characteristics of genes
(i) genes are specific sequence of bases in a DNA molecule
(ii) a gene does not code for proteins
(iii) in individuals of a given species, a specific gene is located on a particular chromosome
(iv) each chromosome has only one gene
(a) (i) and (ii)
(b) (i) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)
Answer : B

Question. Exchange of genetic material takes place in
(a) vegetative reproduction
(b) asexual reproduction
(c) sexual reproduction
(d) budding
Answer : C

Question. Two pea plants one with round green seeds (RRyy) and another with wrinkled yellow (rrYY) seeds produce F1 progeny that have round, yellow (RrYy) seeds. When F1 plants are selfed, the F2 progeny will have new combination of characters. Choose the new combination from the following.
(i) Round, yellow (ii) Round, green
(iii) Wrinkled, yellow (iv) Wrinkled, green
(a) (i) and (ii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (i) and (iii)
Answer : B

Very Short Answer Type Questions

Question. How does the creation of variations in a species promote survival?
Answer : Variations lead to adaptation of an organism, which is then able to survive in difficult and unfavourable environmental conditions. It means variations promote survival.

Question. In order to ensure than Mendel gets pure breeding plants in his experiments, what did he do?
Answer : Mendel allowed each variety to self fertilise for several generations

Question. What is DNA?
Answer : 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.

Question. All the variations in the species do not have equal chances of survival. Why?
Answer : All variations will not be able to survive in the environmental changes, which is an essential requirement for survival.

Question. What is a gene?
Answer : 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.

Question. A Mendelian experiment consisted of breeding pea plants bearing violet flowers with pea plants bearing white flowers. What will be the result in F1 progeny?
Answer : Violet flowers

Question. What is heredity?
Answer : Heredity is a process in which traits are passed onto the offspring from parents.

Question. Name the human traits which show variations.
Answer : (i) Colour of eye, (ii) Height, (iii) Colour of skin

Question. Why do mice whose tails were surgically removed, continue to produce mice with tails?
Answer : It is because cutting of tail is done surgically and it is not inherited.

Question. Name the plant on which Mendel performed his experiments.
Answer : Garden pea (Pisum Sativum)

Question. A tall plant is crossed with a dwarf plant. What will be the ratio of homozygous tall and heterozygous dwarf plant in F2 generation?
Answer : The ratio of homozygous tall and heterozygous dwarf plant in F2 generation is 3 : 1.

Question. What type of traits are phenotype?
Answer : The traits which are visible to us are called phenotype, e.g. tall or dwarf (short) are phenotype traits of the plants.

Question. Where is DNA found in the cell?
Answer : DNA is found in the nucleus of the eukaryotic cells and cytoplasm of prokaryotic cells. It is also present in plastids and mitochondria.

Question. A man with blood group ‘A’ marries a woman with blood group ‘O’ and their daughter has blood group O. Is this information enough to tell you which of the traits-blood group A or O is dominant? Why or why not?
Answer : The information is not sufficient because information is confined upto two generation. We need information about third generations to predict the dominant trait.

Question. Give an example where sex determination is regulated by environmental factors.
Answer : In snail, sex is determined by temperature i.e., environmental factor.

Question. State Mendel’s first law of inheritance.
Answer : 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’.

Question. What are sex chromosomes?
Answer : 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.

Question. State Mendel’s second law of inheritance.
Answer : 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’.

Question. No two individuals are absolutely alike in a population. Why?
Answer : Sexual reproduction involves combination of genes from two parents, the genetic material gets exchanged between pairs of chromosomes before formation of their gametes which cause changes.

Question. What is variation?
Answer : Variation is the difference in characteristics or traits among the individuals in a particular species.

Question. Name the term used for traits that are exhibited externally.
Answer : Phenotype.

Question. What is monohybrid cross?
Answer : Monohybrid cross is the cross between two individuals with one pair of different traits

Question. Give the respective scientific names used for studying:
(i) the mechanism by which variations are created and inherited by organism from the previous generation.
(ii) the development of new type of organisms from the existing ones.
Answer : (i) Heredity (ii) Evolution

Question. What is the scientific name of man and garden pea?
Answer : Man — Homo sapiens
Garden pea — Pisum sativum

Question. Why is it that asexual reproduction produces exact copies but sometimes minor variations are seen in next progeny?
Answer : Bio-chemical reactions are not fully reliable, therefore, it may cause slight difference which causes these variations.

Question. Name the genetic material that is responsible for the inheritance of traits.
Answer : Gene or DNA

Question. How can chromosomes be identified?
Answer : Individual chromosomes can be identified by their lengths, position of centromere and binding pattern of staining and shape

Question. Mendel observed a contrasting trait in relation to position of flowers. Mention that trait.
Answer : Terminal flowers have recessive trait whereas axial flowers have dominant trait.

Question. Name the information source for making proteins in the cells.
Answer : DNA

Question. What is sex of the baby that inherits Y-chromosome from the father?
Answer : Male

Short Answer Type Questions

Question. What is genotype?
Answer : 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’.

Question. What indication do we get by appearance of dwarf plant in F2 generation?
Answer : After obtaining F2 progeny from F1 generation in the dihybrid cross, Mendel concluded that when two pairs of traits are combined in a hybrid, one pair of character segregates independently from the other pair of character.

Question. If a trait ‘A’ exists in 10% of the population of asexually reproducing species and trait ‘B’ exists in 60% of the same population, which trait is likely to have arisen earlier?
Answer : Trait ‘B’ is likely to have arisen earlier, because it has higher percentage in the population. In asexual reproduction, the traits which are present in parent generation are carried over to next generation with minimum variation.

Question. Do genetic combination of mothers play a significant role in determining the sex of a new born?
Answer : 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.

Question. What are chromosomes? Where are they located?
Answer : Chromosomes are long thread like structures which contain genetic information of an individual.
These are located in the nucleus of a cell.

Question. ‘DNA is the carrier of the genetic information’. Justify.
Answer : Most of the characters or traits of an organism are controlled by the genes. Genes are actually segments of DNA guiding the formation of proteins by the cellular organelles. These proteins may be enzymes, hormones, antibodies, and structural components of different types of tissues. In other words, DNA (genes) is responsible for structure and functioning of a living body. Genotype of an individual controls its phenotype.
In human beings there are 23 pairs of chromosome. We inherit half of our chromosomes (genes) from our two parents. This means half of DNA in each cell has been inherited from each parent, hence we resemble both of them or their side of family members

Question. How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?
                                                                       OR
‘‘It is possible that a trait is inherited but may not be expressed.’’ Give a suitable example to justify this statement.
Answer : Some traits that are inherited may not express themselves. Such hidden traits are known as recessive traits. Mendel explained this phenomenon with the help of monohybrid cross. In a monohybrid cross performed by Mendel, a tall plant was crossed with a dwarf plant which produced all tall plants in F_l progeny However, when these F₁ tall plants were crossed with each other, ‘dwarf’ trait, which was not observed in the F_l generation, reappeared in the F₂ progeny.

By this, it is concluded that dwarfness is a trait of parent pea plant that was not lost. It was suppressed in the F₁ generation by the tallness trait and reappeared in the F₂ generation. So, we can say that a trait that is inherited may not be always expressed in an organism.

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