CBSE Class 10 Science Heredity And Evolution Notes Set B

Heredity and Evolution

PART I

Heredity 

1. Offspring produced by asexual or sexual methods of reproduction resemble each other but they differ to some extent which may be easily marked or may be difficult to observe and identify.

• Heredity refers to the similarities among the parents and the progeny.

• Variations refer to differences among individual related by descent.

• Genetics is the study of heredity and variation.

2. Variations are caused by (a) environmental factors and (b) genetic differences.

• Changes in the somatic or non-reproductive cells are not heritable.

• Heritable genetic variations are produced due to inaccuracies in DNA duplication during cell division.

• Genetic variations are transmitted from parents to progeny leading to diversity among the progeny.

3. In organism reproducing by asexual methods these variations are transmitted to their progeny to a limited extent.

• The variations spread faster among the progeny of organisms reproducing sexually because of recombination ofcharacters.

• Recombination of characters occurs due to:

(a) Fusion of gametes arising from different parents.

(b) Exchange of DNA between paternal and maternal chromosomes during meiosis.

Accumulation of variations

4. Variations in an individual may be advantageous, disadvantageous or neutral for an individual in the existing environment.

• Advantageous variations are selected by nature (environment) in successive generations.

• Disadvantageous variations are lost or remain in low frequency.

• The progeny retains neutral variations as such.

• With changes in the environment frequency of these variations is affected leading to diversity in the forms of life adapted to different environments.

• Accumulation of these variations leads to evolution.

Mendel’s Experiments

5. Gregor Johann Mendel proposed the laws of inheritance in 1866.

• Mendel was an Austrian Monk and also a teacher of science and mathematics.

• He applied his knowledge of science and mathematics to establish principles of genetics.

6. Mendel performed experiments on garden pea – Pisum sativam to study the mechanism of inheritance of characters.

• Pea plants are easy to cultivate and have large flowers, so that cross-pollination can be easily performed under controlled conditions.

• Pea plants are self-pollinated in nature.

• Artificial cross-pollination gives rise to seeds that also survive and reproduce successfully. 

• Pea plants exhibit well marked differences (discrete variations) in characteristics in different varieties e.g. tall or dwarf plants; produce purple or white flowers; produce round or wrinkled seeds.

• Intermediate expression of these characteristics is not observed in pea plants.

• He selected tall and short pea plants for crossing and called them parental generation (P).

• He obtained the progeny by cross-pollinating them and called the progeny of this cross F1-first filial generation.

• F1 progeny was allowed to self-pollinate and called as F2-second filial generation.

7. He found that when two plants with the opposing trails were crossed; one of the traits appeared in the F1 progeny. For example, when the tall plant was crossed with short plant F1 progeny constituted of all tall plants.

• When F2 generation was obtained by selfing F1 progeny, hidden trait (short plants) reappeared in one fourth of the total number of progeny.

• This showed that both the characters were present in F1 progeny but only tallness was expressed.

8. Mendel used the term dominant character for the character that expressed itself in F1 generation and recessive for the character, which did not express itself in F1 but reappeared in F2.

• Mendel concluded that each of the character is controlled by two factors in an individual and called these unit factors or alleles.

• Mendel represented dominant traits with first alphabet of the word in capital form. For example, tallness was represented by T and the recessive trait was represented with the same letter in small form, i.e. shortness by t.

• He also used the term genotype for genetic constitution of an individual. For example, TT, Tt or tt.

• He used term phenotype for external expression of the genotype. For example, tall or short.

• A tall plant can be homozygous (having both alleles of same type TT) or heterozygous (having the two alleles of different types Tt).

• Short plants having both recessive alleles are always homozygous.

• A cross considering one pair of contrasting characters is called dihybrid cross.

9. On the basis of dihybrid crosses Mendel gave the Principle of Independent Assortment.

• In a cross between tall plants having round seeds and short plants with wrinkled seeds, the progeny was all tall with round seeds.

• Self-pollination in these F1 plants resulted in progeny of tall plants with round seeds.

• Self-pollination in these F1 plants resulted in progeny of tall plants with round seeds, tall plants with wrinkled seeds, short plants with round seeds and short plants with wrinkled seed in the ratio of 9:3:3:1.
• It indicated that height of pea plant and shape of seeds are independently inherited.

• The principle of independent assortment thus states that each pair of alleles segregates independently.

Expression of traits

10. A section of DNA carrying the information for a structural or functional protein is called gene.

• A gene is thus a sequence of nucleotides, which forms a molecule of DNA.

• Cellular DNA contains information for the sequence of amino acids in a protein.

• Structural and functional proteins determine the expression of a character, e.g. tallness of pea plant depends on the synthesis of enzyme which in turn regulates secretion of hormone gibberellin.

• If the gene is functional the enzyme is produced, in turn the hormone is produced which makes the plant tall.

• If the gene for tallness is altered the protein enzyme is not synthesized or synthesized inefficiently which affects the growth and the plant remains short.

• Every individual has two sets of genes.

• One set of genes is passed to each gamete by meiosis.

• Each individual gets one set of genes from both parents all the time of fertilization.

• Equal distribution of genes occurs because these genes are located on chromosomes, which occur in pairs of homologues.

• Man has 23 pairs of chromosomes in body cells.

• The gametes contain a set of 23 chromosomes bearing the entire set of genes.

11. Sex determination is the mechanism that controls expression of sex of an individual in a species.

• Factors that determine sex of an individual can be (a) environmental factors and (b) genetic factors.

• In some animals environmental factors determine the sex.

• In some lizards, if fertilization eggs are kept at high temperature maleness is induced and at low temperature femaleness is induced.

• Some molluscs become female if reared alone and become male if reared in the company of a female.

12. In human beings sex is genetically determined.

• Human beings have 23 pairs of chromosomes.

• 22 pairs are called autosomes.

• The chromosomes in each pair of autosomes are similar in size and structure.

• 23rd pair is called pair of heterosomes or sex chromosomes.

• It determines the sex in human beings.

• In female both the sex chromosomes called X chromosome are alike.

PART II

Evolution

1. Evolution is a gradual and orderly process of change in the forms of life from simple to complex that has given rise to the existing diversity of life.

• Charles Darwin gave the concept of evolution as descent with modification.

• Charles Darwin was a British naturalist who went for a voyage of word exploration for five years when he was only 22 years old.

• He observed the diversity of life in a number of continents and islands.

• His observations on the birds of Galapagos Islands situated off the coast of South America stimulated him to think about evolutionary process.

• Darwin observed that variations existed in nature; and those, which helped the organism to adapt it to the environment, were passed on to the next generations.

• He published his book, “On the origin of Species by means of Natural Selection”. He explained the way population gets better adapted to the changed environments by natural selection.

• He also conducted breeding experiments on domestic animals and also studied the role of earthworms in soil fertility after his return back home.

• Darwin did not have an idea of how these variations arose.

2. Variations arise as a result of (a) errors in DNA copying or (b) sexual reproduction.

• Variations are of two types (a) acquired and (b) inherited.

• Acquired variations are changes that occur due to the environmental factors in the lifetime of an individual.

• Acquired variation in somatic traits like decrease in average weight of animals due to starvation or changes in skin colour due to exposure to sunlight are not passed from generation to generation.

• August Weismann removed the tails of mice by cutting them off for twenty-one generations but progeny born always had a tail; proving that changes in body cannot pass from generation to generation or change the genes of the germ cells.

• The changes in DNA of non-reproductive tissues cannot be inherited because such tissues do not participate in sexual reproduction.

• Inherited variations are changes in the genes of the germs cells and are passed down to successive generations.

• The experiences of an individual during his lifetime cannot be passed onto his progeny, and cannot direct evolution.

• Inherited variations may give survival advantage, may decrease chances of survival or may not have any survival advantage of disadvantage.

3. Evolution occurs due to changes in frequency of some inherited traits (genes) in a population over a period of time.

• During natural selection, frequency of a gene having a survival advantage increases.

• Increase in the frequency of such genes, results in evolution of populations that are better adapted to existing environment.

• In small populations accidents can change the frequency of some genes, which do not have any survival value.

• Change in the frequency of some gene caused by chance factor alone is called Genetic drift.

• Genetic drift creates diversity without any adaptations.

4. Microevolution constitutes small but significant changes in common characteristics of population of a species.

Speciation

5. Speciation is the origin of new species from the existing ones.

• Speciation takes place when variation is combined with geographical isolation.

• A species consists of similar organisms capable of interbreeding among themselves to produce fertile offsprings.

• Population is the group of individuals of species living in a particular area.

• Interbreeding among individuals of different populations maintain a free flow of genes in these populations.

• Sub-populations of a species sometimes get separated due to a geographical barrier like a river or mountain; the gene flow between them decreases; natural selection and genetic drift result in accumulation of differences in these isolated sub-populations.

• Members of the two sup-populations gradually become incapable of reproducing with each other and become different species.

Origin of life

6. In 1929, J.B.S Haldane speculated that life must have originated from simple inorganic molecules, which were present on the Earth when it was formed.

7. Stanley L. Miller and Harold C. Urey provided experimental evidence in favour of this idea in 1953.

• They simulated the conditions of ancient Earth in an experimental setup.

• They observed that methane, ammonia and hydrogen sulphide and water vapours, when subjected to electric discharges in a reducing atmosphere for a few days; could give rise to amino acids that form protein molecules.

Evolution and classification

8. Classification is to place diverse forms of life into groups and sub-groups on the basis of similarities and differences among them.

• It provides us an idea of diversity of life, interrelations among living beings and the order of evolution of life.

• Classification refers to a particular form or function of organisms which is used to describe, identify and classify them.

• All organisms share some basis characteristics the cell is the basic unit characteristics. For example, the cell is the basic unit of life in all organisms.

• Most, but not all organisms share the next level of classification. For example, presence or absence of a well-defined nucleus in the cell.

• Cell is the basic unit of structure and function of living beings, therefore, it is prokaryotic or eukaryotic.

• Other characteristics of the organism depend upon the characteristics of cells which form tissues, organs, organ system and the organism.

• The basic characteristics for separation of major groups of organisms are:

(a) Organisation of the cell, if it is prokaryotic or eukaryotic.

(b) Orgnisation of the body f it is unicellular or multicellular.

(c) If there is division of labour in multicellular body.

(d) If they produce their own food by photosynthesis.

(e) The way organs of the body develop and become specialized.

• Characteristics of body design are used for making sub-groups.

9. Hierarchy is the framework of classification in which these groups are arranged in the order of increasing or decreasing levels of similarities.

• Organisms that originated first and have not changed much during the course of evolution are called primitive or lower organisms, e.g. bacteria.

• Organisms that originated later and have changed much during the course of evolution are called advanced, e.g. primates.

10. Evidence of Evolution

Different organisms have similar characteristics because they are either inherited from a common ancestor or these appear similar because they perform a common function.

• Homologous organs are organs having same basic structure modified to perform different function in different organisms. Limbs of amphibians, reptiles, birds and mammals have same basic structure but they are modified to perform different functions.

• Homologous characteristics also help us to identify an evolutionary relationship between apparently different species. Mammals are related to amphibians, reptiles and birds at some stage of evolution because of similarity in basic structure of their limbs.

• All similarities in structure of body do not exhibit common ancestry.

Wings of bats are formed by folds of skin stretched between elongated fingers while feathers all along the form wings of birds.

• Analogous organs are organs which look similar because they perform same function, but they do not have some origin and basic structure.

Wings of butterfly, wings of a bat and wings of birds look similar because they perform same function of flying but they do not have same origin and basic structure.

• If we go in further details of anatomical structure of the wings of birds and wings of bats are formed by modification of limbs, which indicates common ancestry at an earlier stage.

Fossils

Fossils provide us direct evidence of the types or organisms (plants, animals and microbes) that existed at a particular geological time and help us to reconstruct the evolutionary process.

• Fossils are preserved remains or impressions of organisms that lived in the past.

• Age of fossils can be found out from the ratio of isotopes in the fossil containing rocks.

• Some fossils provide us links between existing groups of plants and animals for example feather imprints preserved along with dinosaur’s bones indicate that birds have evolved from reptiles.

Evolution by Stages

11. Evolution of complex organs has taken place through a series of DNA changes (mutations), created by bit-by-bit over generations.

• Structure of eye evolved through a series of stage.

(a) Planaria is a flatworm having rudimentary eyes consisting of a few photosensitive cells which detect light.

(b) Insects have well-developed eyes with mosaic vision.

(c) Octopus (Mollusca) and vertebrates also have well-developed eyes.

(d) The structure of the eye in each of these organisms is different, having separate evolutionary origin.

• Structure of wings also evolved through a series of stages.

(a) Feathers originated in dinosaurs for the first time and provided insulation in cold weather.

(b) Birds adapted the feathers to flight later.

(c) Fossils of a small dinosaur from the Dromaesaur family have imprints of feathers along bones of forearm and head.

(d) This indicates the evolution of feathers in the initial stages were not useful for flying which means that a character that originated for one function evolved later to perform another function.

(e) Fossils showing intermediate stages form connecting links between two groups and provide a direct evidence of evolution.

12. The process of evolution is visible through living examples of cultivated plants.

• Man started cultivation of wild cabbage as a food plant two thousand years ago.

• Plants with desired characters were selected and multiplied.

• This process of artificial selection led to evolution of a number of different looking new varieties like cabbage, broccoli, cauliflower, kohlrabi, kale, Brussels sprout and red cabbage.

(a) Cabbage has condensed apical bud with tightly packed leaves.

(b) Broccoli has fleshy green flower heads arranged in a tree-like fashion on branches sprouting from a thick, edible stalk.

(c) Cauliflower has condensed apical bud, which forms a spherical cluster of immature densely packed white flowers buds.

(d) Kohlrabi has swollen, round stem that forms the edible part.

(e) Kale has large green leaves. Central leaves do not form a head. It is closer to wild cabbage than all other domesticated forms.

(f) Red cabbage is just like a cabbage. It has a red or purple coloured leaves due to a pigment called anthocyanin.

(g) Brussels sprout has small (2-4 cm in diameter) leafy green heads resembling miniature cabbages.

13. Evolutionary relationships can be traced very accurately by comparing the DNA of different species.

• More distantly related organisms accumulate greater number of differences in their DNA therefore it gives a direct estimate of how much the DNA has changed during the formation of these species.

• Molecular phylogeny is now extensively used to define evolutionary relationships.

Evolution as the Process of Creation of Diversity

14. Diversity of forms of life has emerged as a result of environmental selection over time.

• Evolution has progressed from simple to complex body designs with passage of time. But it is not the process of replacement of a lower form of life by another higher form.

• Natural selection and genetic drift have led to the formation of populations that are reproductively isolated from the original populations.

• Evolution cannot be compared with technological progress wherein an old design is replaced by a new better design due to conscious human effort.

• Evolutionary sequence can be represented in the form of a tree having many branches at different stages rather than a straight ladder.

• Human beings are not at the top of evolutionary ladder. They represent a species originated quite recently in the diversity of evolving life.

• Human beings and chimpanzees have great similarities at even at molecular level. It does not mean humans have evolved from chimpanzees.

• Human beings and chimpanzees have a common ancestor a long time ago.

• The two species evolved in their own separate ways to give rise to the existing forms.

Human Evolution

15. Studies on human evolution provide evidences that all human beings have evolved from common ancestors.

• Techniques like excavation, time dating, study of fossils and comparisons of DNA sequences are used to study human evolution.

• According to available information humans are descendents of single species Homo sapiens that evolved in Africa and spread across the world in stages.

• Some of the descendents spread across Africa, others slowly migrated spread across the planet, from Africa to West Asia, then to Central Asia, Eurasia, South Asia and East Asia.

• They travelled down the islands of Indonesia and the Philippines to Australia, and crossed the Bering land bridge to the Americas.

Heredity: Transmission of genetically based characters from one generation to the next generation is called heredity.

Gene: It is a specific segment of DNA on a chromosome occupying specific position and determines the hereditary characters.

Traits: The alternative forms of a character are called traits.

Genetics: It is the branch of biology that deals with the study of heredity and variations.

Monohybrid cross: A breeding experiment which involves the alternative traits of one single character is called monohybrid cross.

Dihybrid cross: A cross between two pure breeding individuals taking into consideration alternative traits of two different characters is called a dihybrid cross.

Variation: It is the differences in the traits shown by the individuals of a species and also by the offsprings of the same parents are called variations.

Sex determination: The mechanism by which sex of an individual is determined when it begins its life.

Question. Branch of biology which deals with heredity and variation is called
(A) Paleontology
(B) Evolution
(C) Genetics
(D) Ecology
Answer : C

Question. Which one of the following is not one of Mendel’s laws ?
(A) Law of dominance
(B) Law of segregation
(C) Law of incomplete dominance
(D) Law of independent assortment
Answer : C

Question. Mendel conducted his hybridization experiments on
(A) chick pea
(B) garden pea
(C) pigeon pea
(D) wild pea
Answer : B

Question. The factors which represent the contrasting pairs of characters are called
(A) Dominant
(B) Recessive
(C) Determinants
(D) Alleles
Answer : D

Question. The botanical name of pea is -
(A) PISUMSATIVUM
(B) Pisum sativum
(C) Pisum Sativum
(D) Pisum sitavum
Answer : B

Question. Heterozygous tall plants were crossed with dwarf plants, what will be the ratio of dwarf plants in the progeny?
(A) 50%
(B) 25%
(C) 75%
(D) 100%
Answer : A

Question. In snapdragon, when red flowers crossed with white flowers then ratio of pink flowers and white flowers in the F₂ progeny is -
(A) 10% Red
(B) Red & white in 3 : 1 ratio
(C) Pink and white in 1:1 ratio
(D) Pink & white in 2:1 ratio
Answer : D

Question. Purine & Pyrimidine nitrogen bases respectively are -
(A) A,T and G,C,U
(B) A,C,T & G,U
(C) A,G & C,T,U
(D) A,G,T & C,U
Answer : C

Question. Linkage is an exception of which mendel’s law ?
(A) Law of segregation
(B) Law of independent assortment
(C) Law of domanance
(D) None
Answer : B

Question. How many years ago and in which environment life originate on the earth ?
(A) 3.5 million years and reducing
(B) 3.5 billion years and reducing
(C) 4.5 million years and reducing
(D) 4.5 billion years and reducing
Answer : B

Question. Theory of “inheritance of acquired characters” was written by -
(A) Darwin
(B) Lamarck
(C) Weismann
(D) Aristotle
Answer : B

Question. Which one of the following statements did not explained by Mendel’s law of dominance ?
(A) In a pair of factors, one is recessive and other is dominant
(B) Factors are in pairs
(C) The contrasting character is controlled by separate unit called as factors.
(D) There is no blending of characters & both characters appears as it is in F₂ gen.
Answer : D

Question. Incomplete dominance discovered in
(A) Antirrhinum majus
(B) Mirabilis jalapa
(C) Both (A) & (B)
(D) Pisum sativum
Answer : C

Question. The sequential arrangement of stages in evolution is known as -
(A) Palaeontology
(B) Geneology
(C) Atavism
(D) Ecology
Answer : B

Question. On which island and name of ship by which Darwin made an extensive study of the flora and fauna in South America ?
(A) Galapagos & H.M.S. Beagle
(B) Galapagos & M.H.S. Beagle
(C) Both
(D) None
Answer : A

Question. Finch birds of Galapagos island represent its development due to -
(A) Mutation
(B) Bio-geographical area
(C) Environment
(D) None
Answer : B

Question. Homo sapiens (modern man) evolved from -
(A) Peking man
(B) Java man
(C) Cro-magnon man
(D) Neanderthal man
Answer : C

Question. The forelimbs of man, cat, bat and whale are
(A) Analogous organs
(B) Homologous organs
(C) Missing link
(D) Fossil
Answer : B

Question. The process of formation of new species is -
(A) Evolution
(B) Speciation
(C) Both
(D) None
Answer : B

Question. In pea plant, yellow seeds is dominant over green seeds. If heterozygous plant having yellow seeds crossed with plant having green seeds then what is the ratio of yellow & green plant in F₁ gen ?
(A) 50:50
(B) 9:1
(C) 1:3
(D) 3:1
Answer : A

Short Answer Type Questions

Question. In a monohybird cross between tall pea plants denoted by TT and short pea plant by tt, Sehaj Anant obtained only tall plants denoted by Tt in F1 generation. However in F2 generation she obtained both tall and short plants. Using the above information explain the law of dominance.
Answer : 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.

Question. Where are genes located? What is the chemical nature of genes?
Answer : Genes are located on chromosomes at fixed positions. They are made of nucleic acid (DNA) therefore acidic in nature

Question. (a) What is genetic constitution of human sperm?
(b) Mention the chromosome pair present in zygote determining the sex of male child.
Answer : (a) 22 + Y chromosome or 22 + X chromosomes.
(b) (22 + X + 22 + Y) = (44 + X + Y) chromosomes

Question. Study the following cross showing self pollination in F1, fill in the blank and answer the question that follow:
Parents RRYY                                  ×            rryy
                             Round, yellow                 Wrinkled, green
F₁                         Rr Yy                    ×      ........... .......
What are the combinations of character in the F2 progeny? What are their ratios?
Answer : F₁ progeny is Rr Yy-Round, Yellow
Combinations of character in the F₂ progeny are:
(i) Round yellow — 9 (Both dominant traits)
(ii) Round green — 3 (One recessive, one dominant
(iii) Wrinkled yellow — 3 (One dominant, one recessive)
(iv) Wrinkled green — 1 (both recessive traits)
The ratio is 9 : 3 : 3 : 1

Question. How can we say that change in genes can be brought by the change in DNA?
Answer : 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

Question. An angiosperm plant having red flowers when crossed with the other having same colour flower produced 40 progenies, out of which 30 were red coloured flowers, 10 plants were with white flowers.
Find out:
(i) What is the possible genotype of parent plants?
(ii) Which trait is dominant and recessive?
(iii) What is the cross called and what is phenotype ratio?
Answer : (i) Rr and Rr
(ii) Red colour of flower is dominant trait while white colour is the recessive trait.
(iii) Monohybrid cross, phenotype ratio is 3 : 1.

Question. In a pea plant, find the contrasting trait if
(i) The position of flower is terminal.
(ii) The flower is white in colour
(iii) Shape of pod is constricted.
Answer : (a) Axial position of flower.
(b) The flower is violet in colour.
(c) Shape of pod is inflated.

Question. Sex determination in man depends upon 23rd pair of chromosomes, called the sex chromosomes. If it were a homologous pair-XX, it would be a female. If it were a heterologous pair – XY, it would be a male. Based on this statement, answer these questions:
(i) How many types of eggs/ova and how many type of sperms female and male human being will produce respectively ?
(ii) Which parent’s contribution of sex chromosomes determines sex of the child?
Answer : (i) Male produces two genetic types of sperms, half with X and other half with Y chromosome. The female produces only one genetic type of ova, all carrying X chromosome.
(ii) Sex of children will be determined by what they inherit from their father, and not from their mother

Question. Explain the Mendel’s concept of heredity, by giving three points.
Answer : (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.

Question. ‘‘It is a matter of chance whether a couple will have a male or a female child.’’ Justify this statement by drawing a flow chart.
                                                                  OR
In humans, there is a 50% probability of the birth of a boy and 50 % probability that a girl will be born.
Justify the statement on the basis of the mechanism of sex-determination in human beings.
Answer : 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.

Question. In a pea plant, the trait of flowers bearing purple colour (PP) is dominant over white colour (pp).
Explain the inheritance pattern of F₁ and F₂ generations with the help of a cross following the rules of inheritance of traits. State the visible characters of F₁ and F₂ progenies
Answer : 11. Let purple trait be represented by: PP, White trait be : pp
                          parents: PP X pp
                                         ↓
                          F1 Pp X Pp (Selfing)
                                         ↓
                         Gametes  P     p
                               P      PP    Pp
                               p       Pp    pp
Visible characters of F₁ progeny has all flowers purple coloured and in F₂ progeny 3 are purple coloured and 1 is white coloured flower.

Question. How do Mendel’s experiments show that traits are inherited independently?
Answer : 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.

Question. Mention the total number of chromosomes along with the sex chromosomes that are present in a human female and a human male. Explain how in sexually producing organisms the number of chromosomes in the progeny remains the same as that of the parents.
                                                                           OR
What are chromosomes? Explain how in sexually reproducing organisms the number of chromosomes in the progeny is maintained.
Answer : 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.

Question. What do you understand by the following terms:
(i) Phenotype (ii) Genotype
(iii) Dominant trait (iv) Recessive trait
(v) Factors or Genes
Answer : (i) Phenotype: The set of characteristics observed in an individual as a result of interaction of its genotype with the environment.
(ii) Genotype: The genetic make up of the cell which determines the characteristics (phenotype) of an organism.
(iii) Dominant trait: The trait of the parent that expresses itself in the offspring.
(iv) Recessive trait: The trait of the parent appearing in the offspring which cannot express itself in the presence of dominant trait.
(v) Factors or Genes: The units of inheritance responsible for a trait in an individual.

Long Answer Type Questions

Question. How do Mendel’s experiments show that
(a) traits may be dominant or recessive?
(b) inheritance of two traits is independent of each other?
Answer : (a) 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 thefirst 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.
(For diagram refer to Ans. 10, Short Answer Type Questions-II) 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.

(b) 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.

Question. (a) What is the law of dominance of traits? Explain with an example.
(b) Why are the traits acquired during the life time of an individual not inherited? Explain
Answer : (a) 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.
(b) Acquired traits even if cause genetic changes in non-reproductive tissues, can not be passed on to next generation.
Genetic changes in reproductive tissues can only be passed on to next generation.

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