CBSE Class 10 Science – Chapter 9: Heredity And Evolution – Detailed Notes & Explanation

Define: Heridity

Heredity is the passing of traits to offspring (from its parent or ancestors). This is the process by which an offspring cell or organism acquires or becomes predisposed to the characteristics of its parent cell or organism.

Through heredity, variations exhibited by individuals can accumulate and cause some species to evolve. The study of heredity in biology is called genetics, which includes the field of epigenetics.

What Is Genetics :

Study of Heredity and variations is be known as genetics.

The term genetics was first of all used by William Bateson in 1905.

An Austrian monk namely Gregor Johann Mendel was the first person to study genetics. He was therefore regarded as the `Father of Genetics’. Modern genetics is concerned with study of genes.

Genes are the units of heredity that control the characteristics of organisms. Geneticists investigate how these genes are transferred from generation to the next, what their structure is and how they function to determine the characteristics of living organisms.

Variation

Variation is concerned with the difference between the individuals of same species and also between the offsprings of the same parents.

Variation could be of two types:

• Somatic variation
• Germinal variation

Somatic Variation:

Mutations that occur in the somatic cells are called somatic variations .New mutations occur twice as frequently in sperm as in eggs (germ cells), probably because so many more cell divisions are required to make sperm than eggs.

These variations are non-heritable and affect the somatic cells. They are acquired by the individual during the life time. Hence these are also called acquired variations.

They are produced by three kinds of factors : environment, use and disuse of organs and conscious efforts. e.g., muscular body of the wrestler (use of muscles), boring of pinna, beard and moustache style (conscious efforts).

These variations are the basis of Lamarck’s theory of evolution.

1. Environment : This includes the factors that affect the organisms such as food, air, pressure, humidity, water etc. Environment affects all the organisms however they also affect the plants because they cannot move or hide themselves.
   • Light : Strong sunlight affects the human skin by increasing the dark pigment melanin in the epidermal cells. Melanin protects the underlying cells by absorbing the ultra violet rays of the sun. Plants grown in shade become weak and pale and acquire long internodes and broad leaves.
   • Habitat: It also affects the genetic makeup of an individual and leads to variations.
   • Nutrition: It is also one of the various factors that cause variations.
2. Use and disuse of organs: Continuous use of an organ makes it better developed whereas constant disuse makes it reduced.
3. Conscious efforts: Conscious efforts by man produce somatic variations in humans themselves, in domestic animals and plants.

GERMINAL VARIATION:

Germinal variation is the type of variation that occurs only in germplasm of organisms. So, only the variations from the germplasm and the gametes are passed on the offsprings.It may be apparent from the birth of the individual or may develop at any stage of life of the organism.

They are produced by new combination of characters, crossing over, change in number of chromosomes, radiations and chemicals. e.g., haemophilia, blood groups, colour blindness, baldness, eye colour etc.

Germinal variations could be of two types :

1. Continuous variations: (Fluctuating variations) Continuous variations formed the basis of   Darwin’s theory of evolution (natural selection). 
   Continuous variations are small, indistinct differences from the normal conditions (average) and are called fluctuations. These are not stable and non-heritable.
2. Discontinuous variations : Such variations are large distinct differences from the parents and are termed as mutations or sports. These are stable and heritable.

Causes of discontinuous variations:

• Modification in structure of chromosomes.
• Alteration in the chemical nature of genes.
• Change in the number of chromosomes.
• Radiations and chemicals may also cause mutation.

Significance Of Variation:

• Variation enables the organisms to adapt themselves to the changing environment.
• It forms raw material for evolution
• It enables the organisms to face the struggle for existence in a better way.
• It helps men in improving the races of useful animals and plants.
• It is the basis of heredity.
• It also leads to the existence of new traits.

Heredity And Variation In Asexual Reproduction

Heredity (L. hereditas = heirship or inheritance; Spencer, 1863) is the transmission of characters from parents to offspring.

There are organisms in which reproduction occurs by asexual means. These include Bacteria, Amoeba, Euglenas, fungi etc., many plants such as rose and sugarcane, lower animals namely Hydra, Planaria etc.

This asexual reproduction is mono parental and the organism produced by it inherits all the traits of its single parent. It is almost a carbon copy of the parent and is known as its clone. It is also called as clonal reproduction. Here, one thing to be noted is that the term “offspring” is not used in case of asexual reproduction.

The clones may develop variations:

• By environmental factors
• By mutation

The variations caused due to environmental factors are not transferable but those variations which are caused by mutation are stable and inheritable.

Offspring or siblings (offspring of the same parents but at different births, e.g., brothers and sisters) are the product of sexual reproduction. The monozygotic twins are exactly similar in their genetic make up whereas dizygotic twins are not exactly similar, they possess genetic variations. But, the paragenesis of asexually reproducing organisms are exactly similar because they are formed by mitosis.

The individuals which are carbon copies of one another and/or the parents are called ramets, while the group of such individuals is known as clone. Identical twins are also clones of each other.

Difference Between Clone And Offspring

Clone	Offspring
Clone is product of asexual reproduction.	Offspring is a product of sexual reproduction.
Meiosis does not occur.	Meiosis occurs before the formation of gametes
Clone exactly resembles the parent.	Offspring differs from parents.

Heredity And Variations In Sexual Reproduction

In sexually reproducing organisms, the new individual receives half of the genetic information from its mother through the egg and half from its father from his sperm.

Sexually produced offspring resemble, but are not identical to, either of their parents. Some reasons for these variations between sexually reproduced offspring and their parents include crossing over when gametes are formed in each parent and genetic recombination, which is the combining of the genetic instructions of both parents into a new combination in the offspring when fertilisation occurs.

The science of genetics explains how the characteristics are inherited by the offsprings from the parents. The pioneering work in this regard  was carried out by an Austrian priest, Gregor Johann Mendel (1822-1884). He worked on garden pea plant (Pisum Sativum) and studied the inheritance of a number of contrasting pairs of characters (like tall and dwarf seeds) for a number of generations. From his studies, he concluded that the contrasting characters in pea plant are controlled by certain units, which he called ‘‘factors’’

That each pair of contrasting characters is controlled by one factor. Thus, the character plant height (tall or dwarf) is controlled by one factor.

The factor are the carriers of hereditary information from one generation to the next.

Mendel, thus, explained the principles of inheritance for the first time.

Mendel’s ‘‘factors’’ were identified as ‘genes’ by later scientists. The genes are present on the chromosomes. The term gene was given by the Danish botanist Johansen in 1909.

Basis of Heredity :

Mendel (1866) proposed that inheritance is controlled by paired germinal units or factors, now called genes.

They are present in all cells of the body and are transferred to the next generation through gametes. Factors or genes are thus physical basis of heredity.

Genes or factors are passed from one generation to the next or from one cell to its daughter cells in the form of chromosomes – chromosomal basis of heredity.

The genetic material present in chromosomes is DNA. Genes are segments of DNA called cistrons. Therefore, DNA is the chemical basis of heredity

Mendal’s Experiments And Laws Of Inheritance

Development of Mendel’s experiment’s

• Gregor Johann Mendel (1822-1884) was a monk (religious man) in Austria in 1843. In 1847, he became the head of Augustinian monastery at Brunn.
• In 1851, he went to the university of Vienna where he studied natural history and mathematics for two years.
• Mendel returned to Monastery in 1858. He also worked as a teacher of physics and natural history in a school from 1856 to 1865.
• Mendel conducted breeding experiments on garden pea in the garden of his monastery. Luckily he formulated the laws of heredity by his experiments and published his finding in a journal in 1866.
• His laws from the basis of the science of genetics and, therefore, he is remembered as the father of genetics.
• At the time of publication of Mendel’s research paper, Darwin’s theory of evolution was also in the air. So nobody concentrated on Mendel’s work. Mendel died in 1884 without getting any appreciation.
• In 1900, the Mendel’s work was rediscovered by three biologists de Vries of Netherland, Carl Correns of Germany and Erichron Tshermak of Austria. A few years later, W. Bateson et al. confirmed Mendel’s work and found that same laws can be applied to animals also.

In human beings, the rules for the inheritance of traits are related to the fact that both the father and the mother contribute practically equal amounts of genetic material to the child. Each trait in the child is influenced by both paternal and maternal DNA (deoxyribo nucleic acid). In other words, for each trait, there will be two versions in each child, one from father and another from mother. Gregor Johann Mendel was the first scientist to work out the basic rules of such inheritance of traits more than a century ago. This involved the transfer of characteristics from parents to offspring. He did this by using different varieties of pea plants (Pisum sativum) which he grew in his garden.

General Terms

Genotype : The internal genetic constitution of an organism is termed genotype. Environment has no effect on it.

Filial generation: The generation of offsprings is termed filial generation.

First filial generation (F1): The first generation of offsprings produced form parent generation.

Second filial generation (F2): The second generation of the offsprings.

Monohybrid cross : A cross between parents differing in one trait or in which only one trait is studied.

Dihybrid cross :  It is a cross between two organisms of the same species which is made to study inheritance of two pairs of factors or genes.”

Phenotype:  It refers to externally visible characters of an organism. It is the net result of interactions between genotype and environment. Individuals of the same genotype breed alike termed phenotype.

Genome:  A complete set (n) of chromosomes inherited as a unit from one parent is termed genome.

Gene: A hereditary determiner specifying a biological function; a unit of inheritance (DNA) located in a fixed place on the chromosome is called gene. Gene represents factor used by Mendel.

Factor: Discrete heredity information called as factor by Mendel.

Dominant gene: A gene which expresses itself in the presence of its contrasting gene in a hybrid is termed dominant gene.

Recessive gene:  It is that gene whose expression is suppressed in the presence of dominant gene e.g., In a hybrid (Tt) tall plant, t gene for dwarfness is recessive and T gene for tallness is dominant.

Alleles:  Two alternative genes at the same locus in homologous chromosomes are termed alleles. It is an abbreviated form of allelomorph.

Heterozygous:  An individual having two different alleles for a particular character is called heterozygous or hybrid (Tt).

Homozygous:  An individual having two identical alleles is called homozygous (TT or tt).

Mendel’s Experimental Plant:

Mendel selected garden pea plant (Pisum sativum) for series of hybridization experiments because it had the following special features:

1. It had a short life cycle and, therefore, it was possible to study a number of generations quickly.
2. Garden pea plant had distinct, easily detectable contrasting variants of features. For instance, some plants were tall and some dwarf; some had violet flowers and some had white flowers; some plants had round seeds and some had wrinkled seeds and so on. Mendel, in fact, noted seven pairs of such contrasting characters in garden pea plant. The characters which always appear in two opposing conditions are called contrasting characters.
3. All the contrasting traits existed in every generation because plants had bisexual flowers and normally resorted to self-pollination.
4. In these bisexual plants, artificial cross-fertilization could easily be achieved. It was done by removing the stamens (male part) before maturity of the female part of flower and later dusting the pistil (female part) of this flower with the matured pollens from a desired plant.
5. Each pea plant produced many seeds in one generation.
6. The garden pea plants could easily be raised, maintained and handled.

Crossing Technique Employed By Mendel:

Since garden pea is self − fertilizing, the anthers have to be removed before maturity.

This operation is called as emasculation. The stigma is protected against any foreign pollen with the help of a bag.

The pollens then at the dehiscence stage, is brought from the plant to be used as male parent and is dusted on the feathery stigma of the emasculated flower. At the time of pollination, the pollens should be mature and the stigma should be receptive.

Emasculation 
          ↓ 
Bagging [for protection] 
          ↓ 
Dehiscence [by dusting pollens] 
          ↓ 
Pollination

Traits chosen by Mendel for his experiment: There are seven traits which Mendel has chosen, they are as follows:

S.No.	Characters	Dominant	Recessive
1.	Stem height	Tall	Dwarf
2.	Flower colour	Violet_	White
3.	Flower position	Axial	Terminal
4.	Pod shape	Inflated	Constricted
5.	Pod colour	Green	Yellow
6.	Seed shape	Round	Wrinkled
7.	Seed colour	Yellow	Green

Mendel performed experiments in three stages:

• He made sure that, the plant which he had chosen must be a true breeding plant, by letting the plant to undergo self – fertilisation.
• He performed the process of cross pollination of alternate forms of traits. The resultant generation obtained was termed as hybrid and these hybrids formed are called F1 generation i.e, first filial generation.
• He allowed the hybrids to self pollinate up to five generations and these generations are subsequently termed as F2, F3 F4 and so on.

Precautions taken during experiment

While performing his cross breeding experiments, Mendel took a number of precautions.

1. He always focused on the inheritance of the specific traits under consideration and simply ignored others. For instance, in his crosses, only one trait, i.e., size of plant (tall or dwarf) or colour of flower (violet or white) etc. was considered. He called such crosses as monohybrid crosses. Mendel also conducted crosses considering two or more contrasting traits of garden pea plant simultaneously. He designated such crosses as dihybrid crosses, trihybrid crosses and so on.
2. In cross breeding experiments, most important precaution required is to avoid self-pollination between two varieties or traits of plants. Mendel removed the anthers (male parts) of the flowers well before the maturity of the female part, i.e., gynoecium of the flowers. This process is called emasculation. Such flowers were covered to avoid entry of any foreign pollen grain from outside by wind or animals. For making desired cross, mature pollen grains from the anther of the flower of the desired plant were transferred on the stigma (female part) of the emasculated mature flower. The seeds formed by such crosses were collected. These seeds belonged to the first filial generation or F1generation. To draw effective conclusions, Mendel used the seeds of F1 generation to raise the F2 generation by self pollination and also the F2 seeds for raising F3 generation by self-pollination.
3. He maintained all the records of his experiments.

Result’s of Mendel’s Experiments:

• When the self pollination was made and F1 generation was obtained, it was found that the generation would express only one of the trait and not the other. The trait which is being expressed is called as dominant, whereas the one which is not expressed is called as recessive trait.
• In the F1 generation obtained by self pollination, the dominant and the recessive traits obtained were in the ratio of 3 : 1 i.e. 75% of the off springs which appeared in F1 generation had dominant trait, while 25% had recessive trait. This ratio of 3 : 1 is also said to be known as Mendelian monohybrid ratio.

• Mendel further found that the phenotypic ratio of 3 : 1 of dominant to actually a genotypic ratio of 1 : 2 : 1 of pure dominant, hybrid and pure recessive forms traits which remain hidden in F1 generation got expressed in F2 generation. This was later proved in F3 generation.

Reasons for Mendel’s success:

• He selected true breeding (pure) pea plant for his experiment.
• He studied single trait at a time.
• He kept an accurate mathematical record of his breeding experiments and noted down the number of each type of offspring produced in each cross.
• He was lucky enough to select the seven traits, as the gene for these traits are located on four different chromosomes.

Mendal’s Laws Of Inheritance

On the basis of the experiments performed and the results obtained Mendel formulated four laws. They are:

• The Principle of Paired Factors :Each character in an individual is governed by two factors called as gene. The alternative form of gene is called as alleles or alleleomorphs. If an individual consists of similar types of alleles, they are called as homozygous e.g. TT, tt while those having different types of alleles are called as heterozygous e.g. Tt etc.
• The Principle of Dominance or Law of Dominance :When two homozygous individuals with one or more sets of contrasting characters are crossed the characters that appear in the F1 hybrids are dominant characters and those which do not appear in F1 are recessive characters.
• The Principle of Segregation or Law of Segregation : (Law of purity of gametes) The law of segregation states that when a pair of contrasting factors or genes or alleles are brought together in a heterozygous condition, the two remains together without being contaminated but when gametes are formed from them the two separate out from each other. This is also known as Mendel’s first law of heredity.
• The Principle of Independent Assortment or Law of Independent Assortment :If the inheritance of more than one pair of characters is studied simultaneously, the factors or genes for each pair of characters assort out independently. It is called as Mendel’s second law of heredity.

Mendel’s Experiments

1. Monohybrid cross : Experiments with garden pea for single pair of contrasting characters.
   • Procedure : Mendel crossed pure tall and dwarf plants. The plants belonged to F1generation all tall were self-pollinated. The plants of F2generation were both tall and dwarf, in approximate 3:1 ratio phenotypically and 1:2:1 genotypically. On, self-pollination, the tall plants of F2 only 1/3–rd breed true for tallness, the rest 2/3–rd produced tall and dwarf in the ratio of 3:1(F3generation). It means F2 generation consisted of three types of plants (instead of apparent two types) – 
     Tall homozygous (Pure) 
     1: 25% TT
     Tall heterozygous (Hybrid)
      2: 50% Tt
     Dwarf homozygous (Pure) 
     1: 25%tt 
     Hence it is to be said that in F2 generation 50% plants passes parental combination while 50% are new combination.
   • Mendel’s explanation : Mendel explained above results by presuming that Tallness and dwarfness are determined by a pair of contrasting factors or determiners (now these are called genes). A plant is tall because it possesses determiners for tallness (represented by T) and a plant is dwarf because it has determiners for dwarfness (represented by t). These determiners occur in pairs and are received one from either parent. On the basis of this behaviour the tallness is described as dominant character and dwarfness as recessive (law of dominance).The determiners are never contaminated. When gametes are formed, these unit factors segregate so that each gamete gets only one of the two alternative factors. When F1 hybrids (Tt) are self pollinated the two entities separate out and unite independently producing tall and dwarf plants (law of segregation).
2. Dihybrid cross : (Crosses involving two pairs of contrasting characters)
   • Procedure : Later on Mendel conducted experiments to study the segregation and transmission of two pairs of contrasting characters at a time. Mendel found that a cross between round yellow and wrinkled green seeds (P1) produced only round and yellow seeds in F1 generation, but in F2 four types of combinations were observed. These are
     Round yellow : 9 Parental combinations
     Round green : 3 Non-parental combinations
     Wrinkled yellow: 3 Non-parental combination
     Wrinkled green: 1 Parental combination. 
     Thus the off springs of F2 generation were produced in the ratio of 9 : 3 : 3 : 1 phenotypically and 1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1 genotypically. This ratio is called dihybrid ratio. The results can be represented as follows:

Mendel represented round character of seed by R and wrinkled by r. Similarly he designated the yellow character by Y and green by y. Therefore, it was a cross between RRYY and rryy.

• Mendel’s explanation : Mendel explained the results by assuming that the round and yellow characters are dominant over wrinkled and green so that all the F1offsprings are round yellow. In F2-generation since all the four characters were assorted out independent of the others, he said that a pair of alternating or contrasting characters behave independently of the other pair i.e., seed colour is independent of seed coat.
  Therefore, at the time of gamete formation genes for round or wrinkled character of seed coat assorted out independently of the yellow or green colour of the seed. As a result four types of gametes with two old and two new combinations i.e., RY, ryRy, rY are formed from the F1 hybrid. These four types of gametes on random mating produced four types of offsprings in the ratio of 9:3:3:1 in F2 generation (Law of Independent Assortment).

What are Heredity and Evolution?

• Genetics is the process of transmission of body features from parents to offsprings and the laws related to transmission. In other words we can say Genetics deals with study of both heredity and variations.
• The word “ Genetics “ was coined by William Bateson in 1906.

• Variations are mainly seen during sexual reproduction due to the following reasons :
• (i) Crossing over during meiosis process.
• (ii) Alterations in genetic material due to mutations.
• (iii) Mixing of female and male gametes that come from two different individuals i.e., father and mother
• Gregor Johann Mendel is considered as “Father of Genetics.” He had formulated the Laws of Inheritance by performing hybridisation experiments on Pisum sativum – Garden pea plant.
• He had studied seven contrasting pairs of characters in pea plants.

(1) Height of the plant – Tall or short	(2) Colour of flower – Purple or white
(3) Shape of seed – Round or wrinkled	(4) Colour of seed – Yellow or green
(5) Colour of pod – Green or yellow	(6) Shape of pod – Inflated or constricted
(7) Position of flower –Axial or terminal

• The cross between the two pure breeding varieties of an organism taking into account only a single character at a time is called monohybrid cross.
• Example of monohybrid cross : A pure tall pea plant is crossed with pure dwarf pea plant. In F1 generation tall pea plants are produced with heterozygous condition i.e., they carry both dominant and recessive alleles. When these plants are crossed they give tall and dwarf plants in the ratio 3 : 1 which is also called phenotypic ratio and their genotypic ratio is 1 : 2 : 1.

• The cross between the two pure breeding varieties of an organism taking into account two characters at a time is called dihybrid cross.
• When a pure pea plant with round yellow seeds is crossed with pure pea plant having green wrinkled seeds, in F1 generation hybrid plants with round yellow seeds are produced. But again when these plants are crossed they produce round-yellow, round-green, wrinkled-yellow, wrinkled-green in the ratio 9 : 3 : 3 : 1, which is the phenotypic ratio.

• Based on Mendel’s breeding experiments three Laws were deduced.
• Law of Dominance : The phenomenon of appearance of only one of the two contrasting traits in F1 generation is called dominance. The other character remains suppressed which is known as recessive character
• Law of Segregation : At the time of reproduction when gametes are formed the factors segregate so that each gamete receives only factor of each character. This is called Law of purity of gametes.
• Law of independent assortment : At the time of reproduction, two pairs of factors of each of the two traits in a dihybrid cross segregated independently during gamete formation and randomly formed combinations in F2 generation. Inheritance of factors controlling a particular trait in an organism is independent of the other.
• The mechanism by which sex of an individual is determined when it begins its life is called sex determination.
• In human beings, sex is determined by XX-XY mechanism. Males have XY sex chromosomes whereas females have XX sex chromosomes.
• In some reptiles sex is determined by environmental factors.
• Gene is a fragment of DNA molecule that has a particular nucleotide sequence which encodes for a particular protein.
• Evolution is the constant process of gradual change occurring in an organism since the origin of life which gives rise to variety of complex organisms on the surface of earth.

1. Heredity: Transmission of genetical characters from one generation to the next generation is called heredity.	5. Monohybrid cross: A breeding experiment which involves the alternative traits of one single character is called monohybrid cross.
2. Gene: It is a specific segment of DNA on a chromosome occupying specific position and determines the hereditary characters.	6. Dihybrid cross: A cross between two pure breeding individuals taking into consideration alternative traits of two different characters is called a dihybrid cross.
3. Traits: The alternative forms of a character are called traits.	7. 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.
4. Genetics: It is the branch of biology that deals with the study of heredity and variations.	8. Sex determination: The mechanism by which sex of an individual is determined when it begins its life.