Biology For Class X - Chapter No. 6 - Inheritance- Question Answers

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CHAPTER 6: INHERITANCE
Questions Answers

Q.1: Define the following terms: (i) Heredity (ii) Genetics (iii) Inheritance (iv) Variation
Ans: HEREDITY:
The characteristics of the offspring's to resemble their parents is called heredity.
In other words:
"Heredity is the way by which genes transmit characters from parents to offspring".

GENETICS:
The branch of biology which deals with the study of heredity and Variations is called genetics.

INHERITANCE:
The process by which characters are transmitted from parents to off springs is called inheritance.

VARIATION:
The difference in characters such as height, colour, etc. among individuals of same species are called variation.
Explanation:
Living things when reproduce sexually, their offspring differ somewhat from their parents. They show resemblance with one another as well as with their parents but they are not their exact copies, even than the brothers and sisters of one parent, neither resembles exactly with one another nor with their parents. Similarly puppies of the same female dog are not exactly alike. The same is true about plants, such differences are called variations.

Q.2: Genes control the inheritance of characters. Explain?
Ans: GENES CONTROL INHERITANCE OF CHARACTERS:
Life of every organism; every character of his structure; function and behaviour, at all levels of biological organization is programmed and controlled by a set of instructions. These instructions are in the form of coded biological information called genes. Gene is the smallest segment of DNA which is located at specific position on chromosome. It is the basic unit of biological information. It synthesize a protein which work as enzyme.
This biological information to offspring is passed through egg and sperm (gametes) which are the sex cells. We have also received our genes from our parents and they received their genes from their parents. Thus we and our brother and sisters share many genes of our parents and even of our grandparents. So the inherited characters are determined by genes which are transmitted from generation to generation. Therefore we can say that "Heredity is the way by which genes transmit characters from parents to offspring".

By Mrs. Ayesha Arif Vice Principal
(Jauhar Progressive School)

Q.3: Write a detailed note on Chromosomes.
Ans: CHROMOSOMES:

• The thread like structure present inside the nucleus and are bearer of hereditary character in the form of genes is called Chromosomes.
• Each kind of organism has specific number of chromosomes. These numbers and shapes remain constant in all individual of a species generation after generation.
• The genes are located at chromosomes and inherited through chromosomes during sexual reproduction.

EXAMPLE:
All human contains two sets of 23 chromosomes. One haploid set (n=23) is paternal in origin, which is contributed by sperm while the other haploid set (n=23) is maternal which comes from egg.

HOMOLOGOUS CHROMOSOMES:
The two chromosomes which are similar in their shape, size and position of centromeres present in a cell make one homologous pair of chromosome. These homologous sets of chromosomes provide the physical basis of heredity of an organism.

HETEROLOGOUS CHROMOSOMES:
A heterologous chromosome is a chromosome that contains different set of genes for a given trait.

COMPOSITION OF CHROMOSOMES:

• The chemical material that makes up chromosomes is called chromatin.
• Chromatin is basically a nucleo-protein (Deoxyribo Nucleoprotein) which is composed of 40% of DNA and 60% of Histone (special protein).
• DNA (Deoxyribo Nucleic Acid) is made up of billions of units called Deoxyribo Nucleotides.
• Each nucleotide is made up of deoxy ribose sugar (C₅H₁₀O₄), Phosphoric acid (H₃PO₄)and nitrogenous bases.
• The DNA of a chromosome exists as one very long double stranded, a duplex, which extends unbroken through the entire length of the chromosome. This long DNA (more than 7 feet or 2 meter) is coiled to fit into a much smaller spaces.
• The histone is positively charged while DNA is negatively charged. This beads of histone and DNA is called nucleosome.
• When the string of nucleosomes wraps up into high order coil called super coil. This super coiled chromomers form chromosome.

Diagram:

Q.4: What is DNA?
Ans: Deoxyribo Nucleic Acid (DNA):

• The DNA of a chromosome is one very long double stranded fiber, duplex which extend unbroken through the entire length of chromosome.
• A typical human chromosome contain about 140 million nucleotide in its DNA.

Size:

• If a strand of DNA from a single chromosome were laid out in a straight line, it would be more than 7 feet long (2 meter). This is too long to fit into a cell. Thus this long DNA is coiled to fit into a much smaller space.

Nucleosome:

• DNA resembles a string of beads. This bead is made up of histone and DNA and is called Nucleosome.

Q.5: Describe the Waston and Crick model of DNA?
Ans: Watson and Crick Model Of DNA:
The model of DNA was suggested by James Watson and Francis Crick, in 1953. This model is based on X-Rays, Photographs and chemical analysis of DNA. According to this model:

• Double Helix:
  (i) DNA is a double helical structure.
• Backbone of DNA:
  (ii) Each helix is a polynucleotide chain which are twisted around each other.
  (iii) The outer part is called Upright, which are made up of deoxyribose sugar and phosphate. The inner part called Rungs, made up of paired nitrogenous bases.
• Pairing of Bases:
  (iv) Both helix are complementary to each other which are held together by hydrogen bonds.
  (v) The complementary helix have complementary base pairs i.e Adenine pairs with Thymine and Cytosine always pairs with Guanine.

STRUCTURE OF DNA

Q.6: What is gene and how its work?
Ans: GENES:

Gene is the smallest segment of DNA which has genetic information in the form of code to synthesize a protein. Life of every organism, every character of his structure, function and behaviour, all the level of biological organization is programmed and controlled by a set of instructions. These instructions are in the form of coded biological information called 'Genes'. Thus:
Genes are the basic unit and structure of Biological information. They are unit of inheritance which is copied and inherited from generation to generation.

LOCATION:
Each gene is located at specific position on the chromosomes .This position of gene on chromosome is called Gene Locus (loci).

FUNCTION:

• Gene is the localised part of DNA which has a coded information to synthesize a protein which works as enzyme.
• Each gene has specific function e.g. a gene determines the height of plant; another gene determines the colour of their petals etc.
• The coded information may change due to any reason which is called genetic variation caused by mutation. This genetic variation among off spring lead to tremendous variety in their traits.
• The variation in the genes of a trait give rise two or more than two alternative forms of a gene. These alternative form of gene are called alleles or allomorph.
• All the inherited characters are determined by genes which are transmitted from generation to generation.

Q.7 Explain how gene is the unit of hereditary information with reference to replication of DNA?
Ans: A UNIT OF INHERITANCE:
Gene is a Unit of inheritance which is copied and Inherited to the next Generation.

Importance Of  The Complementary arrangement Of Nucleotide:

• The complementary arrangement of nucleotide is of great importance in DNA molecule.
• If the sequence of bases in one strand is known, the sequence of bases in the other strand will be automatically known due to specific base-pairing.
• This property of the two strands of double helix makes DNA a unique molecule best suited to store, copy and transfer genetic information from generation to generation.

REPLICATION OF DNA:
"DNA is able to make its exact copy itself in nucleus of cell before cell division. This process of duplication of DNA is called DNA replication."
Gametes are haploid cell that carry one copy of replicated DNA. This copy carry genes from parents to offspring through these gametes.
The Watson and Crick Model of DNA ideally proposed the mechanism of self replication:
The hydrogen bond that hold together the double helix of DNA are broken up from one end to another end like a zip. The double helix gradually “unzip” along its length and each new nucleotide of the complementary nucleotide would be added to separate chains sequentially.

REPLICATION OF DNA

Q.8: What do you understand by Central dogma of Protein synthesis? How gene expresses describe in detail?
Ans: CENTRAL DOGMA:
"A set of beliefs where a gene expresses by synthesizing a protein is termed as central dogma."
This protein is work as enzyme, which carry down a chemical reaction to produce a metabolic product. This metabolic product either develops a character or lead to develop a character.

RNAs:
DNA is located in the nucleus of eukaryotic cell while the protein synthesis and metabolic reactions occur in cytoplasm under the instructions of DNA (genes). Therefore DNA requires some other molecules to carry its genetic information from nucleus to cytoplasm. These molecules are different types of RNAs (Ribo Nucleic Acid).

GENE EXPRESSION:
All functions in the body of an organism are controlled by genes. A function expressed or performed by a gene is called gene expression.

Process of Gene Expression:
Genetic information flows from DNA to mRNA and then to ribosomes in cytoplasm. This is a two step process of gene expression for protein (enzyme) synthesis.

1. Transcription
2. Translation

1. Transcription:
The step of protein synthesis where information which contained in a specific segment of DNA (gene) is copied in complimentary from (genetic codes) i.e. RNA. This RNA carries information of DNA sequences to ribosome from nucleus to cytoplasm are called messenger RNA or mRNA. The process of copying DNA information to mRNA is called Transcription.

2. Translation:
In the second step two other types of RNA called tRNA(transfer RNA) and ribosomal RNA(rRNA) translate the information of mRNA into the specific sequence of amino acids which help to synthesize the proteins. This process is called Translation.

Q.9: What lead Mendel to present laws of genetics?
Ans: MENDEL AND HIS LAWS OF INHERITANCE:
Gregor John Mendel was an Austrian Monk. He explained the mechanism of  hereditary. He is known as the Father of Genetics.

Mendel’s Experiment:

• Mendel carried out breeding experiments on garden pea plant (Pisum sativum). He studied seven traits in pea plant one by one. These traits were height, seed shape, pod shape, seed colour, flower colour, flower position and pod colour.
• He has chosen to study the contrasting pairs of these above traits because they were easily distinguishable like Tall and Dwarf.
• These varieties continued to produce offspring identical to their parents generation after generation, as long as they were self-pollinated. Such varieties are called true breeders and their offspring are pure- breeds.
• Pure-breed means that if an organism is self fertilized, the offspring always look exactly like parents e.g. if a parent pea is tall and have purple flowers perform self pollination to produce next generation, the offspring will always be tall and have purple flowers.
• Mendel study was based on these seven pairs of contrasting characters but considering the inheritance of single pair of contrasting characters at a time.
• He counted and recorded the parents and offspring of each of his cross.

Laws Of Inheritance:
Based on experiments and observations and keen interest in Mathematics, Mendel had formulated certain hypothesis which led him to presents Laws Of Genetics or Law Of Hereditary or Laws Of Inheritance. These laws are:

1. Law of complete Dominance
2. Law of Segregation
3. Law of independent assortment.

Q.10: State the Mendel's Law of complete Dominance and also explain the experiment and its conclusions.
Ans: LAW OF COMPLETE DOMINANCE:
Statement: According to this law:

"When a pair of contrasting characters (heterozygous) is brought together in cross , only one factor will express in the off spring while other will be masked."

It means that in heterozygous condition one factor is completely dominant over the other. Law of dominance is also known as the first law of inheritance.

EXPERIMENT:
Selection Of Parents On The Basis Of Gene:
Mendel took gene as factor, he told that both parents donate one factor as a trait.

• Homozygous:
  Mendel explained that if both parents donate same factor of the trait, the organism is pure or homozygous for this trait like Pure tall i.e. TT or the pure short (dwarf) i.e. tt.
  
  
• Heterozygous:
  Mendel explained that if both parents donate different factors (allele) of a trait, the organism is heterozygous or hybrid for the trait i.e. Tt for tall.

Crossing Over:

• Mendel took pure tall stem and short stem varieties and crossed them.
• All the seeds were collected and allowed to grow.
• He observed that all the plants were of tall stem, no plant with intermediate stem were found to grow in that generation.
• He repeated his experiment on pea plants with different traits having contrasting characters, found same result than he presented the law of complete dominance.

Conclusion:
In first filial generation (F1) all plants are tall (Phenotype) with (Tt) Genotype.

Q.11: Define following terms:
(i) Dominant (ii) Recessive (iii) Phenotype (iv) Genotype (v) Homozygous (vi) Heterozygous (vii) Parental generation (P1) (viii) Filial one Generation (ix) Second Filial Generation
Ans: (i) DOMINANT:
The factor (gene) which express and masks the expression of other factor in heterozygous condition is said to be Dominant factor (allele). Mendel represent it with capital letter like T for tall.

(ii) RECESSIVE:
The factor which is unable to express or masked in heterozygous condition is said to be recessive. It represents by small letter of same alphabet like t for small (dwarf).

(iii) PHENOTYPE:
It is the physical appearance of the trait on the basis of inherited genes like tall, dwarf, round seed, wrinkled seed etc.

(iv) GENOTYPE:
The genetic makeup or genetic constitution of a trait like TT, Tt, tt etc.

(v) HOMOZYGOUS:
An individual having same factors (allele) of a trait called homozygous. For example, TT (factor for tallness).

(vi) HETEROZYGOUS:
An individual having different factors of a trait called heterozygous. For example, Tt (factors of tallness and dwarfness both).

(vii) PARENTAL GENERATION (P1):
The original true breeding organisms were called parental generation or P1 by Mendel.

(viii) FIRST FILIAL OR FILIAL ONE GENERATION (F1):
The offspring of true breeding organism were called First Filial Generation or F1.

(ix) SECOND FILIAL GENERATION:
The offspring of filial one generation produced by crossing self-fertilization were called Second Filial generation or F2.

Q.12: State and explain the law of Segregation Or law of Purity of gametes of Mendel's.
Ans: LAW OF SEGREGATION (MENDEL’s FIRST LAW) OR LAW OF PURITY OF GAMETES:
Statement:

"In an organism, the factors (genes) exist in pairs but during gametes formation these factors segregate cleanly from each other, so each gamete has only one factor of a trait."

Experiment:

• Mendel when crossed pure tall and dwarf plants, he got all tall plants in F1 generation but unlike the P1 tall, these plants of F1 generation were heterozygous (Tt).
• Mendel did not stop his experiment here. He planted all the seeds of F1 generation and allowed them to self pollinate.
• He observed that in the generation (F2), both Parental type i.e. tall and dwarf stem varieties were produced. When he counted these, he found that in F2 generation, the two types of individuals were present in the ratio of 3:1, tall and dwarf respectively.

Results:
Mendel perceived from these results that

• When the plants of F1 generation form their gametes, the factors (allele) separate or segregate again in such a way that only one of the two alleles enters each gamete.
• On the basis of these observation, results and conclusions from experiment he presented the law called the "Law of segregation or law of purity of gametes".

Q.13: State and explain the law of Independent assortment OR Explain the crossing over of dihybrid by law.
Ans: LAW OF INDEPENDENT ASSORTMENT:
Statement:

"The genes of assorted traits are independent in their inheritance."
OR
"Members of one pair of genes separate from each other during gametes formation."

Dihybrid - Cross And Dihybrid Ratio:
Mendel crossed between two individuals (pea plant) differing in two traits. He called it Dihybrid - Cross and the ratio obtained in F2 generation is called Dihybrid-ratio.

Inheritance of two traits:
The results achieved as a consequence of dihybrid-cross is known as Inheritance of two traits.

Experiment:

• In his experiments Mendel crossed between two individual (pea plants) differing in two or more character traits as well.
• Mendel crossed yellow cotyledon and rounded seed containing plant with green cotyledon and wrinkled seed containing plant.
• The F1 generation had all plants with yellow cotyledon and round seeds.
• This proved Mendel previous finding that allele for round seed was dominant over wrinkled while yellow cotyledon over green.
• Mendel self fertilized the F1 generation and expecting any of the two following possibilities:
  i) If the genes for round seed and yellow colour are inseparable (dependent on each other), as well as those for wrinkled shape and green colour, then in F2 generation ³/₄ of the seeds will be round and yellow and ¹/₄ will be wrinkled seed and green.
  
  ii) If the genes for seed shape and cotyledon colour are separable (independent) and distributed to the gametes independent to each other, then in F2 generation some plants will produce round seeds with green cotyledon and some will have wrinkled seeds with yellow cotyledon along with parental combination.

Result:
Mendel actually obtained four different kind of phenotypes in F2 generation. Out of a total 556 seeds, he obtained four different phenotypes i.e.
Phenotype:

• Round and Yellow seeds = 315
• Wrinkled and Yellow seeds = 101
• Round and Green seeds = 108
• Wrinkled and Green seeds = 32

Genotype:

• Genotypes for round and yellow seeds RRYY, RRYy, RrYY, RrYy.
• Genotypes for round and green seeds RRyy, Rryy.
• Genotypes for wrinkled and yellow seeds rrYY, rrYy.
• Genotypes for wrinkled and green seeds rryy.

Dihybrid-cross ratio:
These numbers are in a ratio of about 9:3:3:1 for the four phenotype.

Conclusion:
It showed that the genes for seed colour and shape are independent in their inheritance. They do not necessarily stay together in the combination in which they occurred in parents.

Q.14: Explain the concept of multiple allele with reference to human blood groups.
Ans: ALLELE:
"The alternative form of gene developed as a result of variation is called allele or allomorph."
Some gene may have as many as 300 alleles. A diploid cell thus have any two of these multiple alleles while a haploid have one allele. Alleles are described as either dominant or recessive depending on their associated traits. 

MULTIPLE ALLELES:
A gene controlling a trait may two or more than alternative forms (alleles). These traits are called Multiple Allelic Trait or  Multiple Alleles. ABO blood group system in human population was the first discovered multiple allelic trait.

EXAMPLES OF MULTIPLE ALLELES:
Human Blood Group
A well-known example of multiple alleles in humans is the inheritance of ABO blood group system.
Karl Landsteiner in 1901 discovered ABO blood group system. There are four type of phenotype due  to the presence or absence of two specific antigen on the surface of the RBCs.
The genetic basis of ABO system was explained by Bernstein in 1925.
Antigens:
There are two types of these antigens:

• Antigen A and
• Antigen B

Blood Groups:

• "A": A person with antigen “A” has blood group "A".
• "B": A person with antigen “B” has blood group "B".
• "AB": With both antigens A and B, a person has blood group “AB”.
• "O": If there is none of the two antigens, the blood group of a person will be "O".

Gene "I":
The ABO blood group system is controlled by a polymeric gene "I" on chromosome 9. The Gene "I" contains three alleles represented by the symbols I^A, I^B and i.

• The allele I^A is responsible for production of antigen A.
• The allele I^B produces antigen B.
• While the third allele i does not produce any of the antigens.

OR

Blood Group	Genotype	Phenotype	Antigen
Group A	IA IA  IA i	A—Homozygous  A—Heterozygous	A
Group B	IB IB  IB i	B—Homozygous  B—Heterozygous	B
Group AB	IA IB	AB—Heterozygous	AB
Group O	i i	O—Homozygous	NO

R^h Blood Type ( R = Rhesus):

• The R^h blood group is one of the most complex blood groups known in humans.
• This protein was first time observed in rhesus monkey the another mendelian gene found in human.
• This gene produce a protein which is found on the surface of R.B.C as well.
• This trait has 2 allele a dominant R^h and r^h recessive.
• The R^h protein produce R^h protein therefore person is called R^h +ve whereas r^h gene is unable to produce R^h protein so the person is called R^h -ve, will be r^hr^h.

Q.15: Discuss the role of antibodies for AB and Rh in blood transfusion.
Ans: ANTIBODIES:
The chemical cells or organs of an organism work as antigen to other organism. When these antigens enter in the body of other organism, this organism produce defense protein against this antigen to protect itself. These proteins are called antibodies.

Role of antibodies for AB and R^h in blood transfusion:

• The blood group alleles (I^A, I^B, i) start their expression at embryonic stage and keep expressing till death.
• A person also produces anti-A, anti-B antibodies during first few months after birth.
• The person of A-blood group produces anti-B antibodies.
• Person of B-blood group produces both anti-A and anti-B antibody.
• Person with AB blood group does not produce any of these antibodies.
• Person with AB blood serum containing antibodies called antibody. The blood serum containing antibodies called antiserum.

Role in transfusion:

OR

Blood group	Blood group recipient	Reason
A B AB	A and AB B and AB AB only	Recipient donot have antibodies A.  Recipient donot have antibodies B.  Recipient neither have antibody A or antibody B.
O	O, A, B, AB	* Blood group have antibody A and antibody B so it can receive only O blood group.  * O blood can be donated to A, B and AB because donor’s antibodies are quickly absorbed by other. Tissue or diluted in recipient blood. Therefore O blood is called universal donor.

Preventive Measures For Transfusion Of Blood:
For ABO Blood Group system:

• The person of A blood group which contain anti-B antibodies if given blood of B-group.
• The R.B.C of this B-blood group will work as antigen in the person of A-blood group and its anti-B antibodies will work against the donor R.B.Cs of B-blood group and clumped cells called agglutination.
• It leads to a serious problem as these clumped cells can not pass through fine capillaries. Therefore during transfusion of blood the people must be careful about blood group.

For R^h Factor:

• Anti-R^h antibodies do not produce automatically. The production of anti-R^h antibodies require stimulus which is R^h human protein itself. It work as antigen.
• If an R^h -ve person receive R^h protein (antigen) through wrong R^h +ve blood transfusion, he will begin to produce anti-R^h antibodies against R^h antigens.
• R^h-ve blood does not contain any R^h protein (antigen) so it can be transferred to R^h +ve person (recipient).

Q.16: Explain the concept of co-dominance and incomplete dominance?
Ans: CO-DOMINANCE:
The phenomenon of inheritance where both alleles are dominant and expressed equally is called Co-Dominance.

Explanation:
According to Mendel’s law of dominance only one factor is expressed in heterozygous condition but it was also found that some traits has alleles that are equally dominant and are both expressed themselves in a heterozygous condition ( in phenotype) independently.
As a result the heterozygous offspring displays the phenotype of both the homozygous parents and none of the two alleles masks the appearance of the other.

Example:
AB blood group, where both alleles I^A and I^B express and produce antigen A and B both.

INCOMPLETE DOMINANCE:
The term ”incomplete dominance” was proposed by Carl Correns in 1899.
Definition:
"The phenomenon of inheritance where both alleles of a trait  is expressed in heterozygous conditions and their expression get blended to produce new phenotype called incomplete dominance or partial dominance."

Experiment:

• Carl Corren crossed a pure breed red flowered Japanese 4 O’clock plant (Mirabilis jalapa) with pure breed white flowers.
• He found pink flowered plants in F1 generation , these result were against the law of dominance. This new phenotype was intermediate between those parents.
• When Correns self-pollinated F1 pink flowered plant, the F2 generation showed all three phenotypes of flowers in the ratio 1 red, 2 pink and 1 white flowers, i.e. 1:2:1.
• Red and white are homozygous for their respective alleles.
• It was also found that when alleles for red and white were present together (heterozygous) in the same plant, neither of them masked the effect of other, therefore expression of both genes blended to produce new phenotype, i.e. pink.

Q.17: What do you mean by similarities? OR Define the species which have similarities in features?
Ans: SIMILARITIES:
Living things exists in great variety.  Species are interrelated in some ways. They have many features in common. For instance, there are fundamental similarities in their chemical structure. Not in in chemistry, but there exist similarities in morphology also. Thus:
"Similarities suggest that species are more closely interrelated in some ways, some species are more closely interrelated than the others."
Example: Such as rats with mice, turtle with tortoise, frog with toads etc.

Q.18: Explain the concept of variation with types and causes.
Ans: VARIATIONS:
"The differences in characters such as height, colour, etc. among individuals of same species are called variations." Variation may be caused either by the effect of environment or by the changes in the genetic material.

TYPES OF VARIATIONS:
On the basis of causes variation can be classified into the following types:
1. Environmental variation:
Variations caused by environmental factors among the members of same species, are called environmental variations . They are not inherited to the offspring.
e.g. Development of muscles in athletes, loss of some body parts due to accident or diseases etc.

2. Heritable Variations:
The variation caused due to changes in genetic material is called genetic or heritable variation. This type of variation is consider as raw material for evolution.

On the basis of effects there are other two types of variations.
1. Continuous Variation:
These variations refers to small differences in characters of the members of a species. Such variations are neither purely genetical nor purely environmental. They appear to be the combination of both factors.
e.g. Height, Skin colour, Intelligence, Eye colour etc.

2. Discontinuous variations:
These variations are sudden and sharp differences among the members of same species. They are heritable and purely caused by genetic material.
e.g. Blood group, Six fingers in hand or foot, tongue rolling etc are common examples in man.

CAUSES OF VARIATIONS:
Some important causes of variations are as follows:
1. Mutation:
The phenomenon of producing sudden changes is called mutation. It can be defined as the sudden changes in the genetic material {genome) of an organism. It is the major source of heritable variation among the organisms which is considered as the starting point of new species.
e.g. Some person has six fingers in hand or foot

2. Crossing over:
The process of mutual exchange of segments of chromatids between non-sister chromatids of homologous pair of chromosome which occurs during prophase of 1^st meiotic division is called crossing over. The maternal and paternal alleles are mixed and segregated in this process So the indefinite combinations of alleles are made.
e.g. Due to this crossing over and segregation, offspring of same parents become variable.

3. Environment:
Number of environmental factors affects the body cells to cause variations in body cells.
e.g. the changes in the pigmentation of skin due to extent of exposure to sunlight or development of muscles due to exercise etc.

4. Independent assortment of chromosomes:
During metaphase of first meiotic division homologous chromosomes come together in the form of pairs and subsequently segregate during anaphase I into the daughter cells independently. This produce a wide variety of different gametes.

5. Fertilization:
The set of alleles carried by chromosomes of each gametes is unique and always differ from each other. There are number of male gametes available to fertilize a single female gametes. Thus different combination of characters in an individual are possible as a result of fertilization.

Q.19: Define Evolution and organic evolution? Explain the Darwin theory of natural selection.
Ans: EVOLUTION:
Evolution is a process where descends become better than ancestors.

ORGANIC EVOLUTION:
The organic evolution explains that present day living things are modified from simple ancestral forms during the course of time through a gradual and continual process of modification.
Theories of organic evolution not explain that living things have become so varied, but it also explains 
their basic similarities as well.

THEORY OF NATURAL SELECTION:
Charles Darwin (1809 - 1882) was an Englishman. He studied different plants and animals, collected new specimens and categorized them. In the year, 1859, he published a book “Origin Of Species” where he proposed the "Theory of Natural Selection", in which he presented the mechanism of origin of species.

FACTORS INVOLVED IN NATURAL SELECTION OR MAIN POINTS OF DARWIN'S THEORY:
The main points of the Darwin's theory are as follows:
1. Over Production:
Living organisms reproduce rapidly so that the number of their offspring could increase rapidly.

2. Struggle for Existence:
Due to the limited available resources of food, shelter, etc. the offspring of species compete not only with each other but also with the members of different species to share these resources. In this struggle a large number of individuals of each species are eliminated. As a result the population remains stable.

3. Heritable Variation:
Individual of a species differ from each other in their ability to obtain resources, withstand environmental extremes etc. These differences in character are called minor variations.
Darwin concluded that survival in struggle for existence is not random but depends upon in part on the heredity constitution of the surviving individuals. Those individuals whose inherited characters fit them best to their environment would survive and produce more offspring than less fit individuals who will vanish.

4. Natural selection:
Nature selects the fittest individuals to survive and reproduce. Therefore only the favourable variations are preserved through their inheritance to new young ones.

Conclusion:
Over millions of year of variations, natural selection and inheritance might have led to the accumulation within the species a number of characters with survival values. As a result, a species may slowly change to a better new species. 
Number of evidence are reported in support of evolution from comparative anatomy, homologous organs, analogous organs, vestigial organs and fossils.

Q.20: Write a short note on artificial selection?
Ans: ARTIFICIAL SELECTION:

"Artificial selection is the cross breeding of domestic animals and plants to produce specific desirable features."

Example:
Various breeds of dogs, pigeons, sheep, horse, cattle, cow, buffalo, hen etc. among the animals have been developed through artificial selection.

Explanation:
Darwin was very much convinced by Artificial selection. He convinced biologists that the process of artificial selection could produce so many changes in a species in relatively short period of time, then natural selection should be capable of considerable modification of species over thousands of generations.

Source: Special Thanks To Sir Syed Arif Ali