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Indian Institute of Technology Bombay

Genetics

Heredity and variations

It is commonly seen that members of a species are largely alike. An elephant resembles other elephants, a rose plant looks alike other rose plants, and children resemble their parents, even grandparents or great grandparents. This resemblance among the individuals of a species has given rise to a general truth `like begets like’ which implies continuity of life. It is, however, not absolutely true as the members of a species are seldom exactly alike. For instance, in human beings, the children often have some individual characters in which they differ from one another, and also from their parents. In fact, their differences are as marked as their resemblances. This is true about other species as well.

The similarities and differences among the members of a species are not coincidental. They are received by the young ones from their parents. The hereditary information, in fact, is present in the gametes (egg and sperm) which fuse to form the fertilized egg or zygote during sexual reproduction. The zygote then develops into an organism of a particular type. For instance, fertilized eggs of sparrows hatch into sparrows only and the fertilized eggs of pigeons hatch into pigeons only. Similarly, a cow gives birth to calves only. The wheat plant gives rise to seeds which, in turn, develop into wheat plants.

Heredity :- The transmission of characters [or traits] from one generation to another generation.

or

The transmission of characters from the parents to their offspring.

Variations :- The differences in the characters [or traits] among the individuals of a species are called variations. e.g. Plant height Tall, dwarf & middle.

Ear lobe in human being :- The lowest part of our ear is called earlobe.

  • In most of the people,the ear lobe is hanging and it is called free earlobe.
  • In some people, the earlobe is closely attached to the side of the head and it is called attached ear lobe.
Accumulation of Variations During Reproduction

Heredity involves inheritance of basic body design (similarities) as well as subtle changes (variations) in it from one generation to the next generation, i.e., from parents to the offspring. When individuals of this new generation reproduce, the offsprings of second generation will have the basic body design, the differences that they inherit from first generation as well as newly developed differences.

Asexual reproduction involves single parent. When a single individual reproduces asexually, the resultant two individuals again after sometime reproduce to form four individuals. All these individuals would be similar. However, there would be only very minor differences between them. These very minor differences arise due to small inaccuracies in DNA copying.

Sexual reproduction, on the other hand, generates even greater diversity. This is so because sexual reproduction involves two parents (father and mother) and every offspring receives some characters of father and some characters of mother. Since, different offsprings receive different combination of characters of their parents (father and mother), they show distinct differences (variations) among themselves as well as from their parents. The variations accumulate and pass on to more and more individuals with each generation.

During sexual reproduction the variation caused by

(i) Chance separation of chromosomes during gamete formation (gametogenesis).

(ii) Crossing over during meiosis.

(iii) Chance coming together of chromosomes during fertilization.

(iv) Mutations, i.e., alterations in the genetic material.

All the variations in a species do not have equal chances of surviving in the environment in which they are generated. Depending upon the nature of variations, different individuals would have different kinds of advantages. For instance, bacteria that can withstand heat will survive better in a heat wave than the others. In other words, environmental factors select the variants and this selection forms the basis of evolution.

Gregor Johann Mendel(1822 – 1884)

Mendel was born on 22 July 1822 at Heinzendorf in Austria at Silesia village. Mendewas worked in Augustinian Monastry as monk at Brun city, Austria.

In 1856-57, he started his historical experiments of heredity on pea(Pisum sativum) plant. His experimentawork continued on pea plant til1865 (19th century).

The results of his experiments were published in the science journal. “Nature For Schender Varein” in 1866.

This journal was in German language. Title is ‘ Verschue Uber pflangen hybridan’.

This journal was published by ‘NaturaHistory society of Bruno’.

A paper of Mendel by the name of Experiment in plant Hybridization published in this journal.

Mendel was unable to got any popularity no one understand of him. he died in 1884 (due to kidney disease (Bright disease)) without getting any credit of his work.

After 16 years of Mendel’s death in 1900 Mendel’s postulates was rediscovered. Mendel experiment remain hidden for 34 years.

Rediscovery by three scientist independently.

1. CarCorrens – Germany – (Experiment on Maize)

2. Hugo deVries (Holland) (Experiment on Evening Primerose)

3. Erich von Tschermak Seysenegg – (Austria) (Experiment on different flowering plants)

Character

A recognizable feature of human beings or any other organisms are called characters.

eg. (i) Height

(ii) Complexion

(iii) Shape of hair

(iv) Colour of eyes

(v) Shape of nose

Traits :- Various forms of a character are called traits.

S. No. Character Dominant Recessive
1. Length of Plant Tal Dwarf
2. Flower position Axia Terminal
3. Shape of pod Inflated Constricted
4. Colour of pod Green Yellow
5. Shape of seed Round Wrinkled
6. Colour of cotyledon Yellow Green
7. Colour of flower Violet White

Mendel’s Monohybrid cross

A breeding experiment dealing with a single character is called a monohybrid cross.

Mendel first selected `pure line’ plants (i.e., the plants that produced similar traits generation after generation). He, then, cross pollinated such plants having the contrasting traits, considering one trait at a time. For instance, in one such cross breeding experiment, he cross bred garden pea plant having round seeds with plant having wrinkled seeds. In this mono hybrid cross, the pollen grains from the flower of the desired plant raised from round seeds were transferred over the previously emasculated flower of a plant raised from wrinkled seeds or vice-versa. After the transfer of pollen grains, the cross pollinated flower was properly covered and seeds produced were allowed to mature. All the seeds of F1 generation were carefully observed. Mendel observed that all the seeds
of F1 generation were of round type and there were no intermediate characteristics.

He raised plants from F1 seeds and allowed the flowers to self-pollinate to produce the seeds of F2 generation. The flowers were kept covered from the beginning to avoid unwanted pollens to reach these flowers. In F2 generation, Mendel observed the appearance of both round and wrinkled seeds in approximately 3 : 1 proportion.

Mendel’s Di-hybrid Cross

A cross involving two pairs of contrasting characters.

or

A cross in which two pair of contrasting characters are studied at a time.

In one such cross, Mendel considered shape as well as colour of the seeds simultaneously. He selected pure line plants and then cross pollinated flowers raised from seeds of round shape and yellow colour with those from wrinkled seeds and green colour. Mendel observed that in F1 generation all seeds had the features of only one parental type, i.e., round shape and yellow colour. He raised plants from F1 generation seeds and allowed the flowers to self pollinate to produce the seeds of F2 generation. These flowers were kept covered from the beginning. In F2 generation, Mendel observed the appearance of four types of combinations. These included two parental types (round shaped and yellow coloured seeds, and wrinkled shaped and green coloured seeds) and two new combinations (round shape d and green coloured seeds, and wrinkled and yellow coloured seeds) in approximately same proportion.

Some Important definitions

Dominant gene :- The gene which decided the appearance of an organism even in the presence of an alternative gene.

Recessive Gene :- The gene which can decide the appearance of an organism only in the presence of another identical gene.

Chromosomes :- A thread – like structure in the nucleus of a cel formed of DNA which carries the genes.

Geno-type :- The genetic constitution of an organism.

or

The description of genes present in an organism e.g. TT, tt, Tt.

Phenotype :- External and morphological appearance of an organism for a particular character.

Allele :- Alternative forms of a gene which are located on same position [loci] on the homologous chromosome.

F1 Generation :- When two parents cross or breed to produce progeny [or offsprings], then their progeny is called F1 generation or first filia generation.

or

The offspring produced by the parental generation.

F2 generation :- When the first generation progeny cross or breed among themselves to produce second progeny, then this progeny is called second filia generation or F2 generation.

or

The offspring produced by the F1 generation

Hybrid :- A new form of plant resulting from a cross or breeding of different varieties of a plant is known as a hybrid.

Pure-breeding :- Characteristics that appear unchanged generation after generation.

Dominant characters :- Any character that appears in the F1 generation offspring from a cross between parents possessing contrasting characters such as tallness & dwarfness in pea plants.

Recessive characters :- Any character present in the parental generation that does not appear in the F1 generation but reappears in the F2 generation.

Homozygous :- A condition in which the 2 members of an allelic pair are similar. e.g. TT, tt.

Heterozygous :- A condition in which the 2 members of an allelic pair are dissimilar. e.g. Tt.

Offspring :- Organisms produced as a result of sexual reproduction.

Homologous chromosomes :- Al chromosomes found in pair & chromosomes of a pair are called homologous chromosomes.

Non-homologous chromosomes :- Chromosomes of different pair are called non-homologous chromosomes.

Genes :- Unit of heredity which transfers characters from parents to their offsprings during reproduction.

Gene → Protein synthesis → Enzymes [Controls phenotype of a character]

Laws of Mendel

On the basis of Mendel’s work, 3 basic laws of inheritance were proposed.

(i) Law of Dominance

(ii) Law of Segregation

(iii) Law of Independent Assortment

Law of dominance :- In crossing between organisms pure for contrasting characters of a pair, only one character of the pair appears in the F1 generation. This character is termed dominant while the one which does not express itself in F1 generation is termed recessive.

Law of segregation :- Allele or genes remain together and segregate at the time of gamete formation. This means that the alleles do not mix in the hybrids [Non-mixing of alleles]

This is also known as the Law of Purity of Gametes.

Law of Independent Assortment :- This law states that – when individuals differing in 2 or more than 2 pairs of contrasting characters are crossed, the inheritance of any one pair is not affected by the presence of the other.

e.g. The inheritance of tall character is not way related to the smooth character of the seed. Rather, the 2 characters are inherited independent of each other.

 DEOXYRIBONUCLEIC ACID (DNA)

The expanded form of DNA is deoxyribonucleic acid. It was first isolated by the scientist Frederick Meisher from the nucleus of the pus cells in 1869. He named it as ‘nuclein’ or nucleic acid because of its acidic nature. Later, it was experimentally proved by the scientists Griffith (1928), Avery, McLeod and McCarty (1944) that DNA is the carrier of the genetic information from generation to generation. It transmits the hereditary characters in a coded language from parents to the offsprings (i.e., from one generation to another).

DNA is a macromolecule or polymer. It is made of very large number of `nucleotide’ units and hence is termed polynucleotide.

Each nucleotide unit in a DNA molecule is made up of three components

1. Deoxyribose sugar :- It is a pentose sugar.

2. Nitrogenous base :- Each nucleotide unit has a nitrogen containing base. In a DNA molecule, nitrogenous bases are of two types :

(a) Purines :- The purines in a DNA molecule are — Adenine (A) and Guanine (G).

(b) Pyrimidines :- The pyrimidines in a DNA molecule are — Cytosine (C) and Thymine (T).

3. Phosphate group :- The phosphate group contains one phosphorus atom and four specifically linked oxygen atoms.

Double Helical Mode of DNA

J. D. Watson and F.H.C. Crick proposed the double helical mode of DNA in 1953. They were awarded the Nobel Prize for this discovery in 1962. The important features of the double helical mode are

(i) DNA molecule is made up of two long polynucleotide strands forming a double helical structure (double helix) just like a spiral staircase. Each helical turn of the DNA molecule is 3.4 nm in length in which ten nucleotide base pairs are present.

(ii) Deoxyribose sugar and phosphate molecules are joined alternately to form the backbone of each polynucleotide strand. The nitrogenous base of each nucleotide is attached to the sugar molecule and projected towards the interior of the double helix.

(iii) In the interior of double helix, the nitrogenous bases of two polynucleotide strands form a pair with the help of hydrogen bonds. Adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C).

Thus, the two polynucleotide strands of the DNA molecule are joined by hydrogen bonds between specific nitrogenous bases. Such a specific pairing of the bases of the opposite strands of the DNA molecule is called complementary pairing. Adenine (A) and thymine (T) are complementary to each other. Similarly, guanine (G) and cytosine (C) are complementary to each other. The hydrogen bonding between the specific nitrogenous bases keeps the two strands to hold together. Therefore, al the base pairs remain stacked between the two strands.

Blood groups

Four different types of blood groups in human beings are : A, B, O, AB

O → universal donor

AB → universal recipient

Sex Determination

How is the sex of newborn individual determined ? In human beings, the sex of the individual is largely genetically determined. In other words, the genes inherited from our parents have assumed that similar gene sets are inherited from both parents. If that is the case, how can genetic inheritance determine sex ?

Al human beings these are 23 pairs of chromosomes are present out of these 23 pairs, are autosomes (similar in males and females) and 1 pair is called sex chromosome (different in males and females). In males sex chromosome are XY and in females sex chromosome are XX.

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So, it is clear from the above that male is responsible for producing male child.

POINTS TO REMEMBER

  • The term Genetics comes from “gene” word _ means “to grow”.
  • The term “Genetics” coined by Bateson.
  • Study of heredity and variations is called Genetics.
  • Father of Genetics – Gregor Johann Mendel.
  • Father of Modern Genetics – Bateson.
  • Father of Experimental Genetics – Thomus Hunt Morgan.
  • “Heredity” term coined by Spencer.
  • Bateson coined terms Genetics, Allele, Homozygous, Heterozygous, F1 and F2 Generation
Section 1Introduction to Genetics
Section 2Heredity and Variations
Section 3First and Second Law of Inheritance
Section 4Dihybrid Cross
Section 5Mutation – Types, Application and Significance