Mendel's Contributions - Science Class 10

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. 
  Example: 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. 
  Example: 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 i.e. from the parents to their offspring.
• Variations: The differences in the characters, or traits, among the individuals of a species, are called variations.
  Example: Plant height (Tall, dwarf & middle).

In some people, the ear lobe is hanging and it is called the free earlobe whereas others have closely attached to the side of the head and it is called the attached ear lobe.

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

Accumulation of Variations During Reproduction

• Heredity involves the 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 offspring of the second generation will have the basic body design but they will have the differences that they inherit from first-generation as well as newly developed differences.
• Asexual reproduction involves a single parent. When a single individual reproduces asexually, the resultant two individuals again after some time reproduces 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.
  Variation
• 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 offspring receive a 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 variations are caused by: 
(i) Separation of chromosomes during gamete formation (gametogenesis).
(ii) Crossing over during meiosis.
(iii) Chance of 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. 
  Example: Bacteria that can withstand heat will survive better in a heatwave than the others. In other words, environmental factors select the variants and this selection forms the basis of evolution.

Heredity

Gregor Johann Mendel(1822 - 1884)

• Mendel was born on 22 July 1822 at Heinzendorf in Austria at Silesia village. Mendel worked in Augustinian Monastery as a monk at Brun city, Austria.
• In 1856-57, he started his historical experiments of heredity on the pea (Pisum sativum) plant. His experimental work continued on pea plant till 1865 (19th century).
  The results of his experiments were published in the science journal "Nature For Schender Verein" in 1866. 
• This journal was in the German language. Title is ' Verschue Uber Pflangen Hybridan'. This journal was published by 'Natural History society of Bruno'. A paper of Mendel by the name of "Experiment in plant Hybridization" was published in this journal.
• Mendel was unable to get any popularity as no one could understand his work. He died in 1884 (due to kidney disease (Bright disease)) without getting any credit for his work. After 16 years of Mendel's death in 1900, Mendel's postulates was rediscovered. Mendel's experiment remained hidden for 34 years.
• These postulates were rediscovered by three scientists independently:
  (i) Carl Correns - Germany - (Experiment on Maize)
  (ii) Hugo de Vries (Holland) (Experiment on Evening Primrose)
  (iii) Erich Tschermak, Edler von Seysenegg - (Austria) (Experiment on different flowering plants)

Rules for the Inheritance of Traits: Mendel's Contributions

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

Examples: Height, Complexion, Shape of hair, Colour of eyes, Shape of the nose.

Traits: Various forms of a character are called traits. 

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 contrasting traits, considering one trait at a time. 
  Example: In one such cross-breeding experiment, he cross-bred garden pea plant having round seeds with the plant having wrinkled seeds. 
• In this monohybrid 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 the F₁ generation were carefully observed. 
• Mendel observed that all the seeds of the F₁ generation were of a 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 the F₂ generation. The flowers were kept covered from the beginning to avoid unwanted pollen grains to reach these flowers. 
• In the F₂ generation, Mendel observed the appearance of both round and wrinkled seeds in approximately 3:1 proportion.

Mendel's Monohybrid Cross

Mendel's Dihybrid Cross

• A cross involving two pairs of contrasting characters i.e. two pair of contrasting characters are studied at a time.
• In one such cross, Mendel considered the shape as well as the 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 parent 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-parent types (round-shaped and yellow coloured seeds, and wrinkled shaped and green coloured seeds) and two new combinations (round-shaped and green coloured seeds, and wrinkled and yellow coloured seeds) in the approximately same proportion. (giving the ratio of 9:3:3:1)

Some Important Definitions

Dominant gene: The gene which decides 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 cell formed of DNA that carries the genes.

Chromosome

Genotype: The genetic constitution of an organism i.e. description of genes present in an organism.
Example: TT, tt, Tt.

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

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

F₁ Generation: When two parents cross or breed to produce progeny [or offspring], then their progeny is called F₁ generation or first filial generation.
The offspring produced by the parental generation.

F1 and F2 generation

F₂ generation: When the first generation progeny crossbreed among themselves to produce second progeny, then this progeny is called second filial generation or F₂ generation.
The offspring produced by the F₁ generation.

• Hybrid: A new form of the plant resulting from cross-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. 
  Example: TT, tt.

Homozygous and Heterozygous

• Heterozygous: A condition in which the 2 members of an allelic pair are dissimilar.
  Example: Tt.
• Offspring: Organisms produced as a result of sexual reproduction.
• Homologous chromosomes: All chromosomes found in a 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 offspring 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

1. Law of Dominance

• In crossing between organisms having contrasting characters, 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 Dominance

2. 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.

3. 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 anyone pair is not affected by the presence of the other.
  Example: The inheritance of a tall character is in no way related to the smooth character of the seed. Rather, the 2 characters are inherited independently of each other.

Law of Independent Assortment