Mendel's Laws of Inheritance

INHERITANCE: HEREDITY AND VARIATIONS

Heredity : 

It is the transmission of genetic characters from parents to the offsprings.

It deals with the phenomenon of "like begets like", e.g., human babies are like human beings in overall characteristics.

About 200 characters are found to be hereditary in man.

Variations are common in sexually reproducing organisms.

Asexually reproducing organisms are monoparental, hence exhibit no genetic variations.

Concept Builder

Variations are of following two types:

(1) Somatogenic Variations: These are acquired variations and are non-inheritable in nature. The ability of an organism to alter its phenotype in response to environment is called phenotypic plasticity.

(2) Blastogenic Variations: These are germinal variations and are hereditary in nature. They are again of two types :

(a) Continuous Variations: These are the fluctuating variations and can not give rise to new species. These are further of two types :

(i) Substantive: Variation in size, shape and colour of organism.

(ii) Meristic: Variation in number of parts, e.g., number of grains in an ear of wheat.

(b) Discontinuous Variations: Also known as mutations, sports or saltations. These variations are responsible for formation of new species and the organism thus formed, is called' a mutant.

Types of discontinuous variations :

(i) Substantive variations: These influence shape, colour, size etc., e.g., hairless cat, short legged ancon sheep.

(ii) Meristic variations: These affect number of parts, e.g., polydactyly in humans. Variations are of great significance in evolution as they make the organism better suited to modifying environmental conditions, produce new trait in organism and provide raw material for evolution.

Branches of Genetics

(i) Transmission genetics or Classical genetics, e.g., It is the study of Mendelian genetics and non-Mendelian genetics.

(ii) Forward genetics : It is the identification of mutated gene using the mutated phenotype.

(iii) Reverse genetics : It is the study of genes whose protein products are unknown.

(iv) Cytogenetics: It is the study of various aspects of chromosomes.

(v) Molecular/biochemical genetics : It is the study of structure and functions of genes.

(vi) Population or biometrical genetics : It is the study of the behaviour and effects of gene in population using mathematical models.

(vii) Behavioural genetics: It is the study of interaction of genes with the environment to produce a particular pattern of behaviour.

Pre-Mendelian Ideas About Inheritance or Theories of Blending Inheritance

The science of genetics arose with the rediscovery of Mendelism in 1900. Early philosophers, thinkers and workers have presented various theories to explain the phenomenon of inheritance.

These are called theories of blending inheritance. Some of these theories are:

(a) Moist Vapour Theory (Pythagoras : 500 B.C.) : Various body parts emit certain vapours, which get aggregated to form new individual.

(b) Reproductive Blood Theory (Aristotle: 384-322 B.C.) : According to him, menstrual fluid and semen are kinds of highly purified blood. Menstrual fluid provide inert substance for embryo formation and semen provide form and shape to embryo.

(c) Preformation Theory or Homunculus theory (J. Swammerdam) : According to this theory miniature form of individual is already present in sperm or egg called "homunculus". Fertilization is required to stimulate its growth.

(d) Theory of Pangenesis (Darwin 1868) : According to him, each part of body produces minute particle called gemmules or pangenes, which aggregate into gamete. On fusion they give rise to new individual.

(e) Theory of Epigenesis (K.F. Wolff) : According to this idea,· neither egg nor sperm had a structural homunculus but the gametes contained undifferentiated living substance capable of forming the organized body after fertilization. This suggested that many new organs and tissue which were originally absent, develop structurally de novo due to mysterious vital force.

Theory of pangenes was disproved by Weismann.

A.Weismann proposed his theory of germplasm, according to which the changes which affect the germplasm are heritable and the changes which affect somatoplasm are non heritable.

Objections to blending inheritance:

1. Unisexual traits

2. Skin colour in humans

3. Atavistic character

GENETIC TERMS AND SYMBOLS

 

MENDELIAN INHERITANCE

Mendel was born on July 22, 1822. He worke

d on Pisum sativum (Garden pea or Edible pea) for 7 years by taking 7 pairs of contrasting traits.

The results were read out in two meeting of Natural History Society of Brunn in 1865.

His paper "Experiments on plant Hybridisation" was published in 4th volume of "Proceedings of Natural Science Society of Brunn" in 1866.

Mendel was first to apply statistical analysis and mathematical logic.

He selected 14 true breeding pea plant varieties. He died due to kidney disorder in 1884.

Mendel selected following characters in pea plant for carrying out hybridisation experiments:

Mendel failed to produce same results in Hawkweed (Hieracium) and Beans (Lablab). Detailed investigation by S. Blixt on pea plant led to locate Mendel's seven characters on 4 different chromosomes, i.e., 1, 4, 5 and 7.

However, Mendel's work did not receive any recognition, it deserved, till 1900.

Mendel's work remained unnoticed and unappreciated for several years due to following reasons:

(1) Communication was not easy in those days and his work could not be widely publicised.

(2) His concept of stable, unblending, discrete units or factors for various traits did not find acceptance from the contemporaries.

(3) His approach of using mathematical and statistical analysis to explain biological phenomena was totally new and unacceptable to many of the biologists of that time.

(4) He could not provide any physical proof for the existence of factors. It was rediscovery of his work by a Dutch -Hugo de Vries, a German -Carl Correns and an Austrian botanist -Erich von Tschermak, independently in 1900, that brought Mendel to limelight. Correns raised status of Mendel's generalisations to laws.

Selection of pea plant : The main reasons for adopting garden pea (Pisum sativum) for experiments by Mendel were as follows :

(1) Pea has many distinct alternative traits (clear contrasting characters).

(2) Life span of pea plant is short.

(3) Flowers show self (bud) pollination, so are true breeding.

(4) It is easy to artificially cross-pollinate the pea flowers. The hybrids thus produced were fertile.

Concept Builder

1. Term allele was given by Bateson, term homozygous and heterozygous were given by Bateson and Saunders; genotype, phenotype, gene and pureline by Johannsen.

2. Father of genetics -Mendel; Father of Modem genetics -Bateson

3. Isoalleles : Alleles that produce similar phenotypes but are distinguishable amongst themselves through changed optima, e.g., .

4. Pseudoalleles : Genes are present together side by side and they produce related phenotypes. These are distinguished from true alleles through rare crossing over, e.g., star (dominant) and asteroid (recessive) traits in Drosophila.

5. Pure lines (pure breeding line) : A population obtained by continuous inbreeding over many generations, such that each individual has essentially the same genome as every other member of the inbred line and that all (or most) loci are homozygous.

Mendel's Work and Results

Mendel has made cross between parents having contrasting traits.

Firstly he made monohybrid cross (cross between parents, differ from each other in one character) followed by dihybrid cross (cross between parents, differ from each other in two characters) and then trihybrid.

The F₁ hybrids were self crossed to give rise to F₂ generation.

Mendel also carried out the reciprocal crosses and found that reciprocal crosses gave the same result.

(Reciprocal cross means opposite cross, i.e., the parent which provides male gamete in one cross, in second experiment it provides the female gamete and vice versa).

The result of reciprocal cross proves that both gametes produce the same effect and it does not matter which parent provides male and which one provides female gamete.

On the basis of his experimental crosses, he formulated four postulates.

Postulate I:

According to this postulate characters are controlled by a pair of unit factors. The two factors are now called alleles or allelomorphic pair.

Postulate II :

If two dissimilar unit factors are present in an individual, only one expresses itself. The one which expresses itself is known as dominant factor, while the second which does not express at all is known as recessive factor.

Postulate III :

According to this postulate, two contrasting alleles responsible for contrasting traits present in an individual do not get mixed and get separated from each other at the time of gamete formation by F₁ hybrid and due to their recombination, four combinations can be obtained in equal frequency.

All the above three postulates are based upon Mendel's monohybrid cross or one gene interaction.

Law of dominance and law of segregation can be explained on the basis of monohybrid cross or one gene interaction.

(1) Law of dominance :

This law states that when two contrasting alleles for a character come together in an organism, only one is expressed completely and shows visible effect.

It is called dominant and the other allele of the pair which does not express and remains hidden is called recessive.

This law is not universally applicable.

Plant height is controlled by two alleles -Dominant allele (T) and Recessive allele (t)

These two alleles can be present in three forms

Tt -Dominant allele expressed] Heterozygous (Hybrid for character)

Mendel crossed two pea plants, one homozygous tall (TT) and another homozygous dwarf (tt).

He observed that all the F₁ progeny plants were tall, like one of the parents, none were dwarf.

He made similar observations for the other pair of traits and found that F₁ always resembled only one of the parents, and that the trait of other parent was not seen in them.

(2) Law of segregation or Law of purity of gametes:

This law states that both parental alleles (recessive and dominant) of F₁ separate and are expressed phenotypically in F₂ generation. This law is universally applicable.

The F₂ generation was produced by allowing F₁ hybrid to self pollinate, to find out segregation or separation.

It was observed that both dominant and recessive plants appeared in 3 : 1 ratio. Thus, F₂ progeny shows both parental forms.

On the basis of F₂ generation, following observations can be made:

(i) An organism generally has two alleles for each character. These alleles may either be similar or dissimilar. Organism with similar alleles of a pair is called pure or true breeding for that character. If the organism contains dissimilar alleles of a pair, the organism is impure or hybrid.

(ii) An organism receives one of the two alleles from the male gamete and the other from the female gamete. The gametes fuse during fertilization and form a zygote. Zygote develops into an organism.

(iii) Each gamete (male or female) has only one allele of the pair. Thus, each gamete is pure for a trait. That is why this law is often called as Law of purity of gametes.

(iv) The fusion between male and female gametes to produce zygote is a random process.

The plants obtained in F₂ generation show 3 (tall) : 1 (dwarf) phenotypic ratio. Of these three tall plants, one is pure or homozygous dominant and the remaining two are heterozygous (tall in this case). There is only one plant that shows recessive character (dwarf in this case). Dwarf is pure or true breeding, being homozygous recessive.

 

Postulate IV :

This postulate was made on the basis of dihybrid cross or two genes interaction.

He postulated that inheritance of one character is independent of the inheritance of another character.

On the basis of this postulate, Mendel proposed the "Law of independent assortment".

(3) Law of independent assortment :

The law of independent assortment states that when a cross is made between two individuals different from each other in two or more characters, then the inheritance of one character is independent of the inheritance of another character.

Because of their independent assortment, besides the parental types, recombinants are also obtained.

In dihybrid cross, these combinations are obtained in the ratio of 9 : 3 : 3 : 1. e.g., He crossed homozygous dominant round and yellow seeded plant (RRYY) with homozygous recessive wrinkled and green seeded (rryy) plant.

The F₁ hybrids were all heterozygous, showing yellow and round seeded plants. This law is not universally applicable.

Phenotypic ratio: round yellow : round green : wrinkled yellow : wrinkled green

9/16 3/16 3/16 1/16

Genotypic ratio : 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1

If the phenotypic ratio of each pair of alleles (e.g., yellow and green colour of seed) is considered, it shows 12(9 + 3) yellow seeded plants and 4(3 +1) green seeded plants.

This comes to 3 : 1 ratio; similar to one obtained in F₂ generation of monohybrid cross showing segregation.

The same is true for another pair of alleles involved, i.e., round and wrinkled seeded plants. So, the results of each character are similar to the monohybrid ratio.

SUMMARISED ACCOUNT OF MENDEL'S EXPERIMENTS:

Back Cross and Test Cross

Back cross:

F₁ hybrids are obtained by crossing two plants of parental generation.

Mendel devised a cross where F₁ hybrid is crossed with anyone of the two parents, i.e., homozygous dominant and homozygous recessive.

Thus, there would be two possibilities:

(a) F₁ hybrid (Tt) is crossed with homozygous dominant (TT)

(b) F₁ hybrid (Tt) is crossed with homozygous recessive (tt)

Both these crosses collectively are called as back cross. If F₁ is crossed with dominant parent, it is called out cross.

Test Cross:

Out of the two types of back crosses, a cross between F₁ hybrid (Tt) and its homozygous recessive parent (tt) is called test cross.

This cross is called test cross because it helps to find out whether the given dominant F₁ phenotype is homozygous or heterozygous.

A monohybrid test cross between F₁ tall plant (Tt) and its homozygous recessive parent (tt) will produce 50% heterozygous tall (Tt) and 50% homozygous recessive (tt), i.e., 1 : 1 ratio, for both phenotype and genotype.

If a test cross with two characters, i.e., dihybrid test cross is made, it gives four types of plants in 1 : 1 : 1 : 1 ratio.

The phenotypes obtained are similar to those found in F₂ generation of dihybrid cross.

Thus, a dihybrid test cross between F₁ yellow and round seeded plant (YyRr) and its homozygous recessive parent green and wrinkled (yyrr) would give following combinations:

1 yellow, round (YyRr) Parental combination 25%

1 yellow, wrinkled (Yyrr) Recombinants 25%

1 green, round (yyRr) Recombinants 25%

1 green, wrinkled (yyrr) Parental combination 25%

If this ratio is obtained, it would be confirmed that F₁ hybrid with dominant phenotype is infact heterozygous.

The parental combinations (50%) are equal to the frequency of recombinants (50%).

Trihybrid cross.

Mendel crossed two pea plants which differed in 3 characters and observed independent assortment of genes in them.

He crossed two pea plants pure in three traits viz., height of stem, form of seed and colour of cotyledon of seed.

He crossed homozygous tall, round and yellow (TT RR YY) plant with dwarf, wrinkled and green (tt rr yy).

All the F₁ individuals produced were tall, round and yellow (Tt Rr Yy) and are called trihybrids.

On selfing trihybrids, F₂ phenotypic ratio is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1. The ratio for a trihybrid test cross is 1 : 1 : 1 : 1 : 1 : 1 : 1 : 1 .