Principle of independent assortment or Mendel's second law of inheritance



As per the principle of independent assortment, two factors of any character in the Fi hybrid assort independent of the two factors of any other character present together at the time of gamete formation as per the law of probability and get randomly rearranged in the offspring.

Thus, the two factors for seed colour (Y and y) and the two factors for shape of the seed (R and r) present together in the Fi hybrid assort independently and randomly during gamete formation so that a gamete contains only one of the two factors of a character. As a result four types of gametes are formed,viz. RY, Ry,rY and ry.

These four types of gametes unite randomly as per probability to give rise to four types of F2 offspring. Hence inheritance of two or more factors or genes is independent of each other. However, it is now well established that genes or factors located very close to one another on the same chromosome arc linked and are not assorted independently.

Only those factors or genes located on different chromosomes or on same chromosome but distantly apart from one another assort independently.

Back Cross: back cross is a cross between the Fi hybrid with any one of the homozygous parents. When Fi hybrid is crossed with the homozygous dominant parent then all the offspring will be with dominant phenotype.

When Fi hybrid is crossed with Recessive parent, both dominant and recessive phenotypes appear in equal or 1:1 proportion.

Test Cross: This cross is actually a back cross employed to know the genotype of the dominant phenotype. A dominant phenotype can be a homozygous dominant (TT) or heterozygous dominant (Tt). If the dominant phenotype with unknown genotype is crossed with a recessive one thenthe cross is known as test cross.

After crossing if the F2 gives all dominant phenotypes then the test plant is a homozygous dominant. If the F2 gives equal proportion of dominant and recessive phenotypes then the test plant is a heterozygous dominant. Theis was a powerful tool employed by Mendel to know the genotypes of dominant phenotypes.

Some Dominant and Recessive traits in humans.

The chromosomal basis of inheritance was advocated after the behaviour of chromosomes in mitosis and meiosis were discovered.

In 1900 Mendelism was reestablished by three workers, namely Hugo de Vries, Correns and Tschcrmark. Correns coined the term factors for hereditary unit, Which Mendel referred as "elemcnte". An American graduate student, Walter.S.Sutton and a German biologist, Theodcr Bovcry observed close parallelism between the Mendclian factors

and the behaviour of chromosomes during gamete formation and fertilization. Basing on their observations', Sutton and Bovcfv in ^902 independently put forward the chromosomal basis of inheritance. The parallelism between chromosomes and Mendelian factors are summarized below:


Chromosomes occur in homologous Pairs in the diploid organisms.

During gamete formation( meiosis) homologous chromosomes separate.

Each gamete receives only one of the two chromosomes of a homologous pair.

After fertilization in a diploid cell chromosomes again occur in two sets one contributed by father and the other by mother.

Mendelian Factors

Mendelian factors also occur in pairs in diploid organisms.

Mendelian factors also segregate during gamete formation.

Each gamete receives only one of the two alternative factors.

After fertilization, Mendelian factors also occur in pairs, one each contributed by father and mother.

The chromosomal basis of inheritance advocates that

Chromosomes are the bearer of hereditary units or genes.

As chromosomes occur in pairs, each pair of Mendelian factors is carried by a pair of chromosome separately.

The Mendelian factors or genes have specific loci on chromosomes.

Factors segregate due to the segregation of chromosomes during meiosis

Later on in 1909 Johannscn coincd the term gene for Mendelian factor the two alternative forms of genes occur in two homologous chromosomes and correspond to the same loci on the two homologous chromosomes

Other patterns of inheritance:

In all his experiments, Mendel observed that the homozygous dominant as well as heterozygous genotypes were showing the same phenotypes.

This meant, in a paired allele controlling alternative forms of characters, one allele is completely dominant over the other in its expression. Subsequent workers discovered that such a simple dominant-recessive relationship was not always true. Some
ease studies revealed intermediate phenotypes in the hctcrozygotcs and some hetero/.ygotes even had the equal expression of both the alleles.

These findings laid to the establishment of incomplete dominance and codominance respectively. Further it was observed that one character might be controlled by more than one gene (polygenic inheritance) and one gene might control more than one character (Pleiotropic effect).

A. Incomplete Dominance:

Incomplete or partial or mosaic dominance is the phenomenon where there is absence of complete dominance so that in the hetcrozygote condition an intermediate phenotype is observed.

The Fi hybrid shows intermediate character of the two alternative forms. This is not a blending or mixing of characters as the F2 again shows the parental types. Carl Correns, for the first time reported incomplete dominance in the petal colour inheritance of Mirabilis jalapa (Four 0" Clock plant).In Mirabilis jalapa and Antirrhinum majus (Snapdragon or Dog flower) the cross between red and white flower varieties yielded all pink flowered Fl hybrids. In Fi hybrids neither the red nor the white trait was dominant rather the hybrid showed intermediate colour.

This can be explained by using genetic symbols. The pure red (RR) is crossed with pure white (rr). Both the forms produce only one type of gametes,ie, either R or r. The Fi receives one R allele and one r allele and is heterozygote (Rr).

F2 Generation

Here each parent is diploid and thus receives two alleles for petal colour. In red flower variety the parent has two functional alleles (RR), both producing mRXA for the translation of neccssary enzymes involved in the synthesis of red pigments. Where as in pink hybrids only half of the mRXA are transcribed by the only functional allele(R), so insufficient red pigments are synthesized and the flower becomes pink (intermediate) in white flower plants both the alleles are defective. So no red pigments arc synthesized.

The defective allele may give rise to defective proteins or even may not be transcribed at all due to defcct in the promoter.

Andalusian fowls also show incomplete dominance. There arc two pure forms of fowls as black and white. When these two forms are crossed the hybrid Fi are blue in colour.F2-generation shows 1 pure black, 2 hybrid blue and l pure white.

1J.Multiple Allclism: Alleles arc alternative forms of a gene. In any diploid organism two copies of a particular gene occur one each on each homologous chromosome occupying the same loci.

The alternative forms of a gene arise due to mutation in the original or wild gene. In some cases there may not be any alternative forms at all. Mutation that completely eliminates a gene is called a null mutation. But sometimes the mutation may not have any effect at all. This type of mutation is known as silent mutation.

Null mutation or any other kind of mutation that impedes gene function but do not eliminates it completely result in loss of function and gives rise to alternative allele. Thus it is possible thatmutation can occur in different directions in a wild gene to give rise to many alternative alleles. More than two alternative forms of a gene present on the same locus are known as multiple allelism.

But one should remember that a single organism can have only two alleles. Multiple alleles occur in a population. Further in case of multiple alleles, different pairs of alleles may show different dominant-recessive relationships. Some may be completely dominant over others, some incompletely dominant and some may be co-dominant.

There are many examples of multiple allelism. The gene for coat colour in rabbit has four alleles and human blood group gene has three alleles. An extreme example is the wild gene controlling eye colour in Drosophila. So far over 100 different alleles of this gene have been identified. Characteristic feature:

1. There are more than two alleles of a gene in a population.

2. Any diploid individual contains only two alleles as a chromosome contains only one allele of the group.

3. Multiple alleles occupy the same locus on a chromosome or its homologous chromosomc.

4. There is no crossing over between the members of a multiple alleles group.

5. The wild allele whose mutations give rise to the multiple allele series maybe always the dominant alleles and the mutant alleles may show co-dominance, partial dominance or even complete dominance among themselves.

Human gene that determines the blood group is an example of multiple allclism showing both complete dominance and co- dominance among the alleles. It is important to note that co-dominance is cases where both the alleles of a hetcrozygote have equal phenotypic expressions.

There are four different blood groups found in human population. These are A, B, AB and O. The different blood groups are defined by the presence of different antigens on the surface of the erythrocytes. 'I" he genes responsible for producing these ccll surface antigens is called I.

This gene has three alleles: and co dominant, and both are dominant over encodes an enzyme that adds galactosaminc to the surface of RBC. 1B encodcs the enzyme that adds galactose to the surface of RBC.I0 or i code a protein that does not add any sugar to the ccll surface of RBC. The different combinations of three alleles produce four different phenotypes:

1. Blood group A individuals arc either IAIA homozygotes or IiA hcterozygotes. They have only galactosaminc added to the cell surface of RBC.

2. Blood group B individuals are cither IBIB homozygotes or IBi heterozygotes. They have only galactose added to the cell surface of RBC.

3. Blood group AB individuals have both sugars added to the surface of RBC and they are always IAIB heterozygotes.

4. Blood group O individuals have neither sugar added to the surface of RBC and are always homozygotes ii/I°I°

The four different cell surface phenotypes are called ABO blood groups. If type A receives blood transfusion from type B then recipient's immune system identifies the "foreign" antigen galactose on RBC of received blood and attacks donated blood cells causing agglutination or clumping.

The same thing happens if type B individual receives blood from type A individual. This also happens if donated blood is type AB to either type A or B. If the donated blood is type O then no agglutination occurs as the RBC in this case has no cell surface antigen. Therefore, O group is considered as universal donor. However, AB individuals can receive blood from type A, type B as well as type O. But for this another factor called Rh should match between recepient and donor. Rh negative (-) cannot receive from Rh Possitive (+). Hence O (negative) is universal doner and AB (Positive) is universal acceptor.

Another example of multiallelism is the coat colour gene in- rabbit. The gene for coat colour has four alleles: wild type C% Chinchila Cch, Himalayan Ch and Albino C. The wild type is dominant to all other alleles. The Himalayan allele is dominant to albino but recessive to all other. Chinchila is partially dominant to Himalayan and completely dominant to albino but recessive to wild.