Sunday, May 20, 2012


Types of Gene Interaction
1. Dominance.

2. Lethal Genes.

3. Pleiotrophy.

4. Duplicate genes.

5. Complementary Genes.

6. Supplementary Genes.

7. Epistasis

Complementary Genes - In this type of gene interaction, the production of one phenotype requires the presence of dominant alleles of both the genes controlling the character. When anyone of the two or both the genes are present in the homozygous recessive state, the contrasting phenotype is produced.

Thus anyone of the two dominant genes is unable to produce the phenotype when it is alone. But the dominant alleles of the two genes complement each other to produce the concerned phenotype when they are together. This type of interaction shows a ratio of 9: 7.
For example, in sweet pea, the development of purple coloured flowers requires the presence of two dominant genes C and B. When either C or R (e.g. CCrr, ccRR) or both of them (e.g. ccrr) are present in homozygous recessive condition, purple coloured flower cannot be produced. As a result, white flowers are obtained.
Supplementary Genes - In supplementary genes, the dominant allele of one gene produces a phenotypic effect. The dominant allele of the other gene does not produce any phenotypic effect on its own. But when it is present with the dominant allele of the first gene, it modifies the phenotypic effect produced by the first gene.

That is, the dominant allele of one gene is necessary for the development of the concerned phenotype, while that of the other gene modifies the phenotypic expression of the first gene. This interaction shows a ratio of 9:3:4.For example, the development of aleurone colour in maize is governed by two completely dominant genes Rand P.
A plant producing purple coloured grain (RRPP) is crossed with a plant producing white coloured grain (rrpp). The FJ plants produce purple colour. In F2' 9 out of the 16 zygotic combinations will have at least one dominant allele either R or P.

They will develop red colour since the recessive allele has no effect on colour production. Three other zygotes will be homozygous rr but will have the dominant allele P. These seeds will be white since rr is unable to produce colour and P does not produce any colour. The remaining four zygotes will be homozygous recessive for both the genes (rrpp) and will produce white seed.
Lethal Genes - Genes which affect the viability of an organism are called as lethal genes and the phenomenon is called as lethality. If the lethal effect is dominant and immediate in expression, all individuals carrying the gene will die and the gene will be lost.

Some dominant lethal, however, have a delayed effect so that the organism lives for a long time, e.g. Huntington's disease. Recessive lethal carried in the heterozygous condition nave no effect but may come to expression when mating between carriers occurs. The phenotypic ratio is 2: 1.
For example, in mice, yellow colour is dominant over agouti. But when a cross takes place between two yellow coloured mice, the result was 2 yellow and 1 agouti (2: 1) instead of 3: 1. This was because the yellow colour is a dominant lethal.

The existing yellow coloured mice were in heterozygous conditions. All the yellow coloured mice in homozygous condition died at the embryo stage.

Duplicate Genes - The presence of a single dominant allele of anyone of the two genes governing the trait produces the dominant phenotype. The recessive phenotype is produced only when both the genes are in the homozygous recessive state, i.e., the dominant genes when present together or alone will produce the same phenotype, but when they are in recessive state together, they will produce different phenotypes. This gene interaction produces a ratio of 15:1.

.For example, the non-floating habit in rice is controlled by two dominant genes DWI and DW2' When a non-floating rice strain with the genotype (DW1' DW1' DW2' DW2) is crossed with a floating strain (dw1' dw1' dW2' dw), the Fl is non-floating.
In the F2generation, on an average 9 plants will have at least one dominant allele of both the gene, 3 plants will have at least one dominant allele of one of the two genes (homozygous for other gene) and 3 other individuals will have one dominant allele of the other gene.

All these 15 individuals will have non floating habit, and only one of the 16 possible zygotic combinations will be homozygous recessive for both the genes, dW1 and dW2'.

Plelotrophy - The phenomenon in which a single gene affects two or more characteristics is called plelotrophy. For example, in humans a rare genetic disorder, phenylketonuria, occurs in individuals homozygous for a defective recessive allele.

These people lack the enzyme necessary for the normal metabolism of the amino acid phenylalanine. In addition, they possess smaller head and somewhat lighter hair.
Incomplete Dominance Gene Interaction - In incomplete dominance, alleles may produce the same product, but in lesser quantity as compared with the dominant allele. In heterozygous condition, the total product is intermediate between that of the dominant and recessive alleles.

The phenotypic ratio is 1: 2: 1. For example, in snapdragons, the homozygous dominant for flower colour will show red colour and homozygous recessive will be white. In contrast, the heterozygous will have pink colour.

This is against Mendel's theory of dominance, as a heterozygous plant should show the characteristic of dominant, i.e., red.
Gene Interactions
The genes of an individual do not operate isolated from one another, but obviously are functioning in a common cellular environment. Thus, it is expected interactions between genes would occur. Bateson and Punnett performed a classical experiment that demonstrated genetic interactions. They analyzed the three comb types of chicken known to exist at that time:
Chicken Varieties
Rose Comb
Pea Comb
Single Comb
                                                   Rose             Pea 


Result: The F1 differed from both parents and two new phenotypes not seen in the parents appeared in the F2. How can this result be explained? The first clue is the F2 ratio. We have seen this ratio before when the F1 from a dihybrid cross is selfed (or intermated). This observation suggests that two genes may control the phenotype of the comb. The gene interactions and genotypes were determined by performing the appropriate testcrosses.
A series of experiments demonstrated that the genotypes controlling the various comb phenotypes are as follows.
It was later shown that the genotypes of the initial parents were:
Rose = RRpp
Pea = rrPP
Typically the cross was:

The development of any individual is obviously the expression of all the genes that are a part of its genetic makeup. Therefore, it is not an unexpected conclusion that more than one gene could be responsible for the expression of a single phenotype. We will now discuss this situation. First let's give a definition.
Epistasis - the interaction between two or more genes to control a single phenotype
The interactions of the two genes which control comb type were revealed because we could identify and recognize the 9:3:3:1. Other genetic interactions were identified because the results of crossing two dihybrids produced a modified Mendelian ratio. All of the results are modifications of the 9:3:3:1 ratio.

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