12th Class Biology Principle Of Inheritance And Variation Interaction Of Genes

Genes interaction is the influence of alleles and non-alleles on the normal phenotypic expression of genes. It is of two types :

(1) Inter–allelic or intra–genic gene interaction : In this case two alleles (located on the same gene locus on two homologous chromosomes) of gene interact in such a fashion to produces phenotypic expression e.g., co-dominance, multiple alleles.

(i) Incomplete dominance or Blending inheritance (1: 2:1 ratio) : After Mendel, several cases were recorded where \[{{F}_{1}}\] hybrids were not related to either of the parents but exhibited a blending of characters of two parents. This is called incomplete dominance or blending inheritance.

Example : First case of incomplete dominance or blending inheritance was reported in 4-O’clock plant, (Mirabilis jalapa) by Carl Correns (1903) when plants with red flowers (RR) are crossed with plants having white flowers (rr) the hybrid \[{{F}_{1}}\] plants (Rr) bear pink flowers. When these F₁ plants with pink flowers are self pollinated they develop red (RR), pink (Rr) and white (rr) flowered plants in the ratio of 1:2:1 (\[{{F}_{2}}\] generation). Snapdragon or dog flower (Antirrhinum majus) is a other example of in complete dominance. 

(ii) Codominance (1:2:1 ratio) : In codominance, both the genes of an allelomorphic pair express themselves equally in \[{{F}_{1}}\] hybrids. 1:2:1 ratio both genotypically as well as phenotypically in \[{{F}_{2}}\] generation.

Example : Codominance of coat colour in cattle, Codominance in andalusian fowl and Codominance of blood alleles in man.

Differences between incomplete dominance and codominance

(2) Non–allelic or inter-genic gene interaction : Here two or more independent genes present on same or different chromosomes, interact to produce a new expression e.g., epistasis, complementary genes, supplementary genes, duplicate genes, inhibitory genes, lethal genes etc.

(i) Complementary genes (9 : 7 ratio) : The complementary genes are two pairs of nonallelic dominant genes (i.e., present on separate gene loci), which interact to produce only one phenotypic trait, but neither of them if present alone produces the phenotypic trait in the absence of other.

(ii) Supplementary genes (9 : 3 : 4 ratio) : Supplementary genes are two independent pairs of dominant genes which interact in such a way that one dominant gene will produce its effect whether the other is present or not. The second dominant when added changes the expression of the first one but only in the presence of first one. In rats and guinea pigs coat colour is governed by two dominant genes.

(iii) Epistasis (Inhibiting genes) : Epistasis is the interaction between nonallelic genes (Present on separate loci) in which one-gene masks, inhibits or suppresses the expression of other gene. The gene that suppresses the other gene is known as inhibiting or epistatic factor and the one, which is prevented from exhibiting itself, is known as hypostatic. 

Dominant epistasis (12:3:1 or 13:3 ratio) : In dominant epistasis out of two pairs of genes the dominant allele, (i.e., gene A) of one gene masks the activity of other allelic pair (Bb). Since the dominant epistatic gene A exerts its epistatic influence by suppressing the expression of gene B or b, it is known as dominant epistasis. Example – Dominant epistasis in dogs

Similar phenomena have been seen in fruit colour in cucurbita as summer squash and coat colour in chickens.

Recessive epistasis (9:3:4 ratio) : Epistasis due to recessive gene is known as recessive epistasis, i.e., out of the two pairs of genes, the recessive epistatic gene masks the activity of the dominant gene of the other gene locus. The dominant A expresses itself only when the epistatic locus C also has the dominant gene if the epistatic locus has recessive gene c, gene A fails to express.

(iv) Duplicate genes (\[15:1\] ratio) : Sometimes two pairs of genes located on different chromosomes determine the same phenotype. These genes are said to be duplicate of each other. The dominant triangular fruit shape of Capsella bursa pastoris (shepherd’s purse) is determined by two pairs of genes, say A and B. If any of these genes is present in dominant form, the fruit shape is triangular. In double recessive forms the fruits are top shaped and thus we get a 15 (triangular) : 1 (top shaped) ratio in F₂ generation.

Example : Coat colour of mice.

(v) Collaborator genes : In collaboration two gene pairs, which are present on separate loci but influence the same trait, interact to produce some totally new trait or phenotype that neither of the genes by itself could produce.

Example : Inheritance of combs in poultry, where two genes control the development of comb.

Pleiotropic effect of genes

Lethal genes : Lethal factor were first of all reported in mice body by of French geneticst 'Cuenot'. Certain genes are known to control the manifestation of some phenotypic trait as well as affect the viability of the organism. Some other genes have no effect on the appearance of the organism but affect the viability alone. These genes are known as lethals or semilethals depending upon their influence. Lethal factors in case of plants were reported first of all in snapdragons (Antirrhinum majus) by E. Baur (1907).

Dominant lethals : The dominant lethal genes are lethal in homozygous condition and produce some defective or abnormal phenotypes in heterozygous condition. Their most serious effect in heterozygous may also cause death. Following are the examples of dominant lethal genes.

Example – Yellow lethal in mice : A well known example of such lethals is from mice, given by Cuenot. He found that the yellow mice never breed true. Whenever the yellow mice were crossed with yellow mice, always yellow and brown were obtained in the ratio of 2:1. A cross between brown and brown mice always produced brown offsprings and a cross between brown and yellow produced yellow and brown in equal proportions. Yellow mice never present homozygous condition.

In 1917, Stiegleder concluded that yellow mice are heterozygous. The homozygous yellow (1/4^th of the total offsprings) dies in the embryonic condition. When there unborn ones are added to the 2:1 ratio of yellow and brown, these form typical 3:1 ratio. Cuenot suggested that gene Y has a multiple effect. It controls yellow body colour and has a dominant effect. It affects viability and acts as a recessive lethal. Other examples are Inheritance of sickle cell anaemia in man, Brachyphalangy, Huntington’s chorea in man.

Recessive lethals : The recessive lethals produce lethal effect only in homozygous condition. Their heterozygotes are normal. Therefore, recessive lethals remain unnoticed in the population but are established in the population because female are carrier for lethal gene. These are detected only when two heterozygous persons get married. Example : Tay Sach’s lethal

Qualitative inheritance : Qualitative inheritance or monogenic inheritance is that type of inheritance in which one dominant allele influences the complete trait, so that two such allele do not change the phenotype. Here dominant allele is monogene.

Quantitative/Polygenic inheritance : Quantitative inheritance or polygenic inheritance can be defined as, two or more different pairs of alleles which have cumulative effect and govern quantitative characters. The quantitative inheritance is due to incomplete dominance.

Examples : Ear size in maize, White spotting in mice, Grain colour in wheat.