Interaction of Genes

Interaction of Genes

Interaction of Genes

In independent assortment, we consider two genes controlling two characters, Consequently, assortment of genes was presumed from the assortment of characters. It is, however, not necessary that a single character is controlled by one gene only, instead it may be controlled by more than one gene. In such a situation two or more than two genes may interact to give rise to a particular phenotype. For instance if gene A is responsible for phenotype A and gene B is responsible for phenotype B, both A and B when present together may give rise to a phenotype C. Nevertheless, these genes still obey the principles segregation and independent assortment.

If A and B are two genes, both dominant over their respective recessive alleles a and b, then the interaction will depend upon (i) the presence of both dominant alleles A and B, (ii) the absence of A, (iii) the absence of B or (iv) the absence of both A and B.

As indicated above, the interaction is possible between more than two genes also, but due to complexity involved in those cases, interactions involving only two genes will be considered in detail in this chapter, and only a summary of interactions involving more than two genes will be presented.

Abbreviated genotypic ratio-

Depending upon the four alternatives outlined above, we can give an abbreviated genotypic ratio expected in F, i.e. 94B: 3 Ab: 3aB: lab. In this ratio, AB means those individuals which have both A and B either in homozygous condition or in heterozygous condition (1AABb: 2AaBb). Similarly, Ab means absence of B, but presene of A in homozygous or heterozygous condition (1AAbb : 2Aabb). Likewise, aB includes two genotypes (1aaBB : 2aaBb) and ab will include only one genotype (1aabb).

The abbreviated genotypic ratio as outlined in summary chart shown in Table.1 is very useful in predicting phenotypic ratio for a particular kind of interaction. If the abbreviated genotype is understood and its use practised, no checker boards or forked line methods will be needed for working out phenotypic ratios. In the following text some cases of interaction of genes will be discussed using only the abbreviated genotypic ratios, since checker board can be easily dispensed with.

Table. Genotypes and abbreviated genotypic ratio obtained in F2 from F2 (AbBb)

Genotypes Ratio Phenotypes Phenotypic ration
AABB

AaBB

AABb

AaBb

1

2

2

4

AB 9
AAbb

Aabb

1

2

Ab 3
aaBB

aaBb

1

2

aB 3
aabb 1 ab 1

Two gene pairs affecting same character-

In dihybrid ratio, the two genes independently controls separate characters. Therefore, there was independent assortment of the genes as well as of the characters.

In case of comb shape in poultry, two genes are responsible for this character. While one gene gives rise to rose comb, another gives rise to pea comb. Each of these two is dominant over single comb. However, when both are brought together, a new phenotype walnut appears.

As worked out by W. Bateson and R.C. Punnett, when both dominant alleles are present, walnut phenotype appears and when both recessive alleles are present single comb appears. Rose and pea phenotypes each appears due to the presence of different but single dominant alleles. If pea (rrPP) and rose (RRpp) are crossed or if walnut (RRPP) and single (rrpp) are crossed, the F1 individual in both cases will have walnut phenotype and will be heterozygous for both genes (RrPp). These individuals when crossed among themselves will give four phenotypes in 9:3:3:1 ratio.

Table: Abbreviated genotypic ratio and phenotypic ratio obtained in F2 derived from F1 RrPp (walnut)

Abbreviated genotypic ratio RP

9

Rp

3

rP

3

rp

1

Phenotypic ratio Walnut

9

Rose

3

Pea

3

Single

1

It the above 9:3:3:1 ratio it represent four phenotypes of the same character.

Epistasis

Epistatic interaction means that one genes masks or changes the effect of another gene. In other words it does not allow the other gene to express itself fully. This is a counterpart of dominance. While dominance works at interallelic but intragenic level, epistasis works at intergenic level. Epistasis can be of the following types-(i) due to recessive allele: a masks the effect of B. (ii) due to a dominant allele: A masks the effect of B. In some cases, this may be in one direction only meaning that either A masks the effect of B or vice versa. In other cases, if can be in both the directions also, which means that both genes are mutually epistatic to each other.

Contrary to the above definition of epistasis, in population and quantitative genetics, epistasis refers to all non-allelic interactions including complementary and duplicate interactions.

In mice, the wild body colour is known as agouti, characterized by banding of individual hairs. The agouti colour is controlled by a gene, say A which is hypostatic to recessive allele c. The dominant allele C in the absence of A gives coloured mice. Moreover, in the presence of dominant allele C, A gives rise to agouti. Therefore, CCaa will be coloured and ccAA will be albino When coloured mice (CCaa) are crossed with albino (ccAA), agouti mice (CcAa) appear in F₁. The details of the genotypes and the phenotypes obtained in F2 are given in Table.

As can be realized, CC masks the effect of AA in this example and is therefore epistatic. Consequently, ccAA is albino. However, ccaa is also albino but not due to epistasis but due to the absence of both the dominant alleles.

Table. Abbreviated genotypic ratio and phenotypic ratio obtained in F₂ derived from F1 CcAa (agouti)

Abbreviated genotypic ratio CA

9

Ca

3

cA

3

ca

1

Phenotypic ratio Agouti

9

Colored

3

Albino

4

Complementary genes-

A classical example of interaction of genes is the complementation between two genes meaning that both genes are necessary for the production of a particular phenotype. W. Bateson and R.C. Punnett observed that, when two white flowered varieties of sweet pea, Lathyrus odoratus were crossed, F1 progeny had coloured flowers. When F₂ progeny obtained from F, was classified, plants with coloured flowers and those with white flowers were obtained in 9:7 ratio. This is again a modification of 9:3:3:1 ratio, where only one character i.e. flower colour is involved and only two classes recognized (coloured and white flowers). The abbreviated genotypic ratio given in Table will explain the kind of interaction involved.

Table. Abbreviated genotypic ratio and phenotypic ratio obtained in F2 derived from F1 CcPp (purple).

Abbreviated genotypic ratio CP

9

Cp

3

cP

3

cp

1

Phenotypic ratio Colored flowers

9

White flowers

7

It is obvious in the above example that both the dominant alleles, ‘C’ and ‘P’ are necessary for the production of pigment in the flowers. Each of the two parents lacks one of the two dominant alleles and, therefore, both bear only white flowers. The two dominants are brought together in F1 generation and therefore coloured flowers are produced.

Duplicate genes-

A modified ratio 15: 1 (derived from standard ratio 9:33: 1) was observed by G.H. Shull from his studies on the plant known as Shepherd’s purse belonging to the genus Capsella. Two kinds of phenotypes with respect to fruit shape were known- (i) triangular capsules and (ii) top shaped capsules. When races with these phenotypes were crossed, in F1 generation only triangular capsules were observed. When such F₁ individuals with triangular capsules were intercrossed among themselves, in F₂ progeny, plants with triangular capsules and top shaped capsules were obtained in 15: 1 ratio. Obviously the top shaped capsules result from double recessive genotype.

If ‘A’ and ‘B’ are two genes, top shaped capsules will be obtained on plants with the genotype aabb. Plants with the triangular capsules can be AABB, AAbb or aaBB and other genotypes with heterozygosity. It shows that even a single dominant gene is enough to give rise to triangular capsules, Presence of both dominants has the same effect.

Table. Abbreviated genotypic ratio and phenotypic ratio obtained in F2 derived from F1 AaBb (triangular).

Abbreviated Genotypic Ratio AB

9

Ab

3

aB

3

ab

1

Phenotypic Ratio Triangular

15

Top shaped

1

The above examples, which are only few from a large number of them demonstrating interaction of genes, indicate that the basic dihybrid ratio, 9:3: 3:1 can be modified giving rise to 9: 3:4, 12:3:1, 13:3,9:7 and 15: 1 ratio depending upon the kind of interaction involved: Such an interaction may also be possible between three gene pairs and ratios, which are modifications of 27:9:9:9:3:3:3:1, will be obtained.

Additional Interactions Involving Two Gene Pairs- As we have seen in the previous examples, the abbreviated genotypic ratio 9AB: 3Ab: 3aB: 1ab gets variously modified, because two or more of these abbreviated genotypes give the same phenotype. We have thus found that 9 3 3 1 ratio gets modified into 9: 3:4, 97 and 15: 1 in three situations of epistasis, complementation, and duplication. One can think of several other possibilities giving ratios like 13:3, 12:3:1, 9:6: 1. Some of these examples are summarized in table.

Table. A summary of some interactions involving two gene pairs.

Kinds of interaction Abbreviated genotypes Phenotypic ratio
One gene pair (A), when dominant epistatic on another (B) AB

Ab

white 12
Example: fruit color in summer squash aB

ab

Yellow

Green

3

1

A= white, dominant over color (green) and epistatic on B
B= yellow dominant over green
Interaction, Dominant A (white) masks the effect of yellow or green

One gene pair (A), when dominant epistatic on another (B) AB

Ab

ab

White 13
Example: fruit color (fowl) aB Color 3
A= color inhibitor (epistatic on B)
B= color dominant to white
Interaction. A does not allow B to express color
Interaction between two AB Disc 9
Dominant to give new phenotype AB

aB

Sphere 6
A, B= Sphere shape dominant over long shape
Interaction. A and B, when present together give disc shape

Interaction Between More than Two Gene Pairs- Several genes may be involved in the expression of same character and may interact in a rather complex manner. This may be understood if we realize that a phenotypic character is the end product of several steps involved in a biochemical pathway, and each step may depend on a gene. One such example is the study of coat colour in guinea pigs, which involves a number of gene pairs including the following-

  1. EE, black; ee, yellow; eP ep, tortoise shell (yellow and black).
  2. AA, agouti (wild type, black and yellow banded hairs) aa, no agouti, only black.
  3. BB, black, bb, brown.
  4. CC, no colour dilution : Ck, Cd, Cr, Cd, various degres of colour dilution: CKCK, yellow, CaCa, albino.
  5. PP, no dilute; pp, dilution of black or brown, but not yellow.
  6. SS, no spotting; ss, spotting; Ss, occasional spots.
  7. SiSi, unsilvered; sisi, silvered effect.

The above different genes act at different stages of pigment development and interact in a complex manner.

Modifiers, Suppressors and Pleiotropic Genes- Although we normally ascribe the control of phenotype pertaining to a specific character to one particular gene, it is normally influenced in various ways by several genes. Although some of these influences have been characterized in terms of complementary genes, epistatic genes, duplicate genes, etc., in several other cases, the influence cannot be characterized in such definite terms. Such genes, which modify the effect of other genes without any characteristic form, are sometimes described as modifiers. These modifiers normally change the phenotypic effect of other genes in quantitative manner. Many genes responsible for dilution of body colour may belong to this category. There may always be present modifiers with minor effects on quantitative characters like yield or height, etc., but these genes cannot be easily identified.

There are also modifiers, which will not allow mutant allele of another gene to express either fully or partially. These modifiers have been called suppressors and result in wild phenotype, so that one will have to determine whether it is due to reversion of mutant to wild type or due to a suppressor. Some of the examples of suppressor genes are su-h (suppressor for hairy wings) and su-S (suppressor for star eye shape).

Modifiers may also influence the degree of dominance expressed by another gene. In Abraxas grossulariata, a moth, it could be possible, to establish lines with same gene (lutea=yellow colour) dominant in one case and recessive in the other. This was proved to be due to the effect of modifiers.

There are certain other genes, which in addition to their main effect, may also act as modifiers for another but entirely different gene. Such genes having more than one effect are called pleiotropic genes. Some examples of pleiotropic genes are as follows-

  1. Some genes for bristles, eye and wing may influence the number of facets in Bar-eyed
  2. Gene for white eyes may affect shape of sperm-storage organs in females and also some other structures.

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