These are – rose comb found in Wyandotte breed, pea comb in Brahma breed, single comb in, Leghorn breed, and Walnut comb found in Malay breed of fowls.
The first three types were known to be related to each other while the fourth one was thought to be unrelated to the other three.
Each of the types mentioned above breed true (they can exist in the homozygous condition). A cross between rose and single combed birds showed that, the latter was recessive to the former and in the F2, they segregate into typical 3:1 ratio.
Similarly in a cross between pea comb and single comb, the latter again acts as recessive to the former with a typical 3:1 ratio in the F2 generation. When the rose combed birds were crossed with pea combed, however an interesting result was obtained.
The F1 was Walnut, previously thought unrelated to any other three types of combs. When F, Walnuts were bred in between them, the F2 generation showed (see the checker board) walnut (9), rose (3) pea (3) and single (1) comb in the ratio of 9:3:3:1.
Explanation for the experiment:
The fact that single comb acts as recessive to both pea and rose indicates that it (single) must be having recessive alleles for both rose and pea. Again, rose and pea must have dominant alleles, and these alleles must be having something common between them as otherwise they could not cross with single.
The fact that single has recessive alleles for both rose and pea indicates that it must have more than one pair of alleles controlling the comb shape. The fact that rose and pea both are dominant to single is an indication that in them, the dominant genes present must exist together with recessive genes.
The production of walnut in F1 is a clear indication that the dominant alleles of both (rose and pea) have interacted.
This clearly illustrates the operation of two pairs of genes. In the present case rose comb is produced by R gene together with pp gene (the genotype can be RRpp or Rrpp), the pea comb is controlled by P gene together with recessive rr (the genotype can be rrPP or rrPp) genes.
Single comb is produced by both gene pair P and R in their recessive state (rrpp). Both P and R present together will, produce the walnut comb by their interaction (the genotype can be RRPP, RrPP, RRPp or RrPp).
What is important to produce walnut comb is the presence of P and R either in homozygous or heterozygous state the following checker board will give the results of this cross. In the above checker board, giving 16 probable types of progenies when the mating between four types of gametes is random – the phenotypes of progenies are -1,2,3,4,5,7,9,12,13 are walnut, 6,8,14 are rose, II, 12 and 15 are pea and 16s single giving a ratio of 9:3:3:1 respectively.
The basic mode of inheritance of the genes in reality does not differ from the Mendelian pattern. The main difference being the F does not resemble either parent and new character appears in the F2 in the form of single.
Obviously the walnut is the result of interaction between two independently inherited dominant genes, the recessive alleles of both producing the single comb. The single comb produced in F2 is double homozygous recessive as can be proved by the fact that on self breeding it produces only single comb.
This type of genetic interaction is due to the action of collaborative genes. These may be defined as two different on allelic genes present on separate loci, but influence the same trait and interact to produce a new trait that neither of them could produce alone.
2. Flower Color in Sweet Pea (Complementary Genes):
In sweet peas (Lathrus odoratus) the flower color is of two types – purple and white. Bateson and Punett conducted several crosses and obtained two strains of white races which bred true.
But when these two races are crossed with each other, the F1 progeny had purple colored flowers. The F on self pollination produced in the F2 generation both purple and white flowers in the ratio of 9:7 as against the Mendelian ratio of9:3:3:1
According to Bateson and Punnett, the development of the purple color is dependent on the presence of two independent dominant genes C and P; both of which interact to produce the anthocyanin pigmentation gene C is responsible for the formation of the colorless compound chrortlogen.
This produces an enzyme that catalyzes the formation of raw materials necessary for the formation of anthocyanin. Gene P controls the conversion of the raw material 7into anthocyanin. Hence both C and P are necessary for purple color.
If there is no C the first step is blocked and pigment is not formed and if there is no p the second step is blocked and the raw material does not get converted into anthocyanin. In a way C and P act as complementary to each other’s functions.
In order to produce the color, they should be present either in the homozygous condition CC or PP or in the heterozygous condition (Cc or Pp). The fact that the two pure breeding white races produce a purple color is because each of them has either C or P and not both. In the FC and P come together to produce color.
Since two independent non allelic genes are involved, it goes without saying that the recessive alleles of either C or P must be present in white flowered plants together with one of the dominant genes (either C or P).
The F1 hybrid (purple) forms four different types of gametes which on random mating produce 16 possible types of offspring. In the above checker board wherever both C and P are present together, either in the homozygous or in the heterozygous condition the flower colour is purple.
In the checkerboard, the 16th, which has all the recessive genes is also white, for there is no question of color in the absence of both C and P.
The genes C and P here are called complementary genes. They can be defined as two or more separate (non allelic) dominant genes present on different loci interacting to produce a phenotypic trait, neither of which can produce the trait alone
A similar example of complementary genes has been reported in Indian corn where the grains or either white or red in color. In order to produce red color both R and C are required. The absence of both or any on dominant allele, the grain will be white in color.
When a homozygous red is crossed with homozygous white the F) progeny is red as expected in a Mendelian ratio, but when the F is selfed to produce F a 9:7 ratio of red and white grained plants are produced. The cross takes place as follows:
In the above checker board, 1,2,3,4,5,7,9,10 and 13 are red and 6,8,11, 12,14,15 and 16 are white in the ratio of 9:7.
3. Coat Color in Mice (Supplementary Genes):
In the common house mouse, there are several variations in the skin (coat) color like Agouti, black, albino etc. The agouti color is a mixture of red and grey and has two pigments in the fur.
This coat color is found in wild rodents such as Norway rat, meadow vole, guinea pig, grey squirrel etc. From this wild or agouti color, several variations have arisen in domestic rats. The most common variation is the albino.
In albinos the coat color is white with pink eyes. Albinos breed true and are recessive to all other color variations. Another variation in coat color is black. This is produced by the removal of grey pigment from the coat. Black is recessive to agouti but dominant over albino. The black mice breed true.
When a pure breeding black mouse is crossed with albino, the F1 will be agouti. When the F agouti hybrids are crossed among themselves in the F2 generation, the following varieties are produced. Agouti 9/16, Black 3/16 and 4/ 16 albinos
This is a modification of the 9:3:3:1 ratio in that the last two categories have joined to form the third category of albinos. The results can be explained by assuming that the coat color is due to two independent dominant genes namely C and A.
The gene C by itself either in the homozygous condition (CC) or heterozygous condition (Cc) can produce black color provided A is absent. However when both C and A are present they interact to produce agouti color. When there is no C, irrespective of the fact whether there is dominant A gene or not, the coat color will be albino. C gene is necessary to produce color. The following gives the details of the cross.
In the above checker board 1,2,3,4,5,7,9,10 and 13 are agouti 6,8 and 14 are black, while 11,12,15 and 16 are albinos.
Supplementary genes may be defined as two independent pairs of genes one of which is capable of producing a phenoty.pic trait, provided the second gene is absent. When both the dominant genes are present together they interact and produce a different trait.
In the above cross, C will produce black color, but when supplemented (supported) by A the color agouti is produced.
4. Plumage Color in Poultry (Epistasis):
In some of the poultry birds, the plumage is either colored or white and it seems to have a peculiar type of inheritance. The white plumage of white Leghorn birds is dominant over the black plumage of barred and other colored varieties.
However there are other birds with white plumage such as the Wyandotte’s or Plymouth rocks which are recessive to the colored plumage. Obviously the white plumage color in Leghorns and Wyandotte’s is controlled by different genes.
In order to find the genotypes of the colored plumage and white plumage of both leghorns and Wyandotte’s, a cross can be made between the two varieties of white bird’s viz., Leghorns and Wyandotte’s.
The pure lines of such birds when crossed produce an F, with a white plumage with dark flecks (resembling an F, from a cross between white Leghorns and black birds). An interesting result waits when these F, birds are bred together. Both white and colored plumage appears in the F2 in the ratio of 13:3. The white birds resemble the F, bird.
It is assumed that the white Leghorns contain a color gene C, but also an inhibitor gene I. Which does not allow C to function and produce colouration? Then the Leghorns are genetically colored birds but have two dominant genes one of which suppresses the expression of the other.
It is because of this, that Leghorns are dominant over both black and white Wyandotte’s birds. The white Wyandotte birds on the other hand are genetically white in the sense that they do not have a colored gene. What is the effect of the inhibitor gene I in the absence of the colored gene C is none, because there is no gene to be suppressed by it.
The plumage color in this instance will be white because there is no colored gene (C). This is a peculiar phenomenon where two dominant non allelic genes interact but the interaction is not coordination, complementary or supplementary.
One dominant gene (C) by itself can bring out a morphological trait and the sole function of the other (I) is to suppress it whenever it is present. The gene I do not seem to have any function in the absence of C.
This phenomenon is called Epistasis. Epistasis may be defined as a phenomenon in which one non allelic dominant gene suppresses the action of another dominant gene. The two genes are situated on different loci. The gene that suppresses another gene is called Epistatic (I) and the dominant gene that is suppressed (C) is called hypostatic.
In the above cross the genes may be designated as follows. I epistatic gene, i = recessive allele C hypostatic gene, c = recessive allele the following is the checker board representation of the cross.
Polymeric genes (Additive genes)
In some of the plants like Cucurbita pepo the fruit shape or of different types in different varieties the fruit shapes may be long, spherical or discoid. The shape of the fruit seems to be governed by two pairs of interacting factors.
When two individuals with spherical fruits are crossed only discoid fruits are produced in all the progeny on the selfing of the Ft, in the F2 progeny discoid, spherical and long types of fruits are produced in the ratio of 9:6:1.
As has been pointed already two pair’s genes are responsible for the fruit shape. When both the gene pairs are dominant the fruit shape is discoid; when both gene pairs are recessive the fruits are long.
If one pair of genes is dominant and the other recessive spherical fruits are produced (in a gene pair even if one gene is dominant the recessive gene is suppressed. In other words one gene pair is dominant means at least one of the genes is dominant in the pair).
The genes involved in this are called polymeric genes and the phenomenon is known as polymerism. The following checker board will give the details of the above explanation.
In some of the poultry and plants (Capsella bursa pastoris) more than one pair of genes seem to affect a single trait. These are non allelic but to produce a particular trait both the dominant genes should be present or at least one of them should be present either in the homozygous condition or in the heterozygous condition.
We can take an example in poultry to illustrate this. Certain breeds (Langshans) in poultry have feathers on their shanks, while others (buff rocks) have clean shanks (no feathers).
Feathered shanks are dominant over clean shanks. When two birds – one feathered and the other clean (both pure line) are crossed, the F, bird is feathered. When these F, feathered birds are self bred, in the F progeny 15 birds will be feathered and one bird will have clean shanks in the ratio of 15:1.
5. Duplicate Genes:
In some of the poultry and plants (Capsella bursa pastoris) more than one pair of genes seem to affect a single trait.
These are non allelic but to produce a particular trait both the dominant genes should be present or at least one of them should be present either in the homozygous condition or in the heterozygous condition.
We can take an example in poultry to illustrate this. Certain breeds (Langshans) in poultry have feathers on their shanks, while others (buff rocks) have clean shanks (no feathers). Feathered shanks are dominant over clean shanks.
When two birds – one feathered and the other clean (both pure line) are crossed, the F, bird is feathered. When these F feathered birds are self bred, in the F2 progeny 15 birds will be feathered and one bird will have clean shanks in the ratio of 15:1.