The following points highlight the fourteen types of inheritance in Plants. The types are: (1) Simple Inheritance (2) Incomplete Dominance or Blending Inheritance (1:2:1 Ratio) (3) Simple Interaction (9:3:3:1 Ratio) (4) Epistasis (5) Supplementary Gene or Recessive Epistasis (9:3:4 Ratio) (6) Inhibitory Genes (13:3 Ratio) (7) Lethal Genes (8) Complementary Genes (9:7 and 9:6:1 Ratios) (9) Supplementary and Complementary Genes (27: 37 Ratio) (10) Multiple Factors (11) Multiplication of Factors (12) Pleiotropy (13) Modifying Genes and Polygenes and (14) Cytoplasmic Inheritance.
Inheritance Type # 1. Simple Inheritance:
These are the cases where inheritance is found to be as simple as observed by Mendel. Hundreds of examples of this type have been found after Mendel’s own classical experiments. Drosophila or the fruit-fly is an insect which has proved to be a gold mine showing numerous instances of this and other types of inheritance.
Among human beings, brown eye and blue eye form an allelomorphic pair of characters where brown eye is dominant.
Inheritance Type # 2. Incomplete Dominance or Blending Inheritance (1:2:1 Ratio):
The classic example of this is that of the Mirabilis jalapa flower (four-o’ clock plant) found by Correns (1903). When the red- flowered variety (rosea) is crossed with the white- flowered variety (alba) the F2 hybrid is pink and the F2 ratio is 1 red: 2 pink: 1 white. This shows that there is no complete dominance in the heterozygous plant which bears pink flowers. If the pair of alleles controlling flower colour be named Rr, then red flowers develop on RR plants, white on rr plants, while the heterozygous Rr plants develop pink flowers and not red ones which would have been the case if there was complete dominance of R over r (Fig. 849).
This is also called blending inheritance as if the two colours have blended in the heterozygote. But this does not necessarily mean actual blending of two gene particles and may well be caused by the simultaneous action of two genes, one of which may ‘dilute’ the other.
There are many other examples of this type. In some rice, when a short-glumed variety is crossed with a long-glumed one, the F1 shows glumes of intermediate length. This is a case of quantitative inheritance where we may suppose that the dominant gene increases the length while the recessive retards it. Then the heterozygous F1 should be intermediate between the double dominant and the double recessive.
Inheritance Type # 3. Simple Interaction (9:3:3:1 Ratio):
Among fowls, four types of combs are commonly observed—pea (e.g., Brahma fowl), Jose (in Wyandotte fowl), walnut (previously known in Malay fowls—not connected with the other three breeds) and single (in Leghorn fowl). Beteson and Punnett found that these are caused by two allelomorphic pairs of genes which may be symbolized as Pp and Rr (capital letters signify dominant genes while small letters signify their recessive alleles).
When the dominant gene p is present alone (i.e., without the simultaneous presence of the other dominant gene r) the comb is of the pea type. When r is present alone the comb is rose. But when both p and r are present, there is an interaction and the comb becomes walnut. When no dominant gene is present, the comb is of the double recessive single type. Thus if one crosses a pure-breeding pea-combed fowl with a pure-breeding rose-combed one (Fig. 850) the F1 will show a comb of walnut type and, when selfed (sib-crossing), the F2 will segregate in the ratio 9 walnut: 3 pea: 3 rose: 1 single.
The above is a simple case of interaction where the normal dihybrid ratio (though with unexpected phenotypes) 9: 3: 3: 1 is obtained.
The modes of interaction discussed below are more complex and involve complex ratios. The F2 segregation in all such cases of interaction, however, will be understood if one remembers that in a dihybrid ratio where two allelomorphic pairs, Aa and Bb, are involved, the segregation ratio is 9 A-B-: 3 A-bb: 3 aaB: 1 aabb, The phenotypic expression will vary according to the type of interaction and the ratio will vary accordingly.
Inheritance Type # 4. Epistasis:
There are numerous cases where a factor of one pair masks the expression of the factors of another pair. The former is then said to be epistatic over the latter. The latter may, then, be designated as hypostatic. Epistasis is thus the same effect between factors of two different allelomorphic pairs as dominance is between two alleles of the same pair. Epistasis may be of different types. The epistatic gene may be a dominant gene or a recessive one or both.
Epistasis of Dominant Gene (12: 3: 1 ratio) in the coat colour of dogs, there are two allelomorphic pairs which may be named Aa and Bb.
They interact as follows:
A—B— and A—bb = white
aaB—= black
aabb = brown
Thus in a brown (aabb) X white (AABB) cross, the F1 will be white (AaBb) and the F2 segregation will be 12 white (A-B—and a—bb): 3 black (aaB—): 1 brown (aabb). This is a case of dominant epistasis as the dominant gene A (white) is epistatic over both B and b.
Inheritance Type # 5. Supplementary Gene or Recessive Epistasis (9:3:4 Ratio):
The anthocyanin pigmentation (red, magenta, blue, etc.) of flowers and the melanin pigmentation of animals present very interesting examples of gene action. The red-type anthocyanin colour of many flowers is caused by two alleles which may be termed Aa and Bb.
In the snapdragon (Antirrhinum) flower:
A—B – = magenta flower
A—bb = ivory flower
aaB — and aabb =white flower
Thus, in an ivory (AAbb) X white (aaBB) cross (Fig. 851), the F, (AaBb) is magenta and the F2 segregation ratio is 9 magenta (A—B—): 3 ivory (A—bb): 4 white (aaB— and aabb). This may be explained as a case of recessive epistasis by saying that the recessive factor aa suppresses the expression of all colour. But a much more plausible explanation is the supplementary factor hypothesis.
It has been experimentally shown that the ivory flowers contain anthoxanthin and magenta flowers anthocyanin pigments. It is known that anthoxanthin and anthocyanin are synthesised out of the same raw materials, the former in the first stage and the latter when some additional chemical reactions take place.
It is, therefore, reasonable to suppose that the pigment- producing chemicals in the flower are brought to the anthoxanthin stage by the gene A. When the gene A is supplemented by the gene B, this reaction proceeds to a higher stage producing the anthocyanin pigment. Thus, the magenta anthocyanin pigment may only be produced in two stages.
The first stage is completed by A and the end product is ivory (anthoxanthin). The second stage (anthocyanin) is brought about by B, Thus B is a supplementary gene having no effect by itself (i.e., without A). Flowers with neither A nor B (aabb) are white.
An exactly similar situation is met with in rat colour. Here
R—C— = black
rrC— = cream
R—cc and rrcc = albino white
This also may be explained in the same way by considering R as a supplementary gene: The development of pigments in animal skin is now known to follow the same pattern as the development of anthocyanin pigments. The black pigment melanin is the final oxidation product of a series of chemical reactions. One may suppose that C causes the basic reactions while R causes the final reactions for the complete synthesis of melanin.
Inheritance Type # 6. Inhibitory Genes (13:3 Ratio):
In some rice variety the presence of the gene P causes its leaves to be coloured deep purple. But if a gene I is present the purple colour is inhibited and the leaf becomes normal green. The I gene may be considered as epistatic over P.
I—P—, I—pp, iipp = green
iiP— = purple
Thus in a green (IIpp) x purple (iiPP) cross the F1 will be green (IiPp) and the F1 segregation will be 13 green (I—P—, I—pp and iipp): 3 purple (iiP—) as the presence of P in the absence of I only may cause the purple colour.
Similar inhibitory genes are known in many other plants and animals. For example, the purple grain colour of maize may be inhibited by an I gene and the white leghorn fowl owes its colour to the inhibition of a colour gene by an I gene.
Inheritance Type # 7. Lethal Genes:
These usually occur as gene mutations when the presence of the mutated gene I or L (more commonly the former) causes death of the organism. An example is the ‘xantha’ (yellowness) character of rice. The character develops as a mutated recessive gene 1. The mutant plant is then heterozygous with the constitution LI. The seedlings segregate as 1 LL (normal green): 2 L1 (heterozygous, i.e., segregating green): 1 11 (lethal yellow or xantha), i.e., 3 normal: 1 xantha. The xantha plants cannot develop chlorophyll and die within a week or two after germination.
Another lethal yellow type of pigmentation is found in rats. Here, yellow and black pigmentation form an allelomorphic pair where yellow is dominant but the double dominant dose of yellow is lethal so that the yellow rats exist only in the heterozygous hybrid state. If the pair of genes be signified by Y and y.
YY = lethal
Yy = yellow
yy = black
Thus if two yellow rats be crossed the offspring will show the Fs ratio of 1 YY (dies): 2 Yy (yellow): 1 yy (black), i.e., only yellow and black rats in the ratio 2:1.
Inheritance Type # 8. Complementary Genes (9:7 and 9:6:1 Ratios):
In some rice varieties, the following interaction of two pairs of genes has been noted:
A—B— = red grain
aaB—, A—bb and aabb = grey grain
Thus in a grey (AAbb) X grey (aaBB) cross, the F1 is red (A—B—) and the F2 segregation is 9 red (A—B—): 7 grey (aaB—, A—bb and aabb).
The phenomenon may be explained by the hypothesis of complementary factors. This supposes that A and B are two complementary genes which have no effects by themselves but when they combine together, the red colour is produced.
As in the case of supplementary factors, one may suppose in this case that the production of the red pigment requires two stages brought about independently by the factors A and B, But the effect of any one factor is not outwardly manifested.
Similar instances are known in many other cases. Bateson obtained hybrid pea plants with red flowers on crossing two white-flowered plants because of similar gene action:
C—P— = purple flower
ccP—, C—pp and ccpp =white flower
Thus, Bateson obtained red F2 plants (CcPp) from a white (CCpp) x white (ccPP) cross.
There are other types of complementary gene which have some expression even singly.
For example, the shape of the common gourd (Cucurbita pepo—also called squash in some countries) is controlled by two pairs of genes:
A—bb and aaB— = rounded or spherical
A—B— = discoid
aabb = elongated
Fig. 852 shows the segregation resulting after crossing two round type plants of composition AAbb and aaBB. The F2 segregation is 9 discoid: 6 rounded: 1 elongated and not 9: 7 as we are concerned here with three phenotypes and not two.
Inheritance Type # 9. Supplementary and Complementary Genes (27: 37 Ratio):
We may now consider an interesting case of gene interaction involving three pairs of genes instead of two. Emerson found that seed-coat colour in maize is controlled by three pairs of genes. C and R are two complementary genes which have no action independently but together cause a brownish green colouration of the stem while the grain remains colourless.
If the gene A is added to C and R some plant parts as well as the grain-coat become purple by the development of anthocyanin. We may suppose that the anthocyanin pigment is developed in two stages: C and R are complementary in forming the brown pigment and when A is supplemented the full anthocyanin pigment is developed. A is, therefore, a supplementary gene.
A-B-C- = purple seed
A-B-cc, A-bbC-, aaB-C-, aabbC-,
aaB-cc, A-bbcc, aabbcc = colourless seed
Thus, a cross, purple seed (AABBCC) X colourless-seed (aabbcc) will produce a purple F1 plant (AaBbCc) while the F2 ratio will be 27 coloured; 37 colourless. If the cross involves other genotypes, other ratios are expected, viz., AABBcc (colourless) X aabbcc (colourless) should show the complementary F2 ratio of 9 coloured: 7 colourless.
Many of the interactions illustrated above explain some instances of reversion or atavism which were problems to the classical geneticists. Thus, a red pea flower from the cross between two whites, purple-seeded maize from the cross between two colourless ones, a walnut fowl comb from the cross between a pea and rose type might well be taken as cases of reversion or atavism, i.e., sudden reappearance of some lost ancient character, but, these phenomena are now well explained by gene interaction.
Inheritance Type # 10. Multiple Factors:
A large number of instances have been found where characters are caused not by single allelomorphic pairs but by a large number of such pairs of genes. This is specially true in the case of quantitative characters, i.e., where the characters may be measured in units as in the case of height, weight, etc. Usually, these genes have additive or cumulative effects. This is also called polymery. Hybrid plants involving such multiple genes are also polyhybrids.
The simplest case of multiple factors is that involving only two pairs of genes. In wheat each of two genes Ct (of the allelomorphic pair C1c1) and C2 (from C2c2) may cause light red colour but C, and C3 together cause deep red. Thus, in a suitable cross, the F2 segregation is 9 deep red (C1-C2-) : 6 light red (C1-c2c2 and c1c1C2-) : 1 white (c1c1c2C2).
It can be easily realised that this is a case of complementary action of two genes as in the case of Cucurbita pepo shape but this is considered under multiple factors as this is a case of quantitative inheritance involving measurement, viz., the intensity of colouring.
The above is a case of only two factors having an additive effect. There may be more factors having such cumulative effects. For example, there are cases of plant height being controlled by a number of genes each of which add say, 2 cms to the height.
When a large number of such factor pairs are involved, there will be a large number of segregated phenotypes and there will be such gradation among them that sometimes it may be impossible to distinguish between them.
Multiple factors, which are caused by a large number of factors, each having a small effect, may also be affected by minor polygenes (explained below) and even by the environment, e.g., plant size, controlled by a large number of genes, may be affected by the availability of nutrition; human skin colour, also controlled by a large number of genes, may also be affected by exposure to sunlight.
Inheritance Type # 11. Multiplication of Factors:
In some cases there may be two or more allelomorphic pairs causing the same character. Such genes have no additive effect.
In the case of a rice variety, the awn is caused either by the gene A1 (from A1a1) or by its duplicate gene A2 (from A2a2). Thus, in a suitable awned Xawnless cross the F1 will be awned while the F2 segregation will be 15 awned (A1-A2-, A1-a2 a2 and a1 a1 A2): 1 awnless (a1 a1 a2 a2).
Similar duplicate genes are known for rice apiculus (lemma and palea tips) colour where the presence of two pairs of genes, Ap1ap1 and Ap2ap2, caused a 15 coloured; 1 green F2 ratio. Duplicate genes point out the presence of duplicate chromosomes (present in polyploids—explained in connection with mutation) and, therefore, suggest a polyploid origin of rice.
Instances of triplicate genes (also suggesting polyploidy) are also known. Here the expected F2 ratio is 63: 1.
Inheritance Type # 12. Pleiotropy:
It has now come to light, specially in studies with Drosophila, that one gene may control two or more different characters simultaneously. This is called pleiotropy. For example, anthocyanin genes is plants cause colouration in different parts; the ‘vestigial wing’ gene in Drosophila also causes development of bristles.
Inheritance Type # 13. Modifying Genes and Polygenes:
Modifying genes are minor genes which modify other characters. Polygenes are similar minor genes which act together in modifying quantitative characters. Ordinarily, the individual effect of such genes is very insignificant and may even go undetected. But, in the long run, they may play an important role in the evolution of new species.
Inheritance Type # 14. Cytoplasmic Inheritance:
In this book, it has hitherto been taken for granted that hereditary characters are controlled by genes located in the chromosomes. But in the formation of a zygote a big amount of egg cytoplasm takes part. This cytoplasm (along with the plastids contained in it) may cause the transmission of certain maternal characters just as it may transmit any virus disease particle contained in it. Some chlorophyll deficiencies or variegations are known to be inherited in this way. Correns showed long ago that the albomaculatus type leaf variegation in Mirabilis jalapa is inherited in this way.
Recently, some instances have been found where cytoplasm seems to play a more complicated part in inheritance. For example, in the unicellular animal Paramecium, Sonneborn has shown that the inheritance of a certain character (‘killer’ character) is partly controlled by some particles contained in the cytoplasm.
The ‘killer’ Paramecium contains a gene K in its nucleus and also particles called kappa particles in its cytoplasm which are capable of regeneration. The kappa particles are made of DNA and they secrete a substance called paramecin which kills all ‘sensitive’ Paramecia. i.e., those possessing kk genes.
The ‘killer’ character is ineffective unless both the K gene and the kappa particles are present. These kappa particles must come through the physical inheritance of some cytoplasm containing these. Such heredity controlling particles in the cytoplasm are termed plasmagenes (sometimes also called cytogenes) as opposed to the common nuclear genes.
Other instances of inheritance through maternal Cytoplasm are well-known. The dextral and sinistral coiling of the snail Limnaea is determined not by its own genes but by the genes of its mother. Wettstein noted that when the moss Funaria hygrometrica is crossed with F. mediterranea the appearance of the F1 plant is like its mother whichever way the cross was made.