Divergence From Mendelism or Neomendelism
We have already discussed about the Mendel's laws in previous articles. in such events whose results deviate from Mendel's laws , called Neomendelism. Following are the divergence from Mendel's laws -
- Incomplete Dominance Or Intermediate Inheretance
- Codominance
- Lethal Gene
- Multiple Allelism
- Pleiotropic Gene
- Complementary Gene
- Supplementary Gene
- Pseudoallelic Gene or Duplicated Gene
- Epistasis
- Dominant epistasis
- Recessive Epistasis
1 . Incomplete Dominance Or Intermediate Inheretance
When hybridization is done between one dominant and one recessive parent, none of the traits obtained in the first generation (F 1) progeny are dominant, but the traits obtained in the offspring of the F 1 generation are those of both the parents. This phenomenon is called incomplete dominance or intermediate inheritance. It was discovered by Carl Correns in 1903.
Example – 1: By crossing two varieties of Blue Andalusian chicken (homozygous state) one black and one white, we get blue colored chickens in the first generation. As a result of self-fertilization between the offspring of the first generation, we get one black, two blue and one white cock in the F 2 generation. Hence their ratio is 1 : 2 : 1 in the F 2 generation.
Example – 2: On crossing two homozygous varieties of mirabilis jalapa or rose bamboo, one with pure red flowers and the other with pure white flowers, in the first generation (F 1) we get plants with pink colored flowers with different characteristics from the parents. occur, which are in the heterozygous state. As a result of self-pollination between these plants, in the second generation (F 2) we get plants with red, pink and white flowers, which have a ratio of 1:2:1.
Ashoka's Dhamma : Subjects Mentionedin Pillar Edicts2 . Codominance
When the opposing genes controlling a trait are equally expressed in the next generation and neither gene can reduce the effect of each other, then this type of phenomenon is called codominance.
Usage
The skin color of short horned animals is controlled by homozygous dominant and homozygous recessive genes. When there is hybridization between them, then in the next generation we get the offspring of starry cover color, that is, in the offspring of the next generation, white and black spots are found in the skin. Which shows that both the symptoms have appeared together in the child.
3 . Lethal Gene
When certain genes are present in the homozygous state, the embryo dies during pregnancy or in plants such homozygotes do not germinate or the plants die as soon as they germinate. Such genes are called lethal genes. In this the ratio is 2 : 1
Example: In the domestic rat, Mus musculus, the yellow color is due to a gene Y, which is dominant on all colors. This gene is found in heterozygous state in rat. When two heterozygous species of mice are crossed, we get only a ratio of 2:1 in the next generation, which is less than the normal ratio of 3:1. The reason for this is the presence of YY gene in a child in the embryonic stage due to the death of the child in the embryonic stage itself. Thus the YY gene is recessive lethal in mice. This Y gene is called lethal gene.
4 . Multiple Allelism
More than two different genes can occur at the same location on a chromosome, but not exactly at the same time. “When a mutation in a gene has the potential to produce more than two alternative characters, the phenomenon is called polyallelicity.”
Sickle Cell Anaeamia : Genic Mendelian Abnormalities5 . Pleiotropic Gene
Some genes are capable of affecting more than one character, they are called polymorphic genes.
Example : The gene that determines the white eyes in Drosophila fly affects the sperm-storing organ and other structures in the female.
6 . Complementary Gene
When two pairs of genes that produce a trait are located on different chromosomes, but either pair of genes is unable to express that trait separately, then such genes are called complementary genes. In complementary genes, one pair of genes act as the actual determinant while the other pair of genes act as the starter or anchor.
Example : If two varieties of sweet pea (Lathyrus odoratus) are crossed with white flowers, then plants with all colored flowers are obtained in the F1 generation, but as a result of self-pollination in the first generation, plants with colored and white flowers are obtained. A ratio of 9 : 7 is found in the middle , which is a modified form of the ratio 9 : 3 : 3 : 1 obtained from the dihybrid cross .
Bateson and Punnett reported that in one of these varieties, a gene (C) for a colorless substance chromozone is found, while in the other variety, a gene (E) is found for an enzyme, which makes the chromozone purple. When these two genes come together, only colored flowers are obtained.
Surrogacy , Surrogacy Regulation Bill
7 . Supplementary Gene
The interaction of two independent genes in such a way that the typical character in the progeny obtained from them is different from them, then such genes are called complementary genes. In this the ratio is 9 : 3 : 4.Usage
Coat color of guinea pig and rat is controlled by complementary genes. Aguti is a wild-type coat colour, with pigment bands on brown hairs. Another pair of genes (Cc) are involved in the synthesis of melanin pigment. But in the homozygous condition (Cc), this gene inhibits the synthesis of melanin, due to which neither agouti nor black color can appear and the offspring containing the Cc gene are pigmentless (albino).
When a cross is made between a double dominant agouti (BBCC) and a double recessive pigmentless (bbcc), all guinea pig agoutis are obtained in the F 1 generation. But in the F 2 generation, the ratio of 9 : 3 : 4 is obtained between the agouti black and pigmentless progeny.
Usage
8. Pseudoallelic Gene or Duplicated Gene
9. Epistasis
When a gene located at one locus in a chromosome inhibits the expression of a gene located at another locus in the same chromosome, this action is called dominance. In this, the gene that inhibits the expression of another gene is called a dominant gene or epistatic gene or repressor gene, while the gene whose expression is affected is called a recessive gene or hypostatic gene.
It is mainly of two types -
- Dominant Epistasis
- Recessive Epistasis
( i ) Dominant Epistasis
When the dominant gene suppresses the expression of the other gene in its dominant state, then this action is called dominant dominance.
Example: In white color Langhorne and white color Plymouth Rank chicken varieties, gene C is dominant and gene c is recessive. Gene C reveals color. Apart from this, there is a dominant dominant gene I and a recessive gene i, which prevents their colors from appearing.
When a cross is made between a white Langhorne ( CCII ) and a white Plymouth ( ccii ) , all the offspring in the F 1 generation are white . But after self-fertilization, white and colored offspring are obtained in the F 2 generation, whose ratio is 13: 3.
( ii ) Recessive Epistasis
When the dominant gene in its recessive state prevents the expression of another gene from appearing, then this action is called recessive dominance.
Example : Gray color in rat is due to dominant gene G and black color is due to recessive gene g. The expression of genes G and g is inhibited by another gene a. Here gene a is in recessive state, but it will be able to show its effect only in homozygous state (aa).
When fertilization is done between one homozygous gray colored and one homozygous white colored mice, all the progeny in the F1 generation are obtained with gray colour. But in the F 2 generation, grey, black and white colored mice are obtained in the ratio 9 : 3 : 4 respectively.
Hybridization Experiment of Gregor John Mendel and Mendel
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