Quiz 7: Genetic Transfer and Mapping in Bacteria
Epigenetic inheritance is a process, by which, heritable modifications are observed in a gene function, but not by the change in the DNA (Deoxyribonucleic acid) base sequence. The modification of gene function without altering the DNA sequence tells about poor gene control. Generally, the function of a gene is depended on its genealogical historic patters. For example: Parental imprinting and Paramutation. 1. Paramutation is a phenomenon, by which the function of a gene for a normal allele is reduced than the heterozygous allele. A heterozygous gene contains a paramutagenic allele, which obstructs the normal function of a gene. This phenomenon is observed in plant species. 2. Parental imprinting is a phenomenon, by which the function of a gene is dependent on inheritance of the alleles, whether they are inherited from paternal or maternal genome. This phenomenon is observed in mammalian autosomal genes.
Alfred Sturtevant conducted an experiment in snails to explain maternal effect. In snails, the coiling pattern can be dextral, that is, right-handed, or sinistral, that is, left-handed. The dextral pattern is dominant to sinistral pattern. He first crossed a true-breeding dextral female (DD) to a sinistral male (dd). The F 1 offspring were all dextral. When he carried out the reciprocal cross with sinistral female (dd) and dextral male (DD), all the offspring were sinistral. From the results obtained in F 2 and F 3 generation, Sturtevant showed that coiling pattern in snails showed maternal effect. When the Dd heterozygotes of F 1 generation were crossed to each other, the F 2 generation showed dextral pattern. That is, the F 2 generation showed only the maternal characteristics. This is unlike the Mendelian pattern of inheritance, which predicts a 3:1 phenotypic ratio for F 2 generation. The F 1 mother had the genotype Dd and showed dextral pattern of coiling. Hence, all the offspring had dextral coiling, which is the same as the maternal parent. This was true even in the case of the offspring whose genotype was dd. The F 3 generation obtained by crossing the members of F 2 generation had a ratio of 3:1 dextral to sinistral pattern. Again this resembled the genotypes of the maternal parents of the F 2 generation. As the genotypic ratio of the F 2 mothers was 1DD: 2Dd: 1dd, three dextral offspring were produced from DD and Dd females, and one sinistral offspring from the dd female. This clearly explains the maternal effect of inheritance of snail-coiling pattern.
The inheritance of some nuclear genes is such that the phenotype of the offspring is directly determined by the genotype of the maternal parent. The genotype of the mother affects the phenotype of the offspring irrespective of the genotype of the father and the offspring itself. When the maternal effect exists in the recessive form, its detection is possible only when a phenotypically normal mother produces offspring with that are abnormal. A heterozygous female carrying a recessive maternal effect allele can be identified in the following way: The heterozygous female can be crossed with a heterozygous male. All the F 1 offspring have normal phenotypes. However, 1/4 th of the offspring will be homozygous for the recessive maternal effect gene. But these flies would be phenotypically normal because they would have received the gene product of the normal allele from the mother. When the F 1 offspring are crossed with each other, those females which are homozygous will not be able to produce any normal offspring. This indicates that the parental female fly was heterozygous for the recessive maternal effect allele. The homozygous female fly cannot be produce normal offspring and so cannot be maintained for many generations. The only way to maintain them in the laboratory would be to produce them by crossing heterozygous females to heterozygous or homozygous males. This produces homozygous females which can again be identified due to their lack of production of viable offspring.