INTERESTING FACTS OF GENETICS
Dr. ANJU BATTA SEHGAL
Laws of Mendelian Genetics are always not followed by all types of genes in Genetic inheritance. Mendel the father of Genetics gave very important findings that the dominant allele (gene) will express itself in the progeny no matters whether it comes from father or mother. It does not hold good in all types of inheritance. An example of very interesting study regarding deviation of Mendel rule, is the Genetic Imprinting, The Inheritance of IGF2 gene. This gene is responsible for formation of healthy placenta, supply of nutrients, regulation of growth and metabolism in developing Foetus and regulation of Insulin like growth factors for Foetal development. Genomic imprinting is a process of silencing genes through DNA methylation. The repressed allele is methylated, while the active allele is unmethylated. This stamping process, called methylation, is a chemical reaction that attaches small molecules called methyl groups to certain segments of DNA.
In an organism like human beings two sets of chromosomes are present inside nucleus of each cell. One set of 23 chromosomes is inherited from paternal side and one set of 23 chromosomes from maternal side. Out of these 23 pairs, 22 are autosomal chromosomes which don’t play any role in sex determination but the 23rd pair is called sex chromosome, which is responsible for deciding the sex of the progeny. Each autosomal genes are represented by two copies or alleles. One from father and one from mother. As per classical Mendelian Inheritance only the dominant gene out of these two alleles will express in next generation but in case of Genetic Imprinting; paternal IGF2 (Insulin-like Growth Factor 2) gene makes the maternal IGF2 gene silent.
This silencing leads to the development of placenta in female on the command of father’s IGF2 gene. Paternal IGF2 gene, no matters dominant or recessive always makes maternal IGF2 silent. Paternal gene directs mom’s placenta to give best nutrition to the foetus, “I want my child to be fed the best”. Mom’s IGF2 also wants to feed the foetus the best but explaining that “I cannot feed the child at my own’s cost, I want to keep some nutrients in reserve for future breeding”. With the result only paternal IGF2 dominates in expression making maternal as silent. The foetus in order to survive chooses the paternal gene for growth. For the survival of the fittest theory to hold good.
Genetic Imprinting is an epigenetic process that involves DNA methylation and Histone Methylation without altering the genetic sequencing. Genes can also be partially imprinted. Partial imprinting occurs when alleles from both parents are differently expressed rather than complete expression and complete suppression of one parent’s allele. Forms of genomic imprinting have been demonstrated in fungi, plants and animals. In 2014, there were about 150 imprinted genes known in mice and about half that in humans. As of 2019, 260 imprinted genes have been reported in mice and 228 in humans.
This also includes Prader-Willi and Angelman syndromes (the first examples of genomic imprinting in humans). Prader Willi and Angelman syndromes both are caused by identical deletion of very small segment of DNA from the long arm of chromosome 15. Phenotypically both show different manifestations.
Prader Willi syndrome leads to progeny with short stature, mental retardation, small hands and feet, poor suckling leading to foetal death, but obese as they grow.
Angelman Syndromes –Child with frequent uncontrolled movements, uncontrolled laughter, large mouth and seizures.
The cause of phenotypical difference is that Prader Willi syndrome occurs in a child by inheritance of deletion of Chromosome 15 from DAD and Angelman Syndromes occur by inheritance of deleted chromosome1, from MOM.
The majority of imprinted genes in mammals have been found to have roles in the control of embryonic growth and development, including development of the placenta. Other imprinted genes are involved in post-natal development, with roles affecting suckling and metabolism.
Hypotheses on the origins of Imprinting:
A widely accepted hypothesis for the evolution of genomic imprinting is the “parental conflict hypothesis or Genetic Conflict Hypothesis”. Also known as the kinship theory of genomic imprinting, this hypothesis states that the inequality between parental genomes due to imprinting is a result of the interest of each parent in terms of the evolutionary fitness of their genes. The father‘s genes that encode for imprinting gain greater fitness through the success of the offspring, at the expense of the mother. The mother’s evolutionary imperative is often to conserve resources for her own survival while providing sufficient nourishment to current and subsequent later breeding. Accordingly, paternally expressed genes tend to be growth-promoting whereas maternally expressed genes tend to be growth-limiting.
Other conditions involving imprinting include “Beckwith-Wiedemann”, Silver-Russell syndrome, and pseudo hypoparathyroidism. Neonatal Diabetes mellitus can also involve imprinting. The “imprinted brain hypothesis” argues that unbalanced imprinting may be a cause of autism and psychosis.
(The Author is Ex Principal NSCBM Govt. PG College Hamirpur & Former Professor of Botany)
