Anoushka Sachdeva
Centre for Excellence in Basic Sciences, Mumbai

Best science starts with simplest questions which no one bothered to answer as they seemed too obvious.

When mammals reproduce why do they require a male and female parent?

The first thing that strikes out mind is the need to restore normal chromosomal number. But if that’s the only thing then why not 2 eggs or two sperms fuse to give rise to a healthy offsprings?

Quantity isn’t Everything

Prof. Azim Surani, a developmental biologist at the Wellcome Trust/Cancer Research UK Gurdon Institute at the University of Cambridge, demonstrated that there is a need of both male and female pronucleus. In these experiments inbred mouse strains were used to ensure that all fertilized  eggs were genetically identical.

Image Source: www.scilynk.in

This happened because quantity isn’t everything. Maternal and Paternal genomes may deliver the same DNA but they aren’t functionally equivalent. Our genes somehow remember who they come from. This is known as Parent-of-Origin effect. The zygote and its daughter cells can always distinguish between father’s and mother’s chromosomes. This is an Epigenetic Effect.

Another Experiment/Epigenetic Effect

In 1987 another experiment conducted in Prof. Surani’s lab in which an extra piece of DNA was inserted randomly in mice genome and methylation of the sequence was studied in offsprings of 7 lines of mice, in one of these 7 something odd happened. Whether the DNA sequence was methylated or not in parent it was found that if the extra piece was contributed by mother then DNA sequence was methylated in offspring and if contributed by father them it wasn’t.

It’s as if the chromosomes carry little flags, those from sperm says ‘ I’m from DAD’ and those from egg says ‘ I’m from MUM’.

DNA methylation is the fabric that these flags are made from our extra DNA was getting attached anywhere in the genome randomly and this was seen in only 1 of the 7 lines suggesting that not all genome carries the flag.

What happens in humans?

In about 1 in 1500 Human pregnancies there is only placenta and no foetus.

Abnormal placenta, covered with fluid filled grape like lumps. This structure is called a hydatidiform mole. In these pregnancies women gain more weight and undergo more sickness symptoms as these lumps release abnormally high levels of hormones. These moles must be removed after early detection as they potentially form dangerous tumours.

But why these mole are formed? These are formed when an egg which somehow has lost its nucleus gets fertilized. Either one sperm fertilizes this empty egg and copies its own genome to credit diploid condition (80% cases) or two sperms fertilize the empty egg(20% cases).

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