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A-Z of thought: Parthenogenesis

Denis Alexander

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Making babies involves a male and  female. So it may be surprising to learn that thousands of animal species manage without males: their eggs develop without fertilization. They practice parthenogenesis, literally virgin birth. In most animals (and plants), adults have two sets of chromosomes - one each from father and mother. In making sperm or egg cells, special cell division  halves the number of chromosomes. Fertilization restores the double set. In animals that reproduce parthenogenetically, though, like honey-bees, ants or greenfly, eggs develop as a result of a physical or chemical stimulus, rather than the injection of the male chromosome. Offspring have only half the usual adult chromosome number.

Does parthenogenesis ever happen in mammals? Dolly - the sheep born when the nucleus (the gene-bearing part of a cell) of an adult ewe was inserted into an egg cell from which the nucleus had been removed - was a parthenogenetic lamb. But she resulted from a completely different mechanism to that occurring naturally in insects, so she had a double set of her mother's chromosomes. Such procedures have been used in other species, and there are enormous economic gains from being able to produce high milking cows or sheep which secrete a useful drug in their milk.

There seems no scientific reason why humans could not be 'made' in a similar way. But Jesus was not the result of a laboratory operation. Luke tells us that the Holy Spirit 'overshadowed' a young Jewish woman, an expression more suggestive of the Transfiguration than of conventional intercourse. Many doubt this particular virgin birth. Leaving aside the 'Dolly' sort of procedure, a major stumbling block is that a parthenogenetic offspring is inevitably the same sex as the parent. Where did Jesus get his male-determining Y-chromosome from?
Ironically, modern genetics can suggest possible answers. Studies of the genetic code show that the two sex chromosomes (X and Y) share a common ancestor: the Y is an X which has lost most of its genes but gained testis producing genes. Jesus's conception was miraculous. Perhaps he was a virgin birth in which God mediated very rapid change in an X-chromosome. Or perhaps Mary had male sex chromosomes but a mutation which prevented her responding to male hormones - a condition found in around 1 in every 100,000 people. If this gene then changed to its 'normal' form, she might have had a male child.

These speculations are unimportant. We know nothing about Jesus's genetics. It was important for Jesus to have both human and divine parentage. But we can affirm that a male child born to a virgin is not absolutely impossible.

RJ Berry