It is possible for the parents to have a brown eyed child although the
current gene expression model for eye colour does not account for it.
The basics of this model are as follows: There are three genes
involved in eye colour determination. They have found that on these
genes the allele coding for brown eyes is dominant over the alleles
for green and blue colour; and the allele coding for green is dominant
over blue. So if a brown allele is present it dominates over
everything else and the child will have brown eyes. If there is no
brown but there is a green then the child will have green eyes. The
child then can only have blue eyes if all 6 alleles (2 for each gene)
are blue. So the parents, who have blue and green eyes,would only have
blue and green alleles present which means they should only produce
children with blue and green eyes. This unfortunately does not hold,
as it has been seen that blue eyed parents can produce children with
brown eyes. This model also doesn't account for why some people have
blue/green, hazel and grey eyes. So its safe to say that either 1)
there are other genes involved in determining eye colour or 2) a
mutation arose at some point, during pregnancy,which altered the level
of melanin production in the eye (heaps of melanin equals brown eyes,
not alot equals blue). |
