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How can two brown-eyed parents have a blue-eyed baby?

Most of our characteristics, such as eye colour, are encoded by a pair of alleles. These are different forms of the same gene, one inherited from the father and one from the mother. You can get dominant and repressive alleles. Dominant alleles, such as brown-eye alleles, only need one copy of the allele in order for the characteristic to be expressed. Repressive alleles, such as blue-eye alleles, need two copies of the allele to be expressed. If a mother and father both have brown eyes but each carry the repressive blue-eye allele (i.e. they are heterozygous), there is a chance that the offspring may inherit two blue-eye alleles: one each from the mother and father. Thus the child would have blue eyes. We can use punnett squares to work out the probability of the offspring inheriting different allele combinations. 

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