Why are my daughters eyes blue while mine and her fathers brown?

The inheritance of eye colour is a polygenic trait, and so is determined by multiple genes. As a starting point, brown eye colour is typically dominant over blue eye colour.

When two individuals with brown eyes (Bb) have a child, the alleles involved are inherited following 1 of 3 possibilities:

- The child receives two dominant alleles (BB): this child will have brown eyes.

- The child receives two recessive alleles (bb): this child will have blue eyes.

- The child receives one dominant and one recessive allele (Bb): this child will have brown eyes, as the dominant brown-eyed colour is expressed.

In the case of your family, both you and the father must be carriers for the recessive blue-eyed gene (Bb), and so there is a one-in-four chance of each of your children having blue eyes.