Which of the two chromosomes in a pair is expressed?

Question

I have completely dummy question.I have 23 pairs of chromosomes in the cells 23 single coming from my mother and 23 single from my father. So how my cell choose which chromosome, mother's or father's to use to synthesize peptides?

Answer 1

This is actually a really good question! When a cell decides "hey, go transcribe this gene", both alleles are transcribed equally**. "Dominant" or "recessive" actually have little meaning at the molecular level, for this reason. This is true for autosomes, but not sex chromosomes. While females do have two X chromosomes, one of them is permanently deactivated early in development (called x-inactivation). This is done entirely at random, and all descendants of that cell will maintain the same inactive X chromosome. So, in females, if the gene is on X chromosome, the only one that can be transcribed is the allele that's not on the inactivated X chromosome. This is also what gives calico cats their distinctive fur patterning, because the fur color gene happens to be on the X chromosome. A calico cat is technically heterozygous for black and brown fur, but black patches of fur means the "brown chromosome" was inactivated, and brown patches means the "black chromosome" was inactivated. This also explains why calico cats are (almost) always female!

** It wouldn't be biology if there weren't exceptions! There are some genes that are only the paternal or maternal allele are expressed. This is called imprinting. If a gene is imprinted, one allele will be preferentially expressed -- sometimes exclusively -- but what makes this special is the allele that is repressed is entirely dependent on whether it is paternally or maternally derived.