Answers to questions

blueyeqQ. If each parent has one copy of the dominant allele A for brown eyes and one copy of the recessive allele a for blue eyes, what’s the likelihood that the child will have blue eyes?

A. 25% because the only way that the child will have blue eyes is if they have two copies of the recessive allele. But, as all things are in biology, eye color isn’t exactly that simple.  We now know that over 15 different genes contribute to determining eye color, not just one!  (back to post)

Q.The 4 nucleotides that make up RNA, have 64 possible combinations, so why do they only make only 20 amino acids?

A. RNA is made out of the nucleotides A, U, G, and C.  If you combine them in every possible combination of 3 nucleotides (or codons), there are 64 combinations.  But there are only 20 different amino acids plus 3 codons that tell the protein-making machinery to stop.  How does this work?  There is redundancy.  Most amino acids (shown in blue below) are coded for by multiple codons.  Typically, the first two nucelotides of the codon are the same, but the third nucleotide may be one of several nucleotides and still code for the same amino acid.  Let’s take the example of Thr (stands for threonine).   The codons ACU, ACC, ACA, and ACG all code for Thr.  That third nucleotide in the codon is what’s called the “wobble” position, because it can wobble between a few nucleotide and still make the same amino acid.  This also means that mutations in the wobble position may not have an effect on the protein at all – because it will make the same amino acid!!

codon_usage