This is optional extension work on Genetics for when you have finished the Punnet Squares worksheets.
1. Each human body cell (as opposed to gametes) has 23 pairs of homologous chromosomes. One chromosome from each cell is placed in each gamete (sperm or egg), so each one has a random set of 23 chromosomes. When a sperm and an egg combine to form a zygote, the zygote has a full set of (46) chromosomes, so it can grow into a baby.
A. How many different gametes can one person produce?
B. How many different children could one couple produce (if they were fertile for long enough)?
Note: During meiosis, individual genes are exchanged between chromosomes before they separate into different gametes. This results (crossing over) in an even greater (almost infinite) number of possible offspring. Hair colour and eye colour are located on the same chromosome, so that’s why most people who have blonde hair have blue eyes and vice versa, however occasionally this crossing over results in people who have blue eyes and dark hair (or dark eyes and fair hair).
Spider mites have different numbers of chromosomes, (from 4 to 14) however let’s imagine a particular spider mite (male and female) has 4 chromosomes in total. Each pair of chromosomes is shown as a different colour. Clearly four different gametes are possible, so sixteen different offpsring are possible.
Use the information above to explain why it’s unlikely that couples have two children the same (except identical twins). Suggest a reason crossing over might occur, given that it appears unnecessary for the purpose of ensuring variation among offspring.
As another exercise, determine the phenotype ratio of two herterozygous parents for two traits, for example two parents who are right-handed (and carry the gene for left handedness), and have black hair (and carry the gene for blonde hair) have many children. Determine the ratio of
black+right : black+left : blonde+right : blonde+left.