The DNA of organisms is found in the nucleus of their cells in the form of chromosomes. Segments of chromosomes that code for specific information, for example eye colour, are called genes.
Most characteristics of organisms are coded for by two genes, one inherited by the mother and one by the father. These two genes are called alleles and their instructions may be slightly different, even if they code for the same characteristic, e.g. a child may inherit one gene for blue eyes and one for brown eyes by the mother and father, respectively. It is worth reminding you here that we inherit half of our DNA from our father and half from our mother. This passes onto the offspring in the gametes (sex cells, sperm and egg).
Genes are symbolised as letters, any letters. In your exams, you will be given combinations of two letters for the characteristic of an organism. For example, the gene for brown eyes is B and for blue eyes, it is b (Be careful: any letters could be used, so do not be confused by the letter in an exam. Pay attention to what information is given to you). The small letter (b in our case) is recessive, which means that two of those (one from the mother and one from the father) are needed to have blue eyes; the combination would be bb. The person that has this combination is called homozygous, because he/she has two same alleles. A person with BB is also homozygous, but they have brown eyes. The capital B gene is dominant, which means it overtakes the b if they are put together, so the combination Bb would code for brown eyes, even if there is a small b there. A person with two different alleles is called heterozygous.
Punnett squares are used to predict what characteristics are inherited by different combinations of alleles:
The Punnett square above shows what happens when a heterozygous father with brown eyes crosses with a heterozygous mother with brown eyes. The possible offspring are:
- BB, homozygous with brown eyes 25% probability
- Bb, heterozygous with brown eyes 50% probability
- bb, homozygous with blue eyes 25% probability
Overall, there is a 25% probability that their child will have blue eyes and 75% probability that it will have brown eyes. The combination of alleles, e.g. Bb is called the genotype. The way the combination makes the organism look like is called the phenotype, i.e. Bb codes for brown eyes.
The gene coding for sickle cell disease is d. A couple both with Dd genotypes had a number of children.
(a) What is the probability that they had children with sickle cell disease? Use a genetic diagram or Punnett square to aid your explanation.