As mentioned in a previous chapter, DNA is a long molecule that contains the instruction to make a cell work. DNA is found in the nucleus, sometimes as a compact structure called a chromosome. There are 23 pairs in each body cell. The last pair are called sex chromosomes. Males have XY sex chromosomes, while Females have XX sex chromosomes.
Genes are small sections of DNA that code for a specific protein. Proteins are made up strings of amino acids and have a variety of important functions within the cell and body.
Alleles are different forms of the same genes. For example, for the gene for eye colour there can be an allele for brown eyes or blue eyes. Because 23 chromosomes are inherited from each parent, there are 2 sets of alleles for each gene. The allele that is expressed depends on which allele is recessive and which is dominant. Dominant alleles are always expressed, while recessive alleles are only expressed if the person has not inherited any dominant alleles from their parents.
A genotype is the type of alleles someone has. Someone can have a homozygous genotype, where both of their alleles are dominant or both of their alleles are recessive. Or someone can have a heterozygous genotype, where someone has one dominant allele and one recessive allele for a gene. A phenotype is how a genotype is expressed within the body. For example, a heterozygous genotype or dominant homozygous genotype for the hair colour gene are both expressed as brown hair.
Using this information we can predict the likelihood that someone’ is going to have a specific genotype and phenotype using a Punnett square.
The Punnett square below shows the likelihood of a child having brown or blue eyes. The father has brown eyes with a heterozygous genotype, marked black. The mother has blue eyes with a recessive homozygous genotype, marked red.
B = dominant brown eye allele
b = recessive blue eye allele
The Punnett square shows that there is a 50% chance the child will have either a heterozygous genotype or a recessive homozygous genotype. Therefore, there is a 50% (1:1 ratio) chance the child will have brown or blue eyes.
Genetic diagrams are another method to predict the child’s genotype. They work in exactly the same way as a Punnet square but are slightly messier.
The genetic diagram below shows the likelihood that a child is going to inherit the genetic disorder polydactyly. Polydactyly is caused by the dominant allele. Both parents have polydactyly and a heterozygous genotype. The father is marked in black, while the mother is marked in red.
The diagram shows there is a 50% chance the child will have a heterozygous genotype, a 25% chance the child will have a dominant homozygous genotype and a 25% chance the child will have a recessive homozygous genotype in the ratio 2:1:1. However, there is a 75% chance the child will express the disorder polydactyly in the ratio 3:1. This is because the two heterozygous genotypes and the dominant homozygous genotype both have at least one dominant allele, and it is the dominant allele that causes this disorder.