Chromosomes determine the predisposition for hair colour
Chromosomes come in pairs. Which gene (mother or father) they will use depends on whether their genes are dominant (F) or recessive (f). We make cross diagrams to see the distribution. If you have two folding genes (ff) or two dominant genes in the chromosome, they are called homozygous – if they have one dominant and one folding, they are called heterozygous gene pairs.
Freckles have dominant predisposition (F). A father who has F/F (he has freckles) and a mother who has F/F will have children with freckles. If we make a cross diagram we see 4 pieces F/f.
Example: if both mother and father have F/f, 3 out of 4 children will get freckles while the fourth child will not get freckles. In a cross diagram, we see that there is a 75% chance that their children will have freckles.
A predisposition can be due to several genes and then there can be many variations of colours, not just black or white. Environment also plays a role. In the skin there are a lot of colours and the sun makes some people more likely to have more melanin and their skin becomes darker.
Damage to the sex chromosome X causes haemophilia. Girls have two X chromosomes, which means that there will always be a healthy chromosome. But boys have XY chromosomes and if X is damaged, the boy gets haemophilia. This is why girls almost never suffer from this.
Colour blindness comes from the the Y chromosome, which is why girls never become colour blind.
Genes are influenced by both inheritance and environment. Allergy and type 1 diabetes are examples of environmental impacts.