The genetic fault that usually causes colour vision deficiency is passed on in what's known as an X-linked inheritance pattern.
This means:
1) it mainly affects boys, but can affect girls in some cases
2) girls are usually carriers of the genetic fault – this means they can pass it on to their children, but do not have a colour vision deficiency themselves
3) it's usually passed on by a mother to her son – the mother will often be unaffected as she'll normally just be a carrier of the genetic fault
4) fathers with a colour vision deficiency will not have children with the problem unless their partner is a carrier of the genetic fault
5) it can often skip a generation – for example, it may affect a grandfather and their grandson
6) girls are only affected if their father has a colour vision deficiency and their mother is a carrier of the genetic fault
Water-borne diseases, including cholera, typhoid, and dysentery, are caused by drinking water containing infectious viruses or bacteria, which often come from human or animal waste. Water-washed diseases, such as skin and eye infections, are caused by lack of clean water for washing.
When atoms in a covalent bond share electrons unequally (one atoms pulls more than the other), the bond is said to be a polar bond is formed when electrons are unequally shared between two atoms. Polar covalent bonding occurs because one atom has a stronger affinity for electrons than the other (yet not enough to pull the electrons away completely and form an ion).