The explanation is simple, it comes from the law of gene balance. In the law of genetic balance, we know that the frequency of genotype can be inferred from the frequency of gene. The formula is simple: p squared +2pq+q squared = 1. You should have learned this, right? Where the square of p is the proportion of aa in the population, 2PQ is the proportion of heterozygotes in the population, and the square of q is the proportion of AA. Do you know where this formula comes from? Because the law of gene balance does not consider individuals, only gene frequency is considered. For example, if we put A and A in a jar for people to draw lots, where the proportion of A is P and the proportion of A is Q, and a person has to draw A twice to become aa, then according to mathematical knowledge, its probability is the square of P, and becoming Aa is the square of Q. If you want to become AA, there are two possibilities, the first time you draw A, the second time you draw A, and vice versa, so it is 2PQ.

Sex-linked inheritance is much simpler, because men only have one X, so they only need to draw once, while women have to draw twice. Therefore, if the frequency of A is still P and the frequency of A is still Q, then the incidence of male invisible diseases is the probability (Q) that A appears. Women have two X's, that is, they have two lucky draw opportunities, so the probability of women getting sick is the square of the probability of A (the square of Q). In China, the incidence of red-green color blindness is 7% for males and 0.5% for females (the theoretical value is 0.49%), which is a fact. This is just an invisible disease. If it is an obvious disease, the situation is reversed. It can also be calculated that the incidence rate of men is lower than that of women. It's not easy to write formulas here, so forget it.