Hereditary properties of genes

How are the genes inherited?
People have 46 chromosomes in 23 pairs. One of the chromosomes of each pair is inherited from the mother and one from the father. 22 pairs are identical both in males and females - these are called autosomes. The remaining two chromosomes are sex chromosomes and are called X and Y.

The offspring always receives an X chromosome from the mother. Therefore, the gender of the offspring depends on the man. If the sperm provides an X chromosome, it will be a daughter and, if a Y, it will be a son. Because we have two copies of each chromosome and the chromosomes keep our genes, we have two copies of each gene that is contained in the autosomes.

Our properties are manifested depending on the combination of the genes we inherit. By some of them, we look like the mother or the father. Women have two copies of each gene on the X chromosome, while men have only one gene that is inherited from the X chromosome of their mother. Those genes which men inherit from their father on the Y chromosome contain the instructions for the development of the male sex.

How we inherit some diseases?
Many diseases occur as a result of mutations or polymorphism in certain genes. Nevertheless, polymorphism or mutations inherited only from one parent do not mean that there is a risk of a disease for a human being. We have two copies of most genes in our cells – one from the mother and the second from the father. Sometimes only one impaired copy of a gene is sufficient for an outbreak of the disease and in another case two copies are needed. In principle, there are several ways how to inherit diseases and other traits.

X chromosome-linked diseases
Because males have only one X chromosome, they have only one set of genes on the X chromosome. The diseases which are caused by mutations on the X chromosome are called X chromosome-linked diseases.

This imbalance means the differences in how males and females inherit various diseases. For example, when a female inherits one copy of the gene mutation on her X chromosome, she probably avoids the diseases since she still has a normal copy of the gene on the second X chromosome.

Most of all, it is necessary for a woman to inherit a copy of the mutated gene from both parents to create the conditions for a disease. On the other hand, if a male inherits one mutated copy of the gene on his X chromosome, the conditions for development of the disease are created. The reason for this is that he does not have a non-mutated alternative of this gene on the second X chromosome.

Therefore, it is more likely that males, more than females, have X chromosome-linked genetic disorders. Haemophilia (impairment of blood coagulation) and colour blindness are X-chromosome-linked disorders, and therefore they are much more frequent in males compared to females.