Because of having X-, and Y- bearing gametes in equal numbers by males, sex is determined by the Y chromosomes. According to Mendel’s law of Segregation, the sexes should occur in equal proportions. In most of the species, the number of male and female is about equal, but this is not always. For example, in humans different sex ration occurs for different age groups.
Primary Sex Ratio: Sex ratio is the ratio of males to females in a population. The primary sex ratio is the ratio at the time of conception. It is determined from the male and female frequency in abortion’s, miscarriages and still-births along with their live birth records. Instead of expected ratio of 1:1, there is a deviation i.e. 1.6 males : 1 female. The following factors are responsible for this:
- The selective fertilization of the egg with a Y-bearing sperm due to its greater motility than the X-bearing sperm.
- The more favorable environment for the survival of the Y-bearing than the X-bearing sperm in the female ducts.
- The preference given to the Y-bearing than to the X-bearing sperm by the egg.
Secondary Sex Ratio: Secondary sex ratio is the ratio of males to females at time of birth. It is easy to, determine, but it does not accounts for disproportionate embryonic or foetal mortality.
Tertiary Sex Ratio: Tertiary sex ratio is the ratio of mature organisms that has been found in the age group around twenty years. Generally the number of males exceeds females. This trend gradually gets reversed and female population is higher after fifty years on-wards due to increased male death rate in every age category.