Sex Determination and Inheritance Patterns

Sex Determination Mechanisms

Haploid-Diploid Sex Determination

In this system, the total number of chromosomes in a cell determines the sex of a new individual.

X Chromosome to Autosome Ratio

Sex is determined by the ratio between the number of X chromosomes and the total number of autosomes.

Environmental Sex Determination

In this system, individuals are affected by several environmental variables after fertilization. Generally, the most influential of these is temperature. However, other factors, such as the number of hours of daylight, nutrition, humidity, and the pH of water can also affect sex determination.

Inheritance Linked to Sex

Genes found on sex chromosomes are transmitted differently in males and females because these chromosomes differ in each sex.

Recessive Characteristics

A female heterozygous for a recessive characteristic can pass the affected allele to her offspring, even if the characteristic is not expressed in her phenotype. We say that the female carries the trait. Males, having only one allele for the characteristic, cannot be carriers.

Y-Linked Characteristics

Characteristics linked to the Y chromosome can only appear in males. In this case, all carriers show the characteristic.

Genealogical Trees

Genealogical trees allow us to track the transmission of a particular characteristic through generations. Therefore, they also allow us to analyze if illnesses or physical features that occur in families are hereditary.

Blood Groups

The ABO blood group system is a well-known example of discontinuous characteristics. The membrane of our red blood cells can have, or not have, certain carbohydrates that act as antigens when they come into contact with someone else's blood.

• A blood group: Has A antigen and its plasma produces anti-B antibodies.
• B blood group: Has B antigen and its plasma produces anti-A antibodies.
• AB blood group: Has A and B antigens and its plasma does not produce any antibodies.
• O blood group: Does not have antigens and produces anti-A and anti-B antibodies.

In humans, the ABO blood group system follows multiple allele inheritance.