The genes which are located on X chromosome (sex chromosome), are called sex linked genes. These genes show criss-cross inheritance.

When a male has a defective sex linked gene located on X chromosome he transmits the defective X chromosome to his daughter only during reproduction. The female who has this gene transmits it to her son and daughter both in equal probability. So the male passes on his recessive sex linked trait to 50% of his grandsons through his daughter.

The sex linked trait being recessive is not expressed in female but is expressed in males. Therefore males suffer from the genetic defect due to the presence of faulty gene on the single X-chromosome while females are only carriers of these defective genes as they have the other X which masks the effet of faulty gene. The trait shows up in females only both X chromosomes from mother and father have faulty gene.

This type of inheritance of recessive sex linked character from father to daughter and then from the daughter to her sons is known as criss-cross inheritance or sex linked or X-linked inheritance.

Criss Cross Inheritance in Humans

Red green colour blindness and Haemophilia are examples of sex linked inheritance in humans. The defective gene is located on X chromosome. Thus a single defective gene causes disease in male while two defective genes (homozygous condition) only can cause the disease in female.

Females in heterozygous condition are apparently normal but actually the carriers of the disease. Carrier females pass this defective gene to 50% of her sons. The disease is expressed only in males because male does not have the partners of the genes on Y Chromosome to mask the effect of the faulty gene.