Sex determination
Inheritance of biological sex
Diploid human body cells have 23 pairs of The structure made of DNA that codes for all the characteristics of an organism. in the nucleus. Twenty two pairs are known as Chromosomes that control the characteristics of an organism but do not determine its sex., and control characteristics, but the 23rd pair carries genes that determine sex and are known as the sex chromosomes.
- males have two different sex chromosomes, X Y
- females have two X chromosomes, XX.
Chromosomes from a male
These diagrams are known as human karyotypes, and show all the chromosomes aligned in pairs.
The blue box shows the two sex chromosomes - these are different sizes, therefore an X (larger chromosome) and a Y (smaller one).
Chromosomes from a female
The red box shows the two sex chromosomes 鈥 these are the same size, both two X larger chromosomes.
Genetic diagrams
A genetic diagram, like a Punnett square, shows how Different forms of the same gene. may combine in A fertilised egg cell.. The diagram below shows how sex is inherited.
The two possible combinations are:
- an X The structure made of DNA that codes for all the characteristics of an organism. from the mother and an X chromosome from the father - producing a girl (female phenotype from the XX The alleles that an organism has for a particular characteristic, usually written as letters.)
- an X chromosome from the mother and a Y chromosome from the father - producing a boy (male phenotype from the XY phenotype).
All ova have an X chromosome. Half of sperm have an X chromosome and half have a Y.
Because of this the ratio of female to male offspring is 1:1. On average, half of the offspring will be girls and half will be boys. This can also be converted into a The extent to which something is likely to be the case. of 50% (XX) and 50% (XY).