What is sex determination?
- A biological system that determines the development of sexual characteristics in an organism is called Sex determination. Punnett square is used to predict the outcome of the offspring of two
- In the Punnett square, the unique genotype of two different individuals is
Punnett square
- Humans require a male parent and a female parent to give rise to a new
- The human germ cells do not have a pair of sex chromosomes. Each child receives one copy from the mother and one from the father. They together make up a pair and thus the natural pair of the chromosomes in a cell is
XX chromosome pair
- All females have a perfect pair of sex chromosomes in all their cells. It is called the XX chromosome pair. Each chromosome is named with the letter ‘X’. Both these together make up the 23rd pair of
- The male sex chromosome is also made up of two chromosomes: ‘X’ chromosome and ‘Y’
chromosome.
Sex chromosomes
- The 23rd pair of chromosomes of the female parent is represented as ‘XX’ while the 23rd
pair of chromosomes of the male parent is represented as ‘XY’.
- The egg cell and the sperm cell are sex cells and do not have 23 pairs, that is, 46 chromosomes. They have half the number of chromosomes, one from each pair of autosomes and sex
Sex chromosomes
Who decides the sex of the child?
- In the female egg cell, there are two ‘X’ chromosomes. In the male sperm, there can be an ‘X’ chromosome in the 23rd chromosome place or a ‘Y’
- It depends on the sperm genotype which will fertilise the egg
- If the cell containing the ‘Y’ chromosome fertilises the egg cell, the resultant offspring will be a male with ‘XY’
- If the sperm containing an ‘X’ chromosome fertilises the egg, it will result in a female child
with ‘XX’ combination.
- So, the father decides the sex of the
Male and female chromosomes
How is Punnett’s square used in sex determination?
- When fertilisation takes place, there is some probability of the outcome of the offspring’s
biological sex. The Punnett square helps with the outcome.
- In the process of fertilization of two genotypes, there are four possible outcomes. In the leftmost column, the genotype of the female parent – X’ and ‘X’ – is In the topmost row, the genotype of the male parent – ‘X’ and ‘Y’- is written.
The different possible combinations these four chromosomes could display are:
| X | Y | |
| X | XX | XY |
| X | XX | XY |
- Half the offspring will be females who carry the ‘XX’ genotype while the other half will be males who show the ‘XY’
- The Punnett square enables us to see the various possibilities of having either a male or a female offspring, but the pairing of chromosomes is random so the biological sex of the child cannot be
