Sex chromosomes
Definition
Sex chromosomes
The pair of chromosomes that determine biological sex: XX in females and XY in males.
- One of the 232323 pairs of chromosomes is the pair of sex chromosomes.
- Females have two X chromosomes (XXXXXX).
- Males have one X and one Y chromosome (XYXYXY).
- The other 222222 pairs are the same type in both sexes.
Note
The Y chromosome carries the genes that lead to male development.
Hint
Females are XXXXXX and males are XYXYXY: the male has the Y.
How sex is determined
Definition
Punnett square
A grid used to predict the possible genotypes and phenotypes of offspring from a genetic cross.
- All egg cells carry one X chromosome.
- Half the sperm carry an X and half carry a Y.
- If an X sperm fertilises the egg, the child is XXXXXX (female).
- If a Y sperm fertilises the egg, the child is XYXYXY (male).
- So there is a 1:11:11:1 ratio, giving about a 50%50\%50% chance of each sex.
Example
Worked cross: determining sex
- Parental sex chromosomes: mother XXXXXX and father XYXYXY.
- Gametes: every egg carries XXX; half the sperm carry XXX and half carry YYY.
- Possible offspring: XXXXXX, XXXXXX, XYXYXY and XYXYXY.
- Outcome: 222 female : 222 male, which simplifies to a 1:11:11:1 ratio.
- Probability: each fertilisation has a 12\dfrac{1}{2}21 or 50%50\%50% chance of producing an XXXXXX offspring and the same chance of producing an XYXYXY offspring.
Common Mistake
Do not say the mother decides the sex: the sperm carries the X or Y that decides it.
Self review
- Which sex chromosomes do females and males have?
- Which parent's gamete decides the sex of the child?
- What is the ratio of male to female offspring?
- What is the chance that a child is male?
