Sex chromosome: One of the 23rd pair of human chromosomes that determines the biological sex of an individual. Females have two X chromosomes (XX); males have one X and one Y chromosome (XY).
Autosome: Any chromosome that is not a sex chromosome. Humans have 22 pairs of autosomes (44 chromosomes), which carry genes for non-sex-related characteristics.
X chromosome: A sex chromosome present in both males and females. Females have two X chromosomes; males have one. The X chromosome is much larger than the Y chromosome and carries many genes unrelated to sex determination.
Y chromosome: A sex chromosome present only in males. It is much smaller than the X chromosome and carries fewer genes, including the SRY gene that triggers male sex development.
Sex-linked disorder: A genetic condition caused by an allele on a sex chromosome (usually the X chromosome). Because males have only one X chromosome, they are more likely to be affected by X-linked recessive conditions than females, who have two X chromosomes.
Carrier (sex-linked): A female who is heterozygous for an X-linked recessive allele (X^A X^a). She does not show the condition because her second X chromosome carries the dominant allele, but she can pass the recessive allele to her children.
This definitions covers Key Definitions within Sex Determination for GCSE Biology. Sex chromosomes and sex determination mechanisms It is section 5 of 11 in this topic. Make sure you can use the exact wording confidently, because definition marks are often lost through vague language.
Practice questions for Sex Determination
What determines the sex of a human baby?
Use a genetic diagram to show how sex is inherited in humans.
Quick recall flashcards
How do you remember XX = female and XY = male?
XX: the two X's look like chromosomes that pair up perfectly โ females have a matching pair. XY: the Y looks like a little man standing with arms out โ males have the odd one out. 'Why are boys different? Because of Y!'
What sex chromosomes do females and males have?
Females: XX (two X chromosomes). Males: XY (one X and one Y chromosome). These are the 23rd pair of chromosomes in humans.