X and Y Chromosomes

• There are 23 pairs of chromosomes in every human body cell
• 21 of these 23 pairs are matched pairs of chromosomes that just control characteristics
• The 23rd pair


  Labelled XX or XY, they decide your sex - whether you turn out male or female
• Males


  • Have an X and a Y chromosome: XY
  • The Y chromosome causes male characteristics
• Females


  • Have two X chromosomes: XX
  • The XX combination allows female characteristics to develop
• When making sperm


  1. The X and Y chromosomes are drawn apart in the first division in meiosis

  2. There's a 50% chance each sperm cell gets an X chromosome and a 50% chance it gets a Y chromosome
• When making eggs


  The original cell has two X-chromosomes, so all the eggs have one X-chromosome
• When crossing together different genes or chromosomes

  You can use a genetic diagram to show all the possible genetic outcomes
• Genetic diagrams


  • Also called Punnett squares
  • Show the possible combinations of gametes
• There are two XX results and two XY results, so there's the same probability of getting a boy or a girl
• The 50:50 ratio is only a probability at each pregnancy
• Another type of genetic diagram

  • Shows the possible gametes that are formed and the criss-cross lines show all the possible ways the X and Y chromosomes could combine