X & Y Chromosomes

• Chromosomes control whether you're male or female
• There are 23 pairs of chromosomes in every human body cell. Of these 22 are matched pairs of chromosomes that just control characteristics. The 23rd pair are labelled XY or XX
• XY or XX are the 2 chromosomes that decide the sex - whether you turn out to be male or female
• Males have an X and a Y chromosome. The Y chromosome causes male characteristics
• Females have 2 X chromosomes. The XX combination allows female characteristics to develop
• When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis. There's a 50% chance each sperm cell gets a Y-chromosome. A similar thing happens when making eggs. But the original cell has two X-chromosomes, so all the eggs have one X-chromosome
• Genetic diagrams show the possible gamete combination

  
• To find the probability of getting a boy or a girl, you can draw a genetic diagram

  
• This type of genetic diagram is called a Punnett square

  
• Genetic diagrams are just models that are used to show all the possible genetic outcomes when you cross together different genes or chromosomes

  
• Put the possible gametes form one parent down the side, and those form the other parent along the top to make a Punnett square

  
• The pairs of letters in the middle show the possible combinations of the gametes

  
• There are 2 XX results and 2 XY results, so there's the same probability of getting a boy or a girl

  
• The other type of genetic diagram looks a bit more complicated, but it shows exactly the same thing

  • At the top are the parents
  • The middle circles show the possible gametes that are formed. One gamete from the female combines with one gamete from the male
  • The criss-cross lines show all the possible ways the X and Y chromosomes could combine. The possible combinations of the offspring are shown in the bottom circles
  • Only one of these possibilities would actually happen for any one offspring