X and Y chromosomes

• Don't forget that the 50:50 ratio is only a probability of each pregnancy
• XX chromosomes control whether you're male or female
• There's a 20% chance each sperm cell gets an X-chromosome and a 50% chance it gets a Y-chromosome
• How to find the probability of getting a boy or a girl

  1. Drew a genetic diagram

  2. Use genetic diagrams to show all possible genotypic outcomes

  3. Match genes or chromosomes from each parent to find the possible combinations of the gametes
• There are two XX results and two XY results, so there's the same probability of getting a boy or a girl
• Genetic diagrams in exams concentrate on a gene instead of a chromosome, but the principle is the same
• Only one of the possibilities shown in genetic diagrams would actually happen for any one offspring
• Genetic diagrams show the possible gamete combinations
• All eggs have one X-chromosome