Probability

Genetic ratios are really probabilities. A probability is a number from 0 (impossible) to 1 (certain), and can be written as a fraction, decimal or percentage. A 3 : 1 ratio means each offspring has a probability of ¾ of the dominant phenotype and ¼ of the recessive phenotype.Each fertilisation is an independent event. The outcome for one child does not affect the next. If a couple have an affected child, the chance that the next child is affected is exactly the same.Two rules let you combine probabilities.Product rule (AND): the probability of two independent events both happening is found by multiplying.P(A and B) = P(A) × P(B)Sum rule (OR): the probability of one or other of two mutually exclusive events is found by adding.P(A or B) = P(A) + P(B)Worked example: two carriers of cystic fibrosis (Ff × Ff) have children.• P(one child is ff) = ¼
• P(child is a carrier, Ff) = ½
• P(two children both ff) = ¼ × ¼ = 1/16
• P(child is FF or Ff, unaffected) = ¼ + ½ = ¾
• P(child is a boy and ff) = ½ × ¼ = ⅛The same rules apply to gametes: an AaBb individual makes an ab gamete with probability ½ × ½ = ¼. Probabilities are used to predict outcomes and to calculate the expected values for the chi-squared test.