Text lesson

Unit 3 — Variation of Traits in Living Organisms

Science — Third Preparatory

Probability of independent events

Each birth is an independent event.
If two carrier parents have a 25% chance of an affected child, then after a first affected child the chance of the second child being affected is still 25%.
To find the chance of two particular outcomes in two separate children, multiply: chance of two affected children in a row = 1/4 × 1/4 = 1/16.

A pea plant with round seeds is crossed with a plant with wrinkled seeds.
In the offspring, 48 plants have round seeds and 52 have wrinkled seeds (close to 1 : 1).
What are the genotypes of the parents?
A 1 1 ratio appears in a cross between a heterozygote and a homozygous recessive: Rr × rr.
The wrinkled offspring (rr) prove that the round parent had an r allele, so it was Rr.
If two hybrid tall pea plants (Tt × Tt) produce 400 seeds that all grow, about how many plants are expected to be short?
Short = tt = 1/4 of 400 = 100 plants; tall = 300 (of which about 100 are TT and 200 are Tt).
Real counts usually differ a little from the expected numbers because fertilisation is a matter of chance, but with large numbers the results come close to the predicted ratio.
That is why Mendel’s use of hundreds and thousands of plants gave clear results.

Unit 3 — Variation of Traits in Living Organisms · 22 / 24

Probability of independent events