Test Crosses
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Lesson overview
Using a test cross (and back cross) to find an unknown genotype, interpreting offspring ratios, and reading simple family pedigrees.
Go to lesson quiz →Exam guidance · JAMB UTME Speed Lens
Core idea (Mendelian Genetics, DNA & Variation): Define allele symbols clearly, separate gamete formation from Punnett fertilisation, and distinguish genotype ratios (1:2:1) from phenotype ratios (3:1).
Pacing
~40 seconds per MCQ (180 questions in 120 minutes, no negative marking).
How JAMB marks this
Fast dimensional/sign elimination and standard formula shortcuts — skip multi-line algebra when substitution tests options faster.
Watch out for
UTME distractors routinely include the intermediate value (e.g., forgetting to convert cm³ to dm³ or °C to Kelvin).
Lesson content
Overview
A poultry farmer in Ibadan buys a black cockerel and wants to know whether it is pure-breeding for black feathers before using it to breed a whole flock. Looking at the bird cannot tell her: a homozygous black bird (BB) and a heterozygous one (Bb) look exactly the same. A test cross gives the answer by revealing any hidden recessive allele. In this lesson you will explain why a test cross uses a homozygous recessive partner, predict and interpret test-cross results, distinguish a test cross from a back cross, and use family pedigrees to work out genotypes, including deciding whether a trait is dominant or recessive.
Key terms
Test cross: crossing an organism showing the dominant phenotype, but of unknown genotype, with a homozygous recessive individual. Back cross: crossing an offspring with one of its parents (or an individual of the same genotype as a parent). A back cross to the recessive parent is a test cross. Pure-breeding (true-breeding): homozygous for a trait, so that when crossed among themselves all offspring show the same trait. Pedigree: a family tree showing which members of several generations have a particular trait. Squares are males, circles females, and shaded symbols show affected individuals.
Why the test cross works
A homozygous recessive individual produces only recessive gametes, so it cannot mask anything. Whatever allele the unknown parent contributes appears directly in the phenotype of the offspring. If the unknown parent is BB, every gamete it makes carries B, so all offspring are Bb and show the dominant trait. If it is Bb, half its gametes carry b, so about half the offspring are bb and show the recessive trait. One recessive offspring is enough to prove the unknown parent is heterozygous. However, a result of all dominant offspring is strong evidence rather than proof, especially with few offspring: a Bb parent could, by chance, produce a small litter that are all Bb. The probability that n offspring of a Bb × bb cross are all dominant is (1/2)ⁿ, so with 8 offspring it is only 1/256. Pedigrees work like natural test crosses. If two unaffected parents have an affected child, the trait must be recessive and both parents must be heterozygous carriers. If an affected child always has at least one affected parent and the trait appears in every generation, it is likely dominant. Test crosses have real uses. Livestock breeders test-cross a valuable bull or ram before using it widely, so that a hidden harmful recessive allele is not spread through a whole herd. Plant breeders use test crosses to confirm that a new variety is pure-breeding before releasing its seed to farmers, who need every plant in a field to have the same qualities, such as height, maturity date and grain colour. In humans, of course, test crosses are not possible, which is why geneticists rely on pedigrees instead.
Worked example 1: the black cockerel
The black cockerel is crossed with white hens (bb). Of 24 chicks, 11 are white. What is the cockerel's genotype?
Active Worked Example · Predict Step 1
3 stepsWrite down the formula, rule, or first move you would make before revealing Step 1.
Worked example 2: the heterozygous result
Show, using a Punnett square, the expected outcome of the test cross Bb × bb.
Active Worked Example · Predict Step 1
4 stepsWrite down the formula, rule, or first move you would make before revealing Step 1.
Worked example 3 (UTME standard): reading a pedigree
Two parents with normal skin pigmentation have four children, one of whom has albinism. (a) Is albinism dominant or recessive? (b) Give the parents' genotypes. (c) What is the probability that their next child is albino?
Active Worked Example · Predict Step 1
4 stepsWrite down the formula, rule, or first move you would make before revealing Step 1.
Common traps
• The partner in a test cross is always the homozygous RECESSIVE. • One recessive offspring proves heterozygosity; all-dominant offspring only suggest homozygosity. • In pedigrees: two unaffected parents with an affected child means the trait is recessive. • A back cross is to a parent; it is a test cross only if that parent is homozygous recessive.
Summary
• Test cross: unknown dominant-looking individual × homozygous recessive. • All offspring dominant → unknown is probably homozygous (BB); about 1 : 1 → heterozygous (Bb). • Larger numbers of offspring make the conclusion more reliable: P(all dominant from Bb) = (1/2)ⁿ. • Pedigrees: affected child of two unaffected parents shows a recessive trait and carrier parents.
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