Read Mendelian Alleles and Crosses
Mendelian problems become clearer when alleles, loci, gametes and cross types are kept distinct.
Mendelian Alleles and Crosses is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Keep loci separate
Alternative forms at one locus are alleles; traits at different loci must not be conflated. Linked genes can be inherited together, so independent segregation is not universal.
Recognise common crosses
A test cross pairs a dominant-phenotype individual with a homozygous recessive partner to reveal genotype. A dihybrid cross tests two loci; reciprocal crosses swap which sex carries each parental trait.
Interpret classical statements
A gamete carries one allele per gene. A diploid individual contains at most two alleles of a locus, even when more alleles exist in the population. Pure lines breed true; dominance describes phenotype in heterozygotes.
Infer dominance from a cross
A recessive phenotype crossed with a true-breeding dominant individual produces uniformly dominant-phenotype offspring. In textbook Mendelian problems, an all-normal F₁ supports recessive inheritance of the unusual curled-ear trait and a homozygous normal parent.
OlympIQ exam lens
- ✓Yellow/green seed share one locus; seed colour and shape do not.
- ✓Test cross reveals unknown dominant genotype.
- ✓Pure line is homozygous and true-breeding.
- ✓One gamete carries one allele per locus.
- ✓Write parental genotypes before interpreting offspring.
- ✓A true-breeding normal parent crossed with a recessive mutant gives uniform normal F₁.
Independent assortment applies to unlinked loci; do not use “always” when linkage is possible.
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