What is Mendel's Law of Independent Assortment?
Mendel's Law of Independent Assortment states that genes for different traits are inherited independently of one another, as long as they sit on different chromosomes (or far apart on the same one). It explains the classic 9:3:3:1 dihybrid ratio.
The Law of Independent Assortment states that alleles of different genes separate independently during gamete formation, so the inheritance of one trait does not affect the inheritance of another.
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Step-by-step worked examples
A pea plant heterozygous for seed shape and color (RrYy) self-pollinates. What phenotype ratio is expected in the offspring for round/wrinkled and yellow/green seeds combined?
Each gene assorts independently: Rr × Rr and Yy × Yy separately give 3:1 each Combine the two 3:1 ratios: (3:1) × (3:1) = 9:3:3:1 Phenotypes: 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green
Out of 320 offspring from an AaBb × AaBb cross, how many are expected to show both recessive traits (aabb)?
Double-recessive fraction = 1/16 320 × 1/16 = 20 So about 20 offspring are expected to be aabb
A test cross AaBb × aabb is performed. What phenotype ratio is expected?
AaBb parent makes 4 gamete types in equal numbers: AB, Ab, aB, ab aabb parent makes only ab gametes Offspring: AaBb, Aabb, aaBb, aabb — each 1/4 Phenotype ratio = 1:1:1:1
Flashcards
Quick quiz
Q1.What phenotype ratio results from an AaBb × AaBb dihybrid cross?
Q2.How many different gametes can an AaBb organism produce?
Q3.Independent assortment requires genes to be:
Q4.A test cross AaBb × aabb produces what ratio?
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Common mistakes
Assuming independent assortment always applies, even to genes on the same chromosome. — Correct: Genes close together on the same chromosome are usually inherited together (linked), violating independent assortment.
Confusing the dihybrid 9:3:3:1 ratio with the monohybrid 3:1 ratio. — Correct: 9:3:3:1 comes from tracking two genes at once; 3:1 is for a single gene.
Thinking an AaBb individual makes only 2 gamete types. — Correct: It makes 4 equally likely gamete types: AB, Ab, aB, ab.
Forgetting that independent assortment happens during meiosis I, alongside segregation. — Correct: Both segregation and independent assortment occur as homologous chromosomes line up and separate in meiosis I.
FAQ
What is Mendel's Law of Independent Assortment?
It states that alleles for different genes sort into gametes independently of each other, as long as the genes are on different chromosomes.
What is the formula for the Law of Independent Assortment?
A dihybrid AaBb × AaBb cross gives a 9:3:3:1 phenotype ratio.
What are examples of independent assortment?
Mendel's dihybrid pea crosses for seed shape and color, which produced a 9:3:3:1 ratio.
How do you calculate dihybrid cross ratios?
Multiply the separate 3:1 monohybrid ratios together (3:1) × (3:1) = 9:3:3:1, or use a 16-box Punnett square.