What is Mendel's Law of Segregation?
Mendel's Law of Segregation explains how the two alleles an organism carries for each gene separate during gamete formation, so every sperm or egg cell ends up with only one allele. It is one of the founding principles of classical genetics.
The Law of Segregation states that the two alleles for a gene separate during meiosis, so each gamete (sperm or egg) receives exactly one allele, chosen randomly.
- 1↓Diploid parent cell (Aa)The cell carries two alleles for the gene, one on each homologous chromosome.
- 2↓Homologous chromosomes pair upDuring meiosis I, homologous chromosomes line up together.
- 3↓Chromosomes separateMeiosis I pulls homologous chromosomes — and their alleles — apart into different cells.
- 4Gametes formEach resulting gamete carries only one allele, A or a, never both.
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Step-by-step worked examples
A pea plant with genotype Aa (purple flower) self-pollinates. What genotype and phenotype ratios are expected in the offspring?
Each parent produces gametes A and a in equal numbers (segregation) Punnett square: AA, Aa, Aa, aa Genotype ratio = 1 AA : 2 Aa : 1 aa Phenotype ratio = 3 purple : 1 white (A is dominant)
In a cross of Aa × aa, what fraction of offspring is expected to be heterozygous (Aa)?
Aa parent gives gametes A or a (50/50) aa parent gives only gametes a Offspring: Aa, Aa, aa, aa Heterozygous (Aa) fraction = 2/4 = 50%
Out of 200 offspring from an Aa × Aa cross, how many are expected to show the recessive phenotype?
Expected recessive fraction = 1/4 (aa) 200 × 1/4 = 50 So about 50 offspring show the recessive trait
Flashcards
Quick quiz
Q1.According to the Law of Segregation, how many alleles does each gamete receive for a given gene?
Q2.In an Aa × Aa cross, what is the expected phenotype ratio if A is completely dominant?
Q3.During which stage does allele segregation physically occur?
Q4.A pea plant is Aa. What gametes can it produce?
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Common mistakes
Thinking both alleles end up in the same gamete. — Correct: Only one allele per gene goes into each gamete — that's the whole point of segregation.
Confusing segregation with independent assortment. — Correct: Segregation is about one gene's allele pair separating; independent assortment is about different genes assorting independently.
Assuming a 3:1 ratio always appears exactly in every batch of offspring. — Correct: 3:1 is the expected probability ratio; small offspring numbers show random deviation.
Believing segregation happens during mitosis. — Correct: Segregation of alleles happens during meiosis, not mitosis (which produces genetically identical cells).
FAQ
What is Mendel's Law of Segregation?
It states that the two alleles for a gene separate during gamete formation, so each gamete carries only one allele.
What is the formula/ratio for the Law of Segregation?
A monohybrid Aa × Aa cross gives a 1:2:1 genotype ratio and, with complete dominance, a 3:1 phenotype ratio.
What are examples of the Law of Segregation?
Mendel's pea plant flower-color crosses (Aa × Aa → 3 purple : 1 white) are the classic example.
How do you calculate offspring ratios using the Law of Segregation?
Use a Punnett square: cross the possible gametes from each parent (A or a) to find genotype and phenotype ratios.