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D3.2.16 (HL)—Segregation and independent assortment

Segregation separates allele pairs into gametes, while independent assortment separates different chromosome pairs independently during meiosis in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.16
Level
HL

Exam analysis

Chance of appearing5%of analysed past papers
Latest appearanceMay 2021
Most common paperPaper2
Typical marks1–6

Common command terms

  • Outline
  • Determine
  • Identify

Recent exam appearances

May 2021Paper2 ["HL"] · TZ25(a)[ 7 ]D3.2.16 (HL)—Segregation and independent assortment
May 2018Paper2 ["HL"] · TZ23(a)(ii)[ 2 ]D3.2.16 (HL)—Segregation and independent assortment
May 2015Paper1 ["HL"] · TZ225[ 1 ]D3.2.16 (HL)—Segregation and independent assortment
May 2013Paper2 ["HL"] · TZ17(b)[ 6 ]D3.2.16 (HL)—Segregation and independent assortment
May 2013Paper1 ["HL"] · TZ234[ 1 ]D3.2.16 (HL)—Segregation and independent assortment

Coverage 2013–2021 · Updated 16 Jul 2026

Segregation Is the Separation of Homologous Alleles in Meiosis

HL only
1

Before meiosis, an Aa diploid cell carries A on one homologous chromosome and a at the same locus on the other homolog.

2

At anaphase I, homologous chromosomes move to opposite poles. A and a therefore enter different daughter cells; meiosis II separates sister chromatids without reuniting the alleles.

3

If meiotic products survive equally, an Aa parent makes approximately 1/2 A gametes and 1/2 a gametes. Random fertilization then restores two alleles in the zygote.

Mendel's law of segregation is therefore a chromosome-mechanics statement: paired alleles separate because their homologous chromosomes separate during meiosis.

Independent Assortment Comes from Random Bivalent Orientation

HL only

Each bivalent can face either spindle pole at metaphase I. The orientation of the A/a pair does not determine the orientation of a B/b pair on a different chromosome, so maternal and paternal homologs enter daughter cells in different combinations.

Gene positions Assortment expectation
on different chromosomes independent
far apart on the same chromosome with recombination approaching 50% behaves approximately independent
close together on the same chromosome linked; parental combinations exceed recombinants

Independent assortment is not the random separation of sister chromatids. It concerns the relative orientation and separation of different homologous chromosome pairs in meiosis I.

Two unlinked homologous chromosome pairs orient in two equally likely ways at meiosis I, producing AB and ab gametes in one orientation or Ab and aB gametes in the other.

Derive Dihybrid Gametes One Allele from Each Gene at a Time

HL only

Every gamete from AaBb must contain one allele from the A/a locus and one from the B/b locus. For unlinked loci, multiply the independent probabilities.

A-locus choice B-locus choice Gamete Probability
A, 1/2 B, 1/2 AB 1/4
A, 1/2 b, 1/2 Ab 1/4
a, 1/2 B, 1/2 aB 1/4
a, 1/2 b, 1/2 ab 1/4

A valid gamete list has one allele per locus, no duplicate types and probabilities summing to 1. Writing A, a, B and b as four separate gametes loses one gene from each gamete.

The same multiplication rule scales: an individual heterozygous at n independently assorting loci can make 2ⁿ gamete types, before considering linkage or unequal viability.

The Unlinked Model Runs from Meiosis to Dihybrid Ratios

HL only

homologs separate alleles → independently oriented bivalents combine alleles from unlinked genes → AaBb makes AB, Ab, aB and ab gametes equally → random fertilization produces 9:3:3:1 in an F2 or 1:1:1:1 in a test cross

Prediction Required conditions
1:1 allele segregation normal meiosis and equal gamete contribution
four equal AaBb gametes loci unlinked or recombination effectively 50%
9:3:3:1 F2 complete dominance, random fertilization and equal survival
1:1:1:1 test cross double heterozygote × double recessive tester

A database locates the genes; the cross tests their behaviour. If observed offspring depart systematically from the unlinked expectation, inspect chromosome location, parental phase, crossing over and viability before blaming random sampling alone.

Inheritance Models Are Built from Chromosomes and Tested with Offspring Counts

HL only

meiosis predicts gamete combinations → chromosome loci determine whether genes assort independently or travel together → crossing over creates reciprocal recombinants → test-cross counts reveal parental phase and recombinant frequency → chi-squared tests whether departures from a chosen ratio exceed chance expectation

Offspring pattern First model to examine
9:3:3:1 F2 two unlinked genes with complete dominance
1:1:1:1 test cross four equally frequent gametes from an unlinked double heterozygote
two large and two small reciprocal test-cross classes linked loci with crossing over
significant χ² departure assumptions, linkage, viability, fertilization or scoring require review

Keep prediction and evidence separate: define the genetic model first, calculate expected counts from it, compare observations with the expectation, and change the model only when chromosome location or statistical evidence requires it.

Segregation and independent assortment

HL only

[ 6 ]

Outline the relationship between Mendel's law of independent assortment and meiosis.

Concept essentials

  • Segregation gives each gamete one allele from each pair.
  • Independent assortment applies to genes on different chromosome pairs.
  • Linked genes do not assort independently in the same simple way.