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D3.2.1—Haploid gametes + fusion = diploid zygote

Haploid gametes fuse at fertilization to restore diploid chromosome number in the zygote and combine alleles from two parents in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.1
Level
HL

Exam analysis

Chance of appearing1%of analysed past papers
Latest appearanceMay 2018
Most common paperPaper1
Typical marks1

Common command terms

  • Identify

Recent exam appearances

May 2018Paper1 ["HL"] · TZ110[ 1 ]D3.2.1—Haploid gametes + fusion = diploid zygote

Coverage 2018–2018 · Updated 16 Jul 2026

Each Parent Contributes One Allele to a Diploid Zygote

diploid parent (two homologous copies of each autosome) → meiosis → haploid gametes (one copy) → fertilization → diploid zygote (two copies again)

Cell Chromosome sets Alleles possible at one autosomal locus
body cell in a diploid parent 2n two alleles, one on each homolog
gamete n one allele
zygote 2n one allele from each parent

Two alleles does not mean two different alleles. A diploid genotype may be homozygous, with matching alleles, or heterozygous, with different alleles.

Inheritance Predictions Move from Chromosomes to Alleles to Phenotypes

homologous chromosomes carry two alleles in a diploid parent → meiosis separates one allele into each gamete → fertilization restores a two-allele genotype → allele relationship and environment shape phenotype → offspring data test the prediction

For any genetic cross:

  • define allele symbols and their relationship
  • write parental genotypes, not only phenotypes
  • derive gametes before drawing the grid
  • combine gametes, then translate genotype to phenotype
  • state probabilities and the assumptions behind them
If the simple 3:1 expectation fails First question to ask
heterozygote has a third phenotype incomplete dominance?
both products appear codominance?
phenotype changes with conditions environmental or plastic response?
more than two alleles occur in the population multiple-allele locus such as ABO?

Haploid gametes + fusion = diploid

[ 1 ]

For what reason do gametes contain only one allele of each gene?

Concept essentials

  • Gametes are haploid so fusion can restore the diploid number.
  • A zygote receives chromosomes and alleles from both parents.
  • Fertilization connects chromosome number with inheritance across generations.