Genotype Determination Calculators

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Genotype determination is the process of identifying the specific alleles present at one or more gene loci in an individual organism. Knowing an individual's genotype — rather than just its phenotype — is essential in medical genetics (carrier testing, disease diagnosis), plant and animal breeding, forensic identification, population genetics, and evolutionary research. Methods range from classical test crosses and gel electrophoresis to modern molecular techniques including PCR-based genotyping, SNP arrays, and whole-genome sequencing.

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Classical Genotyping: Test Cross

The test cross determines whether an organism showing a dominant phenotype is homozygous (AA) or heterozygous (Aa) by crossing it with a homozygous recessive (aa):

  • AA × aa → all Aa (all dominant phenotype)
  • Aa × aa → 50% Aa : 50% aa (both phenotypes appear)

Any offspring showing the recessive phenotype reveal the parent was heterozygous. This classical approach is still used in plant and animal breeding.

Molecular Genotyping Methods

PCR + Gel Electrophoresis

PCR amplifies a genomic region using primers flanking a polymorphism. Products are separated by gel electrophoresis. For example:

  • Insertion/deletion (indel) polymorphisms: wild-type and mutant alleles produce different-sized bands
  • RFLP (restriction fragment length polymorphism): restriction enzymes cut at specific sequence variants, producing different fragment patterns for each allele

SNP Genotyping Arrays

Microarrays containing millions of probes detect single nucleotide polymorphisms (SNPs) genome-wide. Used in GWAS, ancestry analysis, and agricultural breeding programs. Genotype calls are made by measuring hybridization intensity for each allele variant.

Sanger Sequencing

Direct sequencing of a PCR product reveals the exact nucleotide sequence at a locus. The chromatogram shows one peak per base (homozygous) or overlapping double peaks at heterozygous positions. Gold standard for confirming specific variants.

Next-Generation Sequencing (NGS)

Whole-genome, exome, or targeted panel sequencing detects variants at all loci simultaneously. Used in clinical genetics for comprehensive diagnosis and in population genomics.

Carrier Testing and Prenatal Genotyping

Identifying carriers of recessive disease alleles requires genotyping — carriers have a normal phenotype but carry one mutant allele (Aa). Prenatal testing (chorionic villus sampling or amniocentesis) extracts fetal DNA for genotyping of inherited disease alleles.

Glossary

Test Cross
Mating an individual of unknown genotype (dominant phenotype) with a homozygous recessive (aa). Any recessive offspring reveal the parent was heterozygous (Aa). Used to determine unknown dominant genotype.
RFLP (Restriction Fragment Length Polymorphism)
A PCR-based genotyping method in which restriction enzymes cut DNA at sequence variants, producing allele-specific fragment patterns visualized by gel electrophoresis.
SNP Array
A microarray containing millions of probes that simultaneously genotype single nucleotide polymorphisms (SNPs) across the genome. Used in GWAS, population genetics, and breeding programs for high-throughput genotyping.

Frequently Asked Questions

A test cross mates an individual showing the dominant phenotype with a homozygous recessive (aa). If all offspring show the dominant phenotype, the parent was homozygous dominant (AA). If half show the recessive phenotype, the parent was heterozygous (Aa). The recessive offspring reveal the hidden recessive allele. Test crosses are the classical genetic method for distinguishing AA from Aa genotypes when only the phenotype is visible.

PCR amplifies the genomic region containing the polymorphism of interest using sequence-specific primers. For an insertion/deletion: wild-type and mutant alleles produce different-sized PCR products that are separated on a gel. Homozygous wild-type shows one band; homozygous mutant shows a different band; heterozygous shows both bands. For SNPs, allele-specific PCR, RFLP analysis, or probe-based methods detect single nucleotide differences.

SNP genotyping determines which nucleotide variant(s) an individual carries at single nucleotide polymorphism positions. Arrays containing millions of SNP probes can genotype an entire genome simultaneously. SNP genotyping is used in genome-wide association studies (GWAS) to find disease-associated variants, in population genetics to infer ancestry and relatedness, and in plant and animal breeding for marker-assisted selection of desired traits.

Many heritable diseases are caused by specific alleles that can be identified by genotyping. Carrier testing identifies heterozygous carriers of recessive alleles (e.g., CFTR for cystic fibrosis, HBB for sickle cell) who are clinically unaffected but can pass the condition to offspring. Prenatal genotyping can detect disease alleles in fetal DNA. Pharmacogenomics uses genotyping to predict drug metabolism (CYP2D6, CYP2C19) and tailor drug selection and dosing to individual genotypes.