🧬 Coefficient of Relatedness Calculator
Pick a relationship and read off Wright's coefficient of relatedness (r) — the share of genes two individuals hold in common by descent — with a plain-language reading and the special haplodiploid cases that shape social insect evolution.
🧬 Coefficient of relatedness
Extremely close kin — closer than the parent–offspring bond, the hallmark of haplodiploid sisters that helps drive eusocial cooperation.
Sisters inherit an identical set of genes from their haploid father and share half of the mother's — averaging r = 0.75. The classic (and debated) key to why eusociality keeps evolving in Hymenoptera.
Why r = 0.75 matters for eusociality
Bees, ants, and wasps are haplodiploid: females develop from fertilised eggs and are diploid, while males come from unfertilised eggs and are haploid. Because a father passes on his single, identical set of genes to every daughter, full sisters share all of their paternal genes and, on average, half of their maternal genes — giving a relatedness of 0.75. That is higher than a female's relatedness to her own offspring (0.5). Hamilton argued this asymmetry can make it pay, in gene's-eye terms, for a worker to raise more sisters than to reproduce herself — the classic (and still-debated) explanation for why sterile worker castes and eusociality evolved repeatedly in the Hymenoptera.
What is relatedness — and why it matters
Kin selection theory turns on a single number: how much genetic interest one individual has in another. That number is the coefficient of relatedness. Because natural selection ultimately acts on genes, an animal that helps a close relative reproduce is, in effect, propagating copies of its own genes carried in that relative.
This calculator gives the textbook r for common relationships and highlights the haplodiploid quirk of bees, ants, and wasps, where full sisters can be more related to one another than to their own offspring — a central, and much-argued, thread in the story of how eusociality evolved.
❓ Frequently Asked Questions
What is the coefficient of relatedness?
Wright's coefficient of relatedness, r, is the probability that a gene picked at random in one individual is also present in another by common descent. It ranges from 0 for unrelated individuals to 1 for genetically identical ones (clones or identical twins). Parents and offspring, and full siblings in diploid species, sit at 0.5.
Why are haplodiploid sisters related by 0.75?
Bees, ants, and wasps are haplodiploid: females are diploid, but males are haploid and pass an identical copy of their single gene set to every daughter. So full sisters share all of their paternal genes plus, on average, half of their maternal genes — averaging r = 0.75, higher than the 0.5 a female shares with her own offspring.
How does that explain eusociality?
W. D. Hamilton pointed out that if a female is more related to her sisters (0.75) than to her own young (0.5), a gene can spread further by making her help her mother rear more sisters than by reproducing herself. This haplodiploidy hypothesis is the classic explanation for the repeated evolution of sterile worker castes in Hymenoptera, though modern biologists debate how decisive it really is versus factors like monogamy and ecology.
Are these values exact?
They are the standard expected values for idealised pedigrees. Real relatedness varies with inbreeding, multiple mating by the queen, and chromosomal chance, so a specific pair can differ from the textbook figure. Treat the numbers as the average expectation for that relationship.