Heritability describes how much of the variation in a trait (within a population) can be explained by genetic differences between people. It’s written as h^2 and ranges from 0 to 1.
Multifactorial disorders (such as type 2 diabetes, obesity, and heart disease) are influenced by multiple factors, including genetics, so their heritability is typically somewhere in the middle.
Heritability has historically been estimated using twin studies by comparing incidence in identical (monozygotic) versus fraternal (dizygotic) twins. A disorder with high heritability will show a higher incidence in identical twins than in fraternal twins.
Homologous and non-homologous chromosomes: Homologous chromosomes carry the same types of genes at the same loci (though the alleles may differ). Non-homologous chromosomes carry different sets of genes. For example, paternal chromosome 1 is homologous to maternal chromosome 1, but it is non-homologous to maternal chromosome 3. Homologous chromosomes pair during meiosis I; non-homologous chromosomes do not. Exchange of genetic material between homologous chromosomes occurs by recombination, while genetic exchange between non-homologous chromosomes occurs through translocation.
Two siblings have a 25% chance of being genotypically HLA identical, a 50% chance of being HLA haploidentical (sharing one haplotype), and a 25% chance of sharing no HLA haplotypes. In each cell, one haplotype is paternal and the other is maternal, so there is 50% HLA sharing with each parent.
DNA sharing between relatives
| Relationship | Percent sharing |
| Identical twin | 100 |
| Parent | 50 |
| Sibling | 50 |
| Grandparent, uncle, aunt, grandchild, niece, nephew, half-sibling | 25 |
| 1st cousin | 12.5 |