Heritability measures how well differences in people’s genes account for differences in their traits. It is given by h^2 and ranges from 0 to 1. A heritability close to one indicates that almost all of the variability in a trait comes from genetic differences, with very little contribution from environmental factors, and vice versa. Multifactorial disorders like diabetes type 2, obesity and heart disease, are influenced by multiple factors including genetics and have a heritability somewhere in the middle. Heritability has historically been estimated from studies of twins by comparing incidence in identical versus fraternal twins. Genetic disorders with high heritability will have a higher incidence in identical compared to fraternal twins.
Homologous and non-homologous chromosomes: Homologous chromosomes consist of alleles of the same type of genes in the same loci, whereas non-homologous chromosomes consist of alleles of different types of genes. For example, paternal chromosome 1 is homologous to maternal chromosome 1, while it is non-homologous to maternal chromosome 3. Homologous chromosomes pair during meiosis I, while non-homologous do not. The exchange of genetic material between homologous chromosomes occurs by recombination, while genetic exchange occurs between non-homologous chromosomes 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 that they share no HLA haplotypes. For each cell, one haplotype is paternal, and the other is maternal, which is 50% HLA sharing with parents.
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 |