Every human shares about 99.9% of their DNA — yet that tiny 0.1% difference, plus how it's expressed, accounts for much of what makes you uniquely you. This lesson explores how we vary genetically, and the often-misunderstood concept of heritability.
Learning Objectives
- •Understand the main types of genetic variation
- •Learn what 'heritability' really means (and doesn't)
- •See why heritability varies enormously by trait
How we differ: SNPs and variants
Genetic differences between people arise from MUTATIONS — changes in the DNA sequence. The most common type is a SNP (single nucleotide polymorphism): a single-letter difference at a specific spot, like one person having an 'A' where another has a 'G'. Different versions of a gene are called ALLELES. Humans are ~99.9% identical, so it's the remaining ~0.1% — millions of SNPs and other variants — that underlies our genetic individuality.
Germline vs. somatic mutations
An important distinction: GERMLINE mutations are present in the egg or sperm, so they're inherited and exist in every cell of the offspring. SOMATIC mutations arise in body cells DURING your life (from DNA damage, replication errors) and are NOT inherited — but they accumulate with age and can drive cancer. So you inherit one set of variation from your parents and acquire more, cell by cell, as you live (a link back to the genomic-instability hallmark).
Heritability: a population statistic, not a personal one
Heritability is one of the most misunderstood concepts in all of biology. It is NOT 'how much of MY trait is due to my genes'. It's a POPULATION statistic: the proportion of the VARIATION in a trait across a population that's explained by genetic differences. Height is ~80% heritable — meaning most of why people DIFFER in height is genetic — but that says nothing about how much any one person's height was 'set' by genes versus nutrition.
SNP one-letter difference (most common variation) ALLELE a version of a gene GERMLINE inherited; in every cell SOMATIC acquired during life; not inherited; accumulates with age Humans are ~99.9% identical — the ~0.1% drives genetic individuality.
Why 'heritable' doesn't mean 'unchangeable'
A trait being highly heritable does NOT mean it can't be changed. Height is strongly heritable, yet average heights have risen dramatically over a century — through better nutrition, not changed genes. Heritability is measured WITHIN a given environment; change the environment and outcomes change. Confusing 'heritable' with 'fixed' is one of the most common and consequential errors in thinking about genes.
Genetic variation & heritability, by the numbers
- ▸Humans share ~99.9% of their DNA; the ~0.1% drives individuality
- ▸SNPs (single-letter differences) are the most common type of variation
- ▸Germline mutations are inherited; somatic mutations are acquired during life
- ▸Heritability is a population statistic about trait VARIATION — not a personal one
If a trait is highly heritable, it can't be changed by environment or lifestyle.
Heritability measures variation within a given environment — it doesn't mean a trait is fixed. Highly heritable height rose sharply with better nutrition. Change the environment and outcomes change; 'heritable' is not 'unchangeable'.
Quick Check
What is a SNP?
Quick Check
What does heritability actually measure?
True or False
Somatic mutations are acquired during life and are not passed to offspring.
Summary
- →Humans are ~99.9% identical; SNPs are the most common variation in the other ~0.1%
- →Germline mutations are inherited; somatic mutations are acquired and accumulate with age
- →Heritability is a population statistic about trait VARIATION, not a personal measure
- →Highly heritable does NOT mean unchangeable — environment still matters
How do these variations translate into health and disease? Next: genes, risk, and why genes are rarely destiny.