Genotype and Phenotype: Why the Genes Do Not Settle It
Dominance, penetrance and expressivity — the reasons two people with the same variant differ.
Genotype is what you carry. Phenotype is what is observable — a trait, a characteristic, a condition. The gap between the two is where nearly all the interesting biology lives, and it is much wider than the usual textbook account suggests.
Alleles and the simple case
You carry two copies of most genes. Matching copies make you homozygous, differing copies heterozygous, and the different versions are alleles.
In the simplest pattern one allele is dominant and one recessive: the recessive trait appears only when both copies carry it. That is the pea-plant model, it is genuinely how some traits work, and it is the exception rather than the rule.
The patterns that are not that
- Incomplete dominance — the heterozygote lands between the two homozygotes rather than matching one.
- Codominance — both alleles are fully expressed. The AB blood group is the standard example: not a blend, both antigens present.
- Multiple alleles — a population can carry many versions of a gene even though an individual carries two.
- Pleiotropy — one gene affecting several apparently unrelated traits.
- Polygenic traits — height, skin colour, most disease risk. Many genes each contributing a little, producing continuous variation instead of categories.
Almost everything people actually want to know about is in that last category, which is why single-gene reasoning so rarely answers the question.
Penetrance and expressivity
Two terms that explain most apparent contradictions.
Penetrance is the proportion of people carrying a variant who show the trait at all. If 60% of carriers develop a condition, penetrance is 60% — and carrying the variant is not a diagnosis. Many clinically significant variants have penetrance well below 100%, and some depend on age, so a carrier who is unaffected at 40 is not necessarily unaffected at 70.
Expressivity is how strongly the trait shows in those who do show it. Two people with the same variant, both affected, can be affected mildly and severely. Same genotype, same diagnosis, very different lives.
Together these are why a family can carry one variant and look nothing alike.
The environment is not a footnote
Phenotype is the product of genotype and everything that happens to it. Identical twins share a genome and differ in height, weight, disease onset and lifespan. Phenylketonuria is the clean illustration: a well-understood single-gene condition whose devastating phenotype is largely prevented by dietary change. The genotype is unaltered; the phenotype is not.
Epigenetics, stated carefully
Chemical marks on DNA and its packaging proteins change which genes are active without changing the sequence. This is real, well-evidenced, and how a liver cell and a neuron with identical DNA become different cells.
It is also the most over-claimed topic in popular genetics. That marks respond to environment is established; that specific lifestyle choices produce specific heritable outcomes in humans is mostly not. Treat confident claims in that direction with suspicion.
What this means for a test result
A report saying you carry a variant associated with a trait is a statement about genotype and a probability. Whether it appears depends on penetrance, on the rest of your genome, and on your life. "Carries the variant" and "has the trait" are different sentences.
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