Why Don't Siblings Always Look Alike?

Meiotic recombination: the great lottery

Each child inherits 50% of each parent's DNA — but not the same 50%. At each conception, parental DNA undergoes a process called meiotic recombination: genes are reshuffled almost randomly before being transmitted.

Concretely: a father has two versions of each gene (one from his father, one from his mother). When he passes DNA to his child, he transmits one — picked at random. Over 20,000 genes, that yields a unique combination. The probability that two siblings receive exactly the same versions is essentially zero (1 in 2^20000).

How much DNA do siblings share on average?

On average, full siblings share about 50% of their DNA. But that 50% is a mean with significant variance:

• Extreme cases (rare): 35% to 65% sharing
• Average case (95% of sibships): between 45% and 55%

You can therefore have a brother who shares only 38% of your DNA — about as much as a half-sibling or a close cousin. This variance explains why some sibships look visually homogeneous and others highly contrasted.

Beyond genes: epigenetics and the womb environment

Even with the exact same genetic combination, identical twins (who share 100% of their DNA) can have slightly different faces. How? Through epigenetics — the environment modifies which genes are expressed and with what intensity.

For siblings born years apart, intrauterine conditions differ: maternal nutrition, stress, exposures, parental age. These variables subtly modify the expression of genes both children carry.

The role of grandparents

A child doesn't resemble only their parents — they carry traits from their 4 grandparents (and beyond). At each generation, recessive genes can resurface.

If one child draws paternal traits from the maternal line (genes of the mother's father, passed to the mother, then to the child) and another child draws maternal traits from the paternal line, the two siblings will have different "visual roots" even with the same parents.

Does birth order change anything?

Not directly on the genes — birth order doesn't influence genetic transmission. But it does influence the epigenetic environment: a first-born is conceived in a "fresh" womb, a fourth-born in a womb modified by previous pregnancies (vascularisation, hormones, microbiome).

Recent studies (Wadhwa et al.) suggest these differences can mildly affect birth weight, some physical traits, even sensitivity to certain genes. The effect is modest but real.

Measuring resemblance between siblings

Our Look Like Me LLM Engine can compare two people — including two siblings. It's a less obvious use case than parent/child, but often illuminating.

Typical patterns observed:

• Score 60-80%: "easily mistaken for each other" siblings
• Score 40-60%: noticeable resemblance but distinct faces
• Score 20-40%: "they don't look alike at all" — frequent and normal

So the science says: if your children don't look alike, it's not a flaw. It's just genetic diversity expressing itself — which is, on the evolutionary scale, rather a blessing.