More severe: Short rib polydactyly syndrome (usually finger/toe fused together) -- rickets

This Short rib polydactyly syndrome has usually fused finger or toes and is in most cases early before or short after birth.

https://en.wikipedia.org/wiki/Short_rib%E2%80%93polydactyly_syndrome

All types are genetically inherited.

Verma-Naumoff type (with gene DYNC2H1)

https://en.wikipedia.org/wiki/Syndactyly (with genes)

https://en.wikipedia.org/wiki/Webbed_toes

As in many other cases and syndrome, not all genes are identified yet (12/2025)

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Note, that bone growth and deformation is also affected by nutrition and digestion.

https://en.wikipedia.org/wiki/Rickets

It can be very complicated, to find out, what is genetic and what nutrition in bone problems.

And some things gone wrong in childhood may not easy be corrected later.

Note, that the severity can have a wide range, dependend also from body shape.

Body Health, especially spine, is also heavily dependent on muscular strength.