There are few ectomorphs anymore per the general population.
Some exist..but not as many.
At some point, you will have an endomorph marrying or a mesomorph marrying.
If it's endo-endo, the child will be endo unless some genetic parental mixture is endo-meso /endo- ecto...in which case, the child could be meso-ecto...but that would carry a 25% chance.
Same with meso. As the American pool of endomorphs and mesomorphs becomes greater, the frequency of ectomorphs in their pure form gets smaller.
Take Elinor Purrier. She's fast and will represent us in 2021, i am sure...but she's not *small " yet she is one of the fittest runners I have seen.
Maybe we're evolving into people who can be fitter at a better rate than BMI numbers can show.
If, at 5'8" 181, I can still run a 5k at below 7 per mile at age 45 last year and if i gained about 2 pounds all year just walking instead of running, why do we worry about old charts and go by what us right for our bodies?