I'm a student in biology and just recently learned about Lamarck's theory on evolution and how it has been disproved. While I understand that you cannot alter your genes, I'm still having a hard time understanding some parts of it, mainly how inheriting strength (such as working out) will not be passed down to my offspring. If someone was obese, exercised to the point of no longer being obese, will this person's offspring still start off as obese?
Indeed, whether you work out or not will not change how strong your offspring will be. This is exactly why Lamarck's was wrong.
I would note however that Behaviour can be heritable and Your behaviour can affect your kids environment
Behaviour can be heritable
The probability of you working out is affected by your genes and so if you have a genetic predisposition for willing to work out, this will be passed on.
Assuming there are genetic predisposition for liking to work out, the fact that you worked out would not increase the probability of your kid to work out via genetic inheritance but the fact that you have a genetic predisposition for liking to work out will increase the probability that your kid will have the same preference and will work out
By the way, if you want to learn more about the concept f heritability you should have a look at Why is a heritability coefficient not an index of how “genetic” something is?
Your behaviour can affect your kids environment
Working out won't affect your genes but it will likely affect the environment in which the kid will grow. Here are three examples
If you eat healthy, you will probably teach your kids to eat healthy too.
- Womb environment
Also, if you eat healthy and are a woman, you will likely give the right nutrients to your offspring while still in the womb (assuming you are a lady).
Your environment will cause epigenetic changes (changes not in DNA but in the very close environment of the DNA such as typically histone tail methylation). These epigenetics modifications will be directly passed to the foetus.
A post your question is almost a duplicate of...
A post on heritability
And finally an answer that talks about non-genetic inheritance