How do human chromosomes in sex cells can be damaged by external factors?
Many mutagens will not reach the gonads – but those that enter your bloodstream (eg the toxins in cigarette smoke) can reach them. If they get there, it is possible for them to mutate the DNA in your gametes.
Other mutagenic circumstances, like exposure to ionising radiation, can also directly damage the DNA in your gametes: X-rays and gamma rays can do this. UV rays and alpha and beta particles won’t be able to penetrate far enough into your body to do this.
As far as alcohol goes, it is the same as for cigarette smoke, except it is the "impurities" in the drinks which can be carcinogenic and mutagenic (like the polycyclic hydrocarbons which might give a whiskey its "smoky" flavour). Alcohol itself is a toxin, but isn’t mutagenic.
Such germinal line mutations induced by alcohol consumption and cigarette smoking are highly unlikely. If such mutations were possible these two mutagens would have been banned long ago. Before any new variant hereditary state can be ascribed to mutagenesis, recombination and segregation must be ruled out in both the somatic and germinal mutations. Repair mechanisms are such that substitution, alteration or damage to the base pairs in chromosomes is 1 in ten billion per base pair per cell cycle. -Basically toxins you take in can damage sex cells such as mercury or chemicals from pesticides and plastics. ——— Also Radiation can cause damage. Such as from nuclear fall out or UV exposure.