How does the reduction in the strength of the earth's electromagnetic field affect genes.?

What you heard is not true.  Strong magnetic fields can affect the building of proteins, including DNA, but it would have to be many, many times the strength of the earth's magnetic field - much stronger than it has ever been known to be.

I'm not savvy about such things, but there's another thought to consider on this.  I certainly agree that the Earth's magnetic field is, in itself, too weak to cause damage to humans.

But the Earth's magnetic does help protect us from the damaging short wavelength (i.e. x-rays, gamma rays) electromagnetic radiation generated by our sun and universe in general.  This field deflects much of this radiation, so if the field strength were reduced, there would be an increase in genetic mutations, cellular damage, cancers..and if reduced enough, the end of human existence.