A heterozygous right handed mom and a homozygous left handed dad (recessive) mate. What percent of children are expected to be left handed?

50% chance will left handed.

Explanation:

Right handed = dominant (R)

Left handed = recessive (r)

Heterozygous right handed mom = Rr

Homozygous left handed dad = rr

According to punnett's square:

Rr x rr

=2/4 Heterozygous (Rr)

= 2/4 homozygous recessive (rr)

So, there will be 50 % or 2/4 chance of left handed children.