Tall tomato plants are dominant over dwarf (gene D). Hairy stems are
dominant over hairless stems (gene H). A dihybrid tall, hairy plant is
testcrossed. The F1 progeny include 118 tall hairy: 121 dwarf
hairless: 112 tall hairless: 109 dwarf hairy. What are the genotypes
of the parents?
a) Dd Hh x dd hh 
b) Dd HH x dd hh 
c) Dd Hh x DD HH 
d) DD Hh x dd hh 
e) Dd Hh x Dd Hh

Hi Operator,

Our options here are as follows: DDHH, DDHh, DdHH, and DdHh (tall,
hairy); DDhh and Ddhh (tall, hairless); ddHH and ddHh (dwarf, hairy);
and finally ddhh (dwarf, hairless).  These are the four different
phenotypes and nine different genotypes available for our use.

The tricky part of this question is realizing that the numbers are so
close to each other that you essentially have a 1:1:1:1 ratio of
phenotypes.  Remember, as I mention in another answer, that
statistically the results of a cross approach perfection if you've got
enough progeny, but that they can be off a little.

So, in essence you have four sets of phenotypes equally represented. 
This indicates that you've got one heterozygotic parent (Dd Hh) and
one homozygotic recessive parent (dd hh), for answer A.  This will
give us "equal" amounts of DdHh, ddHh, ddhh, and Ddhh - which we're
seeing in the offspring.

Look at it this way.  You know neither parent can be homozygous
dominant - or all the progeny would display the dominant
characteristic regardless of what the other parent was (this hearkens
back to the black spaniels).  So answers b, c, d are all wrong.  If
both parents were heterozygous you'd get the "standard" ratio of
9:3:3:1 where for every 16 plants you have 9 that are tall/hairy, 3
that are dwarf/hairy, 3 that are tall/hairless, and 1 that is
dwarf/hairless.  Since that ratio is clearly not in operation here, we
can also knock out answer e.  That leaves us with a.

Librariankt