Question

A short-haired black guinea pig and a long-haired brown guinea pig.
Photos by Katrina and Ilovebutter

. . . the genes assort independently.

short, black fur

long, black fur

short, brown fur

long, brown fur

Answer 1

Let S = short (dominant), s = long (recessive); B = black (dominant), b = brown (recessive).
If the short, black parent is heterozygous for both (SsBb) and the long, brown parent is ssbb, the cross is:

SsBb × ssbb

Gametes from SsBb: SB, Sb, sB, sb. Gametes from ssbb: sb only. Combining gives four equally likely offspring genotypes/phenotypes:

- SB × sb → SsBb → short, black (25%)
- Sb × sb → Ssbb → short, brown (25%)
- sB × sb → ssBb → long, black (25%)
- sb × sb → ssbb → long, brown (25%)

So you get a 1:1:1:1 ratio — 25% each phenotype. (If the short/black parent were homozygous at one or both loci the ratios would differ.)