Final answer:
In the cross of a homozygous black male rat with a heterozygous female rat, the genotype would be 1:1 BB to Bb and the phenotype would be 100% black rats. This follows the principle of dominance where the black fur allele is dominant over any other allele.
Step-by-step explanation:
When you cross a male homozygous black rat (which we can represent as BB for black fur being dominant) with a female heterozygous rat (Bb), the offspring's genotype would be 1:1 ratio of BB (homozygous black) to Bb (heterozygous black). The phenotype would be 100% black rats since both BB and Bb genotypes express the black fur trait due to dominance.
This is similar to a test cross, which is a cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; however, in this question, both the genotypes of the parents are known. The expected outcome in a test cross can help determine if the unknown parent is heterozygous or homozygous for the dominant trait.
For an X-linked trait example, if you cross an X-linked recessive white-eyed male fruit fly with a homozygous red-eyed female, all F1 females would be heterozygous red-eyed and all males would be red-eyed. The homozygous dominant parent would pass the dominant allele to all offspring, masking the expression of the recessive allele in the next generation.