Aspecies of mice can have long or short tails. if you crossed a male mouse which is heterozygous and has a long tail with a female mouse with a short tail, what would the expected phenotypic outcome be and why?

The expected phenotypes will be long tail and short tail

Explanation:  

Suppose L for the character "long tail" and l for "short tail, the genotype of the male for that character would be Ll (dominant heterozygous), and that of the female ll (homozygous recessive). When these individuals intersect, the progeny of the F1 (first generation) would be genotypically Ll or ll, therefore the expected phenotypes will be long tail and short tail. Those with long tails will be heterozygous and those with short tails will be homozygous. This is because the male has a dominant and a recessive allele giving the chance of two possible combinations of alleles, while the female being homozygous, only has one allele option to combine with any of the two from the male. When a recessive allele meets a dominant one, the dominant one is expressed in the phenotype. A recessive allele will only be expressed if it is combined with another recessive.

a unit of heredity that is transferred from a parent to offspring and is held to determine some characteristic of the offspring.