Final answer:
Performing a test cross can determine if a dominant-expressing organism is homozygous or heterozygous for the trait. By crossing it with a homozygous recessive organism, the resulting offspring will indicate the genotype of the dominant-expressing organism.
Step-by-step explanation:
A test cross can be performed to determine whether an organism expressing a dominant trait is homozygous or heterozygous. In a test cross, the dominant-expressing organism is crossed with a homozygous recessive organism. If the dominant-expressing organism is homozygous, all the offspring will be heterozygous for the dominant trait. However, if the dominant-expressing organism is heterozygous, the offspring will exhibit a 1:1 ratio of heterozygotes and recessive homozygotes.
To determine the genotype of a mouse with a dominant phenotype, perform a test cross with a homozygous recessive mouse; offspring phenotypes will reveal the genotype as homozygous or heterozygous.
To determine if a mouse with a dominant phenotype (P-) is homozygous (PP) or heterozygous (Pp), you would perform a test cross with a mouse that is homozygous recessive for the trait. In this cross, the mouse with the dominant phenotype is mated with a mouse that has the recessive phenotype (pp). If the offspring all have the dominant phenotype, the original mouse is likely homozygous (PP). If some offspring have the recessive phenotype, the original mouse is heterozygous (Pp).
Option 3) Cross it to a mouse with the recessive trait is the correct method. This technique is a classic genetic tool used to determine an unknown genotype when an organism expresses a dominant trait. The outcomes of this test cross can reveal the genetic composition of the dominant parent by observing the phenotypes of the offspring. If any of the offspring display the recessive trait, it indicates that the dominant parent must have contributed a recessive allele, confirming heterozygosity.