A testcross is a cross performed to test the genotype of an organism. The test cross is performed when the nature of the phenotype that is if the phenotype is dominant or recessive is known.
We know that a recessive trait is the result of the homozygous condition or phenotype is expressed when the genotype has two recessive alleles. But the genotype of the dominant trait could be determined when the genotype has either both the dominant alleles (homozygous dominant) or the either one of them is dominant allele (heterozygous condition).
So to determine if the dominant phenotype is determined by homozygous genotype or the heterozygous genotype, testcross is performed between the homozygous recessive genotype and the unknown dominant phenotype organism.
Thus, Homozygous recessive individual is the correct answer.
Oxygen levels are generally thought to have increased dramatically about 2.3 billion years ago. Photosynthesis by ancient bacteria may have produced oxygen before this time. ... In addition, early plants and algae began to release oxygen at a faster rate. Oxygen levels then showed a dramatic increase.
