There are all sorts of ways to reconstruct the history of life on Earth. Pinning down when specific events occurred is often tricky, though. For this, biologists depend mainly on dating the rocks in which fossils are found, and by looking at the “molecular clocks” in the DNA of living organisms.
There are problems with each of these methods. The fossil record is like a movie with most of the frames cut out. Because it is so incomplete, it can be difficult to establish exactly when particular evolutionary changes happened.
Modern genetics allows scientists to measure how different species are from each other at a molecular level, and thus to estimate how much time has passed since a single lineage split into different species. Confounding factors rack up for species that are very distantly related, making the earlier dates more uncertain.
These difficulties mean that the dates in the timeline should be taken as approximate. As a general rule, they become more uncertain the further back along the geological timescale we look. Dates that are very uncertain are marked with a question mark.
Answer:
This question is incomplete, the options are:
A. Two copies of the allele that determines phenotype whenever the allele is present
B. Two copies of the allele whose effect is hidden unless the other allele is absent
C. One copy of the allele that determines phenotype when present and one copy of the other allele
D. One copy of each of two alleles that both contribute equally to determining phenotype
The answer is B.
Explanation:
According to Gregor Mendel, a Gene comes in two alternative forms called ALLELES. One allele called the DOMINANT ALLELE is capable of masking the phenotypic expression of the other called the RECESSIVE ALLELE. The dominant allele will always express itself whenever it is present. However, a recessive trait will only be expressed when two alleles for recessiveness occur in the gene.
An individual whose genotype is for the recessive allele will have two copies of the allele whose effect is hidden unless the other allele is absent. This means that in that particular gene, the two present alleles will be recessive alleles.
Carbon is the correct answer hoped i helped
