Answer:
D. they are usually dominant or recessive.
Explanation:
alleles are pairs of genes found on a particular spot a chromosome. If one is dominant over the other it give rise to a particular trait.
The invention of the microscope allowed the first view of cells. English physicist and microscopist Robert Hooke (1635–1702) first described cells in 1665. He made thin slices of cork and likened the boxy partitions he observed to the cells (small rooms) in a monastery.