Which of the following statements about homologous chromosomes is correct?
a. each gene is at the same locus on both chromosomes.
b. they are two identical copies of a parent chromosome which are attached to one another at the centromere.
c. they always produce identical phenotypes.
d. they are chromosomes that have identical genes and alleles.

The gene that causes sickle-cell disease is present in a higher percentage of residents of sub-saharan africa than among those of african descent living in the united states. even though this gene causes sickle-cell disease, it also provides some protection from malaria, a serious disease that is widespread in sub-saharan africa but absent in the united states. discuss an evolutionary process that could account for the different percentages of the sickle-cell gene among residents of the two regions.

Describe the response the kidneys have to dehydration and excessive water intake. what happens to the concentration of urine in each case ?

Below, the levels of organization in a multicellular organism are shown from least to most complex. which level of organization can be described as several different types of tissues working together to perform a common task?