Which of the following can be observed in a karyotype?
a. a change in a dna base
b. an extra chromosome
c. genes
d. alleles

B. 
karyotypes are just pictures of the chromosomes. you don't have great enough resolution to see a change in dna base, as bases are extremely small compared to a chromosome. similarly, genes and alleles are not visible as individual sets. technically, you can see all the genes of the chromosome on the karyotype, but that's like saying you can see all humans on earth with a picture from a satellite. alleles are just different variations of a gene (for example, the gene for eye colors has variants of blue, green, brown, etc; these colors are alleles; though really eye color is more complex than that). thus, alleles are not visible either.