4) All the offspring of a cross between a black-eyed Mendelian and an orange-eyed Mendelian have black eyes. What is the expected phenotypic ratio of a cross between two orange-eyed Mendelians?

0 : 1 ( black eyed : orange eyed )

Explanation:

When black eyed-Mendelian is crossed with orange-eyed Mendelian, the entire progeny has black eyes. This means that the allele for Black color is dominant to the color for orange color and it masks its effect completely in the next progeny.

BB X bb = Bb ( All black )

So orange Mendelian is a recessive trait and will have genotype bb. When two orange eyed Mendelians are crossed:

bb X bb = bb ( All orange since there is no B allele )

Hence, the phenotypic ratio will be = 0 : 1 ( black eyed : orange eyed )

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.