Darwin suggested that within any species of animal, minor variations exist perhaps ini, color, shape, strength, or some other minor trait. In the struggle to find food and escape
from predators, such traits might give that animal an advantage, making it more likely to
survive: If the animal can pass on that trait to its offspring, they too will have this survival
advantage. Darwin's finches provide a good example. If a bird has a beak that help it obtain
food, it has a better chance of survival, so in time, only finchen with long beaks remained
where the cactus grew. But where only seeds could be found for food, the strong, wide
beaked finches thrived. Darwin called this natural selection Othern called it survival of the
fittest. It simply means that organisms that best adapt to a changing environment survive,
while those that don't lose out, and eventually die off.
Which of the following is an example of natural selection?
A
Over time, the Earth's crust started to solidify about one billion years after the
planet was formed.
B
Over time, a species of cricket became green because green crickets were better
able to hide among the leaves.
С
In the winter, as food becomes more scarce, some species of birds fly to new
habitats in the south.
D
In June, the Northern Hemisphere gets more direct sunlight due to the tilt of
Earth's axis as it revolves around the sun.
Mi

"do a testcross with a homozygous recessive plant."

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.