4. Gene Pool is not a mechanism, thus your answer!
Because electrons are “pulled” towards the oxygen.
The pulmonary vein carries oxygenated blood from the lungs to the left atrium
The effect nervous system has on the heart rate is
Stimulation by parasympathetic nerves causes the heart rate to slow down.
The two branches of the autonomic (involuntary) nervous system regulate heart rate. The parasympathetic nervous system and the sympathetic nervous system (SNS and PNS) (PNS). To increase heart rate, the sympathetic nervous system (SNS) produces the catecholamines epinephrine and norepinephrine. The hormone acetylcholine is released by the parasympathetic nervous system (PNS) to reduce the heart rate. Your heart rate may briefly increase due to stress, coffee, and excitement, whereas it may temporarily decrease due to meditation or deep, steady breathing. Any amount of exercise will raise your heart rate, which will stay up as long as you keep exercising.
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Answer:
Let the "barred" allele be caller B and the "non-white" allele b. Since chickens use chromosomes 
and 
to determine sex, hens would have chromosomes 
, and roosters would have chromosomes 
. A Z-linked gene is represented as a superscript on the chromosome, 
for the dominant allele and 
for the ressesive allele.
A barred hen would have a copy of B on its Z chromosome, a non-barred rooster would have both copies of b on both Z chromosomes. Using Punnet squares to represent the crosses we get the following cases:
That is a ratio of two barred heterozygote roosters to two non-barred hens. Crossing them we get:
That is a ratio of one barred heterozygote rooster to one barred hen to one non-barred rooster to one non barred hen.
