Let us continue our interesting story about the baby. We have learned that as the early body cells are dividing, a few of them are set aside, unchanged, for a special purpose, and that these are called germ cells. Now what happens to these germ cells?
Germ cells have central controller call the nucleus. This nucleus being the living, active part of the germ, is made up largely of a substance called chromatin. This chromatin is that part that has to do with heredity. Half of this come from our paternal cell and the other half from our maternal cell which are united upon which the general make-up of the individual, both physically and mentally, is made.
Chromatin materials form into chromosomes. Humans have 23 pairs of chromosomes--22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, X and Y. Each parent contributes one chromosome to each pair so that offspring get half of their chromosomes from their mother and half from their father.
In females, at puberty there are about thirty- five thousand ova in each ovary; and there are forty-six chromosomes in humans but, instead of the chromosomes each splitting into two equal parts, they at once saperate into two equal groups. One of these groups is then thrown out of the cell, and is lost. So the resulting cell has only twenty-three chromosomes instead of forty-six A similar thing also happens with the male germ cell remaining only half of the chromosomes. So each remaining cell is really only a half cell, and needs a complementary union of a male's twenty-three chromosomes to form a complete forty-six Chromosomes offspring. This is what occurs during fertilization.
In these Chromosomes are the factors for all the family characteristics that have been handed down from both parents, as tallness or shortness; brown eyes or blue; straight hair or curly; dark or fair skin; tendencies to weakness or strength, to caution or recklessness, to moral integrity or instability. Nothing in nature can be more wonderful than this in its infinite complexity.
We will continue this study a little further. We have seen that in the chromosomes of each body cell are all the factors that have come down from the father and the mother ( one half from each parent). Now, in every complete set of chromosomes there is a gene or genes influencing every possible hereditary characteristic. These genes are always in pairs- ( one from the paternal side and the other from the maternal). The genes of any pair may be alike or they may be very unlike.
Take, for example, a pair of genes that has to do with height. If the parents are both short, the evidence is that the height genes in the body cells of their offspring will carry shortness, and their influence will be to make the offspring short. Likewise, if both father and mother are tall, the height genes in the body cells of their child will carry tallness, and the child will tend to develop into a tall individual.
However, if one of the parents is tall and the other short, a child born of them may carry in each one of his body cells a pair of height genes- one of which will have an influence for tallness, the other an influence for shortness. Tallness, we will say, is positive, or dominant, trait and thus will have a stronger influence than shortness, which is a negative, or recessive trait. This being the case, the child will tend to grow tall in spite of his short genes.
One very important and interesting thing we should note no person ever passes on to any one of his reduced germ cells both a gene he has received from his mother and its partner gene received from his father. He may have only the mother gene or the father gene, never both. Isn't this a wonder of all wonders?
Nothing in all nature is more thrilling than to watch these life processes under the microscope, or to study their outcome in the future offspring. The way these Chromosomes behave in the cells, the marvelous and, to us, still mysterious way in which they move with all the mechanical precision of the planets; the way they behave and grow, carrying various characteristics of the ancestry among the descendants. All this to my mind is the most inspiring and exciting series of events which it has ever been the privilege of the human mind to contemplate.