As soon as the sperm has entered the egg, the nucleus of the sperm and the nucleus of the egg unite to form one nucleus. The nucleus is the central, important part of the sperm and the egg. Under the microscope it looks like a large dark dot. Within the nucleus lie the chromosomes, the genes, and other necessary elements of life.
Chromosomes are threadlike structures within the nucleus of the sperm and egg. They carry the genes, which are responsible for what the new child will look like, how he will act, and all the rest of his traits and characteristics. The chromosomes and genes are responsible for a child resembling one parent more than the other. They are responsible for a child having blue eyes, or brown eyes, or green eyes. They determine how short or tall a child will grow to be, whether his nose will be long or short, narrow or wide, and whether he will inherit various personality traits that his parents have.
Most important is that the chromosomes of the sperm decide whether the new child will be a boy or a girl. The chromosomes of the egg have nothing to do with the child being a boy or a girl! It was your father-not your mother-who supplied the chromosome that determined that you are a girl or a boy. Here’s how it works:
Each and every normal sperm and egg carries twenty-three chromosomes. When the sperm and egg unite, these chromosomes unite and form pairs.
Here, we see the egg, already fertilized by a sperm, beginning to divide into many cells. This takes place while the egg is traveling down from the fallopian tube to the uterus. After the egg buries itself into the wall of the uterus it grows extremely rapidly.
responsible for the inheritance of a different trait or characteristic. Some scientists think that about 17 million possible combinations of chromosomes and genes can occur when a sperm unites with an egg. Thus, even though brothers and sisters may look or act alike in many ways, they may also differ tremendously from one another. They can have entirely different personalities and characters, or may not look alike at all, depending upon the combination of genes that results from the union of the sperm and the egg.
Some genes are called dominant. This means they are so strong that they are likely to appear in most of the children of one set of parents, and they probably will appear in grandchildren, too. That is why we sometimes see a girl or boy who not only looks like a parent but like a grandparent. Other genes are called recessive. This means that they are not very strong and are not likely to appear in many of the children of one set of parents. A recessive gene may produce a characteristic such as blue eyes in a family where everyone has brown eyes. A recessive gene may not happen again for a long time. Thus, a blue-eyed woman who came from a brown-eyed family may have all brown-eyed children and grandchildren.
Sometimes a child inherits genes that make him look like his father and at the same time inherits genes that give him his mother’s personality and character.
After the sperm and egg have united to form one cell with one nucleus, it immediately begins to multiply and divide. First it splits into two, then the two into four, and so forth. This happens even while the fertilized egg is traveling down the Fallopian tube toward the uterus. In the three to four days it takes to reach the uterus, the fertilized egg will have multiplied many, many times so that it will have become a cluster of many cells. And as each cell of the fast-growing egg divides, its chromosomes split into half, one half going to each of the two new cells. This means that every single new cell will contain the same chromosomes that were in the original sperm and the original egg.
Also, as the fertilized egg travels down to the uterus, it begins to develop a sac containing fluid around it. This fluid sac is called the amnion. On about the seventh day after the egg has been fertilized, the sac will attach itself to the lining of the uterus and will begin to absorb nourishment produced for it in the uterine lining. The egg and its sac will then bury itself in the lining, a placenta will develop, and the egg now called an embryo–continues its rapid growth.