Genes, Gametes, and Conception
The conception process is very complex and important to understanding human development. Conception begins with the fusion of an egg cell and a sperm cell, or gametes. At birth, a female has all the immature eggs that she will use throughout the course of her life. A male human being starts producing sperm when he reaches puberty (around 1,000 sperm in a second, but this rate slows as the male ages).
Though a female is born with all her eggs, not all these eggs will reach maturity. Approximately only 1 in 5,000 of a female's eggs reach maturity. When an egg reaches maturity, that egg is then able to produce offspring.
From 200 to 600 million sperm are released in the average ejaculation but a rare few make it to the actual egg, and only one sperm is needed to form a zygote (or a fertilized single-celled egg, the earliest form of human beings). As soon as that one lucky sperm begins to penetrate the jellylike outer coating of the egg cell, the egg becomes defensive and the surface of the egg cell hardens to block out any other sperm cell from penetration. The sperm uses digestive enzymes to work its way through the egg's surface.
Once the zygote is formed, the cell goes into the division process. The first division is called mitosis. In mitosis, the zygote divides to form two identical daughter cells. Later, the cell begins another form of division called meiosis. Meiosis produces four daughter cells, each daughter cell containing half the chromosomes of each original parent cell. Meiosis is necessary to keep the chromosome number constant from generation to generation. The divisions will continue, until a human being is formed. The cells move, or migrate, in relation to other cells, forming the first shape of the embryo; this migration is called morphogenesis.
Each gamete (egg and sperm) has twenty-three chromosomes, and when the human is completely developed, she will have forty-six chromosomes. Your genes are located on your chromosomes. A gene is a small piece of one chromosome; it is a code for a specific sequence of amino acids in a protein. Each code is different and very complex, which brings about many different traits.
Most people are familiar with genes as a transportation of hereditary traits. Genes can affect whether or not you will be born with attached earlobes, freckles, or a widow's peak. Genes are the hereditary codes that are passed on to an offspring. Traits, which are caused by your genotype (or genes for a particular trait), can be dominant or recessive. Dependent upon the alleles carried on the chromosomes you received from your parents, your appearance will develop accordingly.