Genetics and Heredity
If scientists would translate the chemical code inside the tiny fertilized egg, they could tell whether the child is to be a boy or a girl, blue or brown –eyed, blond or dark-haired. The storehouse of this information lies in the chromosomes. Under a microscope, chromosomes appear as dark strings in the nucleus of each body cell.
With the exception of the sex cells (the sperm and the egg), 46 chromosomes are in each cell of the human body. Egg and sperm cell each have 23 chromosomes. In fusing, they give the fertilized egg the total of 46 chromosomes.
Whether the fertilized egg develops into a boy or girl is determined by two special chromosomes-the sex chromosomes.Every egg bears a chromosome known as the X chromosome and every sperm bears either an X or a Y chromosome. When the sex chromosomes meet in the fertilized egg, an XX baby will be a girl and an XY baby, a boy.
When an egg cell is fertilized by a sperm cell, it starts to grow. First, it divides into two cells; then into four cells; then into eight cells, and so on-until it produces all the cells in a baby’s body. During each cell division, the 46 chromosomes in a cell duplicate themselves so that each new cell gets an identical group of 46 chromosomes. The 46 chromosomes are composed of two sets-one set of 23 from the individual’s father and one set of 23 from the mother. Each chromosome in one set of 23 can be matched to a particular chromosome in the other set. In a girl, the 23 pairs of chromosomes look alike. In a boy, one pair of chromosomes, unlike the other 22 pairs, does not look alike. In a girl, the two sex chromosomes (XX) look alike. In a boy, the chromosomes (XY) do not look alike.
Genes determine hereditary traits. Hundreds of tiny particles called genes are arranged in a line along the length of each chromosome. Genes control a child’s inherited traits, such as the blood group, blood-clotting ability, sensitivity to certain testes and hair color.
DNA (deoxyribonucleic acid) is the key chemical compound of a gene. DNA is a molecule that consists of two threadlike strands that are connected by crosspieces. The two strands are wound around each other. The structure (called a double-helix) looks like a rope ladder twisted into a spiral. A DNA molecule makes an exact duplicate during cell division. Except in certain instances of sex-linked traits, there are two genes for every trait-one gene on a chromosome in the set of 23 from the individual’s mother and one gene on the matching chromosome in the set of 23 from the father. In many traits, the action of one gene overpowers the action of the other. The more powerful gene is called dominant and the other gene is called recessive.
The gene for brown eyes is dominant over that for blue eyes. If a child gets a gene for brown eyes from the father and a gene for blue eyes from the mother, the child will have brown eyes. Except in certain instances of sex-linked traits, it takes two recessive genes to make a recessive trait show up. Both chromosomes must carry the gene for blue eyes for a child to have blue eyes. The individual with one gene for blue eyes and one for brown may transmit either gene to the next generation, because only one chromosome from the pair (and therefore only one gene), goes into the sperm or egg cells.
Genes are not always either dominant or recessive. This situation is called incomplete dominance. Three genes-which may be designated “A”, “B” and “a”-control the heritance of blood- group types. (Even though there are three genes, the most any person can have is two.)”A” and “B” are dominant to “a”, but they are incompletely dominant to each other. If a child inherits a dominant “A” gene from each parent or a dominant “A” gene from one parent and a recessive “a” gene from the other, the child has blood type A. If the child inherits a dominant “B” gene from one parent and a recessive “a” gene from the other, the child has the blood type B.
However, if the child inherits as “A” gene from one parent and a “B” gene from the other, the child’s blood type is neither A or B. It is a type AB because of incomplete dominance between genes A and B. If the child inherits a recessive “a” gene from each parent, the child’s blood type is O.
