Genetics (II)
The formation of gametes (sex cells) is accomplished by the process of cell division called meiosis. It is begun by the replication of chromosomes as in mitosis, but in this case the copies remain together while the homologous chromosomes--the two chromosomes that form a pair--are separated and incorporate to different nuclei. The cell divides giving two cells each one with two copies of just one member of each chromosome pair. These two cells divide in turn, the copies going each one to a different cell, and the final result is four cells where each one has half the number of chromosomes of the progenitor cell. Two of the daughter cells have one chromosome of each pair, and the other two cells have the other chromosome of the pair. The reproduction of animals and of some plants is done by the union of gametes to form a cell called zygote that is the first cell of a new organism. The zygote has half the chromosomes of the mother and half the chromosomes of the father.
Each chromosome has always the same genes, but there may be more than one form of a gene. These forms are called 'alleles.' The characteristic (or trait) that is influenced by a gene will have corresponding forms called 'phenotypes,' while the underlying gene structure is called 'genotype.' An allele can be, in turn, of two classes: dominant or recessive. When the pair of instances of a gene in a cell is formed by identical alleles, the genotype is said to be homozygous; otherwise, it is said to be heterozygous.
For example, Mendel's pea plants can exist in two phenotypes: tall and dwarf. There is one gene related to these phenotypes, which consists of two alleles, one causing the plant to be tall, and the other causing it to be dwarf. The first one is dominant and the other recessive. The dominant is labeled T, and the recessive t. As chromosomes are paired, there are in the cell two of these genes, hence the possible genotypes are: TT, Tt, or tt. All cells of the plant will have the same genotype, which corresponds to the genotype of the zygote. The phenotype of the plant will be tall if there are at least one dominant allele in the genotype. Therefore, a tall plant can have a genotype TT or Tt. The genotype tt will produce a dwarf plant.
Genetics is relevant from the point of view of the human being's health. Many human diseases are caused by defective genes that are passed on from generation to generation. One example is phenylketonuria (PKU), a nutritional disorder that if untreated produces mental retardation. There is a gene that is responsible for the metabolism of the aminoacid phenylalanine (a component of milk), which has two alleles: one dominant (P) that synthesizes the required enzyme, and one recessive (p) that does not. If two unaffected persons with genotypes Pp are married, then half of their sperm and egg cells will have the allele P, and half will have the allele p. A child born to this couple may have the genotypes PP, Pp, or pp. The probabilities are, respectively, 25 percent, 50 percent, and 25 percent. Hence, a healthy child will be born in 75 percent of the cases, and a sick one in 25 percent of the cases.
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