Have you ever wondered why children resemble their parents? This question has been asked by many since the beginning of human history. It wasn't until 1865 that an Australian monk called Gregor Mendel discovered that traits are inherited as discrete "factors", which later became known as genes.
Each cell in the body of a plant or an animal has a definite number of chromosomes, with the number being the characteristics of the species. In man, there are 23 pairs and members of each pair are known as homologous chromosomes. In other words, each cell has two complete sets of chromosomes where one set comes from the male parent ( paternal chromosomes ) and the other set comes from the female partner ( maternal chromosomes ). This is the diploid number (2n) of chromosomes.
Chromosomes contain genes which carry instructions for the development of a new organism and the traits or characteristics of an organism are determined by genes. For every gene, a person has two alleles, one inherited from each parent. The combination of inherited alleles represents the genetic makeup, or genotype, of the organism. The way a genotype is expressed in an organism is called its phenotype. For many traits the phenotype is a result of an interaction between the genotype and the environment.
For a specific trait, some alleles may be dominant while others may be recessive. The phenotype of a dominant allele is expressed regardless of what the other allele is, while the phenotype of a recessive allele is expressed only when both alleles are recessive. However, in some cases, one allele is not completely dominant over the other allele, and the resulting phenotype is a combination of each allele's phenotype. This is known as incomplete dominance. In addition, some traits are determined by a combination of several genes, and the resulting phenotype is determined by the final combination of alleles of all the genes that govern a particular trait.
So what is a gene made up of? A gene is made up of a short length of a chemical substance called deoxyribonucleic acid (DNA) located on the chromosome. A DNA molecule contains four types of nucleotides which are the building blocks of nucleic acid. Each nucleotides is made up of a five-carbon sugar (deoxyribose), a phosphate group and one of the following nitrogen-containing bases: adenine (A), cytosine (C), thymine (T) and guanine (G). This means that the nucleotides differ only in which kind of bases attached.
The nucleotides joined end to end in DNA and each DNA molecule is made up of two strands of nucleotides twisted around each other in the form of a double helix, much like the twisting of a circular stairway. Base pairing between the two nucleotide strands is not random in all species. An adenine base only pairs with a thymine base and cytosine base pairs only with a guanine base. However, the order of bases in a strand can vary greatly from one species to the other.
Since each organism has a constant number of homologous chromosomes, when carrying out sexual reproduction, the chromosome number of the gametes must be halved before fertilization. This chromosome number is known as haploid (n). Haploid gametes are produced by a type of cell division, known as meiosis.
Meiosis comprises two successive nuclear divisions with only one round of DNA replication. Four stages can be described for each nuclear division.
Prophase I: Each chromosome dupicates and remains closely associated. These are called sister chromatids.
Metaphase I: Homologous chromosomes align at the equatorial plate. Crossing-over can occur during this stage.
Anaphase I: Homologous pairs separate with sister chromatids remaining together.
Telophase I: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.
Prophase II: DNA does not replicate.
Metaphase II: Chromosomes align at the equatorial plate.
Anaphase II: Centromeres divide and sister chromatids migrate separately to each
pole.
Telophase II: Cell division is complete. Four haploid daughter cells are obtained.
During meiosis, the alleles separate. Each gamete has only one member of the pair of alleles. So during the combination of the genes of each gamete, a great deal of variation can be brought about. Further variation of the offspring is brought about by crossing over. The randomness of fertilization is another source of variation.
As a result, each of us in the world is unique. Genetic variation in a species can ensure the survival of the species under unfavorable condition, thus continuity of the species can be achieved.