Genes are the units of heredity. A gene is a specific DNA segment of a chromosome which controls the expression of a character. Genes are located in a linear fashion on chromosomes. What are chromosomes? Chromosomes are the carriers of hereditary units—the genes. These are located within the cell nucleus.
In normal conditions, each gene has two alternative forms of a character producing different effects. These alternative forms are called alleles. All chromosomes are present in pairs within the cell nucleus. Therefore, all genes are also in pairs, each present on one chromosome of the pair. Two alleles of a pair are always located at the same position or locus on the chromosome pair.
During meiosis, a gamete receives only one chromosome of a pair. Hence, each gamete has only one allele of the pair of alleles for a characteristic. The characteristics thus travel from parents to the offspring in the form of genes on chromosomes. They are being transmitted physically. Genes are, thus, the physical basis of heredity.
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Dominant and Recessive Alleles:
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Out of the two alleles of a gene, the allele which masks any phenotypic effect of any recessive allele is called dominant allele. While the allele that is masked is called recessive allele.
For example, the allele controlling the rolling of tongue is dominant allele and the allele which controls the condition in which tongue cannot be rolled is recessive allele.
Genotype and Phenotype:
The genetic constitution of an organism in which the genes are present in various combinations is called genotype. On the other hand, the externally visible expression of genes, which is an inherited feature in an individual’s appearance, is called phenotype. For every phenotype, there may be two conditions, homozygous and heterozygous.
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Homozygous condition:
A condition in which a pair of a homologous chromosomes carries identical alleles on a gene locus for a specific trait.
Heterozygous condition:
A condition in which a pair of homologous chromosomes carries non- identical alleles on a gene for a specific trait.