The genes contained in DNA provide information about the different characteristics of the individual. Each character, determined by a gene, can have several different alternatives or alleles. The genotype is the set of genes of the individual. The expression of this genotype, depending on a certain environment, constitutes the phenotype of the individual. That is, the phenotype is what we see of that individual, if they have light or dark eyes, if they have straight or curly hair, etc. The phenotype is influenced by the environment, since it can modify it. The genotype, on the other hand, is not influenced by the environment.
Interactive activity: Genotype and phenotype: influence of the environment on skin color.
In diploid beings, DNA is grouped into pairs of chromosomes. In each of these pairs, one chromosome comes from the father and one from the mother (homologous chromosomes). Each gene appears on the two chromosomes of each pair, so a certain character is determined by these two genes, which can be the same or different. The alleles are the different types of possible genes that may be for that character. For example, having light eyes is due to the action of one allele, and having dark eyes is due to the action of another allele. An individual can have the same two alleles, on the same pair of chromosomes, or different.
- If the two alleles of the pair are equal, the individual is said to be homozygous or purebred for that trait.
- If the two alleles of the pair are different, the individual is said to be heterozygous or hybrid for that trait.
The homologous chromosomes are a pair of chromosomes, one from the father and one from the mother, which contain the same genes, but may have different alleles.
As we have seen, the genotype is the set of genes of an individual. But not all the alleles that we have are manifested, since each character is determined by two alleles, and one of them may not appear. That is why there are characters that remain hidden, without manifesting themselves, but they are expressed in the phenotypes of grandparents and grandchildren.
Some of the genetic concepts that we have to be clear about are the following:
- Gene: it is a fragment of chromosome responsible for the appearance of a hereditary character . The molecular genetic defines a gene as a DNA fragment (nucleotide sequence) responsible for the synthesis of a protein . Genes are segments of DNA made up of hundreds or thousands of nucleotides.
- Locus. It is the place on the chromosome where the gene is located. In the plural, it is called loci.
- Homologous genes. The homologous chromosomes have equivalent loci. That is, they both have the gene with the information for the same character in the same position. Therefore, in diploid cells, each character is regulated by two genes.
- Alleles. They are each of the different possible genes that can be located at a certain locus. Two alleles of homologous chromosomes may or may not contain the same information .
- Homozygous individual or pure race (for one character). When the two genes on homologous chromosomes contain the same information, that is, they have the same allele .
- Heterozygous or hybrid individual (for one character). When the two genes on homologous chromosomes contain different information, that is, they have two different alleles.
- Genotype and phenotype. The genotype is the set of genes that an individual has. The phenotype of an individual is the set of characters that it manifests. The phenotype is determined by the genotype and influenced by the environment.