Earth Science: Genetics and Heredity – Genetics
The science of genetics examines the mechanisms governing the inheritance of traits by the offspring of living things.
Within the field, a distinction is often made between general or classical genetics, which deals mainly with the formal aspects of heredity, and molecular genetics, which researches the underlying phenomena of inheritance on the molecular level.
Earth Science: Biology – Genetics and Heredity – Genes
The carriers of genetic information are the genes. These are contained in the chromosomes: structures formed from DNA and proteins. The genetic information is represented by the order of nucleotides within the gene.
A gene is a segment of DNA (deoxyribo nucleic acid) within a chromosome that encodes information for the synthesis of a particular protein. In eukaryotes (organisms with a true cell nucleus), most of the organism’s genetic information is contained within the nucleus. In addition, the mitochondria (the cell’s “power plants”), and chloroplasts in plants, also have their own small, separate sets of genes.
In prokaryotes, which lack a cell nucleus, the DNA generally takes the form of a ringshaped molecule within the cell’s cytoplasm. Many bacteria also possess plasmids (smaller, nonchromosomal DNA molecules), which normally do not contain essential genetic information but may carry genes, such as those for toxin production, carbohydrate processing, or resistance against antibiotics.
Species with these genetic additions often have competitive advantages over other bacteria. Taken together, all of the genetic information contained in an organism’s genes constitute its genotype (the genetic constitution of an organism).
The actual appearance of the organism—its phenotype—results from interactions between the genotype and internal and external environmental influences. Thus, organisms with identical genotypes do not necessarily have the same phenotype.
Organisms with double (diploid) sets of chromosomes, such as human beings, have two copies of each chromosome (homologous chromosomes). Therefore, each gene is represented twice. Different occurrences of specific genes are called alleles. Accordingly, humans have two alleles for each gene.
If two series of nucleotides are identical, the genes are called homozygous; conversely, if the pattern is different, they are heterozygous. Genes that occur in more than two different versions are multiple alleles. If an organism is heterozygous for a particular gene, its phenotype may be determined by only the dominant of the two alleles.
The allele whose effect is overshadowed is recessive. If the phenotype is partially or equally influenced by both alleles, this is called intermediate inheritance, and the appearance of the heterozygous organism reflects a mixed form of the two homozygous phenotypes.