Phenotype, Genotype and Mendel's Laws of Heredity
The phenotype includes all the observable characteristics of an individual, whether morphological, functional, biochemical, behavioral, etc., that are present in a living being. Phenotype is largely hereditary, that is, it corresponds to the living characteristics that an individual receives from its progenitors, although phenotype is not only hereditary.
The genotype is the whole set of genes that an individual presents. These genes very frequently determine characteristics that appear in the phenotype; in other cases, some genes do not manifest.
Genetics
Genetics is the science that studies heredity and variation. It consists of the results of the transmission of characters and the segregation of chromosomes during reproduction.
Key Terms
- Phenotype: any observable characteristic or trait of an individual, which manifests in its interaction with the environment where it develops.
- Genotype: represents all the genetic information that an individual has received from its progenitors.
- Homozygous: pairs of genes or alleles that are identical for a given character (AA or aa).
- Heterozygous: pairs of genes or alleles that are different (Aa).
- Zygote: the cell formed when a female and a male gamete fuse during fertilization.
- Gene: a segment of DNA that, when expressed, confers a particular living characteristic.
- Gametes: sexual cells that fuse during fertilization to form a new individual.
- Heredity: the transmission of the genetic content of a cell and of anatomical, physiological, etc. characteristics.
- Allele: each of the alternative forms a gene can take, which differ in sequence and may manifest in concrete modifications of the gene's function.
- Chromosome: each of the small rod-shaped bodies that organize chromatin and are the central element of the cell in mitosis and meiosis.
Dominance and Codominance
Complete dominance occurs when one of the alleles in a heterozygote masks the effect of the other on the character; the dominant allele determines the final metabolic route and the protein structure that defines the trait.
Codominance occurs when neither allele completely dominates the other, and the genotype results in a mixture of both.
Dominant and Recessive Traits
- Dominant trait: characteristics that appear in the first filial generation.
- Recessive trait: characteristics that appear in the second filial generation.
Crosses
A crossing is the mixing of individuals of different species or races. A monohybrid cross is a genetic cross considering only one pair of contrasting characters, for example, flower color. A dihybrid cross considers two pairs of contrasting characters.
Mendel's Laws
- First Law (Uniformity): when two pure-breeding varieties differing in only one character are crossed, all the offspring of the first filial generation are identical and uniform for that character.
- Second Law (Segregation): the genes that determine a character separate during the formation of gametes and rejoin during fertilization.
- Third Law (Independent Assortment): the genes that determine different characters are transmitted independently of each other.