Understanding Genes, Alleles, and Inheritance

A gene is the carrier of an inherited feature that is passed from generation to generation.

Alleles are possible alternatives for a character. Alleles stand at the same site on two chromosomes in an individual, one from each parent. Alleles are equal when it is said that the individual is homozygous or pure-bred for the character. If different, they are heterozygous or hybrid.

A dominant allele is the character that is always represented (capital letters).

A recessive character emerges only in homozygous individuals (lowercase).

Genotype is the joint of the genes, half from one progenitor and the other half from the other.

Phenotype is the set of characters that manifests externally. This is particular, but the genotype influences the environment.

Codominance: both alleles are expressed equally, giving rise to an intermediate phenotype in hybrids.

Mendel's Laws

Rule 1: Law of Segregation

If you cross two pure-bred individuals different for one character, the F1 hybrids are in equal genotype and phenotype. If the phenotype of the F1 is equal to that parent, then the character is dominant. If there is an intermediate phenotype, there is codominance.

Rule 2: Law of Independent Assortment

Separating the couple of alleles in the next generation, if you cross between the F1 hybrids, a second generation arises in which appears the character of the recessive allele that is not expressed in F1.

Rule 3: Law of Independent Assortment

Inheritance of two characters.

Chromosome Theory

• Heredity is located on chromosomes.
• Genes are arranged linearly on chromosomes.
• Genes occupy the same position for the same character.
• In gamete formation, the chromosomes of each pair are randomly assigned.

Mutations

A mutation is any change that affects the genotype and may be transmitted to progeny.

• Gene mutation: directly affecting a gene.
• Chromosomal mutations: affecting the structure of a chromosome (loss, repetition, inversion, or translocation).
• Genomic mutations: changes affecting the number of chromosomes of a kind.

Living cells have a double chromosome call, diploid or 2N. In the human species, this equals 46, with the exception of sex cells, maintaining haploid (N). There are two types:

• Polyploids: when increasing the number of chromosomes in a cell (triploid or 3N, tetraploid or 4N, etc.). These are common in plants.
• Aneuploid: due to variation in the number of chromosomes by any failure. Some of these mutations affect the human species (Down syndrome is a trisomy 21).