Understanding Genetics: From Mendel’s Laws to Mutations
Mendel’s Laws of Inheritance
1st Law: Law of Uniformity
When crossing two purebred individuals that differ in one character, all individuals of the first-generation offspring are identical for that character.
2nd Law: Law of Segregation
The genes determining a character separate during the formation of gametes and are reunited during fertilization.
3rd Law: Law of Independent Assortment
The genes that determine each character are inherited independently.
Key Genetic Concepts
Allele
An allele is each of the alternative forms that a gene can have. Alleles differ in their sequence and can manifest in specific modifications of the gene’s function.
Gene
A gene is an organized linear sequence of nucleotides in the DNA molecule (or RNA in some viruses). It contains the information necessary for the synthesis of a macromolecule with a specific cellular function, usually proteins, but also mRNA, ribosomal RNA, and transfer RNA.
Genotype and Phenotype
Genotype is the set of genes of an organism. Phenotype is the combination of observable traits of an organism.
Population Genetics
Population genetics is the branch of genetics that describes biological variation and distribution to explain evolutionary phenomena. It defines a population as a group of individuals of the same species that are reproductively isolated from other similar groups.
Genetic Mutations
A genetic mutation is an alteration or change in the genetic information (genotype) of a living being. This change produces a modification in the organism’s characteristics, which occurs suddenly and spontaneously and can be transmitted to offspring through inheritance.
Types of Chromosomal Mutations
- Chromosomal rearrangements: These involve changes in the structure of chromosomes, such as duplication, deletion, inversion, and translocation.
- Aneuploidy: An increase or decrease in the number of chromosomes.
- Polyploidy: The presence of extra sets of chromosomes.
Further Details on Chromosomal Mutations
- Aneuploidy can result in monosomy, trisomy, tetrasomy, etc.
- Polyploidy can involve chromosomes of identical or different origins, leading to autopolyploids and allopolyploids, respectively.
- Deletions and duplications can change large segments of the chromosome.
- Inversions and translocations result in little or no loss of genetic information.
Genome Mutations
Aneuploidy
Aneuploidy is the alteration of the number of one type of homologous chromosome.
Homozygous and Heterozygous Genotypes
Homozygous
When an organism is homozygous for a specific gene, it means it has two identical copies of that gene for a given trait on both homologous chromosomes (e.g., a genotype of AA or aa). Such cells or organisms are called homozygous.
- A homozygous dominant genotype occurs when a particular sequence comprises two alleles for the dominant attribute (e.g., AA).
- A homozygous recessive genotype occurs when the sequence consists of two recessive alleles of the attribute (e.g., aa).
Heterozygous
A heterozygous individual is a diploid organism that, for a given gene (locus), has a different allele on each of the two homologous chromosomes (e.g., Aa). This means it has two different forms of a particular gene, each inherited from one parent.