Essential Terminology: Genes, Genotype, and Inheritance

Fundamental Concepts in Genetics

Understanding genetics requires familiarity with key terminology related to DNA sequences, inheritance patterns, and individual characteristics.

Defining Genes and Genetic Makeup

• Gene

  A Gene is an entire nucleotide sequence determined along one side of the chromosome ladder. This sequence may code for proteins or be inhibited, depending on the program assigned to the cells bearing the chromosomes.

• Genotype

  The Genotype is the genetic content (specific genome) of an individual in the form of DNA. It can also be defined as the complete set of genes of an organism.

• Phenotype

  The Phenotype is the observable expression of the genotype in a given environment. Phenotypic traits include both physical and behavioral characteristics. The genotype, along with environmental variation, determines the phenotype of the individual.

Inheritance and Allelic Combinations

• Allele

  An Allele describes each of the genes in a pair, which occupy the same place (locus) on two homologous chromosomes. Alleles exert the same function on a character or trait of the organism.

• Homozygous

  An individual is Homozygous if they have two identical forms of a specific gene, having inherited one copy from each parent.

• Heterozygous Individual

  A Heterozygous Individual is one that, for a given gene in each homologous chromosome, has a different allele.

• Dominant Gene

  A Dominant Gene refers to the allele that manifests itself in the phenotype, whether it is present in a double dose (homozygous combination, receiving a copy from each parent) or a single dose (heterozygous combination, where one parent provided the dominant allele in their gametes).

• Recessive Gene

  A Recessive Gene is the member of an allelic pair unable to express itself when the dominant allele is present. For this allele to be observed in the phenotype, the organism must have two copies—one inherited from the mother and one from the father.

Types of Individuals and Reproduction

• Pure Line

  A Pure Line is a group of individuals consisting of a parent and its progeny through several generations that are uniform in one or more characters.

• Clone

  A Clone is a set of genetically identical individuals descended from a single individual through asexual reproduction mechanisms.

• Monohybrid

  A Monohybrid is an individual whose genotype contains two different alleles for a particular gene (locus). If the locus under study has only two alleles (e.g., A and a), the hybrid genotype is Aa.

• Dihybrid

  A Dihybrid is an individual heterozygous for two loci. This means the genotype contains two different alleles at each locus. If the loci under study are biallelic (e.g., A, a and B, b), the dihybrid genotype is AaBb.

Mendelian Principles

Mendel's Second Law: Segregation of Alleles

Mendel's Second Law, also known as the Law of Separation or Disjunction of Alleles, states that during gamete formation, each allele of a pair is separated from the other member to determine the genetic constitution of the resulting gamete. The possibilities of hybridization are often represented using a Punnett square.