1. Gene: Mendel presumed that a character is determined by a pair of factors or determination present in each cell of an individual. Such an inherited factor determines a biological characteristic of an organism that is known as a gene in modern genetics. A gene is the unit of DNA responsible for the appearance and inheritance of a character. For example, flower color, seed shape, or stem length, each of these characteristics is controlled by a different gene.
2. Genes and Alleles or Allelomorphs: ‘Genes that govern variations of the same character and occupy corresponding loci on homologous chromosomes are called alleles or allelomorphs‘.
An allele is a Greek word that means ‘belonging to one another‘. It refers to one of the two members of a gene pair. These represent alternatives of a character and are present on two separate chromosomes of a homologous pair, but at the corresponding loci.
Genes are self-replicating units and are quite stable. These replicate quite faithfully. But rarely, once out of thousands or millions of times its composition may change. This is called a mutation. The changed or mutant gene produces a changed phenotypic trait and is called an allele or allelomorph.
3. Locus: The term locus is used to designate the location of an allele on a chromosome. If the locus is named such as yellow locus in pea, it also specifies the kind of ‘generic‘ gene controlling a particular characteristic.
4. Genotype and Phenotype: The term genotype designates ‘the genetic makeup of an organism’, whereas phenotype is the appearance of an individual or the expression of genes possessed by an individual.
5. Homozygous and Heterozygous: Every organism possesses two alleles of a gene for every character. An organism with two identical alleles for a particular character is said to be pure or homozygous for that character. The prefix homo means ‘the same‘ and zygous means ‘a pair‘.
6. Pure-Line: Generations of homozygous individuals which produce offspring of only one type form a pure line.
7. Monohybrid Cross: It involves the study of the inheritance of one pair of contrasting characters.
8. Dihybrid Cross: It is the inheritance of two pairs of contrasting characters.
9. Polyhybrid Cross: It includes those crosses in which the inheritance of more than two pairs of characters is considered.
10. Reciprocal Cross: The reciprocal crosses involve two crosses concerning the same characteristics. But with reversed sexes. It means if in the first cross A is a female parent and B is the male parent then in the 2nd or reciprocal cross A will be used as the male parent and B as the female parent.
11. Back Cross and Test Cross: Cross between F1 offsprings with either of the two parents (hybrid) are known as back cross. When F1 offsprings are crossed with the dominant parents all the F2 offsprings develop dominant character.
On the other hand, when F1 hybrids are crossed with the recessive parents, individuals with both phenotypes appear in equal proportion. While both crosses are known as the back cross, the second one is specified as the test cross.
The test cross is a cross between heterozygous F1 hybrid and the double recessive homozygous. The test cross is used to determine whether the individuals exhibiting dominant character are homozygous or heterozygous.