• Characteristics that run in families have a genetic basis, meaning that they depend on genetic information that a person inherits from his or her parents. This is true for all plants and animals.
• Transmission of characters from one generation to the next generation or the phenomenon of the offspring to inherit the parental traits is known as Heredity.
• The differences that exist between the offsprings and their parents are known as variation.
• The study of scientific facts of heredity and variation is referred to as Genetics.
• Our modern understanding of inheritance of traits through generations comes from the studies made by Gregor Mendel, an Austrian monk.
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• When a cross is made between parents with one pair of contrasting characters it is called a monohybrid cross.
• Each gene exists in two alternate forms called alleles. When two similar alleles for a trait are present in an individual it is referred as homozygous alleles, while when two different alleles for a trait are present in an individual it is referred as heterozygous alleles.
• The phenotypic ratio of a monohybrid cross is 3:1 and the genotypic ratio is 1:2:1.
• A test cross help determine whether a dominant phenotype is homozygous or heterozygous for a specific allele.
• Mendel’s law of dominance states that out of the two contrasting characters, one is dominant and the other is recessive.
• According to law of segregation, alleles of traits segregate in the process of inheritance.
• In case of incomplete dominance, the two alleles of a trait are not related as dominant and recessive, but the dominant gene in heterozygous condition reduce expression, so that each of the allele expresses itself partially.
• When both the alleles of parental generation get equally expressed in F1 heterozygote, it is known as codominance.
• When a cross is made between parents with two pairs of contrasting characters, it is called a dihybrid cross.
•According to the principle of independent assortment of alleles, factors or genes controlling a specific trait is inherited independent of each other.
The phenomenon of inheritance of genes together and to retain their parental combination even in the offspring is known as linkage. Genes located on the same chromosome and being inherited together are known as linked genes and the characters controlled by these genes are known as linked characters.
• The phenomenon in which non-parental combinations appear due to interchange of alleles is called crossing over.
• There are different types of inheritance: Sex-linked inheritance (inheritance resulting from a recessive gene in the sex chromosome), extra chromosomal or cytoplasmic inheritance (inheritance of characters controlled by genes present in the cytoplasm) and maternal or uniparental inheritance (transmission of genetic characters only from maternal extra nuclear elements such as mitochondrial DNA). Organisms having more than two sets of chromosomes in each cell are known as polyploids.
• Organisms in which either one or more chromosomes of a chromosome set are either missing or have more copies are known as aneuploids.
• The process of analysing phenotypic variations in the population and to determine genetic constituents (DNA sequences or genes) regulating these variations is called forward genetics. While reverse genetics approach starts with DNA sequence (gene) or a protein sequence with an unknown function.
• Characters are controlled by small DNA segments called genes.
• Collection of all the DNA material in one complete set of chromosomes (including all the genes and other part of DNA) in a cell is referred to as genome. A eukaryotic cell has two types of genome: nuclear genome and organellar genome (such as chloroplast genome and mitochondrial genome).
• One complete set of chromosomes in an individual makes genome of that individual and is represented by the letter n. Most of the organisms carry two sets of chromosomes (2n) and are known as diploid organisms
