Passage of Information from Parents to Offspring

Principles of Inheritance

Inheritance is the transmission of genetic characteristics (traits) from parent organisms to their offspring.
These traits can be determined by a single gene or multiple genes (known as polygenic inheritance).
Each organism inherits two copies of each gene, one from each parent. These copies may not be identical (i.e., they may represent different alleles).
Dominant alleles mask the effect of their recessive counterparts.
Traits can be co-dominant, meaning both alleles are simultaneously expressed in the phenotype.

Gregor Mendel, the “father of genetics,” established the basic laws of inheritance.
Mendel’s principles include the law of segregation (individual alleles segregate during meiosis) and the law of independent assortment (genes on different chromosomes are distributed independently to sex cells).
Classic Mendelian genetics involves organisms that are homozygous (carry two identical alleles) or heterozygous (carry two different alleles) for a certain trait.
Punnett squares are often used to predict the outcomes of simple monohybrid or dihybrid crosses.

Not all inheritance patterns adhere strictly to Mendel’s principles. Some examples include incomplete dominance, codominance, and multiple alleles.
Incomplete dominance occurs when the phenotype of the heterozygote is a blend of both homozygous phenotypes.
Codominance is when both alleles for a gene are fully expressed in a heterozygote.
A single gene may have more than two possible alleles in a population. This is called multiple allelism. An example is the ABO blood type system in humans.

Epistasis occurs when the effect of one gene depends on the presence of one or more ‘modifier genes’. In epistatic interactions, the phenotypic expression of one gene is influenced by another gene.
Polygenic traits are controlled by two or more genes and exhibit a continuous distribution, such as height or skin colour in humans.

The environment can influence the expression of genetic traits — this synthesis of genetics and environment is referred to as the phenotype.
Genotype, the inherited genetic code, and phenotype, the result of both genetic and environmental influences, are both integral factors in understanding inheritance.