Heredity refers to the process by which certain heritable characteristics are transmitted from parent to offspring 

These inheritable factors are DNA sequences called genes and their position on a given chromosome is known as their gene locus

Most sexually reproducing organisms are diploid, with chromosomes existing as pairs (maternal and paternal)

As such there are two alternate copies of each gene, known as alleles

  • If the two alleles for a given trait are identical, the organism is homozygous for that characteristic (also described as a purebred)
  • If the two alleles for a given trait are different, the organism is heterozygous for that characteristic (also described as a hybrid)

The specific combination of alleles will determine the physical characteristics of a given trait and potential inheritance ratios

  • The gene composition with reference to a single trait is known as the genotype (i.e. the allele combination)
  • The observable characteristics of a particular trait (i.e. it's physical expression) is known as the phenotype and is determined by both the genotype and environmental influences

The inheritance patterns of certain traits were first established by a Moravian monk, Gregor Mendel, and are based on two fundamental processes that occur during meiosis (gamete production):

Principle of Segregation

During egg and sperm formation, homologous chromosomes (and hence allele pairs) separate or segregate during Anaphase I of meiosis

The two alleles become incorporated into different egg / sperm (which are haploid)

So for any given trait:

  • A heterozygote may contribute either of the two distinct alleles
  • A homozygote will always contribute the same allele

Principle of Independent Assortment

As the segregation of chromosomes (and hence alleles) is random, there is an equal probability of a gamete having either of an allele pair

As each chromosome pair separates independently of other pairs, the inheritance of genes are typically independent of each other 

This means that different allele combinations should always be equally possible (except when genes are linked)

Punnett Squares

Punnett squares are grids that map out the potential allele combinations that may result from the fusion of male and female gametes

  • In a monohybrid cross, the two alleles are separated for each partner and then recombined to identify expected combinations and potential ratios in offspring
  • In a dihybrid cross, potential allele combinations for two sets of genes must be calculated, leading to more a complex grid and more potential combinations

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