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Mendel's laws

Gregor Mendel (1822-1884) was an Austrian monk who discovered the fundamental principles of genetic inheritance.

Mendel performed genetic crosses on peas to study inheritances.

He formulated two laws of genetic inheritance through analysis of his results. These are the law of segregation and the law of independent assortment.

Before Mendel's discoveries, it was thought that when two organisms mated, their genes blended, resulting in intermediates.

Mendel found that traits such as pea colour did not mix to produce an intermediate. Furthermore, traits that were not present in the parent phenotype, such as dwarf plant size, could be present in offspring.

This indicated that blending was not occurring.

According to Mendel's law of segregation (or Mendel's first law), each parent passes only one of the alleles of a gene to its offspring.

Diploid organisms carry two copies of each gene. One copy of each gene is randomly assigned to each gamete during meiosis. Most mammals are diploid organisms.

If a parent of a diploid organism has the genotype Aa for a specific gene, there is an equal chance of the child inheriting either allele, A or a.

The probability of the allele being inherited is independent of whether it is dominant or recessive.

It is not possible for the offspring to inherit a blend of two alleles on a chromosome. This is because alleles are inherited as a whole.

The law of independent assortment, or Mendel's second law, states that genes that code for separate traits will be inherited independently (independently assorted).

In a pea plant, the gene for colour of the pea flower is inherited independently from the gene for the texture (smooth or wrinkled) of the flower.

This means that the colour of the flower has no bearing on whether the texture of flower is smooth or wrinkled. The inheritance of one allele does not affect the inheritance of the other allele.

The principle only applies to genes on different chromosomes.

In reality, it is only the chromosomes which are independently assorted, not the genes. During meiosis, there is independent segregation of chromosomes.