Mendel's laws of inheritance explain how individual traits are transferred from one generation to another in an organism. An Australian Scientist, Gregor Mendel (1822–1884), established the Mendel's Laws after conducting series of experiments using pea plants between 1857 and 1865. Among the traits that Mendel studied were the colour of a plants' flowers, their location on the plant, the shape and colour of pea pods, the shape and color of seeds, and the length of plant stems.
Mendel's First Law: When a plant with two dominant (DD) alleles is crossed with a plant having two recessive (rr) alleles (top row), the first generation of plants (middle row) will all have one dominant and one recessive (Dr) allele. In the second generation (bottom row), on average one of four plants will have two recessive alleles.
Mendel’s approach was to transfer pollen (which contains male sex cells) from the stamen (the male reproductive organ) of one pea plant to the pistil (female reproductive organ) of a second pea plant. Suppose one takes pollen from a pea plant with red flowers and uses it to fertilize a pea plant with white flowers. What Mendel wanted to know was what colour the flowers would be in the offspring of these two plants. In his second series of experiments, Mendel studied the changes that occurred in the second generation. That is, suppose two offspring of the red/white mating (“cross”) were themselves mated, what colour will the flowers be in this second generation of plants? As a result of the experiments, Mendel was able to state three generalisations about the way traits are transmitted from one generation to the next in pea plants.
Mendel’s Laws of Inheritance can be summarised into three:
Law of Segregation – This states that during the production of gametes, the two copies of each hereditary factor segregate so that offspring acquire one factor from each parent.
Law of Independence – This states that the laws of chance govern which particular characteristics of the parental pairs will occur in each individual offspring.
Law of Dominance – This states that one factor in a pair of traits dominates the other in inheritance unless both factors in the pair are recessive.
Definition of Terms
Allele - one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.
Chromosome - a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes.
Dominant - An allele whose expression overpowers the effect of a second form of the same gene.
Gamete - A reproductive cell.
Heterozygous - A condition in which two alleles for a given gene are different from each other.
Homozygous: A condition in which two alleles for a given gene are the same.
Recessive: An allele whose effects are concealed in offspring by the dominant allele in the pair.
What are Genes?
Genes are the units in which characteristics are passed from one generation to the other. For example, a plant with red flowers will carry a gene for that characteristic.
A gene for any given characteristic may occur in one of two forms, called the alleles of that gene. For example, the gene for colour in pea plants can occur in the form (allele) for a white flower or in the form (allele) for a red colour.
The first step that takes place in reproduction is for the sex cells in plants to divide into two halves, called gametes. The next step is for the gametes from the male plant to combine with the gametes of the female plant to produce a fertilised egg. The fertilised egg is called a zygote. A zygote contains genetic information from both parents.
For example, a zygote might contain one allele for white flowers and one allele for red flowers. The plant that develops from that zygote would said to be heterozygous for that trait since its gene for flower color has two different alleles. If the zygote contains a gene with two identical alleles, it is said to be homozygous.