Mendel's Laws
- Gregor Mendel was an Austrian monk
- In the mid-19th century, Mendel carried out breeding experiments on large numbers of pea plants whilst looking after the monastery gardens where he lived
- He studied how characteristics were passed on between generations of plants
- Due to his extensive work on the understanding of inheritance, he is sometimes called the Father of Genetics
- Mendel artificially pollinated the plants to allow sexual reproduction
- He did this by carefully transferring pollen (the male gamete) from the anthers of one pea plant to the ovary (female reproductive parts) of another
- This technique eliminated any uncertainty from his data since he knew which pollen had fertilized each of the plants
- He collected the pea seeds from these plants and grew them in favorable conditions to find out their characteristics
- He also crossbred offspring peas in order to find out which, if any characteristics would appear in future generations
- Mendel investigated the height of pea plants, the colors of their flowers and the smoothness of their seed coats
Mendel's Breeding Experiments of Pea Plants Diagram
In this cross, Mendel investigated the inheritance of height in pea plants
- Mendel found that characteristics were inherited in a predictable pattern
- All pea plants in the first generation had the same characteristic as one of the parental plants (e.g. they were all tall)
- The offspring plants in the second generation had characteristics of both parent plants in a 3:1 ratio
- Without knowing it, Mendel had discovered genes, he referred to them as 'units of inheritance'
- He also discovered that some genes are dominant and some genes are recessive
- Different forms of the same gene are called alleles
- These observations formed the basis of Gregor Mendel's laws of inheritance
Mendel's First Law - The Law of Segregation
Each trait is determined by two alleles that are randomly separated during gamete formation
- Gametes contain one allele of each gene
- This means there are two possible allele outcomes for each gene in each gamete
- During fertilization, two gametes fuse to restore the diploid number of alleles - two alleles for each gene
- Combinations of alleles in offspring are random due to random segregation and random fertilization
Mendel's Second Law - The Law of Independent Assortment
Genes located on different chromosomes are assorted independently during gamete formation
- There are 23 pairs of chromosomes and each homologous pair contains a different set of genes
- The inheritance of traits on one chromosome does not influence which traits are inherited on another chromosome
- The alleles of a gene are segregated randomly into different gametes and the assortment of alleles of one gene does not affect the assortment of alleles for another gene
- During fertilization, two gametes fuse to restore the diploid number of alleles - two alleles for each gene
- Combinations of alleles from different genes in offspring are random due to random assortment and random fertilization
Exam Tip
Mendel's Laws of inheritance applied only to unlinked genes on different chromosomes e.g. one gene found on chromosome 1 and another found on chromosome 2.
Genes located on the same autosome are called 'autosomal linked' genes
Genes located on the sex chromosomes are called 'sex-linked' chromosomes