- Mendel’s First Law = the law of segregation of characteristics.
- Mendel’s Second Law = the law of independent assortment.
Gregor Mendel was an Austrian monk. He did a number of experiments on inheritance in pea plants. He did not know about genes and chromosomes: what he called “factors” we now call genes. Mendel did not know about cell division so he had not seen diagrams or slides of mitosis and meiosis.
Mendel’s First Law is the law of “Segregation of Characteristics.”
This says that of a pair of characteristics (e.g. blue and brown eye colour) only one can be represented in a gamete. What he meant was that for any pair of characteristics there is only one gene in a gamete even though there are two genes in ordinary cells.
If your eyes are blue, green or grey you have two alleles for blue eyes (bb), then your gametes must have a blue allele (b); if your eyes are brown you might have two brown allele (BB), then your gametes have one allele for brown (B) or you might have one allele of each kind (Bb), in which case you make two kinds of gametes some contain the brown allele (B) and some contain the blue allele (b).
Mendel’s Second Law is the law of “Independent Assortment”.
This says that for two characteristics the genes are inherited independently.
If you had the genotype AaBb you would make four kinds of gametes: they would contain the combinations of either AB, Ab, aB or ab.
Suppose one of your parents had the genotype AABB then you would have inherited AB from this parent. Suppose also that your other parent had the genotype aabb then you would have inherited ab from this parent. The combinations of AB and ab are parental types. Your genotype is AaBb and some of your children will inherit these parental types either AB or ab from you. However, it is also possible for some of your children to inherit new combinations called “re-combinants from you. These are Ab and aB.
Caveat: perhaps you will need to learn these laws if you go on to study “A” Level biology, but don’t worry too much about them for your GCSE exam.