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the study of the pattern of segregation of phenotypes under the control of genetic loci taken one at a time.
Mendel's lawsthe basic laws of inheritance, first published by Gregor MENDEL in 1866 to explain the results he obtained from experiments with the garden pea. Working without detailed knowledge of cell structure or nuclear division he suggested that:
- each character (e.g. height) is controlled by two factors. We would now state this idea as each gene having two alleles, one on each HOMOLOGOUS CHROMOSOME.
- each factor segregates in the egg and pollen grains. We would now state that MEIOSIS separates allelic forms of gene (Mendel's first law - see SEGREGATION).
- factors for different characters show INDEPENDENT ASSORTMENT. We would now state that genes are assorted independently during meiosis (Mendel's second law), unless linked on the same chromosome.
- factors do not cause blending, but are either dominant (see DOMINANCE (1) or recessive. We know now that we can explain dominance in terms of enzyme activity, and that sometimes two alleles code for enzymes giving an intermediate phenotype when together (see INCOMPLETE DOMINANCE).
- the distribution of factors in the egg cells and pollen grains obey basic statistical laws giving ratios in the progeny which are predictable.
- results from crosses are the same whether the dominant form of the character belongs to the female parent or the male parent. We now know that this statement is true only if the gene is located on an AUTOSOME (see SEX LINKAGE).