Heredity — Genetics 685 



From a scientific interpretation of the data, Mendel formulated the 

 following principles known as Mendel's Laws or Mendelism. 



(1) Law of Unit Characters: The inheritance of a pair of characters 

 occurs as a unit and independently of any other pairs of characters. 



(2) Law of Segregation: Each pair of genes for a character segregate 

 (separate) during the formation and maturation (gametogenesis) of 

 gametes, so that no gamete has more than one gene from each pair. 

 Each pair of genes undergoes this assortment independently of every 

 other pair. 



(3) Law of Dominance (Complete): When a gene and its alternative 

 gene are both present at the same time in an individual, the one which 

 expresses itself is called the dominant, while the one which is latent and 

 unexpressed is the recessive. An individual displaying a dominant char- 

 acter may do so because of the presence of two dominant genes (TT) 

 and be homozygous or because of the presence of one dominant and one 

 recessive gene (Tt) and be heterozygous. For a recessive character to be 

 expressed the genes must be homozygous (tt). In this type of inherit- 

 ance the dominant completely masks the recessive. 



Common Illustrations of Dominance and Recessiveness 



