CHROMOSOME THEORY OF INHERITANCE 331 



3. Mendelian Laws of Heredity. 



a. Law of Dominance. When an individual, which is pure as to 

 a particular factor, is crossed with another individual which is pure 

 in the contrasting factor, only one of these factors will appear in 

 the offspring. The character which appears is called dominant; 

 the other which is inherited, but is concealed, is called recessive. 



b. Law of Segregation. 



(1) When hybrids are mated, the contrasting factors are seg- 

 regated out in the proportion of 25 per cent pure dominant, 50 

 per cent hybrid, and 25 per cent pure recessive. 



(2) When hj^brids are mated with pure dominants, there is a 

 segregation of 50 per cent hybrid and 50 per cent pure dominant. 



(3) When hybrids are mated with recessives, there" results a 

 segregation in the proportion of 50 per cent hybrids, 50 per cent 

 recessives. 



c. Law of Unit Characters. Every organism contains many pairs 

 of factors. Each factor, as tallness or shortness, is inherited as a 

 unit, that is, independently of the other and of any other pair of 

 characters. 



These laws were formulated by the study of the external appear- 

 ance of the results of different matings. What caused it all? A 

 difference in the protoplasms of the germ cells has been mentioned. 

 This is true. Recent investigators have formulated a theory that 

 has stood the test of much careful checking. The chromosome is 

 concerned with heredity. It is thought that a certain part of the 

 chromosome called the gene or character-determiner actually 

 directs and controls the development of the individual char- 

 acters. 



Chromosome theory of inheritance. If both parents are pure 

 as regards a particular character, both the egg and sperm will 

 contain the determiner for that character and the offspring will 

 receive a double contribution of the determiner or " gene." For 

 example, the genes of a pure tall pea plant wall carry only tallness ; 

 the genes in a pure dwarf pea plant will carry only dwarfness. 

 The offspring of such pure breds, whether dominant or recessive, 

 cannot help but be pure. The offspring of the tall plants will be 



