Genetics 301 



heterozygous condition is even entirely different from either of the 

 homozygous conditions. One common example of this is found in 

 the inheritance of color in the Blue Andalusian fowl. If the fowl is 

 homozygous for the gene B, the feathers are black; if homozygous for 

 b, the feathers are white. The heterozygous condition, Bb, results in 

 a fowl having the so-called blue color. 



PARENTS FT fi 



bLack white 



BB bb 



B b 



BLUE 



Bb 



B 



B 



B 



^=r^f-^- BB 



Bb 



— ^r- Bb 



bb 



Fig. 91. — A monohybrid cross involving color in fowl. In this cross, B — a gene 



for black and b — a gene for white. 



The Physical Explanation of Segregation. — The formulation of 

 this first law of Mendel's was not based on any cytological knowledge. 

 It was not until many years after Mendel's work that his conclusions 

 were verified by the study of gametogenesis. During gametogenesis, 

 it will be recalled, the chromosomes divided and separated at the first 

 and second maturation divisions. One of each original pair of homolo- 

 gous chromosomes remained in each egg or sperm finally produced. 

 Further, since the chromosome movements were random, the combina- 

 tions of different chromosomes were entirely fortuitous. 



The factors controlling the expression of various characters are 

 the genes. These are located on the chromosomes, and are arranged in 

 a linear fashion. Thus a single chromosome has many genes. The 

 genes concerned with a single character are located in identical posi- 

 tions on homologous chromosomes ; thus it is evident that in meiosis one 

 of each pair of factors will go to each gamete. When the gametes com- 

 bine at fertilization, the homologous chromosomes come together and 

 the pairs of factors are restored; however, again chance plays an im- 

 portant role for the combination of egg and sperm is also a random 

 affair; any one sperm may combine with any one egg. 



