Genetics 



299 



also definitely showed that the characters were separated at each genera- 

 tion. As one more conclusion, he pointed out that the appearance of 

 the organism did not necessarily indicate the appearance of the offspring. 

 It is now known that these factors determining the flower colors in 

 this cross are located on the chromosomes and are known as genes. Rec- 

 ognizing these facts, geneticists have adopted a shorthand method for 

 indicating these crosses. The genes are commonly indicated by letters 

 of the alphabet, with the dominant gene being designated by the capital 

 letter and the recessive gene by the small letter. The letter chosen for 

 the particular gene is customarily the first one of the name of the 

 character determined by the dominant gene. 



PARENTS 



BED WHITE 



RR rr 



RED 



Rr 



RR 



Rr 



WHITE 



rr 



Fig. 89. — A monohybrid cross involving flower colors in peas. In this cross 

 R —a gene for red and r — a gene for white. Red is dominant over white. The 

 F combinations are shown in a typical Punnett square. 



The abbreviations Pi and Fi refer to the first parental and first 

 filial generations, respectively. The actual genetic composition of the 

 various individuals involved can be seen to differ. Some of the red 

 flowers, for example, apparently have the characters for both red and 

 white, while others have only those for red. When the genes for a single 

 character are identical (RR or rr), the individual is said to be homozy- 

 gous for that character. If the genes are unlike (Rr) the individual is 

 said to be heterozygous. The genetic composition of the individual (Rr, 

 RR, or rr) is known as the genotype, while its appearance (red or 

 white) is the phenotype. In this cross, the F2 generation has a geno- 

 typic ratio of 1 :2 :1, but a phenotypic ratio of 3 :1. 



Due to the fact that these factors are randomly distributed to the 

 gametes, large numbers of individuals must be studied and counted to 



