GENETICS 227 



would ordinarily be written the same way; they are written in opposite 

 orders here merely to show that the gene which came from the egg in 

 one came from the spermatozoon in the other. 



Choice of the letters W and w to represent this particular pair of 

 genes is in accord with a generally accepted convention that the name 

 of the newer character should suggest the symbol. Without much doubt 

 there were colored guinea pigs before there were white ones, hence white 

 is the newer color. In accord "with another convention the small letter 

 is used for the recessive gene, the capital for the dominant. 



To describe other types of matings, it is desirable to provide names 

 for certain of the genetically different types of individuals. An organism 

 whose two genes for any particular character are alike (TFTl^ or ww) is 

 called a homozygote; one whose genes are different (Ww) is a heterozygote. 

 The same animal may be, and usually is, homozygous for some genes, 

 heterozygous for others. 



F. Black X White Fj Black x Black 



Ww WW 



Eggs _3li;ir= W Sperm Eggs 



erm 



B.C. 



)iWw /iww 

 Black White 



BC 



WW /^Ww 

 Black Black 



Fig. 195 Fig. 196 



Fig. 195. — Backcross of a heterozygous black guinea pig with a white animal. 

 Fig. 196. — Backcross of a heterozygous black guinea pig with a homozygous black 

 animal. 



Backcross. — Not always do e.xperiments proceed from an Fi gener- 

 ation to an F2. A very useful kind of cross is that between an Fi animal 

 and another like one of its parents. Such a cross is a backcross. Essen- 

 tially it is a mating of a heterozygote with a homozygote. Such a cross 

 might well be made between a heterozygous Fi black guinea pig and a 

 white one exactly like the white parent. Figure 195 shows what happens 

 when that is done. The heterozj^gous parent produces two kinds of eggs, 

 in equal numbers, the white parent only one kind of spermatozoon (w). 

 Consequently there are two kinds of offspring, heterozygous black (Ww) 

 and white (ww) in equal numbers. 



The backcross may also be made betw^een an Fi and the black parental 

 type, as in Fig. 196. There are two kinds of offspring as before, with 

 respect to their formulas; but they all look alike (black). The difference 

 between these two backcrosses is that one was made to the recessive 

 parental type, the other to the dominant type. The former cross is 



