184 Outline of Genetics 



type of sperm has effected fertilization. In the cases where the 

 male has an unequal pair of sex chromosomes, that member of the 

 pair which is similar to the equal pair of the female is known as 

 the X chromosome, while the other chromosome of the male is 

 the Y chromosome {XX is female, XY is male). 



Furthermore, although the male is usually the heterozygote 

 for sex, there are some cases in which the female is the heterozy- 

 gote. In such cases the sperms are all alike; two types of eggs 

 are produced in equal numbers, and the sex of the offspring depends 

 upon which type of egg has effected fertilization. This is the 

 situation in the birds and in the Lepidoptera group of insects, while 

 in practically all of the other known cases it is the male that is the 

 heterozygote for sex. 



When genes are located on the X chromosome their method of 

 inheritance is characteristic, being known as sex-linked inheritance. 

 (This term should not be confused with sex-limited inheritance, 

 which applies to cases where the genes are carried on the auto- 

 somes in the usual manner, but can express themselves only in one 

 sex, e.g., heavy beardedness in man.) Numerous examples of 

 sex-linked inheritance are to be found in the fruit fly. 



The female fruit fly has a pair of X chromosomes, while the 

 male has an X mated with a F, the two being distinguishable by 

 their shape. Numerous genes are located on the X chromosome, 

 but none have been identified on the F, which seems quite negli- 

 gible in inheritance.^ "Vermilion" {v), a recessive to normal red 

 eye (F), is a gene of this sort. When a vermilion female {iX) 

 {vX) is mated with a red male {VX) F, all of the female progeny 

 are bound to be red {vX) {VX), w^hile all of the male progeny will be 

 vermilion {vX) F, as is explained by fig. 29. This has often been 

 called "zig-zag" inheritance, since the character of the mother 

 passes to the sons, while the character of the father passes to the 

 daughters. All of the other possible matings work out just as one 

 would expect from the mechanism that is involved. 



» Some very recent papers (Castle lo) have suggested that genes 

 may actually be located on the F chromosomes in some animals. A 

 peculiar "one-sided" type of inheritance results, since it is possible for 

 such genes to be present only in the male. The author is not as yet 

 certain that these findings will be "accepted" by other geneticists. 



