"dummy" chromosome. The Y chromosome 

 has very little influence upon the character- 

 istics of the organism, except that, by replac- 

 ing one of the X chromosomes, it plays a 

 deciding role in the determination of the sex 

 of the organism. 



The X chromosome, in contrast, carries 

 many genes (Fig. 26-19). These genes are 

 transmitted along with the sex-determining 

 genes, and consequently a number of hered- 

 itary traits tend to be associated with the 

 sex of the individual. Characteristics for 

 which the genes have loci in the X chromo- 

 somes are said to be sex-linked; and sex 

 linkage influences the inheritance of many 

 characteristics — as shown in the following 

 experiments. 



In DrosopJiiJa, white (w) eye color is a 

 sex-linked recessive character, and the allelic 

 gene for red (IT) eye is dominant. Thus if a 

 white-eyed female (Fig. 26-22) is crossed with 

 a red-eyed male, all the Y x daughters are 

 found to have red eyes, but all the sons have 

 white eyes. Moreover, if these F-, individuals 

 are inbred, the F 2 generation gives 25 per- 

 cent white-eyed daughters, 25 percent red- 

 eyed daughters, 25 percent white-eyed sons, 

 and 25 percent red-eyed sons (Fig. 26-22). Or 

 if the experiment is started oppositely (Fig. 

 26-23) — by crossing a homozygous red-eyed 

 female with a white-eyed male — all the Fj 

 offspring have red eyes; and in the F 2 genera- 

 tion, all the daughters have red eyes, but the 

 sons show an equal distribution of the red 

 and the white eye color (Fig. 26-23). 



The foregoing experiments show that the 

 inheritance of the eye color in Drosophila is 

 linked with the inheritance of sex. It is plain 

 also that the Y chromosome, other than dis- 

 placing the X chromosome, plays no essential 

 role in the determination of the character- 

 istics of the individual. 



In man, several types of abnormality, such 

 as red-green color blindness and hemophilia 

 (p. 323), are sex-linked, recessive character- 

 istics, which are inherited exactly like white 

 eye color in Drosophila. As to color blindness, 

 for example, all the sons of a color-blind 



Heredity - 499 



mother by a normal father will show the 

 defect, although all the daughters will have 

 normal vision. These daughters can transmit 

 color blindness, however, since none is homo- 

 zygous for the normal gene. Thus if we desig- 

 nate the recessive color-blind gene as (£>), and 

 the dominant allelic normal gene as (B), the 

 mother would be (X fj X(,), the father (X B Y), 

 the sons (X 6 Y), and the daughters (X /f X 6 ) 

 as to their respective genotypes. In other 

 words, the inheritance of this type of color 

 blindness follows the same pattern as the 

 inheritance of white eye color in Drosophila 

 (Figs. 26-22 and 26-23). 



Nongenetic Sexual Differentiation. A few 

 species display no genetic difference be- 

 tween the sexes; and in such cases a zygote 

 can give rise to either a male or a female, 

 depending on environmental factors such as 

 food. This type of sex determination is quite 

 rare, however, being encountered in only a 

 few worms and other relatively simple inver- 

 tebrate animals. 



Identical vs. Fraternal Twinning. In a 

 majority of organisms only one individual 

 arises from each fertilized egg; but in a few 

 species (for example, the nine-banded arma- 

 dillo), the embryo regularly divides at an 

 early stage of development, producing two or 

 more genetically identical individuals from 

 the same zygote. This phenomenon of poly- 

 embryony is not common among higher ani- 

 mals. However, polyembryony sometimes oc- 

 curs in man and other mammals, resulting in 

 the production of identical twins and triplets 

 (Figs. 26-24 and 26-25). Identical twins, in 

 contrast to fraternal twins, are always of the 

 same sex. Moreover, identical twins bear a 

 remarkable resemblance to each other, not 

 only as to visible appearance, but also as to 

 characteristics of mind and personality (p. 

 509). 



ASEXUAL REPRODUCTION IN RELATION 

 TO HEREDITY 



Asexual processes of reproduction — exclud- 

 ing meiotic sporulation and parthenogenesis 



