Sex Linkage in Plants 101 



scientists. Some geneticists have considered that two genes for 

 lemophilia have a lethal effect so that potentially hemophilic 

 A'omen would never develop much past the zygote stage of devel- 

 opment. This hypothesis is wholly in accord with other known 

 genetic phenomena, but it has not been proved. 



Other Sex-Linked Genes in Human Beings. Although green- 

 •ed color blindness and hemophilia are the two best-known sex- 

 inked characters in human beings, others have also been found, 

 ^mong them are recessive genes for Gower's muscular atrophy, 

 ibsence of two center incisors, absence of sweat glands, optic 

 itrophy, some cases of nystagmus, some cases of microphthalmus, 

 ;ome types of night blindness, and a dominant gene for defective 

 mamel of the teeth. 



In Fig. 356 ten families are listed which seem to indicate that 

 I certain fingerprint pattern is the result of a sex-linked recessive 

 >ene. This gene produces a radial loop rather than an ulnar loop 

 )n the index finger of the right hand. In eleven matings between 

 )eople who did not have a radial loop on that finger males were 

 produced who had the radial loop although none of their sisters 

 lad it. In the only two instances in which females were pro- 

 luced with a radial loop the fathers had radial loops. Although 

 ill the families are small, the evidence suggests that radial loops 

 •esult from a sex-linked recessive gene and that the female 

 )arents of these families were heterozygous. 



Sex Linkage in Plants 



In most plants both sexes are represented in the same indi- 

 vidual, and sex is a matter of development. In some plants, 

 lowever, the sexes are separate, and sex is controlled by a mech- 

 mism similar to that in many animals. Genes located in these 

 ;ex chromosomes behave in the same way as genes in the sex 

 chromosomes of animals. One of the earliest discoveries of sex- 

 inked genes in plants was made in Lychnis dioica independently 

 3y G. H. Shull and Erwin Baur. In this species, broad leaves 

 ire dominant to narrow. From the way these genes behaved 

 genetically, Baur and Shull concluded that they were located in a 

 ^ex chromosome although cytological observation had not been 

 nade. If homozygous broad-leaved females are crossed with 

 larrow-leaved males, the Fi plants are all broad-leaved. In the 



