104 Genes in the X and Y Chromosomes 



P,: Ww^ X w\ 



red white 



female male 



F,: Ww wew WY weY 



red eosin red eosin 



female male 



F2: Ww X WY = WW + Ww + WY + wY 



Ww X w^Y = Wwe -(- w^w + WY + wY 



w^w X WY = Ww^ + Ww + w^Y + wY 



w^w X u-ey = w^we + w^w -\- w^Y + wY 



Total : 5 red female 



3 eosin female 

 2 red male 



2 eosin male 



4 white male 



Fig. 36. Inheritance of red, eosin, and white ej^es in Drosophila melano- 

 gaster. Red results from gene W (or+), eosin from ir^, and white from u\ 

 These three genes form a series of multiple alleles for they are all found 

 at the same locus of the X chromosome. A cross between a Ww^ female 

 and a white-eyed male produces red and eosin males and females. All 

 four crosses between the two types of females and males result in red and 

 eosin females but red, eosin, and white males. 



Y Chromosome Inheritance 



If a gene is located in a portion of the Y chromosome which 

 is not homologous with any part of the X chromosome the char- 

 acter resulting from its action must be present in both father 

 and son, and must never appear in a female. Y chromosome 

 genes are apparently rare and in Drosophila melanogaster include 

 two genes for male fertility. Four genes reported for the Y chro- 

 mosome in man are genes for ichthyosis hystrix gravior, a skin 

 ailment, for keratoma dissipatum, a skin ailment affecting the 

 hands and feet, for hypertrichosis of the ears, and for webbed 

 toes in certain families. (In other families apparently a similar 

 but different gene is involved.) 



The genetic distribution of a gene on the Y chromosome is 

 indicated in Fig. 37. The male parent has the hypothetical 

 character A which is determined by gene A located in the Y 

 .chromosome. Half the gametes of this male parent have an X 

 chromosome with no A gene, and the other half have a Y chromo- 

 some in which an A gene is located. The female parent has two 

 X chromosomes but no A gene, and all her gametes lack A. 

 The daughters from this cross have two X chromosomes but they 



