Exercise XXII 



INTRODUCTION TO GENETICS OF MAN AND FRUIT FLY 115 



Sex-linked genes in man 



The most common sex-linked human trait is 

 red-green color blindness. This occurs in about 

 8% of the male and 0.5% of the female popula- 

 tion. The recessive gene responsible for color 

 blindness is in the X chromosome, and since 

 men have only one X chromosome, while women 

 have two, a father transmits his X chromosomes 

 to all his daughters but never to his sons, where- 

 as a mother gives one X chromosome to each of 

 her children regardless of sex. It follows that 

 the sons of a color-blind mother are all color 

 blind, but daughters have normal vision if the 

 father has normal vision. The daughters, how- 

 ever, carry the color-blindness trait; if married 

 to men with normal vision, their daughters are 

 normal, but half their sons are color blind. How 

 is a color-blind woman produced? 



Hemophilia, the failure of the blood to clot, is 

 another sex-linked recessive trait, also therefore 

 almost entirely restricted to males. One of the 

 troubles with European royalty is that Queen 

 Victoria, a carrier of the hemophilia gene, tended 

 to have royal descendants who bled for the 

 wrong reasons. 



Attached or free ear lobes 

 Full lips, thin lips 

 Freckles 



DROSOPHILA GENETICS 



The common fruit fly, Drosophila melano- 

 gaster (i.e., "black-belly"), has been highly im- 

 portant in genetics since introduced half a 

 century ago by T. H. Morgan. Its short genera- 

 tion time, ease of handling, large number of off- 

 spring, and convenient size all tended to make 

 this the most widely used organism in genetics. 

 Only lately has it been superseded by micro- 

 organisms, which offer still further conveniences 

 and potentialities for experiment, once one has 

 learned to handle them. 



A further advantage of Drosophila is that it 

 possesses as the diploid number only four pairs of 

 easily identified chromosomes. Also the salivary 

 glands of the larvae contain giant chromosomes, 

 the structures of which have provided important 

 anatomical correlations with genetic linkage 

 maps, and which have furthered the analysis of 

 chromosome functions and rearrangements. 



Other human genetic traits 



You have already typed your own blood. (See 

 page 327 of S. P. T., or pp. 471-472 in ViUee, for 

 a description of genetic aspects of blood types.) 



You may be interested in the following ex- 

 amples of other human Mendelian traits: 

 Blood types Rh+, Rh" 

 Tongue rolling 

 Tongue folding 

 Widow's peak 

 Dimpled cheeks 

 Mongolian eyefold 

 Hyperextension of distal thumb joint 

 Albinism 



Straight hair, curly hair 

 Mid-digital hair on fingers 

 Far-sightedness 

 Near-sightedness 

 Astigmatism 



Overall plan of the experiment 



We have planned an experiment that demon- 

 strates Mendel's laws of segregation and inde- 

 pendent assortment. It involves two recessive 

 mutants, the genes for which are located in 

 separate chromosomes: dumpy {dp) and ebony 

 (e). Flies homozygous for dumpy have truncated 

 wings, only about two-thirds as large as wild 

 type. Those homozygoous for ebony have shiny 

 black bodies, much darker than wild type. 



A week before this laboratory session initial 

 crosses were made between males and virgin 

 females, the flies of one sex taken from a stock 

 homozygous for dumpy body, the other from one 

 homozygous for ebony. The parent flies remain 

 in the vials that you have been given, and will 

 shortly be removed. The eggs already laid by 

 these females will hatch to form the Fi genera- 

 tion with which the experiment will be continued. 



