PROBLEM 2. Differences Between Farents and Offspring 



b ^- Pi 



473 



Fig. 425 Diagram to help you imderstand Vmk- 

 age. The prbiiary sex cell represents a hybrid 

 fruit fly. In one chromosome are genes for ves- 

 tigial wing (v) a?id black body (b). In the 

 other 7nember of the pair are genes for long 

 wing (L) and gray body (G). When the pri- 

 mary sex cell goes through reduction division, 

 the genes (v) and (b) remain together. So do 

 (G) a72d (L). What kinds of offspring would 

 be produced by mating this hybrid to a siinilar 

 one? hi what ratios? 



"pair" in which the two chromosomes 

 were somewhat unHke. These chromo- 

 somes were therefore called sex chromo- 

 so?nes. The cells in a man contain 23 

 pairs of chromosomes which are just like 

 those in the cells of a woman. But in ad- 

 dition to these, in a man there is a pair 

 of dissimilar ones, called X and F, while 

 in a woman there is a pair of similar 

 chromosomes, two X chromosomes. 

 Some of the genes of these chromosomes 

 determine the character, "sex," just as 

 other genes determine other characters. 



(Optional) Sex-linked characters. There 

 are certain characters that appear far 

 more often in one sex than they do in the 

 other. Thev are said to be sex-linked. 

 Red-green color blindness in human be- 

 ings is one of these; another is hemo- 

 philia (the trait of being a "bleeder"). 

 Both of these are far more common in 

 men. From the way each of these char- 

 acters is inherited it is known that the 

 gene that determines the character is in a 

 sex chromosome. See Figures 426 and 427. 



Hemophilia is inherited in the same 

 way as color blindness. It, too, is caused 



Gametes [ X 



X y 



fi 



Fig. 426 Inheritance of color blindness in man. 

 Only the sex chromosomes are shown. In the 

 (F^), which rectangle represents the male? The 

 female? Color blindness is recessive to nornial 

 sight. The chromosome (X) carries the gene 

 for color blindness. The father is color bliiid 

 because the {¥) chromosome carries no gene 

 which can ''dominate'' the recessive color 

 blindness gene. Why do the daughters have 

 normal eyes? Why do the sons have 7Jormal 

 eyes? 



XX 



XY 



Gametes ( X 



©0 



Fig. 427 One of the females of the (F^) 

 (above) is mated with a normal rnale. What 

 are the chances that the sons will be color 

 blind? Will any of the daughters be color 

 blind? 



by a recessive gene which is in the sex 

 chromosome. Can vou do Exercise 19? 

 Why individuals of the same species dif- 

 fer. Your brief study of genetics has ex- 

 plained several important facts: 



1. An individual that has two parents 

 is not exactly like either parent. Under 

 laborator\' conditions, after long and 

 careful breeding, a relatively simple or- 

 ganism like the fruit flv may be made 

 pure in everv^ character. Each of its sex 

 cells, therefore, will be like every other. 

 But except for self-pollinated plants this 

 does not happen normally in organisms 

 that reproduce sexually. 



2. The offspring does not resemble 

 the two parents equally. 



