508 - Heredity and Evolution 



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Fig. 26-27. Diploid set of chromosomes in man, derived from a cell at metaphase. By this time each 

 chromosome has undergone replication, but a complete separation of the sister replicas has not oc- 

 curred. Selection of the paired members requires a precise study of the morphology, including the 

 centromere (constricted point) positions, of the randomly arranged chromosomes of an original prepara- 

 tion (lower left of photo), utilizing a magnification considerably higher than is shown here. Then the 

 paired members are arranged and numbered, forming what is called a karyotype, which may be nor- 

 mal or abnormal. (Courtesy of Alexander G. Beam, The Rockefeller Institute, New York.) 



condition include a malformation of the 

 tongue and a very severe retardation of 

 growth, both physical and mental. 



The Klinefelter and Turner syndromes 

 both are related to abnormalities in the sex 

 chromosomes. The Klinefelter patient is 

 always a male with an XXY constitution. 

 Typically the stature is tall, and usually 

 some degree of mental deficiency is present. 

 Sperm production is reduced to the point of 

 virtual sterility and the breasts show a tend- 

 ency toward overdevelopment. The Turner 

 patient, on the other hand, lacks a Y chromo- 

 some; that is. the genetic constitution is XO. 

 The condition is associated with malforma- 



tions of the aorta, deafness, shortness of 

 stature, and some degree of mental retarda- 

 tion. 



Other Aspects of Human Inheritance. Most 



normal differences among human indi- 

 ct 



viduals — such as size, weight, color of skin, 

 hair, and eyes — are, as in other animals, in- 

 fluenced by many genes; and there are no 

 very closelv inbred stocks in man. All human 

 individuals arc heterozygous for many genes, 

 and consequently it is difficult to forecast the 

 precise distribution of most characteristics 

 among human offspring. Moreover, many 

 characteristics — such as height and weight — 

 are very susceptible to nutritional influences, 



