614 



CONTINUITY OF LIFE 



son of a laborer. Another type of ichthyosis 

 (non-lethal) has been traced through five 

 generations and has successively appeared 

 in the sons but never in the daughters. Fur- 

 thermore, the daughters never transmit it 

 to their offspring, which definitely places 

 the defective gene in the Y chromosome. 



is carried by a dominant gene, although it 

 usually appears in the heterozygous condi- 

 tion. That is to say, a parent may have the 

 disease but usually only certain of his chil- 

 dren, regardless of sex, will show it, indicat- 

 ing that the parent must have been hetero- 

 zygous. Had he been homozygous for the 



Fig. 24-20. There are a great many human anomalies that have been traced to a genetic origin. 

 Certain types of hare-lip are inherited. One of the common types is shown here (upper left). 

 Modern surgery has worked wonders in repairing these unfortunate abnormalities (upper 

 right). 



Many deformities of the hands and feet ore inherited. Here are two cases of "split-hand" 

 (lower). This trait is usually dominant and occurs once in about 90,000 births. More often 

 than not the gene responsible for this abnormality is also responsible for others. Both of 

 these cases were abnormal in other respects. 



This is one of very few cases where a gene 

 has been definitely located in the Y chromo- 

 some in man. 



A type of blindness caused by the cloud- 

 ing, or fogging, of part of the transparent 

 portions of the eye is called cataract. Pedi- 

 grees of families in which this disease oc- 

 curs bear out the contention that the trait 



trait, all of his children would have the 

 defect. Cataract is also acquired by exces- 

 sive exposure to heat, radiations, and con- 

 tusions, but in these cases heredity, of 

 course, is not involved. 



Vastly more important than obvious 

 physical defects are hereditary traits that 

 affect the nervous system, altering the be- 



