Heredity — Genetics 703 



14. Sex-Influenced Traits. — Sex-influenced traits are sometimes cafled 

 sex-modified or sex-controUed traits. Such traits are inherited by genes 

 which are not present in the sex chromosomes (they are autosomal genes) 

 but which are influenced or modified by the sex of the organism. These 

 influences are due in part, at least, to the sex hormones of the male and 

 female gonads and are responsible for differences in the expression of the 

 traits in the two sexes, even though the genes may be the same in both. 



Pattern baldness in human beings is sex influenced, being affected by 

 the sex hormones. There are more bald males than females because only 

 one gene for baldness is required in men, while two genes are required in 

 women. For example, BB causes baldness in both men and women; bb 

 causes no pattern baldness in either sex; however, Bb causes baldness in 

 men but not in women. The same genes (Bb) produce different effects 

 in the two sexes, depending upon the influence of the sex hormones. 



The horns of certain types of sheep are phenotypically different in males 

 than in females. The genes HH produce horns regardless of sex; hh is 

 hornless in either sex; however, Hh produces horns in males, but the 

 same genes produce a hornless female. Hence, Hh expresses itself dif- 

 ferently in the two sexes, so is sex influenced. A similar condition exists 

 in the production of mahogany spots and red spots in Ayrshire cattle. 



15. Inbreeding and Outbreeding. — When closely related individuals are 

 crossed, we call it inbreeding, and when unrelated strains or individuals 

 are crossed, it is known as outbreeding. There seems to be much misin- 

 formation regarding these phenomena. It is commonly thought that 

 inbreeding is harmful and leads to the production of undesirable ab- 

 normalities of various kinds. Inbreeding in itself may not be harmful, 

 but it depends on what is inbred. If the parents have undesirable traits, 

 inbreeding naturally will transmit them, and the chances that both par- 

 ents may possess undesirable traits is greater because of their close heredi- 

 tary relationship. Crossing less closely^ related individuals might prevent 

 the expression of some of the undesirable traits. 



On the other hand, commercial breeders use inbreeding constantly to 

 improve and retain their strains of horses, cattle, dogs, chickens, wheat, 

 fruits, etc., in which cases they capitalize on the good traits possessed by 

 both parents even though they may be closely related. In fact, closely 

 related individuals may be desirable parents providing they possess de- 

 sirable traits (genes). If any stock of plant or animal has undesirable 

 recessive traits, inbreeding may cause some of them to appear, while any 

 stock of plant or animal having desirable traits will transmit them, or 

 may even result in improvements. Many plants (beans, peas, wheat, 



