Heredity — Genetics 699 



The method of producing gametes and of fertihzation, as well as the 

 production of the sex of the offspring, is also similar. Sex is deter- 

 mined at the time of fertilization. If an egg is fertilized by an "X 

 sperm," the zygote will develop into a female, while an egg fertilized by 

 a "Y sperm" leads to an XY zygote which develops into a male. After 

 fertilization, mitosis provides every cell of the developing embryo and 

 the ultimate adult with the original chromosome constitution. 



This explanation seems quite simple and sufficient, but it is only the 

 basis for solving the problem of sex determination and its attendant 

 phenomena. Many of these are too complicated to be considered in 

 detail here, but the following may suggest some of the complications. 

 A human XX zygote or XY zygote develops in a short time into an 

 embryo which is structurally neither male nor female, or rather is both 

 male and female, because the embryonic gonads consist of two parts — a 

 characteristic ovary-like portion, and a characteristic testis-like part. 

 Likewise, a pair of male and a pair of female internal sexual ducts are 

 present in each early, "neutral" embryo. Even the embryonic parts 

 which later develop into external genitalia of either sex are present. 

 After this neutral stage has been reached, the specific genetic sex consti- 

 tution of the embryo begins to differentiate visibly. In the embryo with 

 XX cells the neutral embryonic gonads develop into ovaries, while the 

 embryo with XY cells develops testes. Likewise, the proper types of 

 internal sexual ducts and external genitalia are developed if things de- 

 velop normally. It happens at times that parts of both male and female 

 reproductive systems are present in certain adults who are known as 

 hermaphrodites or intersexes. 



In newborn children, sexual differentiation is not yet complete. Sec- 

 ondary sexual traits (differences apart from actual sex organs) develop 

 during puberty and include differences in larynx and voice, differences 

 in pelvic developments, breasts, hair growth, etc., which are influenced 

 by specific hormones produced by the male and female sex organs. 



13. Sex-Linked Traits. — Besides assisting in the determination of the 

 sex, the sex chromosomes also possess genes for the determination of 

 other traits, which, because of the location of the genes in the sex chro- 

 mosome, are known as sex-linked traits. Two types of sex-linked inherit- 

 ance are possible, depending upon whether the sex-linked genes are in 

 the X chromosome or Y chromosome. Certain genes are known which 

 are always associated with the Y chromosome, while others are associated 

 with X chromosomes. Other genes have been found in males which 

 cross over from the X to the Y chromosome, or vice versa. Very few 



