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Beginner's guide to genetics: Sex and genetics

BMJ 2004; 329 doi: (Published 01 November 2004) Cite this as: BMJ 2004;329:0411400
  1. Adrián J González, interim1,
  2. Luis R Macias, second year resident1,
  3. Regina Gómez-Palacio, second year resident1,
  4. Osvaldo M Mutchinick, chief2
  1. 1medial genetics
  2. 2Department of Genetics, National Institute of Medical Sciences and Nutrition “Salvador Zubirán”, Mexico

In the third part of our series, Adrián J González and colleagues explain the genetic bases of sexual development

Throughout history, humans have tried to discover differences between men and women, and how this determines the nature of sex. In the last century, scientists began to unlock the molecular and genetic mechanisms of sexual development. This process has not been so simple, however; sexual development consists of an orchestrated, ordered, and interrelated cascade of events.

The first step is the establishment of genetic sex (XX or XY). This results when a spermatozoid with an X or Y chromosome (genetic sex) fertilises an oocite, which carries an X chromosome. The second step is development of sex gonads. At the beginning of this step, regardless of genetic sex, embryos develop a bipotential primordium--a structure in the forming embryo that can turn into female or male gonad. This then differentiates to form the testis in XY embryos or the ovary in XX embryos, thereby defining gonadal sex. The last step is phenotypical sexual differentiation, when sexual external and internal genitalia develop due to hormones secreted by the gonad, resulting in the physical characteristics of each specific sex.

Finally, the gender of a person is the rearing assignated sex--the psychosocial implications of sex, which comprises determination and differentiation processes. Sex determination is the genetic events leading to male or female gonadal development; and sexual differentiation is the subsequent steps leading to functional sexuality and secondary sexual characteristics.


Sperm meets egg, sperm loses egg, sperm gets egg in the end

Gonadal sex differentiation

Formation of the genital and urinary systems are highly related. Both arise from the same structure--the intermediate lateral plate of the mesoderm. Before sex determination starts, bipotenial gonads are formed. On the 22nd day of intrauterine life, the primordial germ cells of extragonadal …

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