Re: There is a genetic defect that causes a male child to be born anatomically

Brandi,

What you describe can be either Turner's syndrome or Noonan's syndrome.

Beginning at the beginning, such an abnormality is technically called ambiguous genitalia (or atypical genitalia ). Of course, this is classified as a birth defect (or birth variation) of the sex organs that makes it unclear whether an affected newborn is a girl or boy.

This condition occurs about once in every 4,500 births. The baby seems to have a mixture of both female and male genitalia. For example, they may have both a vulva and testicles. Associated intersex conditions for male babies include hypospadias , where the urethral opening is located in an unusual position such as the underside of the penis.

The causes of ambiguous genitalia include genetic variations, hormonal imbalances and malformations of the fetal tissues that are supposed to evolve into genitals. Tests (including ultrasound, x-rays and blood tests) must be done before the baby's sex can be positively identified. Mild forms of ambiguous genitalia may be characterised by a large (penis-like) clitoris in baby girls or undescended testicles in boys. The latter seems to be the condition in which your question was asked.

A baby's sex is determined at conception (when the sperm fertilizes the ovum). The mother's egg provides an X chromosome and the father's sperm determines the baby's sex by contributing either an X or Y sex chromosome. An XX embryo is female while an XY embryo is male. Both female and male embryos develop in exactly the same way. They have identical gonads and genital parts until around the eighth week of gestation. The sexual determination process includes:

• Girls - the internal genital parts transform into the uterus, fallopian tubes and vagina. The gonads turn into ovaries which start producing female sex hormones. The lack of male hormones is fundamental in allowing the development of female genitalia.

• Boys - the internal genital parts transform into the prostate gland and vas deferens. The gonads turn into testes which start producing male sex hormones. The presence of male hormones allows the penis and scrotum to develop.

Different types of ambiguous genitalia

The different types of ambiguous genitalia include:
The baby has ovaries and testicles, and the external genitals are neither clearly male nor female.
The baby has ovaries and a penis-like structure or phallus.
The baby has undescended testes and external female genitals including a vulva.

A range of causes
For typical genital development, the gender 'message' must be communicated from the sex chromosomes to the gonads. The gonads must then manufacture appropriate hormones and the genital tissues and structures have to respond to these hormones. Any deviations along the way can cause ambiguous genitalia.
Some specific causes include:

Androgen insensitivity syndrome (AIS) - a genetic condition characterised by the foetal tissue's insensitivity to male hormones. This affects genital development. For example, a newborn may have some of the female reproductive organs but also have testicles.

Congenital adrenal hyperplasia (CAH) - an inherited condition that affects hormone production. A child with CAH lacks particular enzymes, and this deficiency triggers the excessive manufacture of male hormones. For example, female genitals are masculinised.

Sex chromosome disorders - instead of having either XX or XY sex chromosomes, a baby may have a mixture of both ('mosaic' chromosomes); or specific genes on the Y chromosome may be inactive; or one of the X chromosomes may have a tiny Y segment attached to it.

Research at the University of California at Los Angeles (UCLA) indicates that ambiguous genitalia can be caused by the doubling up of a particular gene (named WNT-4) on the sex chromosome. This variation will interfere with male sexual development so that a genetically male baby will appear female.

Maternal factors - the pregnant mother may have had an androgen- secreting tumour while pregnant, and the excess of this male hormone affected her baby's genital development. In other cases, the placenta may have lacked a particular enzyme which failed to deactivate male hormones from the baby as a result, both the mother and the female baby were masculinised by the excess of these hormones.

Additional information can be retrieved at:

Turner's Syndrome http://www.hon.ch/cgi-bin/HONselect?browse+ C13.371.820.700.842.309.872

and

Noonan's Syndrome[also called " male Turner's syndrome"]

http://www.medscape.com/viewarticle/410764_3? WebLogicSession=PlveXY55voWW7LX7dY76YF2JclJcocr8e5nnz0ACecF216Z2l7zv|57630919642 71480803/184161395/6/ 7001/ 7001/7002/7002/7001/-1

There is also a Web site for Turner's Society of America, a support group acknowledging that on the basis of statitics it is expected that there are between 112,000 and 225,00 individuals in the United States who may have been born with some degree of this congenital abnormality. http://www.turner- syndrome-us.org/
To address the second part of your question - can one twin have sex reversal while the other doesn't:
There was a twin study reported in 1993 by Reiss et al in which only one of the identical twins had Turner's syndrome. So evidently this disorder is very much individualized.
The following link is to a MedLine abstract of Reiss' article http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi? cmd=Retrieve&db=PubMed&list_uids=8517687&dopt=Abstract