Male Genetics

Biologically being male requires one thing – a Y sex chromosome.  A chromosome is a bundle of DNA that codes genetic information.  The normal number of chromosomes in humans is 46 – 23 from the mother and 23 from the father which pair up.  The 23rd pair are the sex chromosomes (circled below).  Males have an XY pair and females have an XX pair. 


Males will always have a Y chromosome from their father and an X chromosome from their mother.  Females have two X chromosomes, one from the father and the other from the mother.  So it really is the father’s sperm that dictates the gender of the child. However, it is possible for something to go wrong when sperm and eggs are produced and some men can have three or more sex chromosomes rather than just two.

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Sometimes, males can be born with more than two sex chromosomes.  As long as they have at least one Y chromosome, they are biologically male; even if they have two X chromosomes and one Y chromosome.   Unlike all other cells in our body, eggs and sperm are produced by a two-step process called meiosis.  Meiosis allows paired chromosomes to break apart so each parent only donates half of their chromosomes.   For instance, in the first step a sperm will divide first into an X chromosome and a Y chromosome cell.  These cells then duplicate the sex chromosome (making two X or two Y chromosomes) and then divide again into two X sperm and two Y sperm.  Unfortunately, sometimes one sperm (or egg) ends up with two sex chromosomes instead of one.  For example, an egg can have two X chromosomes and a sperm might have two X chromosomes or two Y chromosomes.


When fertilization occurs with the abnormally formed eggs or sperm, the baby will receive two chromosomes from one parent and one from the other—this leads to three chromosomes instead of a pair.  Having a triple set of chromosomes is called “trisomy.” 

Most forms of trisomy will result in a miscarriage because the baby cannot survive.  However, there are a few trisomy conditions where babies can survive.  Perhaps the best known form of trisomy occurs when a baby has three chromosome 21s – we call this Down Syndrome or Trisomy 21.  Babies can survive if they have three sex chromosomes.  Two of these sex trisomy conditions affect males.



47-XXY Syndrome (also called Kleinfelter Syndrome) occurs when a male is born with two X chromosomes and one Y chromosome (XXY).  The 47 in the name of the syndrome comes from having 47 chromosomes instead of 46.  This occurs in about 1 in 1,000 males.  The syndrome can occur due to an egg with two X chromosomes or a sperm with two X chromosomes.  These men often have low levels of testosterone and; therefore, may have a less masculine appearance (lower muscle mass, store fat more in the hips and thighs, less body/facial hair, smaller testicles, and some breast enlargement).  Men with Kleinfelter’s are often prescribed testosterone beginning at puberty to help them have testosterone levels in the “normal” range in order to help combat some symptoms.  Men with Kleinfelter’s tend to be less aggressive due to lower levels of testosterone and some can have some learning difficulties.  In addition, they are usually taller and are often slender and have longer arms.  Men with Kleinfelters can often experience social issues, anxiety, and depression.  In addition, most men with Kleinfelter Syndrome have issues with infertility.  Since Kleinfelter’s can be difficult to spot (especially before puberty), many men aren’t diagnosed until they are adults when they are being seen for infertility issues.  In fact, it is thought that 75% of men with XXY aren't aware that they have it and have never been diagnosed.[i]  

Below is an example of how an XXY boy was conceived from an XX egg and a Y sperm.

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47-XYY Syndrome (sometimes called Jacob’s Syndrome) occurs in about 1 in 1,000 male births as well.  This is a little opposite of Kleinfelter’s Syndrome where men have an extra Y chromosome and make an overabundance of testosterone.  This is caused by a double Y sperm coming from the father. 


Men with Jacob’s Syndrome tend to have severe acne and are larger in stature (taller).  They often have issues with aggression associated with increased hormone levels.  Unlike Kleinfelter’s, these men can father children normally.  However, oftentimes boys with 47-XYY develop more slowly mentally, and often have learning disabilities.[ii] 


All illustrations on this page are are original design