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X Y Chromosomes

Reviewed by Psychology Today Staff

Each human cell contains 23 pairs of chromosomes that carry DNA within their nucleus. The X and Y chromosomes, commonly referred to as the sex chromosomes, are one such pair. They determine the biological sex, reproductive organs, and sexual characteristics that develop in a person. Female (XX) mammals inherit one X chromosome from each parent, but males (XY) receive an X from their mother and a Y sex chromosome from their father.

Sex Differences and Fetal Development

X and Y chromosomes determine not only the sex of an individual, but many broad characteristics as well. From grip-strength to aggression, there are myriad differences between men and women beyond simply their sexual organs. This is known as sexual dimorphism and is found in most species.

Can I choose my baby’s gender?

In vitro fertilization (IVF) technology exists that can allow parents to select their baby’s gender—for a price. It’s possible to separate X and Y-sperms by flow sorting with some degree of accuracy. In the future, fertilization may not be so random after all, but while pre-selection may be important to prevent certain life-threatening, inherited conditions, it involves a slippery ethical slope that requires further investigating.  

What is X chromosome expression?

X chromosomes are expressed differently depending on whether you are male or female. Since females have two X chromosomes, one may be expressed, or manifest in the phenotype, and the other may be silenced. Because males have only inherited one X chromosome, it is sure to be expressed.  

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Understanding Sex-Linked Diseases
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Diseases and conditions found only on the X chromosome are said to be sex-linked, as are rare Y-linked conditions such as male infertility. The X chromosome is larger and commands more regulatory functions throughout the brain and body. While diseases such as color-blindness and hemophilia have long been understood to be X-linked, biologists and behavioral geneticists now know that important brain functions and conditions, from intelligence to autism, may be up-regulated or down-regulated by genes found on the X chromosome.

What is genomic imprinting?

Genomic imprinting is the biological mechanism wherein genes are expressed differently depending on whether they are inherited from the mother or the father. The imprinted brain theory developed by Christopher Badcock and Bernard Crespi hypothesizes that imprinting contributes to a range of psychiatric and neurological afflictions, most commonly autism spectrum and schizophrenia spectrum disorders.

Why would maternal and paternal genes "compete?"

There are evolutionary reasons that maternal and paternal genes might compete for expression. For example, fetal head-size is favorable for a mother as it ensures an easier and safer birth. In fact, smaller head size is associated with a range of conditions that show evidence of maternal imprinting, whereas larger head circumference at birth is correlated with conditions, including autism, that may reflect an over-expression of paternal genes. 

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