X Chromosome, Y Chromosome
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.
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.
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.
The X chromosome carries at least 150 genes linked to intelligence. Since the mother has two X chromosomes to pass along while the father only has one, the mother’s contribution to the child’s intelligence will, on average, be greater than the father’s; this is especially true if the child is male, since his only X chromosome will come from his mother.
Understanding Sex-Linked Diseases
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.
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.
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.
The X chromosome gene XIST can have a huge impact on female mental health. It tends to be overexpressed in the genes of females who have bipolar disorder or major depression . The down-regulation of X genes, on the other hand, can be a predictor of autism spectrum disorders, especially Asperger ’s syndrome. Learning how to reverse some of this abnormal expression of X genes could help treat psychiatric disorders in women.
Explore More About X Y Chromosomes
For a comprehensive understanding of x y chromosomes, read our complete guide: