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X Y Chromosomes
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.
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.
The truth about sex differences, not always accepted at present, is that that they manifest in brain and body almost from the moment of conception. All human beings start as female, neurologically speaking. While the male baby is in the womb, the Y chromosome begins a sequence of masculinizing events in both body and brain, particularly the exposure to androgens, which greatly impacts behavior after birth.
The truth about sex differences is that they affect everyone in varying ways and degrees; however, there are some commonalities. For example, males tend to be stronger, bigger, and have deeper voices. They also enter puberty later than females do and often die younger.
Sex differences can also impact an individual’s behavior. Males are frequently more aggressive than females and have stronger sex drives. They take more risks and have fewer reproduction requirements (since they don’t have to devote the same time and energy during pregnancy as females do).
While the terms “sex” and “gender” are often used interchangeably, they don’t mean the same thing, and both concepts exist on a spectrum. A person’s sex is defined by their physical attributes at birth as either male or female. A person’s gender, on the other hand, is a function of cultural and personal norms and beliefs. It is for this reason that gender is sometimes called a "social" (as opposed to biological) construct. Sometimes, there is a mismatch between the sex a person is assigned at birth and the gender they feel inside.
Most people are born with either male or female sex organs. In rare cases, individuals may be born with ambiguous sex organs, falling under the category of “intersex.”
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.
In the imprinted brain theory, everyone’s brain is configured somewhere on a spectrum between hypomentalism and hypermentalism. In hypomentalism, the mechanistic, paternal genes are over-expressed, creating a baby with a larger head who demands more from the mother; this child is more likely to have autism. In hypermentalism, the mentalistic, maternal genes are over-expressed; the baby is likely to have a smaller head, demand less from the mother, and develop psychosis. The normal brain falls somewhere between the two extremes, ensuring that the child exhibits neither autism nor psychosis.
This rare sex chromosome abnormality, also referred to as Jacob’s syndrome, occurs when a male infant is born with an extra Y chromosome. Some symptoms include being tall and having a proportionally low weight, larger head dimensions, increased likelihood of learning disabilities, emotional problems, and behavioral issues.
Also known as XXY syndrome, this rare condition occurs when male babies are born with an extra X chromosome. These individuals tend to have smaller genitalia, extreme shyness, speech delay, learning disabilities, and dyslexia. However, there are few other obvious signs of XXY syndrome.
Angelman syndrome is caused by an over-expression of paternal imprinted genes and an under-expression of maternal ones on chromosome 15. Children with this genetic disorder tend to be more hyperactive and sleepless.
This genetic disorder occurs when part of chromosome 15 passed down by the father gets deleted. At the same time, the maternal imprinted genes are overexpressed. A child with Prader-Willi is likely to be more sleepy and inactive.
