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.
How do sex chromosomes affect intelligence?
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.
Are sex differences in behavior real?
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.
What are some physical sex differences?
What are some behavioral sex differences?
What is the difference between biological sex and gender?
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.
What is intersex?
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.”
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.
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.
Why are women at greater risk of many mental illnesses than men are?
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.
What causes autism spectrum disorders, according to the imprinted brain theory?
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.
What is XYY syndrome?
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.