Is Penis Size Genetic?

Genetics plays a substantial role in the size of the penis as it does with other physical characteristics like a person's height. Even so, other factors may contribute to the penis size, including nutrition, environmental toxins, and hormones like testosterone.

This article explores how genetics and other factors, such as hormones, influence penis size.

SRY Gene

Penis size is dependent on different genes, starting with chromosomes and genes that determine whether you are biologically male or female.

Your biological sex is determined by the sex chromosomes you receive from your parents. These chromosomes contain all the information—in the form of genes—that establish whether you have male or female characteristics.

Your mother has two X chromosomes, and your father has an X and Y chromosome. To be biologically male, you need to inherit one X chromosome from your mother and one Y chromosome from your father.

Within the structure of the Y chromosome is a specific gene—called the sex-determining region Y (SRY) gene—that directs the formation of the internal male reproductive organs, including the testes (testicles). It also prevents the formation of internal female reproductive organs, including the ovaries, uterus, and fallopian tubes.

AR Gene

While the SRY gene is central to the development of the internal sex organs in males, it doesn't influence the formation of the external genitals like the penis and scrotum.

This is instead directed by another gene called the androgen receptor (AR) gene. Androgens are hormones (including testosterone) that are important for male sexual development.

As cells of an embryo start to specialize, an outgrowth of tissue will form called a genital tubercle. The AR gene directs whether the tubercle (and other structures of the body) has receptors able to respond to androgens. If it does, the tubercle will respond by developing into a penis and scrotum. If it doesn't, the tubercle will invert to form a vagina and clitoris.

Abnormalities in the AR gene can influence the future size of the penis. This is evidenced by a condition called androgen insensitivity syndrome (AIS) in which the androgen receptors are not responsive to testosterone in people who are genetically male. This can lead them to develop female characteristics as well as an abnormally small penis known as a micropenis.

Other Genes

While the AR gene will direct the formation of the external genitals in males, it may only play a partial role in the actual size of the penis in later life.

For this, testosterone plays a more central role—and not only during fetal development but also during a period known as mini-puberty.

Mini-puberty spans the first three to six months of life during which sex hormone levels (testosterone in males and estrogen in females) are exceptionally high. Mini-puberty is the phase in which the sex organs mature and, in males, the eventual length and size of the penis are established.

(Testosterone can also influence penis size during puberty, but its effects are largely focused on secondary male characteristics like body hair, increased stature, and the deepening of the voice.)

Certain genes are thought to influence mini-puberty. This includes the Makorin RING-finger Protein 3 (MKRN3) gene which can significantly increase androgen production and the Kisspeptin receptor (KISS1R) gene which can significantly decrease androgen production.

Other genes can direct how testosterone is utilized in the body. With respect to penis development, testosterone is converted into a more potent androgen called dihydrotestosterone (DHT) that influences the development of both the penis and scrotum.

The conversion of testosterone to DHT is directed by a gene called the steroid 5 alpha-reductase 2 (SRD5A2) gene. A number of mutations are known to render the SRD5A2 gene inactive. When this happens, it can disrupt the development of the penis both before birth and during mini-puberty.

Other Factors

Other factors may influence a person's penis length, girth, and circumference. This includes maternal nutrition during pregnancy and infant nutrition after pregnancy.

It is well known that malnutrition during pregnancy is associated with numerous complications, including low birth weight and delayed development. But, animal studies also suggest that it can negatively impact androgen receptors in the unborn baby and disrupt sexual maturation in early life.

While research into the impact of nutrition on mini-puberty is lacking, a 2021 study in the Journal of Clinical Endocrinology and Metabolism reported that male infants fed soy formula had had a more rapid increase in penile length during mini-puberty than those fed cow's milk or breast milk.

Although there were no differences in androgen levels in the soymilk-fed babies, it does suggest that nutrition may influence the maturation of sexual organs in early life.

Exposure to certain environmental toxins may also influence penis size.

A 2013 study from Mexico reported that male babies born to mothers exposed to the industrial chemical mono-2-ethylhexyl phthalate (used to make plastic) had penises of abnormally smaller length, circumference, and width. Some pesticides are thought to have the same effect.