Male Pattern Baldness Inheritance: X-Linked Genetics and Probability

Androgenetic alopecia (male pattern baldness) has a strong genetic component, with the androgen receptor (AR) gene on the X chromosome being a major factor. Here's a simplified Mendelian model:

Genetic Notation

• X^B = Baldness-associated allele (recessive phenotype in males)
• X = Non-baldness allele
• Y = Y chromosome

Scenario

Woman's father is bald (X^BY) → she inherits his X^B. She is a carrier (X^BX). Man has normal hair (XY).

Punnett Square for Sons

	Mother's XB	Mother's X
Father's X	XBX (carrier daughter)	XX (non-carrier daughter)
Father's Y	XBY (bald son)	XY (non-bald son)

Result

Each son has a 50% chance of inheriting X^BY (bald) and 50% chance of XY (non-bald).

Important Nuances

• Polygenic: Many autosomal loci contribute (e.g., 20p11, 1q21). X-linked AR is major but not sole determinant.
• Incomplete penetrance/variable expressivity: Not all X^BY males go bald; onset/pattern varies.
• Maternal grandfather not sole predictor: Father's side also contributes autosomal genes.
• Genetic testing: Polygenic risk scores available but not deterministic.

This model illustrates the classic X-linked pattern; real-world risk is multifactorial.