Sex linkage and chromosomes

MDCAT Biology · Biology · Inheritance. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All MDCAT Biology topics

The idea

Humans have 22 pairs of autosomes and one pair of sex chromosomes: XX in females, XY in males. All eggs carry an X. Half the sperm carry X and half carry Y, so the sperm decides the sex of the child.

Genes on the X chromosome are X-linked. A male has only one X, so a single recessive allele on it shows in him. A female needs two copies to be affected; with one copy she is a healthy carrier. Haemophilia and red-green colour blindness are X-linked recessive.

Non-disjunction is the failure of chromosomes to separate in meiosis. It gives gametes with one chromosome too many or too few.

Rules to remember

A father gives his X to all his daughters and his Y to all his sons; a son's X always comes from his mother.
Carrier mother × normal father: half the sons affected, half the daughters carriers, no daughter affected.
Affected father × normal (non-carrier) mother: all daughters carriers, all sons normal.
Haemophilia A is a lack of clotting factor VIII.
Down syndrome: trisomy 21, 47 chromosomes. Turner syndrome: 45, XO, a sterile female. Klinefelter syndrome: 47, XXY, a sterile male.
In humans the presence of a Y chromosome gives a male.
Other systems: XO in grasshoppers (male has one X only); ZW in birds (the female is ZW and decides the sex).

Common mistake

An X-linked trait is never passed from father to son, because a son receives his father's Y chromosome, not his X.

Worked example

A woman who is a carrier of red-green colour blindness marries a colour-blind man. What is the probability that a child of theirs is a colour-blind girl?

Genotypes: mother X^CX^c, father X^cY.
Offspring in equal numbers: X^CX^c (carrier girl), X^cX^c (colour-blind girl), X^CY (normal boy), X^cY (colour-blind boy).
One of the four equally likely types is a colour-blind girl.

Answer: 1/4 (and half of all the daughters are colour blind).

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← Beyond simple dominance The heart →Take a MDCAT Biology test