Sex Determination in Humans

Humans have 46 chromosomes arranged in 23 pairs. The 23rd pair are the sex chromosomes. Females carry two X chromosomes (XX); males carry one X and one Y chromosome (XY). It is the sex chromosome inherited from the father that determines biological sex.

Sex determination Punnett square

	X (egg)	X (egg)
X (sperm)	XX (Female)	XX (Female)
Y (sperm)	XY (Male)	XY (Male)

Each pregnancy has a 50% chance of XX (female) and 50% chance of XY (male).

Sex-Linked Traits

Sex-linked traits are controlled by genes located on sex chromosomes, usually the X chromosome (X-linked traits). Because males have only one X chromosome (XY), they only need one copy of a recessive allele to express the trait. Females (XX) need two copies.

Colour blindness (X-linked recessive)

The gene for red-green colour perception is on the X chromosome. Let X^B = normal vision (dominant), X^b = colour blind (recessive).

• X^BX^B — female, normal vision
• X^BX^b — female, carrier (normal vision, can pass on allele)
• X^bX^b — female, colour blind (rare)
• X^BY — male, normal vision
• X^bY — male, colour blind (~8% of males in Australia)

Haemophilia (X-linked recessive)

Haemophilia is a blood-clotting disorder caused by a recessive allele on the X chromosome. A carrier mother (X^HX^h) has a 50% chance of passing the allele to each son; sons who inherit X^hY will have haemophilia. Haemophilia was historically notable in European royal families.

Genetic Disorders

Genetic disorders result from mutations in DNA or abnormal numbers of chromosomes. They can be inherited (passed from parent to child) or arise spontaneously (new mutations).

Autosomal recessive — Cystic fibrosis

Caused by a faulty gene on chromosome 7. Two copies of the recessive allele (ff) are needed to express the disease. Carriers (Ff) show no symptoms. About 1 in 25 Australians is a carrier. Affects the lungs and digestive system.

Autosomal dominant — Huntington's disease

A progressive neurological disorder caused by a dominant allele (H). Only one copy is needed to develop the disease. Symptoms typically appear after age 40, meaning affected individuals may have already had children before diagnosis.

Chromosomal disorder — Down syndrome (trisomy 21)

Caused by an extra copy of chromosome 21 (three copies instead of two), resulting from errors in cell division (non-disjunction). Not inherited in the traditional sense; occurs in all populations. Associated with characteristic physical features and intellectual disability.

Reading Pedigree Charts

A pedigree chart is a diagram showing the inheritance of a trait across generations. It is used to determine whether a trait is dominant or recessive, autosomal or sex-linked.

Pedigree symbols:

• Square = male Circle = female
• Filled shape = affected individual Unfilled shape = unaffected
• Half-filled shape = carrier (typically shown for X-linked recessive)
• Horizontal line between shapes = mating; vertical line = offspring

Clues for identifying inheritance pattern:

• If the disorder skips generations → likely recessive
• If every affected person has an affected parent → likely dominant
• If only males are affected → likely X-linked recessive
• If females are affected too → likely autosomal

Key Vocabulary

Term	Definition
Sex-linked trait	A trait controlled by a gene found on a sex chromosome (usually the X chromosome).
Carrier	An individual who has one recessive allele for a disorder but does not show symptoms; can pass the allele to offspring.
Non-disjunction	The failure of chromosome pairs to separate during cell division, resulting in cells with abnormal chromosome numbers.
Pedigree chart	A diagram showing the occurrence and inheritance of a genetic trait across multiple generations of a family.

Worked Examples

1

A carrier mother (X^BX^b) and a normal-vision father (X^BY). What are the chances of their sons being colour blind?

Step 1: Set up the Punnett square. Mother: X^BX^b; Father: X^BY.

Step 2: Offspring: X^BX^B (normal female), X^bX^B (carrier female), X^BY (normal male), X^bY (colour blind male).

Step 3: Of the two possible sons (X^BY and X^bY), one is colour blind.

Answer: 50% chance of each son being colour blind; 0% chance of daughters being colour blind (though 50% will be carriers).

2

Two carrier parents (Ff × Ff) for cystic fibrosis. What are the offspring probabilities?

Punnett square:

FF (25%, unaffected, non-carrier) : Ff (50%, carrier) : ff (25%, affected with cystic fibrosis).

Answer: 25% chance of inheriting cystic fibrosis, 50% chance of being a carrier, 25% chance of being unaffected and non-carrier. This is the standard 1:2:1 genotype ratio for a monohybrid cross between two carriers.

3

In a pedigree, three unaffected parents have an affected daughter. What does this tell us about the trait?

Observation 1: Both parents are unaffected but have an affected child → the trait must be recessive. Both parents are carriers.

Observation 2: The affected individual is female. If it were X-linked recessive, she would need X^bX^b — meaning her father would also be affected (X^bY). Since the father is unaffected, it is not X-linked.

Answer: The trait is autosomal recessive. Both parents are heterozygous carriers (Aa × Aa), and the daughter has inherited two recessive alleles (aa).

Knowledge Check

Select the correct answer for each question.

Question 1

Which sex chromosome combination determines biological male sex in humans?

XX YY XY XO

Question 2

Why are males more often affected by X-linked recessive disorders than females?

Males have stronger immune systems The Y chromosome causes the disorder Males have more chromosomes than females Males have only one X chromosome, so one recessive allele is sufficient to express the disorder

Question 3

Cystic fibrosis is caused by a recessive allele (f). Two carrier parents (Ff) have children. What is the probability of a child having cystic fibrosis?

25% 0% 50% 75%

Question 4

Down syndrome (trisomy 21) is caused by:

A point mutation in a single gene An extra copy of chromosome 21 due to non-disjunction A deletion of chromosome 21 Inheriting two recessive alleles from each parent

Question 5

A trait appears in every generation of a pedigree, affecting both males and females equally. This pattern is most consistent with:

Autosomal recessive inheritance X-linked recessive inheritance Autosomal dominant inheritance Y-linked inheritance

Human Genetics

Sex Determination in Humans

Sex-Linked Traits

Colour blindness (X-linked recessive)

Haemophilia (X-linked recessive)

Genetic Disorders

Autosomal recessive — Cystic fibrosis

Autosomal dominant — Huntington's disease

Chromosomal disorder — Down syndrome (trisomy 21)

Reading Pedigree Charts

Key Vocabulary

Worked Examples

A carrier mother (X^BX^b) and a normal-vision father (X^BY). What are the chances of their sons being colour blind?

Two carrier parents (Ff × Ff) for cystic fibrosis. What are the offspring probabilities?

In a pedigree, three unaffected parents have an affected daughter. What does this tell us about the trait?

Knowledge Check

Key Concepts Summary

Human Genetics

Sex Determination in Humans

Sex-Linked Traits

Colour blindness (X-linked recessive)

Haemophilia (X-linked recessive)

Genetic Disorders

Autosomal recessive — Cystic fibrosis

Autosomal dominant — Huntington's disease

Chromosomal disorder — Down syndrome (trisomy 21)

Reading Pedigree Charts

Key Vocabulary

Worked Examples

A carrier mother (XBXb) and a normal-vision father (XBY). What are the chances of their sons being colour blind?

Two carrier parents (Ff × Ff) for cystic fibrosis. What are the offspring probabilities?

In a pedigree, three unaffected parents have an affected daughter. What does this tell us about the trait?

Knowledge Check

Key Concepts Summary

A carrier mother (X^BX^b) and a normal-vision father (X^BY). What are the chances of their sons being colour blind?