Sex Determination and Genetic Diagrams

This lesson covers sex determination by sex chromosomes, sex-linked inheritance (X-linked conditions), and family pedigree diagrams. This is essential content for Edexcel GCSE Biology (1BI0) Topic 3: Genetics.

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Sex Chromosomes

In humans, biological sex is determined by the sex chromosomes.

• Humans have 23 pairs of chromosomes (46 in total).
• 22 pairs are autosomes (non-sex chromosomes) — these are the same in males and females.
• 1 pair is the sex chromosomes — these differ between males and females.

Sex	Sex Chromosomes
Female	XX (two X chromosomes)
Male	XY (one X chromosome and one Y chromosome)

How Sex Is Determined

• Egg cells (from the mother) always carry an X chromosome.
• Sperm cells (from the father) carry either an X or a Y chromosome.
• If a sperm carrying an X chromosome fertilises the egg → the offspring is XX (female).
• If a sperm carrying a Y chromosome fertilises the egg → the offspring is XY (male).

Therefore, the sex of the offspring is determined by the father's sperm, not by the mother's egg.

Exam Tip: A common misconception is that the mother determines the sex of the child. In fact, since all eggs carry X, it is the sperm (X or Y) that determines the sex. Make this clear in your answers.

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Punnett Square for Sex Determination

Parental genotypes: XX (mother) × XY (father)

Gametes:

• Mother: all gametes carry X.
• Father: gametes carry either X or Y.

Punnett Square:

	X	Y
X	XX	XY
X	XX	XY

Offspring:

• 2 XX (female)
• 2 XY (male)

Ratio: 1 female : 1 male

Probability of each sex: 50% (1 in 2 chance of being male, 1 in 2 chance of being female).

Exam Tip: This Punnett square is straightforward but often tested. Make sure you set it up correctly with the mother's gametes (both X) on one axis and the father's gametes (X and Y) on the other.

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Sex-Linked Inheritance

Some genes are located on the sex chromosomes rather than on the autosomes. When a gene is located on the X chromosome, it is described as X-linked (or sex-linked).

Why X-Linked Inheritance Is Important

• The X chromosome is much larger than the Y chromosome and carries many more genes.
• Males have only one X chromosome (XY), so they only have one copy of any gene on the X chromosome.
• Females have two X chromosomes (XX), so they have two copies of any gene on the X chromosome.
• This means that a recessive allele on the X chromosome will always be expressed in males (because there is no second X chromosome to carry a dominant allele that could mask it).
• In females, a single recessive allele on one X chromosome can be masked by a dominant allele on the other X chromosome — making her a carrier.

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Colour Blindness (X-Linked Recessive)

Red-green colour blindness is a classic example of an X-linked recessive condition.

Notation

Because the gene is on the X chromosome, we write the allele as a superscript on the X:

Genotype	Phenotype
X^B X^B	Normal-vision female
X^B X^b	Carrier female (normal vision, but carries the recessive allele)
X^b X^b	Colour-blind female (rare — needs two copies of the recessive allele)
X^B Y	Normal-vision male
X^b Y	Colour-blind male (only needs one copy of the recessive allele)

Where:

• X^B = X chromosome carrying the dominant allele for normal colour vision.
• X^b = X chromosome carrying the recessive allele for colour blindness.
• The Y chromosome does not carry a copy of this gene.

Exam Tip: Note that males cannot be "carriers" of an X-linked condition. They are either affected (X^b Y) or unaffected (X^B Y), because they only have one X chromosome. Only females can be carriers (X^B X^b).

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Worked Example: Colour Blindness Cross 1

Cross: Carrier female (X^B X^b) × Normal male (X^B Y)

Parental genotypes: X^B X^b × X^B Y

Gametes:

• Mother: X^B or X^b
• Father: X^B or Y

Punnett Square:

	X^B	X^b
X^B	X^B X^B	X^B X^b
Y	X^B Y	X^b Y

Offspring:

Genotype	Phenotype	Proportion
X^B X^B	Normal-vision female	1 in 4 (25%)
X^B X^b	Carrier female (normal vision)	1 in 4 (25%)
X^B Y	Normal-vision male	1 in 4 (25%)
X^b Y	Colour-blind male	1 in 4 (25%)

Key results:

• 50% of daughters have normal vision (X^B X^B), and 50% are carriers (X^B X^b). No daughters are colour-blind.
• 50% of sons have normal vision (X^B Y), and 50% are colour-blind (X^b Y).
• Overall probability of a colour-blind child: 25% (1 in 4), and this child will be male.

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Worked Example: Colour Blindness Cross 2

Cross: Carrier female (X^B X^b) × Colour-blind male (X^b Y)

Parental genotypes: X^B X^b × X^b Y

Gametes:

• Mother: X^B or X^b
• Father: X^b or Y

Punnett Square:

	X^B	X^b
X^b	X^B X^b	X^b X^b
Y	X^B Y	X^b Y

Offspring:

Genotype	Phenotype	Proportion
X^B X^b	Carrier female (normal vision)	1 in 4 (25%)
X^b X^b	Colour-blind female	1 in 4 (25%)
X^B Y	Normal-vision male	1 in 4 (25%)
X^b Y	Colour-blind male	1 in 4 (25%)

Key results:

• 50% of children are colour-blind (one female and one male).
• This is one of the few crosses that can produce a colour-blind female — the mother must be at least a carrier (X^B X^b) and the father must be colour-blind (X^b Y).

Exam Tip: For a female to be colour-blind, she must be homozygous recessive (X^b X^b). This requires BOTH parents to carry at least one X^b allele. A colour-blind female must have a colour-blind father (X^b Y) and a mother who is at least a carrier (X^B X^b) or colour-blind herself (X^b X^b).

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Family Pedigree Diagrams

A pedigree diagram (family tree) shows how a trait or condition is passed through generations of a family.

Symbols Used

Symbol	Meaning
Square	Male
Circle	Female
Filled (shaded) shape	Affected individual (shows the condition)
Unfilled (empty) shape	Unaffected individual
Half-filled shape	Carrier (heterozygous, does not show the condition)
Horizontal line connecting a square and circle	Mating / partnership
Vertical line below a couple	Offspring

How to Interpret a Pedigree Diagram

1. Identify the pattern of inheritance:
   • If affected individuals have unaffected parents, the condition is likely recessive.
   • If the condition appears mostly in males, it may be X-linked.
   • If every affected individual has at least one affected parent, the condition may be dominant.