Genetics: Sex Determination and Sex-Linked Inheritance

What you'll learn

This topic examines how biological sex is determined through chromosomes and how certain genetic traits are inherited differently in males and females. Understanding sex determination and sex-linked inheritance is essential for CXC CSEC Integrated Science, as questions appear regularly in both multiple-choice and structured formats, often requiring genetic diagrams and probability calculations.

Key terms and definitions

Chromosomes — thread-like structures in the nucleus made of DNA and protein that carry genetic information; humans have 23 pairs (46 total).

Sex chromosomes — the pair of chromosomes (X and Y) that determine biological sex in humans; females have XX, males have XY.

Autosomes — the 22 pairs of chromosomes that are not sex chromosomes and are identical in males and females.

Sex-linked trait — a characteristic controlled by a gene located on a sex chromosome (usually the X chromosome), showing different inheritance patterns in males and females.

Carrier — an individual (usually female) who possesses one recessive allele for a sex-linked condition but does not express the trait due to having a dominant allele on the other X chromosome.

Hemizygous — having only one allele for a particular gene, as in males for X-linked genes, since they have only one X chromosome.

Punnett square — a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross.

Allele — alternative forms of a gene; for sex-linked traits, represented by superscript letters on the X chromosome (e.g., X^H for normal, X^h for haemophilia).

Core concepts

Chromosomal basis of sex determination

Human cells contain 46 chromosomes arranged in 23 pairs. Of these, 22 pairs are autosomes that control body characteristics, while the 23rd pair consists of sex chromosomes that determine biological sex.

Females possess two X chromosomes (XX):

• Inherit one X from mother and one X from father
• All egg cells (gametes) carry a single X chromosome
• Homogametic sex (produce one type of gamete regarding sex chromosomes)

Males possess one X and one Y chromosome (XY):

• Inherit X from mother and Y from father
• Sperm cells carry either an X or a Y chromosome
• Heterogametic sex (produce two types of gametes regarding sex chromosomes)

The sex of offspring is determined at fertilization:

• If an X-carrying sperm fertilizes the egg → XX (female)
• If a Y-carrying sperm fertilizes the egg → XY (male)

The probability of having male or female offspring is approximately 50:50 (1:1 ratio) because males produce equal numbers of X-bearing and Y-bearing sperm.

The Y chromosome and male development

The Y chromosome is significantly smaller than the X chromosome and carries fewer genes. The most critical gene on the Y chromosome is SRY (Sex-determining Region Y), which triggers the development of testes in embryos. Without this gene, the embryo develops ovaries and female characteristics by default, regardless of having a Y chromosome.

Sex-linked inheritance patterns

Sex-linked traits are characteristics controlled by genes located on sex chromosomes. Most sex-linked traits in humans are X-linked because:

• The X chromosome carries approximately 1,500 genes
• The Y chromosome carries only about 200 genes
• Very few traits are Y-linked (mainly related to male fertility)

Key characteristics of X-linked recessive inheritance:

Males are affected more frequently than females because:

• Males need only one recessive allele to express the trait (hemizygous condition)
• Females need two recessive alleles (one on each X chromosome) to express the trait
• Females with one recessive allele are carriers — they possess the allele but don't express the phenotype

The pattern shows:

• Affected males often have unaffected parents
• The mother is usually a carrier
• Affected males cannot pass the trait to sons (sons receive the Y chromosome from father)
• All daughters of affected males are carriers (receive the affected X from father)

Common X-linked conditions

Haemophilia is a blood clotting disorder where blood fails to clot normally due to deficiency of clotting factors. In the Caribbean, haemophilia patients require regular medical attention and imported clotting factor concentrates, creating healthcare challenges for families.

Notation for haemophilia:

• X^H = normal blood clotting (dominant)
• X^h = haemophilia (recessive)
• Possible genotypes: X^H X^H (normal female), X^H X^h (carrier female), X^h X^h (affected female), X^H Y (normal male), X^h Y (affected male)

Red-green colour blindness prevents individuals from distinguishing between red and green colours. This condition affects approximately 8% of males but less than 1% of females globally.

Notation for colour blindness:

• X^C = normal colour vision (dominant)
• X^c = colour blindness (recessive)
• Genotypes follow the same pattern as haemophilia

Duchenne muscular dystrophy causes progressive muscle weakness and affects mainly males. While less commonly featured in CSEC exams than haemophilia or colour blindness, the inheritance pattern remains identical.

Genetic diagrams for sex-linked crosses

When constructing genetic diagrams for sex-linked traits, always:

1. Identify the genotypes of parents using proper notation (X and Y with superscript alleles)
2. Determine possible gametes each parent can produce
3. Use a Punnett square to show all possible fertilization combinations
4. State genotypes and phenotypes of offspring with ratios
5. Calculate probabilities as fractions, ratios, or percentages

Example cross: Carrier female (X^H X^h) × Normal male (X^H Y)

Gametes: Female produces X^H and X^h; Male produces X^H and Y

Offspring:

• X^H X^H (normal female) — 25%
• X^H X^h (carrier female) — 25%
• X^H Y (normal male) — 25%
• X^h Y (affected male) — 25%

Result: 50% females (all normal; half are carriers), 50% males (half affected)

Pedigree analysis

Pedigree charts track trait inheritance through family generations. For sex-linked recessive traits:

• Squares represent males; circles represent females
• Shaded symbols indicate affected individuals
• Half-shaded symbols indicate carriers (female carriers in X-linked conditions)
• Horizontal lines connect mating pairs
• Vertical lines connect parents to offspring

Characteristics that suggest X-linked recessive inheritance in pedigrees:

• More affected males than females
• Affected males connected through carrier females
• No male-to-male transmission
• Affected females only when father is affected and mother is at least a carrier

Worked examples

Example 1: Sex determination probability

Question: In Jamaica, a couple are expecting their third child. Their first two children are girls. What is the probability that their third child will be a boy? Explain your answer using a genetic diagram. (4 marks)

Solution:

The sex of previous children does not affect the probability of subsequent children's sex. Each pregnancy is an independent event.

Parental genotypes: Mother XX × Father XY

Gametes:

• Mother produces: X only
• Father produces: X and Y (in equal proportions)

Punnett square:

	X (mother)	X (mother)
X (father)	XX (female)	XX (female)
Y (father)	XY (male)	XY (male)

Offspring ratio: 2 XX : 2 XY = 1:1 or 50% female : 50% male

Therefore, the probability of a boy is 1/2 or 50% or 1:1 ✓

Mark scheme: 1 mark for correct genotypes, 1 mark for Punnett square, 1 mark for correct ratio, 1 mark for correct probability stated

Example 2: Carrier female and normal male cross

Question: In Trinidad, a woman who is a carrier for haemophilia marries a man with normal blood clotting.

(a) Using appropriate symbols, write the genotypes of the woman and the man. (2 marks)

(b) Draw a genetic diagram to show the possible genotypes and phenotypes of their children. (4 marks)

(c) What is the probability that a male child will have haemophilia? (1 mark)

Solution:

(a) Woman (carrier): X^H X^h ✓ Man (normal): X^H Y ✓

(b) Parental genotypes: X^H X^h × X^H Y

Gametes:

• Woman: X^H, X^h ✓
• Man: X^H, Y ✓

Punnett square:

	X^H	X^h
X^H	X^H X^H	X^H X^h
Y	X^H Y	X^h Y

Offspring genotypes and phenotypes: ✓

• X^H X^H — normal female (not carrier)
• X^H X^h — normal female (carrier)
• X^H Y — normal male
• X^h Y — male with haemophilia

Ratio: 2 normal females : 1 normal male : 1 affected male ✓

(c) Probability of affected male = 1/2 or 50% or 1 in 2 ✓

Mark scheme breakdown: (a) 2 marks for correct genotypes with proper notation; (b) 1 mark for gametes, 1 mark for Punnett square, 1 mark for genotypes, 1 mark for phenotypes with ratio; (c) 1 mark for correct probability

Example 3: Pedigree interpretation

Question: The pedigree below shows the inheritance of colour blindness in a Caribbean family.

        I:    ○—□
              |
        II:   ○—□   □—○
              |         |
        III:  □ ○ ■   ○ □

Key: ○ = unaffected female, □ = unaffected male, ■ = affected male

(a) State the type of inheritance shown. (1 mark)

(b) Explain how you know individual II-1 (the first female in generation II) must be a carrier. (2 marks)

(c) Write the genotype of individual III-3 (the affected male in generation III). (1 mark)

Solution:

(a) X-linked recessive inheritance ✓

(b) Individual II-1 must be a carrier because:

• She has an affected son (III-3) ✓
• She herself is unaffected (normal phenotype) ✓
• She must have passed an X^c chromosome to her affected son

(c) X^c Y ✓

Common mistakes and how to avoid them

Mistake: Writing genotypes for sex-linked traits without indicating the X and Y chromosomes (e.g., writing Hh instead of X^H X^h). Correction: Always show sex-linked alleles as superscripts on the X chromosome. Males have only one X, so write X^H Y or X^h Y, never HY or hY. Females have two X chromosomes: X^H X^H, X^H X^h, or X^h X^h.

Mistake: Believing that having multiple daughters increases the probability of the next child being male, or vice versa. Correction: Each pregnancy is an independent event. The 1:1 sex ratio is a probability for each conception, not a guarantee across siblings. Previous children's sexes do not influence future pregnancies.

Mistake: Stating that all daughters of affected males are affected with sex-linked recessive conditions. Correction: All daughters of affected males are carriers (if the mother is homozygous normal), not affected. They receive one X^h from father and one X^H from mother, giving genotype X^H X^h, which is phenotypically normal.

Mistake: Claiming that males can be carriers of X-linked recessive traits. Correction: Males cannot be carriers of X-linked traits because they have only one X chromosome. They are either affected (X^h Y) or unaffected (X^H Y) — the hemizygous condition means they express whatever allele is on their single X chromosome.

Mistake: Incorrectly showing male-to-male transmission of X-linked traits in pedigrees. Correction: Fathers pass their Y chromosome (not X) to sons, so X-linked traits cannot pass directly from father to son. Affected males can only pass the trait-carrying X to daughters (making them carriers).

Mistake: Confusing sex-linked inheritance with sex-influenced traits or sex-limited traits. Correction: Sex-linked traits are controlled by genes on sex chromosomes. Sex-influenced traits (like pattern baldness) are on autosomes but expressed differently in males and females due to hormones. Sex-limited traits (like milk production) appear in only one sex. CSEC focuses on sex-linked inheritance.

Exam technique for sex determination and sex-linked inheritance

Command words and response strategies:

• "State" or "Name" — Give brief, direct answers (e.g., "X-linked recessive"). Requires 1 mark, approximately 1-3 words.
• "Explain" — Provide reasons or mechanisms. For sex-linked inheritance, mention the location of genes on X chromosomes and the hemizygous condition of males. Requires 2-3 marks, write 2-3 sentences with causal links.
• "Using a genetic diagram, show..." — Must include parental genotypes, gametes, Punnett square, offspring genotypes and phenotypes, and ratios. Typically worth 4-5 marks; omitting any component loses marks.
• "Calculate the probability..." — Show working and express answer as fraction, percentage, or ratio as requested. Worth 1-2 marks; no working shown may lose method marks.

Genetic diagram checklist for maximum marks:

1. Write parental phenotypes and genotypes with correct notation (X^A, X^a, Y)
2. Show gametes produced by each parent in circles or underlined
3. Draw complete Punnett square with all possible combinations
4. List all offspring genotypes
5. State offspring phenotypes with descriptions
6. Give ratios or probabilities as required

Time management: Genetic diagram questions typically carry 4-6 marks. Allocate approximately 5-7 minutes for complete diagrams. Practice drawing Punnett squares quickly and accurately to save time under exam conditions.

Caribbean context awareness: Questions may reference local healthcare challenges (access to clotting factors for haemophilia patients), genetic counselling services in regional hospitals, or implications for small island populations with limited genetic diversity. Apply biological principles to these realistic scenarios.

Quick revision summary

Sex determination in humans depends on sex chromosomes: females have XX, males have XY. The 1:1 sex ratio results from males producing equal numbers of X and Y sperm. Sex-linked traits (usually X-linked) show distinct inheritance patterns because males have only one X chromosome, making them hemizygous. Males need just one recessive allele to express X-linked recessive traits like haemophilia or colour blindness, while females need two. Carrier females (X^H X^h) are phenotypically normal but can pass the allele to offspring. Genetic diagrams must show proper notation with superscript alleles on X chromosomes. In pedigrees, X-linked recessive traits show more affected males, no male-to-male transmission, and carrier females connecting affected relatives.