Sex Linkage (AQA A-Level Biology): Revision Notes
Sex Linkage
Sex chromosomes and sex determination
Humans possess 23 pairs of chromosomes, with 22 pairs being homologous chromosomes that appear identical in both males and females. The 23rd pair consists of the sex chromosomes, which determine biological sex.
In females, both sex chromosomes are X chromosomes and appear similar in size and structure. Males have one X chromosome and one Y chromosome, where the Y chromosome is considerably smaller than the X chromosome.
The significant size difference between X and Y chromosomes is crucial for understanding sex-linked inheritance. The X chromosome is much longer and carries many more genes than the smaller Y chromosome.
Sex inheritance patterns
The inheritance of sex follows a predictable pattern based on which sex chromosomes parents contribute to their offspring.
Female gamete production: Since females have two X chromosomes (XX), all their gametes contain a single X chromosome.
Male gamete production: Males possess XY chromosomes, so they produce two types of gametes in equal proportions - half contain an X chromosome and half contain a Y chromosome.
During fertilisation, offspring receive one sex chromosome from each parent, resulting in a 50:50 ratio of males to females in the population.
Understanding sex-linkage
Sex-linkage refers to any gene located on either the X or Y chromosome. However, since the X chromosome is much longer than the Y chromosome, most sex-linked genes are found on the X chromosome (X-linked genes).
The X chromosome carries numerous genes that have no equivalent on the smaller Y chromosome. This creates a unique inheritance pattern where males, having only one X chromosome, will express whatever alleles are present on their single X chromosome.
Critical Concept: Males are more susceptible to X-linked recessive conditions because they have only one X chromosome. Unlike females, who have two X chromosomes that can compensate for recessive alleles, males will express any recessive allele present on their single X chromosome.
X-linked inheritance using haemophilia
Haemophilia serves as an important example of X-linked recessive inheritance. This condition affects blood clotting, causing slow clotting and potential internal bleeding, particularly in joints.
Allele notation
- H = dominant allele for normal blood clotting
- h = recessive allele for slow blood clotting (haemophilia)
Why males are more affected
Males inherit their X chromosome from their mother and their Y chromosome from their father. Since the Y chromosome lacks genes for blood clotting, males will express whatever allele is present on their single X chromosome.
If a male inherits an X chromosome carrying the recessive h allele, he will have haemophilia because there is no second X chromosome to potentially carry the dominant H allele.
Carrier females
Females can be heterozygous for the haemophilia allele (). These females are called carriers because they carry the recessive allele but do not express the condition. The dominant H allele ensures they produce sufficient functional clotting protein.
Carrier females are crucial in inheritance patterns because they can pass the recessive allele to their children without showing symptoms themselves. This allows X-linked recessive traits to "hide" in families and appear to skip generations.
Inheritance patterns in crosses
Worked Example: Cross 1 - Carrier female × Normal male
Female genotype: (carrier)
Male genotype: (normal)
Offspring ratios:
- 25% normal females ()
- 25% carrier females ()
- 25% normal males ()
- 25% haemophiliac males ()
Worked Example: Cross 2 - Normal female × Haemophiliac male
Female genotype: (normal)
Male genotype: (haemophiliac)
Offspring ratios:
- 50% carrier females ()
- 50% normal males ()
Key observation: Haemophiliac males cannot pass the condition to their sons, as sons inherit the Y chromosome from their father. However, all daughters of haemophiliac males become carriers.
Pedigree charts for sex-linked traits
Pedigree charts provide a visual method for tracing the inheritance of sex-linked characteristics through families.
Pedigree Chart Symbols:
- Squares represent males
- Circles represent females
- Shaded shapes indicate individuals expressing the trait
- Unshaded shapes represent normal individuals
Pedigree analysis helps identify inheritance patterns and can reveal whether traits are sex-linked by showing characteristic patterns such as affected males having unaffected parents but affected maternal grandfathers.
Key features of X-linked recessive inheritance
X-linked recessive traits show several distinctive patterns that help geneticists identify this type of inheritance:
- More males affected than females
- Affected males often have unaffected parents
- The trait can skip generations
- Affected males cannot pass the trait to their sons
- All daughters of affected males are carriers (if mother is normal)
- Carrier mothers have a 50% chance of passing the trait to sons
Key Points to Remember:
- Sex is determined by X and Y chromosomes - females are XX, males are XY
- Sex-linked genes are located on sex chromosomes, mostly on the larger X chromosome
- Males are more likely to express X-linked recessive traits because they have only one X chromosome
- Carrier females are heterozygous and can pass on recessive alleles without expressing the trait themselves
- Pedigree charts help trace sex-linked inheritance patterns through families
- X-linked traits show characteristic patterns: more males affected, skipping generations, and father-to-son transmission is impossible