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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Sex-linked Inheritance
Have you ever wondered why some genetic conditions affect males more often than females? The answer lies in sex-linked inheritance, a fascinating pattern of inheritance where genes located on the sex chromosomes (X and Y) determine traits differently in males and females.
Key Definitions:
Humans have 23 pairs of chromosomes. 22 pairs are autosomes (non-sex chromosomes) and 1 pair consists of sex chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). This difference is key to understanding sex-linked inheritance!
The X chromosome contains many more genes than the Y chromosome. Males have only one X chromosome, so if they inherit a recessive allele on their X chromosome, they will show the trait. Females need two copies of the recessive allele (one on each X chromosome) to show the trait.
Most sex-linked traits follow an X-linked recessive inheritance pattern. This means the allele causing the trait is recessive and located on the X chromosome. Let's explore how this works:
Males have only one X chromosome, which they inherit from their mother. If this X chromosome carries a recessive allele for a condition, the male will show the trait because there's no second X chromosome with a dominant allele to mask it. This is why conditions like colour blindness and haemophilia affect males more frequently than females.
Females, on the other hand, have two X chromosomes. If one X chromosome carries a recessive allele for a condition, the dominant allele on the other X chromosome usually prevents the condition from developing. These females are called "carriers" because they can pass the recessive allele to their children without showing symptoms themselves.
Let's use genetic diagrams to understand X-linked inheritance better. We'll use the example of red-green colour blindness, which is caused by a recessive allele (c) on the X chromosome. The normal allele is represented by C.
In this example, the mother is a carrier (XCXc) and the father has normal vision (XCY).
Gametes:
Mother: XC or Xc
Father: XC or Y
Offspring:
XCXC: Normal female (25%)
XCXc: Carrier female (25%)
XCY: Normal male (25%)
XcY: Colour-blind male (25%)
In this example, the father is colour-blind (XcY) and the mother has normal vision (XCXC).
Gametes:
Father: Xc or Y
Mother: XC only
Offspring:
XCXc: Carrier females (50%)
XCY: Normal males (50%)
Notice that no children will be colour-blind in this cross!
Red-green colour blindness affects about 8% of males but less than 0.5% of females. People with this condition have difficulty distinguishing between red and green colours due to missing or abnormal photoreceptors in the retina.
Haemophilia is a bleeding disorder where blood doesn't clot properly. People with haemophilia can bleed for longer after an injury and may have internal bleeding. It affects approximately 1 in 5,000 male births.
This condition causes progressive muscle weakness. It typically affects boys in early childhood and gets worse over time, affecting mobility and eventually heart and respiratory function.
Geneticists use family trees called pedigrees to track the inheritance of genetic traits through generations. X-linked recessive conditions have distinctive patterns in pedigrees:
One of the most famous examples of X-linked inheritance is haemophilia in European royal families. Queen Victoria of England was a carrier of haemophilia, though neither of her parents had the condition (suggesting a new mutation). She passed the allele to several of her children, who then married into royal families across Europe.
The most notable case was Alexei Nikolaevich, son of Tsar Nicholas II and Alexandra of Russia (Victoria's granddaughter). His haemophilia was a factor that led the imperial family to rely on the mystic Rasputin, who claimed to be able to treat the boy's condition. This relationship with Rasputin contributed to the unpopularity of the imperial family before the Russian Revolution.
Y-linked inheritance is much rarer than X-linked inheritance because the Y chromosome contains far fewer genes. Traits determined by genes on the Y chromosome:
Examples include some forms of hearing loss and certain fertility factors. However, these are not commonly tested in IGCSE exams.
Understanding sex-linked inheritance is important for:
Families with a history of sex-linked disorders can receive counselling to understand the risks of having affected children. For example, if a woman knows she's a carrier of haemophilia, she can be informed that each of her sons has a 50% chance of having the condition.
Recognising the pattern of inheritance can help doctors diagnose conditions. If a condition affects mostly males and follows the characteristic inheritance pattern, doctors might suspect an X-linked disorder.
In exams, you might be asked to:
Always use proper notation, with superscripts for alleles on sex chromosomes (e.g., XC and Xc).
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