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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Sex Determination
Have you ever wondered why some people are born male and others female? It's not just random chance! Sex determination is the biological process that establishes whether an organism develops male or female characteristics. In humans and many other animals, this is controlled by special chromosomes called sex chromosomes.
Key Definitions:
Humans have 46 chromosomes in total, arranged in 23 pairs. Of these, 22 pairs are autosomes, which are the same in both males and females. The 23rd pair consists of the sex chromosomes, which determine whether you're male or female.
Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome is much smaller than the X chromosome but contains a crucial gene called SRY (Sex-determining Region Y) that triggers male development.
Sex determination happens at the moment of fertilisation when a sperm cell carrying either an X or Y chromosome fuses with an egg cell, which always carries an X chromosome.
We can use genetic diagrams to show how sex is inherited. Let's look at a simple Punnett square:
Female (mother): XX
Male (father): XY
The mother can only contribute an X chromosome to her children.
The father can contribute either an X or a Y chromosome.
This gives us a 50% chance of a female child (XX) and a 50% chance of a male child (XY).
| X | X | |
| X | XX (Female) |
XX (Female) |
| Y | XY (Male) |
XY (Male) |
The X chromosome carries many more genes than the Y chromosome. Some of these genes have nothing to do with determining sex but affect other traits. When a gene is located on a sex chromosome, we call it "sex-linked".
Males have only one X chromosome, so if it carries a faulty gene, there's no second X chromosome to provide a working copy. Females have two X chromosomes, so if one carries a faulty gene, the other often provides a working copy. This is why sex-linked disorders affect males more frequently than females.
A female with one faulty gene on one X chromosome and a normal gene on the other X chromosome is called a "carrier". She usually doesn't show symptoms of the disorder but can pass the faulty gene to her children. If a carrier female has a son, he has a 50% chance of inheriting the disorder.
A disorder where blood doesn't clot properly. It's caused by a recessive allele on the X chromosome. Queen Victoria was a famous carrier who passed it to royal families across Europe.
The inability to distinguish certain colours, usually red and green. It affects about 8% of males but less than 1% of females due to being caused by a recessive allele on the X chromosome.
A condition causing progressive muscle weakness. It's caused by a recessive allele on the X chromosome and primarily affects boys, with symptoms usually appearing between ages 3 and 5.
Let's look at how colour blindness is inherited. We'll use XC to represent the normal allele and Xc to represent the colour-blind allele.
Mother: XCXc (carrier)
Father: XCY (normal vision)
Possible offspring:
This shows why sex-linked disorders are more common in males - in this example, none of the daughters have colour blindness, but 50% of the sons do!
Not all organisms determine sex the same way humans do. Here are some fascinating alternatives:
In birds, butterflies and some reptiles, females have different sex chromosomes (ZW) while males have identical ones (ZZ). This is the opposite of our XY system!
In many reptiles like crocodiles and some turtles, the temperature of the eggs during a critical period of development determines whether the offspring will be male or female. Warmer temperatures often produce females, while cooler temperatures produce males.
Queen Victoria of England was a carrier for haemophilia, a sex-linked blood clotting disorder. She passed this gene to several of her children, who then married into royal families across Europe. This led to haemophilia appearing in the royal families of Spain, Germany and Russia.
The most famous case was Alexei Nikolaevich, son of Tsar Nicholas II of Russia. His haemophilia was so severe that it influenced Russian politics, as his mother became increasingly dependent on the mysterious healer Rasputin, who claimed to be able to stop Alexei's bleeding episodes.
This historical example perfectly illustrates how sex-linked inheritance works - Queen Victoria's sons who inherited the gene developed haemophilia, while her carrier daughters showed no symptoms but could pass it to their sons.
Sex determination in humans is controlled by the X and Y chromosomes. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). The Y chromosome contains the SRY gene, which triggers male development.
Sex-linked inheritance occurs when genes located on the sex chromosomes affect traits unrelated to sex determination. Because males have only one X chromosome, they're more likely to express recessive sex-linked disorders like haemophilia and colour blindness.
Different organisms have evolved various methods of sex determination, including chromosome systems opposite to ours and environmental factors like temperature.
Understanding sex determination and sex-linked inheritance helps us predict the likelihood of certain genetic conditions and explains why some disorders affect males and females at different rates.
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