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I/GCSE Biology - Sex-linked inheritance

Sex-linked inheritance

· igcse biology,gcse biology,inheritance,sex-linked,genetics

Have you read the previous post regarding inheritance in I/GCSE Biology? In this post, we'll talk about sex determination and sex-linked inheritance!

Sex determination

Do you remember the difference of girl and boy in I/GCSE Biology?

  • Women have two X chromosomes and men have one X and one Y. The 23rd pair of chromosomes is called the sex chromosomes.
  • When making sperm, the X and Y chromosomes are drawn apart in the first division in meiosis. There’s a 50% chance each sperm cell gets an X-chromosome and a 50% chance it gets a Y-chromosome. All egg cells have one X chromosome.
  • Punnett squares can be used to calculate the probability of the genders of people.
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Sex-linked inheritance

After knowing the determination of opposite sex, we'll talk about the sex-linked inheritance in I/GCSE Biology!

  • Most of our chromosomes come in matching pairs. The two chromosomes in a pair have the same genes in the same places but may have different alleles.
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  • However, the X chromosome is much larger than the Y chromosome. As well as this, there are more genes on the X chromosome than on the Y chromosome. This means that males will only have one copy of most of the genes on the X chromosome.
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  • One gene found only on the X chromosome codes for a substance that clots blood. The normal allele, H, allows the blood to clot. The recessive allele, h, prevents normal clotting and causes a disease called haemophilia.
  • Haemophilia is an example of a sex-linked genetic disorder.
Look at the Punnett squares below to have a clear concept of I/GCSE Biology!
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  • The probability of the inherited alleles can be found the same way as with normal genes, except we must show which chromosomes the genes are on.
  • Each time this couple have a child, the probability that it will be a boy with haemophilia is 1 in 4. This can be written 1:3 or as 25%.
  • The gene for red-green colour blindness is also on the X chromosome. The normal allele of this gene gives normal colour vision, but there is a recessive allele that means a person cannot distinguish red from green.
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And we're done with this topic! Well Done!

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Drafted by Alyssa (Biology)

References:

  1. "New Page 1", https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQaM8aF1YVuFdb_r_74tseSElcQiynC3wXO5O-xPKYFZ9Qzlio8M4zBV7NosgHIr11qZyo&usqp=CAU
  2. "Sex-linked and Mitochondrial Inheritance", https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQh4-__pbTT7EQAdikhN_rDAMqVf0Lk6hb5gKYPk7a16HhoNrK8AMG1q4Sbm5TVDZIWXhA&usqp=CAU
  3. "Why Do Most Humans Have 23 Pairs of Chromosomes?", https://www.23andme.com/static/img/gen101/origins/parents.87606541c2ba.png
  4. "SEX Determination Maleness or femaleness is determined", https://slideplayer.com/slide/2757547/10/images/5/X+and+Y+Chromosomes+X+chromosome+-+Contains+%3E+1%2C500+genes.jpg
  5. "Sex chromosomes & X-linked inheritance", https://cdn.kastatic.org/ka-perseus-images/76600b268da90d33c757b55d51dbd788f5aabdfb.png
  6. "Variations of Inheritance Patterns", https://slideplayer.com/slide/16214378/95/images/7/Red-Green+Color+Blindness.jpg
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