Inheritance

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Inheritance

• Sex Linkage

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What is Sex Linkage?

• In addition to their role in determining sex, the
  sex chromosomes have genes for many characters.
• Genes located on a sex chromosome are called sex
  linked genes.
• In humans the term usually refers to X-linked
  characters genes located only on X chromosomes.
• Fathers can pass X-linked alleles to their
  daughters, but not sons.
• Mothers can pass sex-linked alleles to both sons
  and daughters.

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Recessive alleles

• If a sex linked trait is due to a recessive
  allele
• A female will express the phenotype only if she
  is homozygous recessive.
• If a male receives the recessive allele from his
  mother he will express the phenotype.
• Far more males have disorders that are inherited
  as sex linked recessives than females.
• Examples Colour blindness
• Haemophilia

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Red-green colour blindness

• X chromosome has a locus for colour vision with
  two alleles
• XN Normal colour vision
• Xn Red-green colour blindness
• Y chromosome does not have a colour vision locus.
  
• If a male receives the Xn allele he will have
  impaired colour vision, whereas a female with
  XNXn will not.

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Red-green colour blindness

• Parental
• Phenotypes Carrier Female x Normal Male
• Genotypes XNXn
  XNY
• Gametes
• Offspring 1
• Genotypes

XN
Xn
XN
Y
Female Gametes Female Gametes
XN Xn
Male Gametes XN XN XN XN Xn
Male Gametes Y XN Y Xn Y
Phenotypes Normal Female Carrier Female
Normal Male Colour blind Male 1
1 1
1
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Showing genetic pedigree

• Unaffected female

• Affected female

• Unaffected male

• Affected male

I
1
2
II
2
4
1
3
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III
1
2
3
4
5
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What now?

• Complete WS5 Sex-Linked Genes
• Complete WS6 Extension questions

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