Sex-linked genes

• Sex Determination


  Female: If both sex chromosomes are 'X's

  Male: If one of the sex chromosomes is an X and the other Y

  Half of male's sperm will carry X Chromosome and half will carry Y Chromosome

  Individuals have 2 sex chromosomes which determine their sex

  All eggs produced by female will have X Chromosome
• Sex-Linked Genes


  Many genes on sex chromosomes control other traits that have nothing to do with your sex

  Sex-linked gene: gene found on sex chromosome (X or Y)

  Sex-linked just refers to the location of the genes

  X chromosome longer than Y chromosome and carries more genes

  Most sex-linked genetic traits in humans are carried on X chromosome
• X and Y-Linked Genes

  1. linked genes are found on the X chromosome
  2. linked genes are found on the Y-chromosome

  To write allele for a sex-linked gene, need to include sex chromosome that gene is found on with letter for allele written as a superscript

  Females will have 2 alleles for each X-linked gene as they have 2 X-chromosomes

  Can have dominant or recessive genes

  Because males have only one X chromosome, they only have one copy of each X-linked gene (called hemizygous)
• 1. Linked Genes


  As males only have one allele for each of the sex-linked genes, even the "recessive" allele will result in the phenotype

  Males will always inherit their X-linked genes from their mother

  Females need to inherit the recessive allele from both parents to have the recessive phenotype

  Therefore, more common for males to have a recessive X-linked phenotype
• Colour Blindness


  • Gene for Colour Blindness is recessive and X-linked

  1. Not Colour-blind (Normal vision); n = Colour-blind

  X^n Y: Male – Colour-blind

  X^N X^n: Female - Not Colour-blind

  X^n X^n: Female - Colour-blind
• Carriers


  Females can be carriers who are heterozygous individuals who have one dominant and one recessive allele

  Dominant allele (e.g. Normal vision) will appear as phenotype of carrier but can pass the recessive allele (e.g. colour blind) to an offspring

  Male offspring who receives the recessive allele will show the phenotype
• Punnett Square for Sex-linked Genes


  Must include sex chromosomes in Punnett Square

  E.g. N: Not Colour-blind; n = Colour-blind

  Father: X^n Y – Colour-blind

  Mother: X^N X^n – Normal vision
• Colour Blindness – Scenario 1

  • N: Not Colour-blind, n: Colour-blind

  X^N Y, X^N X^n
• Colour Blindness – Scenario 2


  • N: Not Colour-blind, n: Colour-blind
• Colour Blindness – Scenario 3

  • N: Not Colour-blind, n: Colour-blind

  X^N Y, X^n X^n, X^N X^n
• 1. Linked Genes


  Some diseases are controlled by genes on Y chromosome

  1. linked diseases can only be inherited by sons and only from their father

  Since there are fewer genes on the Y chromosome, there are fewer Y-linked diseases