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Sex-Linked Inheritance. Recall: Homologous pairs. Humans have 23 pairs of chromosomes Each homologous pair contains the same genes, but possibly different alleles 22 pairs of autosomes 1 pair of sex chromosomes In males X and Y form a homologous pair but they do not contain the same g enes.
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Recall: Homologous pairs • Humans have 23 pairs of chromosomes • Each homologous pair contains the same genes, but possibly different alleles • 22 pairs of autosomes • 1 pair of sex chromosomes • In males X and Y form a homologous pair but they do not contain the same genes
Types of Inheritance • Autosomal Inheritance • Occurs when an allele is located on an autosome • Males and females will be affected equally • Examples: Sickle-cell anemia, type I diabetes, PKU • Sex-Linked Inheritance • Occurs when allele is located on a sex chromosome (X-linked or Y-linked) • One sex will be affected more • Examples: hemophilia, colourblindness, pattern baldness
Y-linked Genes • Very few conditions are Y-linked because Y chromosomes is small and contains few genes • Can only be passed from father to son (females never affected) • Examples: spermatogenic failure
X-Linked Genes • Women can be homozygous or heterozygous because they have 2 X chromosomes • Women can be carriers for X-linked recessive conditions • 2 copies of the allele are required for a woman to display trait • Men are ‘hemizygous’ because they only have one X chromosome • Men either have the trait or they don’t, no carriers • X-linked conditions are much more common in men • Would a woman with Turner syndrome be more likely or less likely to show an X-linked condition?
Passing X-linked conditions • Men always pass their X chromosome to their daughters and never to their sons • Women randomly pass one X chromosome to every child
X Inactivation • Everywhere but reproductive organs, only one X chromosome is required • In all female cells, one X chromosome is randomly inactivated • This means female carriers for X-linked conditions can show affected ‘patches’ • Women are “mosaics” because they have cells that rely on different sets of genetic information
Coat Colour in Cats • Black and orange alleles for coat colour are carried on the X chromosome in cats • Male cats can be black (XBY) or orange (XOY)
Female Calico Cats • Female cats can be black if the are homozygous for the black allele XB XB • Female cats can be orange if they are homozygous for the orange allele XO XO • If a female cat is heterozygous (XB XO), They will show orange where XB is inactivated and black where XO is inactivated. • Because of this 99.9% of calico cats are female.
Red-greenColourblindness Example: A normal male and a female carrier of red-green colourblindness are planning a family. What are the expected genotypes and phenotypes of their offspring?
Haemophilia • About 1 in 5 000 males suffer from hemophilia • 2 types of hemophilia: • Haemophilia A: Cannot produce clotting factor VIII • Haemophilia B: Cannot produce clotting factor IX also called “Christmas Disease” • The genes for both forms of haemophilia areX linked • Haemophiliacs do clot their blood, but slowly because there is an alternative pathway via thromboplastin • Treatment involves injections of missing clotting factor concentrates
