Genes and Inheritance III

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Genes and Inheritance III
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Genes and sex

• The examples we have seen so far were genes on
  autosomes, so it didnt matter which parent was
  mother or father
• Many organisms are monoecious - an individual
  can produce both male and female gametes
• Others (including humans, birds, fruit-flies) are
  dioecious so individuals are either male or
  female
• But the way in which chromosomes determine sex is
  different in different species

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Sex determination

• Bees - unfertilised eggs (haploid) produce males
  (drones) and fertilised eggs (diploid) produce
  females (workers, queen) same in ants and many
  other invertebrates
• Grasshoppers - all diploid, males have only one X
  chromosome, females have 2 Xs
• Humans, fruit flies - sperms (haploid) contain an
  X or a Y chromosome, females and males are XX and
  XY
• Birds, butterflies - males are ZZ, and females
  are WZ

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The sex life of bees
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The sex life of bees

• Here is the story
• To be female, bee has to be heterozygous for CSD
  gene
• CSD has 19 different alleles so diploid bee is
  likely to be heterozygous
• Rare homozygous diploid bees develop as sterile
  males and are killed by workers
• Intensive selection by bee-keepers can reduce
  genetic diversity and increase chances of CSD
  homozygosity (which is bad for the bees)

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Functions of sex chromosomes

• In humans (and other mammals) there is a gene SRY
  on the Y, and DAX1 on the X
• Dax1 protein prevents the development of testis,
  allows development of ovary, whereas Sry protein
  overcomes Dax1 and allows testis to develop -
  primary sexual characteristics
• XO individuals are female, XXY male
• Secondary sexual characteristics (body type,
  voice, body hair) are controlled by hormones, not
  by X and Y directly

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Functions of sex chromosomes

• In fruit flies its a bit different
• XO is a sterile male, XXY a fertile female
• It is the ratio of X to Y that determines sex
• There is no gene on the Y corresponding to the
  mammals SRY

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Sex-linked inheritance

• There are very few genes on the Y, but lots on
  the X, and their inheritance shows sex-linkage in
  pedigrees
• White eye colour in fruit flies is sex linked (Dr
  Thomas Morgan again)
• The outcome of a cross depends on which parental
  genotype is from the father and which from the
  mother
• Figure 10.23 in textbook

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Sex-linkage in humans

• There are thousands of genes on the human X, some
  cause diseases like muscular dystrophy and
  haemophilia that mostly affect males
• This can be seen from inheritance in pedigrees -
  for example, colour blindness
• Figure 10.24

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Haemophilia in the Royal Family

• Haemophilia is a hereditary disorder caused by
  defective gene on X-chromosome for a blood
  clotting factor
• Queen Victoria was a carrier her family was
  linked to other European royals
• It did not get passed on to the present Royals.

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Non-nuclear inheritance

• Everything we have seen so far, concerns genes on
  chromosomes in the nucleus
• Mitochondria and plastids are organelles that
  have their own little chromosome
• They may have evolved from bacteria that once
  colonised a primitive eukaryotic cell
• They are found in eggs (but not sperm) so they
  show maternal inheritance - passed from mothers
  to all children

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Mitochondrial (maternal) inheritance