(2) Heritability - adoption/twin/age studies

Question 1

Adoption studies

Answer 1

1. sch. s’s adopted at birth
   - adopted families had rate of sch. present in the general population
   - biological families – i.e. parents and sibs – had “unusually high” incidence of sch.
   - confirmed in follow up replication

Question 2

Twin studies

Answer 2

1. Concordance rate of sch at least 4 x’s higher in monozygotic twins than dizygotic
2. if sch. was transmitted by a single dominant gene, would expect at least 75% of children of two parents with the gene to have sch.
3. if recessive, all children of two sch. parents would have it
   - since rate is much lower than that it means either:
   a. sch. is determined by multiple genes – what most experts believe - or
   b. predisposition or susceptibility for sch. is what is inherited and the expression of it is determined by other factors – also likely
4. the fact that the concordance rate for identical twins is about 50% indicates that while schizo is strongly genetically determined, it is not entirely genetically determined
5. a and b above are supported by the findings of studies of children of sch
   - rate of sch. is identical in the children of monozygotic twins discordant for sch. – 17% for each

• in dizygotic twins discordant for sch.: the rate of sch. in the children of the affected sib also is 17%, and for the children of the unaffected sib at 2% - 2% is higher than population levels estimated at 1%
  6. Indicates that genetics convey a susceptibility

Question 3

Monochorionic vs dichorionic monozyg twins

Answer 3

• dichorionic twinning occurs before day 4 and has separate placenta
• monochorionic twinning occurs after 4 days and has same placenta
• studied twins concordant and discordant for sch.
  a. estimated sch. 60% concordant for monochorionic and 11% concordant for dichorionic
  b. provides evidence for the interaction between genetic and prenatal environmental factors in sch.

Question 4

Risk of schizo increases with age of father

Answer 4

• spermatocytes (cells that produce sperm) divide every 16 days throughout man’s life. So they have divided hundreds of times by the age 50
• oocytes (cells that produce egg) divide only 23 times before birth and once after
• chances of genetic mutation that could cause schizo much greater for father as he ages

Question 5

In contrast to specific gene mutations, epigenetic mechanisms may be involved

Answer 5

• DNA is wound around histone proteins
• When histones are exposed to methyl atoms the DNA wound around them is drawn in more tightly and cannot be translated into mRNA
• Many epigenetic mechanisms – like methylation of histones – are caused by environmental events