Lecture 5- Research Involving Twins: Insights into what it is to be human Flashcards
What are the two types of twins?
• There are two major types of twin pairs – Monozygous (MZ) arising from one egg
– Dizygous (DZ) arising from two eggs
- Monozygous pairs have the same genetic makeup, and are of the same sex
- Dizygous pairs share, on average, half their genes, and can be of the same or different sex
- Twins are of the same age … +/‐ a few minutes
What are some basic facts about twins?
-About 1 in 80 births are twin pairs
~ 2.5% of the population are twins
-Twins tend to be, on average, of lower birth weight and more likely to be premature, than singletons
-On average, twins are delayed in developing language, reading and writing skills cf. singletons
-Some monochorionic MZ twins differ substantially at birth in size and health due to “twin‐to‐twin transfusion syndrome”
‐ sharing a placenta, blood supplies can become connected and shared disproportionately
-are twins different to other humans (to the ones born alone)
-delay in some developmental issues but they do catch up
-the twins compete for the same blood and supply and sometimes one twin gets much more
Why is it useful to conduct research on twins?
- There are many ways studies involving twins can provide insights into what it is to be human, as well as what it is to be a twin
- Twin studies present an “experiment of opportunity”
- Twins are special and can contribute to research in many ways
What are the similarities and differences in twins?
Why are twins similar to one another?
• shared genes or shared “environment”?
• are genetically identical (MZ) pairs more similar than fraternal (DZ) pairs?
Why do some twins differ from one another? • one affected and other isn’t
• one “exposed” and the other isn’t
-how similar and how differnt the twins
-why are twins similar=
-two pairs, one exposed= to envrionmental risk factor, the other is not
What is the Australian Twin Registry?
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Why are twins valuable in medical, health and scientific research?
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What are the twin study designs?
1.
PIC1What was the breast cancer study?
- if the white is there= diffiucult to see tumours
- mammograohy densiry
- amount of white stuff on breats= predicts the likelyhood of cancer
What does mammographic density predict?
-predicts breast cancer risk
• For unaffected women of the same age and BMI, those with greater dense (white) areas on their mammograms are at greater risk of developing breast cancer
• Virtually independent of other known breast cancer risk factors
• Risk gradient is:
– weaker if adjusted for parity and family history – stronger if adjusted for age at menarche
What was the first twin study about breast cancer?
setup:
-Female twin pairs aged 40 to 70 years (without history of breast cancer)
353 MZ, 246 DZ pairs from NSW, Vic or WA recruited through Australian Twin Registry
218 MZ, 134 DZ pairs from North America recruited through media, Ontario Breast Screening Program, Rhode Island Twins Foundation and Twinsburg Festival
-Data collection: Australia
Interviewer‐administered (telephone) risk factor and family history questionnaire
Attendance at BreastScreen for mammogram Original digitised using laser film scanner Sent to Toronto for measurement by Norman Boyd
-Why do women differ so much in mammographic density?
After adjusting for age and BMI, measured lifestyle factors (e.g. number of child births, menopausal status, age at puberty, HRT use) explain only a few percent of variation
If genes explain variation,
then MZ pairs should be highly correlated.
What is the classical twin study?
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Do findings for relatives accord with predictions of classic twin model?
• Studied 800 twin pairs and 600 of their sisters
• Correlation for sister pairs was the same as for DZ pairs (0.28 v 0.29)
• These sister and DZ pair correlations were a little less than one‐half the correlation for MZ pairs (0.59)
• Therefore good consistency with predictions from the additive genetic model
• Since found > 4 genes for mammographic density that also are associated with breast cancer risk
-height correlates highly in MZ twins, DZ and siblings similar but not half of MZ, about 0.6
-the reason why the DZ and siblings have over half correlation as the parents are of similar height, tall people marry tall people etc.
-trait that is highly genetically determined but highly susceptible to environment (due to better nutrition) so just because sth is highly heritable doesn’t mean the environment cannot have an effect
What is the study type when there are difference within twin pairs?
What is the study design: twins discordant for exposure?
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What is the twin study: DZ pairs discordant for genotype?
-DZ twin pairs can differ in genotype
