Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Pleiotropy, Polygeny & Environment
Many human genes affect multiple traits (pleiotropy), while many human traits
are affected by several genes (polygeny). In addition to genes, environmental
factors contribute to human traits and may influence natural selection.
From Science
297 (2002): 852
Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Pea characteristics
studied by Mendel
Mendel chose for study only
characteristics that occurred as
two distinct traits, such as flower
color: purple or white, with no
intermediates
Frequency Distribution for Height of Men
Cummings M.R. (2006)
For a quantitative trait, a frequency distribution shows how
frequently each interval of the trait is observed in a population.
Many (but not all) frequency distributions are bell-shaped. Normal Distribution (Bell Curve)
A normal distribution, here obtained by a multiple coin toss, is
characterized by its mean and standard deviation. A graph of a
normal distribution looks bell-shaped.
Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Monozygotic & Dizygotic Twins
Cummings M.R. (2006)
Monozygotic (“identical”) twins develop from one fertilized
egg, or zygote, which subsequently splits into two embryos.
Dizygotic (“fraternal”) twins result from two eggs fertilized
independently during the same ovulatory cycle. Monozygotic Twins Reared Together (MZT)
Cummings M.R. (2006)
Monozygotic Twins Reared Apart (MZA)
Table 10.1: Monozygotic and Dizygotic Twins
Monozygotic Twins Dizygotic Twins
Definition
derived from one
fertilized egg, or
zygote
derived from two
independently
fertilized eggs
Synonym
identical twins
fraternal twins
Shared
Genetic
Information
100 %
~50 % on average
Boy/Girl Pairs none
50%
Frequency
2 - 50 per 1,000
live births
3.5 - 5 per 1,000
live births
Meiosis and Shared Genetic Info of Siblings
Campbell & Reece (2002)
Because meiosis separates the two homologous chromosomes of
each pair, the chance for dizygotic twins, or other siblings, to
inherit the same homolog of any given pair is 50 : 50. Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Any two individuals showing the same version of a
discontinuously variable (qualitative) trait are said to
be concordant for this trait.
The concordance rate is defined for a sample of
analyzed pairs (siblings, parent-child, etc.) in
which at least one partner shows a (relatively
rare) trait
Number of concordant pairs
----------------------------------- = Concordance rate
Number of analyzed pairs
The heritability of discontinuously variable
(qualitative) traits can be estimated by
calculating the concordance ratio from the
concordance rates for monozygotic twins (MZT)
and dizygotic twins (DZT).
Concordance rate (MZT)
-------------------- = Concordance ratio
Concordance rate (DZT)
Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Table 10.2
Heritability estimates based on
concordance ratios for various human traits
Trait
M Z Conc.
R a t e (%)
DZ Conc.
R a t e (%)
Co ncor da nce
R at io
Source
Epilepsy
72
15
4.8
Cumming s
59
16
3.6
Sutt o n
56
22
2.6
C
59
19
3.1
S
1.0
KK notes
Tuberculosis
Measles
Down synd r o m e
89
7
12.7
C
Diabetes
65
18
3.6
C
Cleft lip
42
5
8.4
C
Rheumatic fever
25
6
4.5
S
Bronchial asthma
88
31
2.83
S
Peptic ulcer
26
13
2.0
S
Eye co l o r
99
28
4.5
C
Handedness (left or right)
79
77
1.0
C
Male homosexuality
52
22
2.4
Bailey &
Female homosexuality
50
25
2.0
Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Normal Distribution (Bell Curve)
A normal distribution, here obtained by a multiple coin toss, is
characterized by its mean and standard deviation. A graph of a
normal distribution looks bell-shaped.
The variance (Vt) of a trait is defined as
Vt = s2 = Σ (yi - m)2/N-1 with yi= trait measurements of sample (i=1, …, N)
m = mean
N = sample size
s = standard deviation
The heritability (H) of a trait in a population is
defined as the genetically caused variance (Vg)
divided by the total variance (Vt)
Vg
H = ——
V t
If
Vt = Vg + Ve
Then
H = Vg / Vt = Vg / (Vg + Ve)
with:
Vt = total variance
Vg = variance caused by genetic diversity
Ve = variance caused by environmental heterogeneity
H = heritability
Standard deviation decreases if environment is
standardized.
Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) Intra-Pair Correlation Coefficient
In this diagram, the x-coordinate represents the height of one monozygotic twin and the ycoordinate represents the height of his/her co-twin reared apart. The incline of the line of
closest fit represents the intra-pair correlation coefficient in MZA twins (rMZA) for height.
rMZA
Vg + Ves
= —————
V t
with
rMZA=correlation coefficient of a measured trait
within pairs of MZA twins
Vg = genetic variance of trait in population
Ves= variance based on shared environmental
factors
Vt = total variance in population
Heritability Estimates Based on MZT & DZT
Data
Intra-pair correlations for MZT and DZT are
rMZT = (Vg+Ves ) / Vt = Vg/Vt + Ves/Vt
and
rDZT = (0.5*Vg + Ves ) / Vt = 0.5* Vg/Vt + Ves/Vt
Subtraction results in
rMZT – rDZT = 0.5*Vg / Vt
or
2 * (rMZT – rDZT) = Vg/Vt = H
Twins, Nature, and Nurture
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It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ) IQ Correlation Coefficients for Pairs of Individuals
Pair Relationship
Reared
Correlation Coeff.
Monozygotic Twins
Together 0.85
Monozygotic Twins
Apart
0.74
Dizygotic Twins
Together
0.59
Siblings
Together
0.46
Siblings
Apart
0.24
Midparent/Child
Together
0.50
Single-parent/child
Together
0.41
Single-parent/child
Apart
0.24
Adopting parent/child
Together
0.20
From Devlin et al. (1997) Nature 388: 468-471
Variance (s2) decreases, and is mostly genetic,
if environmental conditions are standardized.
Twins, Nature, and Nurture
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•
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•
•
It’s Nature and Nurture
Traits May Vary Qualitatively or Quantitatively
Twins Can Be Monozygotic or Dizygotic
Heritability Estimated by Concordance Ratio
Homosexuality Is in Part Heritable
Variance and Heritability of Quantitative Traits Intra-Pair-Correlation Coefficients
Heritability of the Intelligence Quotient (IQ)