devt’l psych handouts, genetics & heritability
1
 46 chromosomes: 23 from each parent
 Chromosomes: long strands of DNA
 Monozygotic twins (MZ): genetically identical
 Dizygotic twins (DZ): same as other siblings
(50% similarity)
-----------------------------------------------------------------------------------------------------------3 Patterns of Genetic Expression
 single gene-pair inheritance (Mendelian)
-
allele: a single pair of genes that determines a characteristic
-
genotype: the genes we have
-
phenotype: the way genes are expressed
-
dominant allele: phenotypic expression
-
recessive allele: phenotypic expression only when both genes in the
pair are recessive
-
incomplete dominance: dominant allele fails to completely mask the
recessive one (e.g. sickle cell anemia)
 sex-linked inheritance – traits determined by genes on the sex chromosomes
(23rd pair)
-
due to recessive genes on X-chromosome
males more at risk
-
e.g. red-green colour blindness, hemophilia, muscular dystrophy
 polygenic inheritance – characteristics influenced by more than one gene pair
- e.g. height, weight, intelligence, skin colour, temperament
-----------------------------------------------------------------------------------------------------------Chromosomal and Genetic Abnormalities
-
some congenital defects are due to abnormal genes and
chromosomes
e.g. too many or too few chromosomes
1 in 200 children
devt’l psych handouts, genetics & heritability
2
 abnormalities in sex chromosomes:
-
XXY, XXXY = Klinefelter’s syndrome (1 in 500 male births)
-
XYY = Supermale syndrome (1 in 1000 male births)
-
XO = Turner’s Syndrome (1 in 3000 female births)
-
XXX, XXXX, XXXXX = Superfemale syndrome (1 in 1000 female
births)
-
Fragile-X syndrome (1 in 1000 children)
 Autosomal Abnormalities:
- affecting chromosomes other than 23rd pair
-
most common: trisomy
-
Trisomy 21 (Down Syndrome)
- less than 1 in 1000 when mother is < 29
- 1 in 220 when mother is > 35
- 1 in 25 when mother is > 45
Risks of any abnormality:
- 1 in 450 for mothers under 29
- 1 in 150 for mothers over 35
- 1 in 12 for mothers over 45
 Aging ova hypothesis
----------------------------------------------------------------------------------------------------------- Genetic Abnormalities due to Mutation
Mutation: change in chemical structure of the gene(s) leading to a
new phenotype.
-
genetic counseling
amniocentesis
chorionic villus sampling (CVS)
ultrasound
devt’l psych handouts, genetics & heritability
3
Research Methods:
 selective breeding research
- Tryon (1940): maze-bright and maze-dull rats
 family study research
- twin study: examine MZ twins raised together versus DZ twins raised
together
- adoption study: compare adoptees with adoptive family versus
biological family
-----------------------------------------------------------------------------------------------------------Estimating Genetic vs. Environmental Contributions:
 concordance rate: the percentages of pairs of people in which, when one
member shows the trait, the other does too.
 correlation coefficient: extent to which a trait in one member of a pair is
related to presence of the trait in the other member.
Heritability coefficient:
H = (r for MZ twins – r for DZ twins) * 2
Non-shared Environmental Influence:
NSE = 1 – r for MZ twins reared together
Shared Environmental Influences:
SE = 1 – (H + NSE)
---------------------------------------------------------------------------------------------------------- IQ is moderately heritable, H = .52
 temperament is heritable, H = .40
Kagan (1989; 1988): identified children as socially inhibited or uninhibited at 21
months, 4 yrs, 5 ½, 7 ½. The first rating predicted subsequent ratings.
Thomas & Chess (1977; 1970): infants can be categorized according to
temperament:
 easy temperament
 difficult temperament
 slow-to-warm-up temperament
devt’l psych handouts, genetics & heritability
4
Adult Personality and Heritability:
 introversion/extroversion heritable, H = .50
 empathy
 generally personality traits are 40% hereditary, 60% environmental
Canalization Principle (Waddington, 1966): genes can limit development to a
small number of outcomes.
 infant babbling is mostly genetic for the first 10 months or so
 sometimes environmental influences over-ride genetic endowment (e.g.
ducks preferring chicken calls)
Range-of-Reaction Principle: genes establish a range of possible responses
that a person will show to different life experiences
-----------------------------------------------------------------------------------------------------------Interaction of Environment and Heredity:
 passive genotype/environment correlations: the kind of home parents
provide for their kids is partly influenced by their genotype
 evocative genotype/environment correlations: a child’s heritable attributes
in turn affects how others treat them.
 active genotype/environment correlations: environments kids seek out are
those that are compatible with genetic endowment.