Heredity

Dear Abbey, My girlfriend tells me that there is no one like me? Do you think that could be true. Hopeful in St. Peter.

Dear Hopeful,

I agree with your girlfriend, and it puts you in a select category with 6 billion other people.

PRINCIPLES OF HEREDITARY TRANSMISSION

The Germ Cells (produce sperm and ova) •  Production of Gametes through Meiosis

–  Duplication of 46 chromosomes –  Crossing-over: adjacent chromosomes break and exchange

segments of genes –  Pairs of duplicated chromosomes segregate into two new cells –  Cells divide, 23 single chromosomes

Figure 3.2 Diagram of the meiosis of a male germ cell.

PRINCIPLES OF HEREDITARY TRANSMISSION

The Germ Cells •  Hereditary Uniqueness

–  Independent assortment – each chromosome pair segregates independently, resulting in genetic uniqueness

Important terms and concepts

Humans have 23 pairs of chromosomes •  22 pairs of chromosomes are homologous (similar in shape and

size), called autosomes •  23rd pair is sex chromosomes, X Y

Meiosis vs. Mitosis Meiosis results in daughter cells containing 23 chromosomes (germ cells).

Mitosis results in daughter cells that are copies with a full complement of 46 chromosomes.

"Crossing Over"

Figure 3.4 These karoytypes of a male (left) and a female (right) have been arranged so that the chromosomes could be displayed in

pairs. Note that the twenty-third pair of chromosomes for the male consists of one elongated X chromosome and a Y chromosome that is noticeably smaller, whereas the twenty-third pair for the female consists of two X chromosomes.

Dominance

Recesssiveness

Homozygosity

Heterozygosity

Phenotype vs

Genotype

Gene interactions

Dominance, recessiveness –  examples of dominant genes: Brown eyes, curly hair, double joints,

limb dwarfing –  examples of recessive genes: baldness, normal fingers and toes

Co-dominance •  Blood antigens A & B

Incomplete dominance •  Sickle Cell

Pleiotropism - single gene has more than one effect •  e.g. PKU, low intelligence and blond hair. •  modifier genes

Can Mendelian genetics explain complex behavior? •  Complex behavior polygenetic, but genetic effects accumulate according

to Mendel’s “laws.” •  Genes can be turned off and on •  Some genes can direct a number or other genes in terms of their actions

Genetic Diseases

Cystic fibrosis - Recessive 1/2,000 white, 1/16,000 black •  Lungs, liver, and pancreas secrete large amounts of thick mucus,

leading to breathing and digestive difficulties.

Phenylketonuria (PKU) - Recessive 1/8,000 •  Inability to neutralize the harmful amino acid phenylalanine,

contained in many proteins, causes severe central nervous system damage in the first year of life.

Sickle cell anemia - Recessive 1/500 black •  Abnormal sickling of red blood cells causes oxygen deprivation,

pain, swelling, and tissue damage. Anemia and susceptibility to infections, especially pneumonia, occur. Example of incomplete dominance.

Genetic Diseases

Tay-Sachs Disease – Recessive 1/3,600 European Jews •  Central nervous system degeneration, with onset at about 6

months, leads to poor muscle tone, blindness, deafness, and convulsions. No effective treatment. Usually die by 4.

Huntington’s Chorea - Dominant 1/20,000 •  Central nervous system degeneration leads to muscular

coordination difficulties, mental deterioration, and personality changes. Symptoms usually do not appear until age 35 or later.

Genes that cause problems often recessive, why is this one dominant?

Figure 3.7 Sex-linked inheritance of red/green color blindness. In the example here, the mother can distinguish

reds from greens but is a carrier because one of her X chromosomes contains a color-blind allele. Notice that her sons have a 50 percent chance of inheriting the color-blind allele and being color-blind, whereas none of her daughters would display the trait. A girl can be color-blind only if her father is color blind and her mother is at least a carrier of the color-blind gene.

Sex Linked Genetic Diseases

Duchenne muscular dystrophy- Recessive 1/4,000 males •  Degenerative muscle disease. Abnormal gait, loss of ability to

walk between 7 and 13 years of age.

Hemophilia - recessive 1/5,000 males •  Blood fails to clot normally. Can lead to severe internal bleeding

and tissue damage.

Diabetes Insipidus - Recessive 1/2,500 males •  A form of diabetes present at birth caused by insufficient

production of the hormone vasopressin. Results in excessive thirst and urination. Dehydration

Sex Chromosome Abnormalities XYY - “Super Male” - 1 in 1,000 male births

•  Typical characteristics are above-average height, large teeth, and sometimes severe acne. Intelligence, development of male sexual characteristics, and fertility are normal

XXX - “Super Female” 1 in 500 to 1,250 •  Impaired verbal intelligence. Affected girls are no different in appearance

or sexual development from normal age-mates,except for a greater tendency toward tallness.

XXY - Klinefelter syndrome 1 in 500 to 1,000 •  Impaired verbal intelligence. Afflicted boys are unusually tall, have a

body fat distribution resembling females, and show incomplete development of sex characteristics at puberty. They are usually sterile.

XO - Turner syndrome 1 in 2,500 to 8,000 •  Impaired spatial intelligence. Ovaries usually do not develop prenatally.

Incomplete development of sex characteristics at puberty. Other features include short stature and webbed neck.

Autosomal abnormality

•  Trisomy 21. Down’s Syndrome. Occurs more often in older women, although 25% inherit problem from father.

•  Aging Ova, Hypothesis –  Mother under 29, <1/1000 (Down’s), 1/450 (any abnormal.) –  Mother over 45 1/25 (Down’s), 1/15 (any abnormal.)

•  Do older fathers have greater risks? –  Fathers over 40 600% increase of having a child with autism,

900% for those over 50. –  Fathers over 40 double the risk of a child with schizophrenia –  Also related to increased risks of other birth defects. –  Older mothers and fathers have increased over the last three

decades.

Amniocentesis, Performed 11-14 weeks, results back in three weeks. Chorionic villi biopsy, Performed 6-8 weeks, Results back in 24 hours.

Nature vs. Nurture

Empiricists - John Locke (1632-1704). •  Children born with a “blank slate,” experience determines

development. •  Psychologist counterpart John Watson

Nativists - Jean Jacques Rousseau (1712-1778) •  Children endowed with innate sense of right and wrong, and with

innate plan for orderly development. •  Psychologist counterpart Jean Piaget

Methods for studying the effects of genes

Inbreeding - mating of related animals Selective Breeding - selecting animals with desired

characteristics to breed

Behavioral Genetics

One of the problems in early studies was the assumption that effects of genetics and environment can be separated in an individual and studied. It cannot.

Modern Behavioral genetics uses groups that differ in genetics and environments to estimate how much each of these determine behavior. •  In the case of groups we can divide them up into the amount of

genetic overlap and experiential overlap, examine how this affects the behavioral variable in question.

Modern Kinship Studies

Twin Studies •  Identical twins – 100% genetic overlap and share environments •  Fraternal twins and siblings - share 50% of genetic material and

share environments •  Identical Twins raised in same home, Identical Twins raised apart •  Siblings raised together and appart

Adoption studies •  Adopted and Related siblings in same home have similar

environments •  Adopted and Related siblings and relation to parents and each

other •  Adopted children-share no genetic overlap with siblings •  Adopted children share genes with natural parents but share

environments with adoptive parents

How similar would IQs be in MZ and DZ Twins if IQ is due entirely to nurture?

00.10.20.30.40.50.60.70.80.9

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MZ twins DZ twins

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How similar would IQs be in MZ and DZ Twins be if IQ is due entirely to nature?

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MZ twins DZ twins

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How similar would IQs be in siblings, siblings reared apart, and unrelated siblings if it were due entirely to nurture?

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Siblings Siblings Apart Unrelated

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How similar would IQs be in siblings, siblings reared apart, and unrelated siblings if it were due entirely to nature?

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Siblings Siblings-Apart Unrelated

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r

.86

.72

.60

.47

.24

.44

.24

.22

.32

.12

Heritability Heritability estimate (0 to +1.0) - estimate of the percentage

of a trait’s variation in a population that is determined by genetics. •  This is not the percentage applied to an individual, e.g. 90% of

your height genetic, but to a population. Would the previous graph suggest to you that heritability is

very high or low? Estimates of the Heritability of IQ

•  Identicals reared Apart +.72 –  Identical twins reared apart direct measure of heritability but

few in number, almost everyone agrees unreliable measure. •  Identical twins together (.86) – Fraternal twins together (.60) times

2 = .52 •  Siblings together (.47) – siblings apart (.24) times 2 = .46 •  Siblings apart (.24) times 2 = .48

Heritability

Can Heritability estimates change? •  Height of Japanese after the war •  What would happen if we created a country in which there was

equal opportunity for everyone? Would heritability for intelligence go up or down?

•  Would heritability for IQ be higher or lower in the upper middle class or in impoverished groups?

Genes and the Environment

Do Genes limit development or provide opportunities for growth?

Range of Reaction

Comments on Range of Reaction

It may in some ways be a matter of semantics. In the case of Freddie his is limited in the ability to develop intellectually given a variety of environments, but in the case of Juan he is given the potential to develop very high intellectual ability. Would you say Juan is limited by his genes or facilitated?

Nature and Nurture

Canalization-the tendency of heredity to ensure the development of some characteristics with little influence from the environment. •  Babbling-Infants will babble regardless of the linguistic

environment around 5 months. •  Humans have a gene for using a language, apes do not. Humans

develop language even under circumstances that make it unlikely. Are humans restricted or enabled in their development of language through this gene?

Berk

“Others argue that heredity does not dictate children’s experiences or development in a rigid way. In one study, boys with a genetic tendency toward antisocial behavior based on the presence of a gene on the X chromosome known to predispose both animals and humans to aggression) were no more aggressive than boys without this gene, unless they also had a history of severe child abuse ( Caspi et al., 2002). Boys with and without the gene did not differ in their experience of abuse, indicating that the ‘aggressive genotype’ did not increase exposure to abuse.” (p. 122)

Genes and Environments are Correlated

Passive Correlation •  Parents provide children with genes and an environment which is

consonant with their genetic makeup. Hence there is a relation between child's genes and the environment experienced.

Evocative Correlation •  Children evoke responses from others because of their genetic makeup,

thus the environment experienced is affected by their genetics. Active Correlation

•  People actively select experiences in their environments based on preferences affected by their genetics, thus the environment is once again influenced by genetics. Niche picking.

Genes “create” environmental effects and determine the way we develop with age.

How would these correlations change with age?

Niche Picking?

Genetics and the Environment

Behavioral Genetics has allowed us to more carefully determine what aspects of the environment are more responsible for traits.

100 % of Human differences in behavior = Genetics + Environment If 50% of the variation in behavior is genetic then other half is due to the

environment. However, the effects of environment is split into two parts.

•  Shared environments. Between family differences. Things that make families different from one another such as SES, education, money, religion, etc.

•  Non-shared environments. Within family differences. Differences in persons within the family, based on the experiences unique to the individual.

Shared or Non-Shared

Your family has an income of $150,000 per year. You are born last in your family of 5? Your father is an exercise nut and leads the family on a five

mile run on weekends. Your math teacher takes you aside and tells you are a very

good student and should major in mathematics. Your mother divorces your father, and moves you and your

siblings to a modest home in the suburbs. In general which would have stronger effects on intelligence

and schooling? Personality?

Estimates of the Environmental Effects

Estimate for IQ •  Nonshared environment = 1.00 – r(identical twins together)

–  100 –.86 =14% •  Shared Environment = 1.00 – (Heritability + NSE)

–  1.00 – (.52 +.14) = 34%

Estimate for Personality •  Nonshared environment = 1.00 – r(identical twins together)

–  100 – .5 =50% •  Shared Environment = 1.00 – (Heritability + NSE)

–  1.00 – (.40 +.50) = 10%

Estimates of the Environmental Effects

Estimates for IQ Nonshared environment = 14% Shared Environment = 34% Estimates for Personality Nonshared environment = 50% Shared Environment = 10%

The contribution to personality traits by: •  Genetics--heritability often .40 + •  Shared family influences-experiences you share with family •  Non-shared family influences-experiences unique to you

Shared vs non-Shared Environmental Effects?

Studies of the resemblance of genetically unrelated children who are adopted together show negligible correlations for personality, psychopathy, and IQ (After childhood).

Since the effects of the shared environment small, it means

that the nonshared environment has a much larger effect. Interesting conclusion. Family members resemble each other

primarily because of heredity and that the environment (NSE) acts primarily to make children in the same family different rather than similar.