Exam II Outline
•The idea that our genes solely determine who we are.
*Two factors responsible for variation in organisms – genes and environment*
•Mendelian traits - trait due to single gene
•Multifactorial traits - traits influenced by environment (environment and genes both
affect each other
•Polygenic traits - due to activity of >1 gene-- more than 2---- Multifactorial goes hand in hand
•Mendelian traits: "either/or" pattern of inheritance- only 2 results
•Ex: Pea plants: purple or white.
People: Huntington’s disease or not.
*Bell Curve distribution of phenotypes
About 180 genes determine height
•Quantitative trait loci (QTLs): the bell shape distribution traits
cardiovascular disease, height, skin color
•It is difficult to predict recurrence for polygenic traits.
Empiric Risk: risk calculations -- estaminets based on risk
•predictions of recurrence based on trait's incidence in specific population
•Incidence – rate at which event occurs
•Prevalence – number of individuals who have a disorder at a specific time
•based on observations/epidemiological studies
Empiric Risk for Cleft Lip
Relationship Recurrence Risk
General population 0.1%
First cousin 0.3%
Niece or nephew 0.8%
Identical twin 40.0%
•% of phenotypic variation due to genes --- H
•H=1 genes only
•H=0 environment only
•Correlation coefficient – proportion of genes relatives share Heritability
•Proportion of people sharing a trait compared to proportion predicted to share trait.
Relationship Degree Percent shared genes
Siblings 1* 50%
Parent and child 1* 50%
Uncle/aunt and niece/nephew2* 25%
Grandparent and grandchild 2* 25%
First cousins 3* 12.5%
^ correlation coefficient
•Example: 100 pairs of siblings.
–Mendelian trait: expect it in 50 sibling pairs.
–If trait is only observed in 37 pairs, the heritability = .37/.50 = .74 or 74%
•Heritability is component of phenotype attributed to genes.
•In lab animals and plants – heritability is easy to determine.
•Humans are not so easy.
•Adopted people share environment but not genes with adoptive family
•Share genes but not environment with biological parents
•Better information than adoption studies
•Concordance - % of pairs in which both twins express the trait
SNP (single nucleotide polymorphism) ---- looking at parts of DNA that are different, 99.9% of
DNAis the same, .1% are the differences
Nucleotide site with more than one allele is a polymorphism.
Site is polymorphic if allele is present in >1% of the population.
•On average between two random individuals, there is one SNP every 1000 bases
=> 3 million differences!
•Studies which compare group of interest (cases) to control group for presence of SNP.
•Controls are matched: age, ethnicity, gender, environment.
•If SNP is present more often in cases than controls, it is associated with trait and implies SNP
may be near gene impacting trait.
Examples of Multifactorial Traits
•Heart disease •Height
•Skin color •Etc .
Example: Body Weight
•Leptin, ghrelin, and other proteins: hormones that control appetite
•Genome-wide study results: weight is multifactorial
•Environment: Example: Pima Indians ofArizona and Mexico
•Behaviors transmitted from parent to child.
–What proportion of behavior is due to genes?
–What proportion of behavior is learned?
•Behavioral genetics: study of genes involved in development and regulation of nervous system.
Brain is composed of neurons
- Sensory neurons bring information to brain
- Motor neurons send information outward to muscles.
Transmitting neuron (presynaptic neuron)
Receiving neuron (postsynaptic neuron)
Genes that affect neurotransmission and signal transduction are good candidate genes for
•understand both genetic and environmental contributions to variations in human behavior
–measurement of behavior
–heritability measures apply only to population studied in particular environment
Evidence Behavior has genetic component
•Species specific behaviors
•Behaviors breed true
•some behaviors run in families
Genotype =/= Phenotype
Cloned Mammals --8---- clone does not mean exact replica
•Dolly the Sheep – first cloned mammal
•Ian Wilmut – 4 cloned sheep “are genetically identical to each other and yet are very different in
size and temperament, showing emphatically that an animal’s genes do not ‘determine’ every
detail of its physique and personality”
Most behavioral traits are multifactorial
•Attention deficit/Hyperactivity disorder (ADHD)
•Siblings of affected child: 3-5x risk than those without affected sibling
•Twins studies: ~ 80% heritability
•Dopamine pathway may be involved Eating disorders are behavioral trait
–psychological perception of obesity and intentional starvation
–psychological perception of obesity and intentional vomiting
–psychological perception of small muscle mass
•Women in U.S. have .5 - 1% lifetime risk
•Risk of mortality: 15-21%
•2.5% risk of second eating disorder.
•10% of cases are males
•Heritability of 0.5 - 0.8
–e.g. 9/16 MZ twins concordant versus 1/14 DZ twins
–gene for leptin – regulates appetite
–gene for serotonin – regulates mood, appetite
–gene for dopamine – emotional response, pain pleasure
•Alfred Binet developed IQ test to predict academic success of developmentally handicapped
•Stanford University modified for white, middle-class Americans
•IQ is normally distributed around a mean of 100.
below 50 severe mental retardation
50-70 mild mental retardation
above 115 above average
•defect on X chromosome, in gene for production of monoamine oxidase A (MAOA).
•MAOAhelps metabolize serotonin, dopamine and noradrenaline
•Those with the defective gene show a decrease in the enzyme's (MAOA) activity.
•Impulsive killers and violent offenders have been shown by Dr. Markku Linnoila, scientific
director of the National Institute on AlcoholAbuse andAlcoholism, to have low serotonin levels.
•Adult mice deficient in MAOAshow a pattern of enhanced aggression
•Violent behavior often runs in families.
DNAStructure and Replication
2 important properties? Discovery of a “transforming principle”
•Griffith’s experiment, 1928
•Pneumonia infects mice - mice die.
•2 types of bacteria:
–R (rough) - no pneumonia
–S (smooth)- pneumonia
What is “transforming principle”?
•Conclusion: DNAis transforming principle. (Avery, MacLeod & McCarty, 1944)
Heat-killed S bacteria “transformed” R bacteria to form that causes pneumonia
Nucleotides are composed of:
DNA: chain of nucleotides
•Base - adenine (A), thymine (T), guanine (G), cytosine (C)
•Sugar - deoxyribose