Genetics

Basics

Heritability

Environment

Modification of gene expression

Genetics I• Every cell has has a copy of our genetic blueprint

– Except for gametes (sperm and egg) and mature red blood cells

• 23 pairs of chromosomes (46 chromosomes total)• Each chromosome is made up of helical DNA

strands• There are approximately 1000 segments – called

genes - in each chromosomal DNA – Approximately 21,000 pairs of genes in humans

• Genes are composed of 4 different nucleotides– Adenine, guanine, cytosine, and thymine

Genetics II

• Genes have two essential jobs– Make copies of themselves that are transmitted over

generations– Direct the assembly of organisms

• Through the production of RNA and, more indirectly, proteins

• Particular proteins are produced when a particular gene is “switched on”; known as “gene expression”

• What is the meaning of life?– We are machines created by our genes for the purpose of

replicating our genes.

Genetics III• Each gene has a partner on the paired chromosome

– Genes can be dominant or recessive

• Genotype: the specific sequence of nucleotides that an organism inherits

• Phenotype: an organism’s observable properties– The result of what happens when genes interact with each other

and the environment

• There are 3 billion nucleotides– 99.9% of the nucleotide sequences are the same in all human

beings• That means that there are 3 million sequences of nucleotides that are

responsible for the differences amongst us

– Chimps and men share 96% of the nucleotide sequences• 120 million sequence differences

Genetics IV• Genotypic variation is created by sexual reproduction and

mutation– A unique genotype is created at the moment of conception

• Meiosis is the process whereby the gametes (sperm in male, egg in female) are formed.– During meiosis in the male, the two members of each pair of

chromosomes fuse, exchange segments, duplicate, then randomly split into 4 sperm.

• Males produce 300 million sperm per day.– The product of meiosis in females is a single egg (the largest cell

in the human body). • Women are born with all their eggs (~ 1.5 million at birth; 40,000 at

adolescence, and 350 to 450 mature during woman’s childbearing years).

• Fertilized cell, known as a zygote, contains the 23 chromosomes, one half of each pair from the father and the other half from the mother

Genetics V• Each parent

contributes a random selection of half of their genes to an offspring

• The probability of an offspring sharing any particular gene with a parent is .5

• Thus, the degree of relatedness between parent and child is .5

Relation

Degree of relatedness

Identical twin 1.0

Fraternal twin .5

Parent/child .5

Sibling .5Grandparent/

grandchild.25

Half-sibling .25

First cousin .125Nonbiological parent/adopted child

0

Heritability vs. heredity

• Heredity is the transmission of characteristics from parents to offspring by means of genes

• Heritability is the degree to which individual differences in phenotype are determined by individual differences in genotype– Heritability: a statistical estimate, for a given trait (like

intelligence), of the percentage of variation within a group that is due to genetic factors

Heritability II

• Both height and intelligence are characteristics that vary across individuals

• How heritable is height?– On the next slide, in the two left-hand columns, there are the

names of six individuals living in six different environments and their corresponding heights

– In the two right-hand columns is the outcome if you could give all six individuals the same genetics.

– Result: Despite living in separate environments, if the six individuals had the same genetics, their heights would be almost identical.

– Conclusion: Height is highly heritable. Differences in environment contribute little to differences in height.

• How heritable is intelligence? Why?

More on heritability: The case of height

Allen 6’ 6” Allen 6’ ½”

Bob 6’ 3” Bob 6’ ½”

Cameron 6’ 1” Cameron 6’

Dan 5’ 11” Dan 6’

Eddy 5’ 9” Eddy 5’ 11½”

Fred 5’ 6” Fred 5’ 11½”

Average 6’ Average 6’

Before holding genetics constant After holding genetics constant

More on heritability: The case of intelligence

Allen 130 Allen 115

Bob 110 Bob 105

Cameron 105 Cameron 100

Dan 95 Dan 100

Eddy 90 Eddy 95

Fred 70 Fred 85

Average 100 Average 100

Before holding genetics constant After holding genetics constant

The heritability of some human traits

Height 90%

Intelligence 65%

Personality 50%

Religious beliefs 30-40%

Vocational interests 30-40%

Determining heritability• Twin studies: compare monozygotic (MZ or identical) twins

raised in the same environment to dyzygotic (DZ or fraternal) same sex twins raised in the same environment.– Holding environment “constant”, but varying the genetics

– If identical twins are more alike on the trait than are fraternal twins, then genetics plays an important role; if not, then environment

• Adoption studies: compare MZ twins raised in the same environment to MZ twins raised in separate environments.– Holding genetics constant, but varying the environment

– If identical twins that are raised together are more similar than identical twins that are raised apart, then the environment plays an important role; if not, then genetics

Environmental Influences• Even among MZ (identical) twins raised together, heritability of

personality characteristics is less than 50%.– Meaning that environment must account for the rest of the variability.– But if identical twins are raised together, how is their environment different?

• Shared vs. nonshared environments– Shared: same parents, same home, same school, etc.– Nonshared: occupy different niches within the family, may have different

friends, have different experiences– Some claim that it is the nonhome, nonshared environment that is critical in

shaping the adult personality

• Also, genetics and environment interact– Reactive: Different genetically determined makeups react differently to the

same environmental stimulation• See MAO and violent behavior example on page 88

– Evocative: Different genetically determined makeups cause people to respond differently

– Proactive: When old enough, children begin to choose their own environments, which in turn will affect their development

Gene Expression Can Be Modified

• Pick your Mouse (Anxious, Hyperactive, etc.)– Knock-out mice: deleting genes to eliminate their effects– Knock-in mice: replace genes in other locations to enhance their

effects

• The Case of Transgenic Mice– Inserting strands of foreign DNA into a developing embryo

• The Promise of RNA Interference– Prevents gene expression by silencing the DNA instructions to

make proteins