AB

CD

(30)

(44)

(28)

(10)

(9)

What is a gene?

A functional group of DNA molecules (nucleotides) that is responsible for the production of a protein or enzyme

Chromosomes and therefore genes are inherited in pairs

The genome is the total collection of genes

http://www.accessexcellence.org/RC/VL/GG/index.html

Familial resemblance

50% of genes are inherited from each biological parent

Therefore, everyone shares exactly 50% of their genes with each parent

Siblings have a 50-50 chance of inheriting the same gene. But, like a coin toss, there’s variation aroung this 50% overlap. Assuming each chromosome is inherited without changes, there is a .00000000000001421 probability (.546) that two siblings will be genetically identical (about 1 in 100 trillion).

On average, people share 50% of their genes with each sibling, but this varies across sibling pairs.

Monozygotic (identical) twins came from the same fertilized egg and share 100% of their genomes; dizygotic (fraternal) twins came from two, separately fertilized eggs and have the same pattern of shared genes as any other siblings.

Genotype (gene) leads to phenotype (characteristic)

Additive combination of two genes: sickle cell anemia

No sickle cell gene, no abnormal proteins; one gene, ½ of proteins are abnormal; two genes, all of proteins are abnormal

Percentage of abnormal genes = percentage of abnormal proteins in an additive, linear way

In families, degree of genotypic relatedness will equal degree of phenotypic similarity

Genotype (gene) leads to phenotype (characteristic)

Additive combination of two genes: sickle cell anemia

Nonadditive combination of two genes: earlobe attachment

Unattached earlobe is dominant; either 1 or 2 unattached genes gives the same result

Attached earlobe is recessive; only 2 attached genes give this result

In families, degree of genotypic relatedness may not correlate perfectly with degree of phenotypic similarlity

Dominant/recessive inheritance and family resemblance

E? ee

Ee ee

Ee

Epistasis and family resemblance: Interaction of multiple genes

20% shared genotype; 0% shared phenotype

60% shared genotype; 0% shared phenotype

Models for studying familial resemblance in personality

Twin studies:

MZ twins compared with DZ twins

Twins reared apart

Family studies:

Parents and siblings

Adoption studies (adopted child compared with biological and adopted parents and/or siblings)

Models for studying familial resemblance in personality

Caveats in interpreting twin studies:

MZ twins appear more similar than most DZ twins (evocative interaction violates the “equal environments” assumption)

“Reared apart” studies may actually mean separation as late as middle childhood

Caveats in interpreting family studies:

There may be correlations between adopting and adoptive family enviroments – placements are not random

Trait Additive geneticNonadditive

geneticShared

environmentNonshared

environment

E .32 .17 .02 .51

A .24 .11 .11 .54

C .22 .16 .07 .55

N .27 .14 .07 .52

O .43 .02 .06 .49

Percent of variance from 4 different sources (Table 5.3, p. 95)

Where is the influence of the shared environment?

Not in personality, but in attitudes and interests: conservatism, religiosity, musical interest all have substantial shared-environment effects

The shared environment is overrated: Siblings may live in the same house with the same family, but they are treated differently within the family (birth order, gender, attractiveness), experience things differently (age at significant events such as divorce or relocation), and of course have different experiences

Genes affect personality through the environment: Parents with “aggressive” genes may engage in verbal or physical aggression toward each other or their children or allow more violent TV or video games, and this environmental correlation, although causal, would come up in statistical analysis as genetic

Where is the influence of the shared environment? Birth order

Firstborn strategies

Laterborn strategies

O (not needed; all niches open)C (be the good kid)E (be socially dominant)A (not needed; just dominate)N (do you need to worry about losing status?)

O (better be creative about finding your niche)C (this niche is probably already taken)E (strength in numbers)A (appeasing is likely to be a good strategy)N (do you need to worry about having low status?)

Where is the influence of the shared environment? Birth order

Makes a firstborn more like a laterborn:

Conflict with parents

Makes a laterborn more like a firstborn:

Long interval between siblings

13 18

N

A

E

C

O

firstborn

laterborn