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Genetics
Heredity
DNA
Chromosomes
Genes
Genome
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Genotype
Phenotype
Traits
Allele
Dominant  Homozygous
Recessive  Heterozygous
 Sex-Linked Traits
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Genetics: Branch of biology that focuses on heredity.
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Heredity: The transmission of genetic traits from parent
to offspring.
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DNA: Deoxyribonucleic Acid, composed of nucleotides.
Molecule within the nucleus that contains genetic
information.
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Chromosome: DNA packaged into thread-like structures
tightly coiled around proteins (histones).
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Gene: Small segments of DNA that code for specific
traits.
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Genome: The complete set of genetic material within an
organism.
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Genotype: The genetic make up of an individual, or
the “types” of genes a person has. All possible
combinations of genes. Example…TT, Tt, tt
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Phenotype: The observable expression of genes,
or the physical characteristics of a person.
Example…tall or short.
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Traits: Distinctive characteristics or behavior
patterns determined by genetics.
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Allele: Different forms of the same gene
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Dominant: The stronger allele
 Dominant traits will appear as the phenotype if at least
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one dominant gene is present.
Recessive: The weaker allele
 Recessive traits show up in the phenotype only when
two recessive genes are present (no dominant allele)
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Homozygous: Two of the same alleles (TT, tt)
Heterozygous: Two different alleles (Tt)
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Sex-Linked Traits: Genes for these traits are
located on the sex chromosomes(pair #23). These
traits are usually recessive and appear more
commonly in males.
To represent alleles, we use symbols:
FF = homozygous dominant
Ff = heterozygous dominant
ff = homozygous recessive
FF or Ff = Free ear lobes
(dominant)
ff = Attached ear lobes
(recessive)
A diagram that predicts the expected
outcome of a genetic cross by considering
all combinations of gametes in the cross.
MONOHYBRID CROSS: Involves ONE PAIR of contrasting traits
ff
FF
X
Parent #1
f
Parent #2
f
F
Ff
Ff
F
Ff
Ff
How many of their offspring will have free earlobes?
4 out of 4 – 100%
(the dominant trait is stronger than the recessive trait)
How many of their offspring have attached earlobes?
0
Are the offspring homozygous or heterozygous?
heterozygous
Ff
Ff
X
Parent #1
F
Parent #2
f
F
FF
Ff
f
Ff
ff
What percent of the offspring are homozygous
dominant?
1 out of 4 – 25%
 What percent of the offspring are heterozygous?
2 out of 4 – 50%
 What percentage of the offspring are
homozygous recessive?
1 out of 4 – 25%
 How many of offspring will have Free Earlobes?
75%
 How many of the offspring will have Attached?
25%
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The concept of heritability plays a central role in the
psychology of individual differences.
Heritability has two definitions.
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Statistical definition: The degree of variance among
individuals that can be attributed to genetic variations.
2.
“Common Sense” definition: The extent to which
genetic individual differences contribute to individual
differences in observed behavior (phenotypic individual
differences).
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The subfield of psychology that studies the
molecular structure and function of genes.
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Tay-Sachs Syndrome: Recessive condition
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Albinism: Recessive condition
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Infant lacks an enzyme to process the amino acid phenylalanine which can build up and poison the
nervous system. Severe, irreversible brain damage unless baby is fed a special diet low in phenylalanine
the first month. (Nature & Nurture)
Huntington’s Disease: Caused by a Dominant gene
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Failure to synthesize or store pigment and involves abnormal nerve pathways to brain. Quivering eyes
and inability to perceive depth or 3D with both eyes.
Phenylketonuria (PKU): Recessive condition
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Produces progressive loss of nervous function and death in a baby
Invovles degeneration of the nervous system. Forgetfulness, tremors, jerky motions, loss of ability to talk,
personality changes, temper tantrums, blindness and death.
Color Blindness: Recessive sex-linked trait located on the X chromosome.
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Found more frequently in males because males only have one X chromosome (XY).
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Down’s Syndrome: Caused by a break in the 21st chromosomal pair,
appears as 3 chromosome 21.
 Mentally retarded, round head, flat nasal bridge, protruding tounge, small
round ears, poor muscle tone and coordination.
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Turner Syndrome: Females have only one X chromosome (X_ or XO)
 Short with webbed neck, lack ovaries, and do not develop secondary sex
characteristics). Normal intelligence with some specific cognitive deficits.
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Kleinfelter’s Syndrome: Males that have XXY sex chromosomes.
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Evident at puberty when secondary sex characteristic do not develop, but breast tissue
does. Typically, very passive males.
Huntington’s Chorea: Caused by degeneration of the basal ganglia.
 Usually does not occur until after age 40, so frequently passed on to next
generation before symptoms appear. Results in muscle impairment
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It is a proportion with a numerical value
Ranges from 0.0 to 1.0
 Genes do not contribute at - genes are the only reason for
differences
 For human behavior, almost all estimates of heritability are
in the moderate range of .30 to .60.
The quantity gives the environmentability of the trait.
 Environmentability has an analogous interpretation to heritability.
 It is the proportion of phenotypic variance attributable to environmental
variance or the extent to which individual differences in the environment
contribute to individual differences in behavior.
 If the heritability of most human behaviors is in the range of .30 -.60,
then the environmentability of most human behaviors will be in the
range of .40 - .70.
There are five important attributes about estimates of heritability.
1. Heritability and environmentability are abstract concepts.
2. Heritability and environmentability are population concepts. They tell us
nothing about an individual.
3. Heritability depends on the range of typical environments in the population
that is studied. If the environment of the population is fairly uniform, then
heritability may be high, but if the range of environmental differences is very
large, then heritability may be low.
4. Environmentability depends on the range of genotypes in the population
studied. This is the opposite of the point made above
5. Heritability is no cause for therapeutic nihilism. Because heritability depends
on the range of typical environments in the population studied, it tells us little
about the extreme environmental interventions utilized in some therapies.
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Therapeutic nihilism is a belief that curing people, or societies, of their ills by treatment is
impossible.