Transcript Genetics - Gordon State College

Chapter 2:
Genetics
Genetic Foundations
Heredity & Environment
Genetic Foundations
 DNA (deoxyribonucleic acid)
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a complex molecule containing the genetic
information that makes up the chromosomes
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has two strands-forming a “double helix”- held
together by bonds between pairs of nucleotides
(spiral staircase)
 Chromosomes
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threadlike structures made of DNA molecules
that contain the genes
Genetic Foundations
 Chromosomes
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Human beings have 46 chromosomes (23 pairs)
Entire DNA code/set of chromosomes is repeated in
each of the cells
Define the limits of species variation; separate
species
Genetic Foundations
 Chromosomes: limits
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“Each of us carries a “genetic code” that we inherited
from our parents. Because a fertilized egg carries
this human code, a fertilized human egg cannot grow
into an egret, eagle, or elephant.”
Genes: Our Biological Blueprint
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Genes
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the biochemical units of heredity that make up the
chromosomes
a segment of DNA capable of synthesizing a *protein
Genome
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the complete instructions for making an organism
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*Proteins are the cell building blocks and bodily
process regulators.
Genetics and Behavior
Nucleus
Cell
Chromosome
Gene
DNA
Genes: Our Biological Blueprint
 Human Genome Project
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Completed about the year 2000
Humans have 20,000 – 25, 000 genes (21,667)
There are far more proteins than genes – 10-20
million
Genes (DNA) are dependent- collaborate with other
sources of information
Gene expression/activity is affected by context or
environment
Context is affected by hormones, light, nutrition, etc.
The Sex Cells
 Sex cells are formed by meiosis rather than
mitosis.
 Gametes (sperm and ova) have only 23
chromosomes total.
 At conception, these two unite resulting in a full
complement of 46 chromosomes (23 pairs).
 A fertilized egg is called a zygote.
Sources of Genetic Variation
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Alleles are normal variations of a gene,
found at the same location.
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A child who inherits the same allele (type of
gene) from both parents is homozygous for
that trait.
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A child who inherits different alleles from
each parent is heterozygous for that trait.
Sources of Variation
Genetic Expression
Influenced by the environment
hormones
light
nutrition
behavior
stress (cortisol may cause a fivefold
increase in DNA damage)
Genetic Foundations
 Genotype – genetic composition
 Phenotype – observable characteristics
 In the eye-color example, genotypes
 BB, Bb, and bB would all have the same
brown-eyed phenotype
Sources of Variation
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Patterns of Genetic Inheritance
Dominant-recessive: the dominant
gene (allele) will determine the
characteristic
The other allele is recessive or
recedes into the background with its
effects not being shown.
Patterns of Genetic Inheritance
Dominant-recessive inheritance
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Examples of dominant genes
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Dark hair, curly hair, dimples, types
A & B blood (vs. type O), traits for
normality in vision, hearing,
pigmentation, etc.
Huntington’s Disease
Patterns of Genetic Inheritance
Dominant-recessive inheritance
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Examples of recessive genes:
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Cystic fibrosis, PKU, Tay-sachs
disease. Sickle-cell anemia
Patterns of Genetic Inheritance
Co-dominance and Additive
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Co-dominance: both alleles contribute
to the phenotype, although not to the
same degree.
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Additive: They contribute about
equally (50%-50%).
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Example of Co-dominance;
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Sickle-cell anemia
X-linked or Sex-linked Inheritance
 Humans have 23 pairs (46 total) of
chromosomes:
 Pairs 1 – 22 are autosomes; they have exactly
the same number of locations for genes.
 The 23rd pair does not. The X and Y
chromosomes do not have equal numbers of
gene locations. Females are XX on this pair,
and males are XY.
X-linked (sex-linked) inheritance
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Female children receive an X chromosome from the
father which matches locations on the mother’s X.
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Male children receive a Y from the father, which does
not have all the gene locations of an X.
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The defective gene on the mother’s X is offset by the
gene on the normal X in females, but not in males.
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So, males will show evidence of the defective gene (e.g.,
hemophilia, RG colorblindness).
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Females will be normal, but carriers of the defective
gene.
Polygenic Inheritance
 Many genes interact to influence the
characteristic
 Most psychological characteristics are
polygenic
 (Where environmental factors are included,
traits are said to be multi-factorial.)
Chromosomal Abnormalities
Usually happen during meiosis
Involve breakage and failure to separate
Usually result in miscarriage
Those most commonly survived are:
Down syndrome (trisomy 21)
Sex-linked abnormalities
Chromosomal Abnormalities:
Down’s Syndrome
 Trisomy 21: extra copy of a chromosome on the
21st pair
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Round face, flattened skull, protruding tongue, extra fold of skin
on eyelids, short limbs
Mental and motor retardation
 1 in 500 live births Most common overall
cause of mental retardation.
 More common to older mothers, rare among
African Americans
Chromosomal Abnormalities
Sex-linked - Fragile X
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Gene damaged on X chromosome
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Most common inherited cause of mild
to moderate mental retardation
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Linked to autism
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More common in males
Sex Chromosome
Abnormalities
XXY (Klinefelter) may have verbal difficulties.
Tall, underdeveloped testes, possible breasts.
1/400 live male births.
XO (Turner) have trouble with math and spatial
skills. Short and have webbed neck; may be
infertile. 1/2500 live female births
XYY (Are they more aggressive, antisocial?)
Gene-linked Abnormalities
 Over 7000 known (most rare), including:
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Cystic fibrosis
Diabetes
Hemophilia
Huntington
PKU (phenylketonuria)
Sickle-cell anemia
Spina bifida
Tay-sachs disease
Twins
Identical
twins
Fraternal
twins
 Identical Twins
 develop from a single fertilized
egg that splits in two, creating
two genetically identical
organisms
 Fraternal Twins
 develop from separate eggs
 genetically no closer than
brothers and sisters, but they
share a fetal environment
Same
sex only
Same or
opposite sex
Multiple Births – fraternal twins
 Dizygotic (two zygotes)
 Share approximately 50% of their genetic
heritage like any two siblings.
 Major causes are maternal age and fertility
drugs.
 Twinning dramatically on the increase since
the 1970s.
Multiple Births – identical
twins
 Monozygotic – one zygote (same fertilized egg)
 Share 100% of genetic heritage
 Occurs about 3 per 1000 live births worldwide
 Factors may include temperature and oxygen
levels and late fertilization
Genetics Research
Behavior Genetics
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 study
of the power and limits of genetic and
environmental influences on behavior
Molecular Genetics
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subfield of biology that studies the molecular
structure and function of genes
Nature-nurture Research
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Molecular genetics (laboratory
biology)
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Human Genome Project
Behavioral genetics (psychology)
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Twin studies
 Equal environment assumptions
Adoption studies
 Concordance rates
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