Transcript Genetics - Gordon State College

Chapter 2:
Genetics
Genetic Foundations
Heredity & Environment
Genetic Foundations
 DNA (deoxyribonucleic acid)

a complex molecule containing the genetic
information that makes up the chromosomes

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.
Genetic Foundations
 99.1% of DNA within the human race is
identical
 98-99% of human and chimpanzee DNA is
identical
Cell Division – Gamete Production
 Mitosis (normal cell division) – the nucleus of
the cell & the chromosomes duplicate and
divide into 2 cells. Each has the same 23
pairs of chromosomes.
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
•
Alleles are normal variations of a gene,
found at the same location.
•
A child who inherits the same allele (type of
gene) from both parents is homozygous for
that trait.
•
A child who inherits different alleles from
each parent is heterozygous for that trait.
Sources of Genetic Variation
 Crossing over – chromosomes pair up and
exchange segments during meiosis.
 Spontaneous mutation
 The probability of genetically identical, nontwin siblings is 1 in 700 trillion.
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
•
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
•
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
•
Co-dominance: both alleles contribute
to the phenotype, although not to the
same degree.
•
Additive: They contribute about
equally (50%-50%).
•
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
•
Female children receive an X chromosome from the
father which matches locations on the mother’s X.
•
Male children receive a Y from the father, which does
not have all the gene locations of an X.
•
The defective gene on the mother’s X is offset by the
gene on the normal X in females, but not in males.
•
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.
Sex-linked Genetic Inheritance
Genetic Imprinting
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Genes are chemically marked so that one member
of the pair is activated regardless of its makeup.
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Important whether the trait is inherited from
the mother or father.
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Examples: asthma, allergies, Huntington’s ,
diabetes
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
•
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
Genetic Counseling – for whom?
 Family history of disease, mental
retardation, physical defects
 History of miscarriages
 Mother over age 35 (rate of abnormality
begins to rise sharply)
Prenatal Diagnostic Methods
May cause miscarriage (except ultrasound,
maternal blood samples)
Is the problem correctible?
Genetic engineering is still in the future.
Often the only decision is whether or not to
abort the fetus.
Prenatal Diagnostic Methods
Chorionic villi sampling (6-8 weeks);
detects genetic defects; risk of
miscarriage (1:100 to 1:200), limb
deformity
Amniocentesis – (11 weeks, best after 15
weeks); detects genetic defects; smaller
risk of miscarriage
Prenatal Diagnostic Methods
Fetoscopy – tube with light inserted into
uterus; 15-18 weeks; limb & facial
defects; some diseases & neural defects;
some risk of miscarriage
Pre-implantation genetic diagnosis –
For a the purpose of choosing embryos to
implant - Associated with in-vitro
fertilization
Infertility
 1 in 6 couples in U.S.
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Waiting too late
Sexually transmitted diseases
 Fertility technology (IVF, donors)
 Adoption
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Babies culturally unavailable
Environmental Influence
Environmental Influence
 Rats reared in an
Impoverished environment
Enriched environment
environment enriched
with playthings show
increased
development of the
cerebral cortex
Environmental Influence
Culture
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the enduring behaviors, ideas, attitudes, and
traditions shared by a large group of people and
transmitted from one generation to the next
Norm
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an understood rule to accepted and expected
behavior
norms prescribe “proper” behavior
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
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Human Genome Project
Behavioral genetics
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Twin studies
 Equal environment assumptions
Adoption studies
 Concordance rates
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Epigenesis – ongoing nature/nurture
exchanges (bi-directional)
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Reaction range
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Canalization
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Genetic-environmental correlation
• Passive
• Evocative
• Active (niche-picking)