Chapter 2: Evolution and Biology

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Chapter 2
Biology and Evolution
Heredity

The transmission of physical
(biological) characteristics from
parent to offspring
Chromosomes
Long Strands of DNA
and a protein found
only in the nuclei of
cells
The Cell’s Three Dimensional Nature
Understanding Physical Anthropology and Archaeology, 8th ed., p. 43
Chromosomes
Each organism has a characteristic
number of chromosomes, usually found
in pairs.
 Humans have 23 pairs.
 Genes, the units of heredity, are
segments of molecules of DNA
(deoxyribonucleic acid) found on
chromosomes.
 Humans have approximately 20,000-25,000
different genes.

Genes and Alleles
• Gene: A Short section of a
Chromosome that codes for a
specific trait
• Alleles: Genes located on a
homologous pair of chromosomes
that may code for different
versions of the same trait
Genotype

The actual genetic
composition of an organism
Phenotype
• the observable physical
characteristics of an organism
• the things you can see
• the detectable expressions of
genotypes
Mendel’s Law of Dominance
and Recessiveness

Dominant alleles are able to “mask”
the presence of recessive alleles.
– allele for type A blood in humans is
dominant to the allele for type O
blood.
– E.g., the blood genotype AO results
in phenotype Type A blood.
– Note: A, B, and O are antigens found
on surface of red blood cells. The
functions of many of the blood group
antigens are not known.



Alleles that are both expressed
when present are co-dominant.
Alleles A and B are co-dominant.
E.g., the blood genotype AB
results in phenotype Type AB
blood.
Sickle-Cell Anemia: Genotypes
• Homozygous Dominant (Hb A, Hb A)
– Genotype: Normal hemoglobin*
• Homozygous Recessive (Hb S, Hb S)
– Abnormal hemoglobin
– Genotype: Sickle-Cell Anemia
• Heterozygous (Hb A, Hb S)
– Genotype: Normal hemoglobin
*Note: Hemoglobin is a protein in red blood
cells carrying oxygen to body cells. (Anemia
Video: 1:04 mins.)
Sickle-Cell Anemia(Homozygous
Recessive genotype)
Note: the amino acid glutamic acid is replaced with the amino acid valine
Clinical Signs and Symptoms of
Sickle-Cell Anemia
• Pain crises
• Acute chest syndrome (a lifethreatening pneumonia-like
illness)
• Cerebrovascular accidents
(Stroke)
• Splenic and renal dysfunction
• Susceptibility to infections
Distribution of Malaria and
Sickle-Cell Anemia
In areas where malaria was a problem, children
who inherited one sickle hemoglobin gene
(Heterozygous genotype) had a survival
advantage.
Children with the heterozygous genotype were
more likely to survive malaria epidemics than
children with the homozygous dominant
genotype.
They more frequently survived the malaria
epidemics they grew up with, had their own
children, and passed on the gene for sickle
hemoglobin.
Population, Gene Pool,
Evolution



Population: a group of similar
individuals that can interbreed.
Gene Pool: Total number of genes
in a population.
Evolution: Change in the
frequency of genes in a gene pool
over time.
Evolutionary Forces
1.
Natural selection a. “Nature” selects those individuals most
fit to survive and reproduce.
b. Changes in the gene pool due to the
differential survival and reproduction of
individuals of a population.
2.
Mutation - A heritable change in DNA that
happens when copying mistakes are made during
cell division.
3.
Genetic drift - the effect of chance events
on the gene pool of small populations.
4.
Gene flow - the introduction of new alleles
from nearby populations.
Skin Color Factors

Melanin – a pigment in skin, eyes, hair

Carotene - a copper-colored pigment

Skin Thickness

Reflection of Blood Vessels
Skin Color Distribution
Why is skin color distributed in
this way?
Natural Selection
Dark Skin near the Equator:
Reduces frequency of skin cancer
 Reduces likelihood of severe sunburn
which interferes with sweating

Melanin screens sunlight passing through
skin.
• The amount of sunlight passing
through skin regulates Vitamin D
Production
• Vitamin D Regulates deposition of
Calcium
• Fair skin near the Equator
• Too Much Calcium > Calcification
of soft tissues
• Dark Skin far away from the Equator
• Too Little Calcium > Rickets;
Reduced Size of Birth Canal
Mutation

The Ultimate Source of Genetic
Variability

During Human Evolution, some
mutations allowed humans to adapt to
the environment better,
e.g., those that led to bipedalism,
increased cranial capacity, reduced
tooth-size.

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