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Human Variation
(Chapter 7)
Human Variation
• Genetics is the study of biological traits. These
traits are coded for in genes, which are parts of
chromosomes.
• An Allele is a variant of a gene. These can be
dominant or recessive, and these are the basis of
inherited traits, both structural and behavioral.
• Chromosomes exist as homologous pairs.
Human Variation
• Somatic Cells - Non-sex Cells. Contain a full
compliment of chromosomes. Characteristic to
their species. Referred to as the diploid number of
chromosomes.
• Gametes - Sex Cells. Cell which carry genetic
information for sexual reproduction. Contain one
half the compliment of chromosomes characteristic
to their species. Referred to as the haploid number
of chromosomes.
Human Variation
• Phenotype
– An organism’s physical traits
• Genotype
– An organism’s genetic makeup
Allele
• Allele: Alternate form of a gene at same position on pair of
chromosomes that affect the same trait.
• Dominant Allele: Capital Letter--O
• Recessive Allele: lowercase letter--o
• Homozygous Dominant--OO
• Homozygous Recessive--oo
• Heterozygous--Oo
a
P
P
Genotype:
PP
a
aa
B
b
Bb
Natural Selection
•
•
•
•
•
•
Variation in population
Variation inheritable
Some individuals survive and reproduce better
than others
Survival and reproduction are tied to variation in
traits among individuals (non-random)
Therefore, these genetic traits become dominant
in a given population.
Due to environmental pressure and natural
selection
Human Variation
• With origins in Africa, modern man has spread around the globe. In
doing so, modern man adapted to the surroundings.
Human Variation
• Arms and legs are longer
and thinner in warm areas of
the planet – shorter and
thicker in cold regions.
• Conserves heat in cold
regions by reducing surface
area
• Skin pigmentation is darker
the nearer the equator to
protect the skin from UV.
Polygenic Inheritance
• The additive effects of two or more genes on a single
phenotype
Polygenic
inheritance
Single trait
(e.g., skin color)
Multiple genes
Visual Summary 9.5
Polygenic Inheritance
• Three genes inherited separately
• The dark-skin allele for each gene
(A, B, and C) contributes one “unit”
of darkness to the phenotype and is
incompletely dominant to the other
alleles (a, b, and c).
aabbcc
(very light)
AaBbCc
Eggs
AABBCC
(very dark)
AaBbCc
Sperm
• An AABBCC person would have
very dark skin
• An aabbcc person would have very
light skin
Figure 9.22
Polygenic Inheritance
• An AaBbCc person would have skin
of an intermediate shade
• As the alleles have an additive
effect, AaBbCc would produce the
same skin color as any other
genotype with just three dark-skin
alleles, such as AABbcc.
aabbcc
(very light)
AaBbCc
Eggs
AABBCC
(very dark)
AaBbCc
Sperm
• The inheritance of these alleles
leads to a wide range of skin
pigmentation in the human
population.
Figure 9.22
Ice age Europe (18,000 years ago)
• Glacial ice 2km thick covers
much of Northern Europe and the
Alps.
• Sea levels are approx. 125m
lower than today and the coastline
differs slightly from the present
day.
• Human populations that began
their migration from Africa
60,000 years earlier were stopped
by the ice.
Ice age Europe (18,000 years ago)
•
Due to the cold and the need
for food, the populations of
the day waited the ice age
out in the three locations
shown on the map.
• These were the Iberian
Peninsula, the Balkans and
the Ukraine.
After the Ice age – 12,000 years
ago
• 12,000 years ago, the ice retreated
and the land has become much
more supportive to life.
• The three groups of humans had
taken refuge for so long their
DNA had naturally picked up
mutations
• These three major population
groups account for approx 80% of
Europe's present-day population
Finally, from 8,000 years
ago
• Peoples from Africa that had
moved to the Middle East
developed the new technology of
agriculture and began moving back
into Europe.
• This was the last migration of
human population into Europe.
• Body shape and skin pigmentation
all changed due to environmental
pressure on the genomes of these
separate populations
Different populations have
different blood groups
• Different populations of people have many different
genetic variations
• The easiest to study is blood type
• Like all other differences, it is all down to the
frequency an allele is passed on during reproduction
and environmental pressure and natural selection
Human Blood Groups
• A, B, AB, and o
• First found during the Crimean war (1854 – 1856)
– British Army Surgeon kept records of successful blood
transfusions
• A to A and B to B worked
• A to B or B to A were always fatal
• Also found that o was the universal donor
– People with this type of blood can give it to anyone
• AB type people can receive blood from anyone
– Universal recipient.
Why does
this
happen?
Figure 7.4
Both type A and type B blood
have specific carbohydrates
which are on the surface of the
blood cells.
AB blood has both carbohydrates
on the surface of the blood cells
o blood has no carbohydrates
Carbohydrates are:
N-Acetylglucosamine, galactose
and fucose
Also known as antigens
Why does
this
happen?
Figure 7.4
Antigen: Molecule that stimulates
an immune response, especially the
production of antibodies by plasma
B cells. Antigens are usually
proteins or polysaccharides.
A person who receives incorrectly
matched blood will make
antibodies against the blood group
antigens.
Blood cells clump together in blood
vessels with fatal results.
Why does
this
happen?
Figure 7.4
Controlled by three alleles
Allele A – dominant
has info for making antigen A
Allele B – dominant
has info for making antigen B
Allele o – recessive
produces neither antigen
AA & Ao gives rise to A type blood
BB & Bo give rises to B type blood
AB is co-dominant - AB type blood
oo is recessive – o type blood
Human
Blood
Groups
Figure
7.3a
At 10-35% frequency in most
populations of the world, the A
blood allele is most common.
The highest frequencies of A are
found in small, unrelated
populations, especially the
Blackfoot Indians of Montana (3035%), the Australian Aborigines
(40-53%), and the Lapps, or Saami
people, of Northern Scandinavia
(50-90%).
The A allele apparently was absent
among Central and South
American Indians.
Human
Blood
Groups
Figure
7.3b
The global frequency patterns
of the type B blood allele.
Note that it is highest in central
Asia and lowest in the
Americas and Australia.
However, there are relatively
high frequency pockets in
Africa as well.
Overall in the world, B is the
rarest ABo blood allele
Human
Blood
Groups
Figure
7.3c
The o blood type (usually resulting
from the absence of both A and B
alleles) is very common around the
world.
It is particularly high in frequency
among the indigenous populations of
Central and South America, where it
approaches 100%.
It also is relatively high among
Australian Aborigines and in Western
Europe (especially in populations with
Celtic ancestors).
The lowest frequency of o is found in
Eastern Europe and Central Asia,
where B is common.
Rh Factor
• There are four blood groups but eight blood types.
• The Rh-factor!!
•
•
•
•
85% Positive (US population)
15% Negative
Genetic factor
Can cause Hemolytic Disease and death of infants.
•
The genetics of the Rh
factor
Another blood grouping system independent of ABo
– the Rh-factor
– Again, three genes (alleles): located very close together
on the same chromosome.
• First C & c, second D & d, third E & e
• Unlike the ABo system there is no co-dominance, c,
d, and e are recessive to C, D, and E.
• ccddee is known as Rh-negative. All others Rhpositive.
Hemolytic disease
• If a child is Rh+, a Rh- Mother can begin to produce
antibodies Rh+ red blood cells
– Rh factor crosses placenta and mother makes
antibodies
• In subsequent pregnancies these antibodies can cross the
placenta and cause hemolysis of a Rh+ Childs red blood
cells.
– Can lead to mental retardation or death
• Prevented by giving Rh- women a Rh immunoglobulin
injection no later than 72 hours after birth. Attacks any of
the babies Abs in mother before her own antibodies are
produced
Hemolytic
disease
Figure 7.5
(1)
Hemolytic
disease
Figure 7.5
(2)
Hemolytic
disease
Figure 7.5
(3)
•Prevented by giving Rh- women a Rh
immunoglobulin injection no later than
72 hours after birth.
•Attacks any of the babies Abs in
mother before her own antibodies are
produced.
Malaria – an agent of natural
Selection
• As any species evolves, biological differences among its
population arise largely through natural selection.
• Diseases are among the selective forces that can result in
genetic differences among populations.
• In disease-ridden areas of the world, natural selection acts to
increase the frequency of alleles that confer partial
resistance to a disease while decreasing the frequency of
alleles that leave people susceptible to a disease.
Malaria – an agent of natural
Selection
• New traits are produced by mutation and are then subject to
natural selection.
• The traits that survive are adaptations.
• Malaria causes 110 million cases of illness each year
– Close to 2 million deaths each year.
• Rare before the invention of agriculture
– Did much to change the selective pressure on human populations
Malaria – an agent of natural
Figure
7.8 (1)
Selection
Malaria – an agent of natural
Figure
7.8 (2)
Selection
Malaria – an agent of natural
Figure
7.8 (3)
Selection
Malaria – an agent of natural
Figure
7.8 (4)
Selection
Malaria – an agent of natural
Figure
7.8 (5)
Selection
Malaria – an agent of natural
Figure
7.8 (6)
Selection
Malaria – an agent of natural
Figure
7.9
Selection
Malaria – an agent of natural
Selection
• Sickle Cell Anemia
• Controlled by intermediate phenotypes at a ratio of
1:2:1
• Red blood cells are not concave
• Normal Hemoglobin (HbA). Sickle cell (Hbs)
• HbA-HbA-normal
Hbs-Hbs – sickle cell
• HbA-Hbs- have the trait
• Therefore, incomplete dominance.
Malaria – an agent of natural
Selection
- Remember mutations? Any change in the
nucleotide sequence of DNA
Normal hemoglobin DNA
Mutant hemoglobin DNA
mRNA
mRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
Figure 10.21
Figure 7.10
A small change in a gene
can have many phenotypic
consequences.
Malaria – an agent of natural
Selection
• Most victims of malaria are young children
• Where malaria occurrence is high, so is the HBs allele
– Odd, as Sickle Cell Anemia is nearly always fatal before
reproductive age
– HBs allele confers resistance to malaria
• So in areas of high occurrence to malaria, the HBs allele
may cause a genetic disorder, but increases the overall
fitness of a population where malaria occurs.
Malaria – an agent of natural
Selection
The concept of racism
• Racism has many meanings:
– All of them come down to the belief that some group of
people are better than others.
• In most cases, the motivation to conquer a region
comes first, the racist ideology comes later
• Came about because people thought that a different
genetic trait was inferior to one(s) they processed.
The concept of racism
• They also believed that their “group identity” was
inherited and could not be changed
– A view which has no basis in genetics
• In the 1940’s the Nazis exterminated 11 million
Jews, gypsies and other groups
– But not before theses groups were declared “inferior”.
• Most people now regard racism as unethical
– Denies basic human rights, results in crime and human
conflicts.
The concept of racism
• Human populations have always been variable.
– adapt and change under selective pressure
• Skin pigmentation is determined by a selective
environmental pressure due to the total amount of
sunlight a population exists with.
• Taught hatred for different populations of people
The end!
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