Human Variation (Chapter 7). Human Variation Genetics is the study of biological traits. These...

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

P a b

Genotype:

P a B

PP aa 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.

• The additive effects of two or more genes on a single phenotype

Visual Summary 9.5

Multiple genes

Polygenicinheritance

Single trait(e.g., skin color)

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).

• An AABBCC person would have very dark skin

• An aabbcc person would have very light skin

Figure 9.22

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

Eggs Sperm


• 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.

• The inheritance of these alleles leads to a wide range of skin pigmentation in the human population.

Figure 9.22

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

Eggs Sperm


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.

Figure 7.4Why does this happen?Both type A and type B blood have specific carbohydrates which are on the surface of the blood cells.

AB blood has both carbohydrateson the surface of the blood cells

o blood has no carbohydrates

Carbohydrates are:N-Acetylglucosamine, galactose and fucose

Also known as antigens

Figure 7.4Why does this happen?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.

Figure 7.4Why does this happen?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 bloodBB & Bo give rises to B type bloodAB is co-dominant - AB type bloodoo is recessive – o type blood

Figure 7.3aAt 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 (30-35%), 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 7.3bThe 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 7.3cThe 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.

Human Blood Groups

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 Rh-positive.

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

Figure 7.5 (1)Hemolytic disease


•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.


• 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

Figure 7.8 (1)Malaria – an agent of natural

Selection


Selection

Figure 7.9Malaria – 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.


- Remember mutations? Any change in the nucleotide sequence of DNA

Figure 10.21

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

Normal hemoglobin DNA Mutant hemoglobin DNA

mRNA mRNA

Figure 7.10A small change in a gene can have many phenotypicconsequences.


• 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.

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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