chapter 2 biology and evolution. heredity the transmission of physical (biological) characteristics...

Chapter 2

Biology and Evolution

Heredity

The transmission of physical (biological) characteristics from parent to offspring

Long Strands of DNA and a protein found only in the nuclei of cells

Chromosomes

Understanding Physical Anthropology and Archaeology, 8th ed., p. 43

The Cell’s Three Dimensional Nature

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.

Mitosis and Meiosis

Mitosis: The process of cell division by which new body cells are produced. Necessary for growth, repair, and replacement Daughter cells have the same number of

chromosomes as parent cell

Meiosis: A kind of cell division that produces the sex cells (gametes) each of which has half the number of chromosomes found in other cells of the organism. Two meiotic divisions

• 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

Genes and Alleles

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

1. Homozygous Dominant (Hb A, Hb A)Genotype: Normal hemoglobin*

2. Homozygous Recessive (Hb S, Hb S)Genotype: Sickle-Cell Anemia

3. 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[Threonine (Thr), Proline (Pro) Glutamic acid (Glu), Valine (Val)]

Clinical Signs and Symptoms of Sickle-Cell Anemia

• Pain crises• Acute chest syndrome (a life-threatening pneumonia-like illness)

• Cerebrovascular accidents (Stroke)

• Splenic and renal dysfunction• Susceptibility to infections

Distribution of Malaria and Sickle-Cell Anemia

In areas where malaria is a problem, children who inherited one sickle hemoglobin gene (Heterozygous genotype) have 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.

Life Cycle of Plasmodium (the parasite that causes malaria)With asexual and sexual reproduction

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 Forces1. 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 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.