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.