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Understanding Heredity: Chromosomes, Genes, and Evolutionary Forces in Human Traits

This chapter explores the fundamentals of heredity, emphasizing the role of chromosomes and genes in the transmission of physical characteristics from parents to offspring. Humans possess 23 pairs of chromosomes, containing approximately 30,000 genes that influence traits. Key concepts include Mendel's laws of dominance and recessiveness, and the implications of sickle-cell anemia in genetic diversity and adaptation to malaria. The chapter also discusses natural selection, mutations, and genetic drift as evolutionary forces impacting gene pools and phenotypic variation, particularly concerning skin color adaptations.

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Understanding Heredity: Chromosomes, Genes, and Evolutionary Forces in Human Traits

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  1. Chapter 2 Biology and Evolution

  2. Heredity • The transmission of physical (biological) characteristics from parent to offspring

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

  4. The Cell’s Three Dimensional Nature Understanding Physical Anthropology and Archaeology, 8th ed., p. 43

  5. 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 30,000 different genes

  6. 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

  7. Genotype • The actual genetic composition of an organism

  8. Phenotype • the observable physical characteristics of an organism • the things you can see • the detectable expressions of genotypes

  9. Mendel’s Law of Dominance and Recessiveness • Dominant alleles are able to mask the presence of recessive alleles. • For example, the allele for type A blood in humans is dominant to the allele for type O blood. • Alleles that are both expressed when present are co-dominant. • An individual with the alleles for type A and type B blood has the AB blood type. 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.

  10. Sickle-Cell Anemia • Homozygous Dominant (Hb A, Hb A) • normal hemoglobin* • Homozygous Recessive (Hb S, Hb S) • abnormal hemoglobin • Heterozygous (Hb A, Hb S) • normal and abnormal hemoglobin • normal hemoglobin more abundant *Note: Hemoglobin is a protein in red blood cells carrying oxygen. (Anemia Video: 1:04 mins.)

  11. Clinical Signs and Symptoms of Sickle-Cell Anemia(Homozygous Recessive genotype) • Pain crises • Acute chest syndrome (a life-threatening pneumonia-like illness) • Cerebrovascular accidents (Stroke) • Splenic and renal dysfunction • Susceptibility to infections

  12. Distribution of Malaria and Sickle-Cell Anemia

  13. 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.

  14. 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.

  15. Evolutionary Forces • Natural selection - • “Nature” selects those individuals most fit to survive and reproduce. • Changes in the gene pool due to the differential survival and reproduction of individuals of a population. • Mutation - A heritable change in DNA that happens when copying mistakes are made during cell division. • Genetic drift - the effect of chance events on the gene pool of small populations. • Gene flow - the introduction of new alleles from nearby populations.

  16. Skin Color Factors • Melanin – a pigment in skin, eyes, hair • Carotene - a copper-colored pigment • Skin Thickness • Reflection of Blood Vessels

  17. Skin Color Distribution

  18. 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

  19. Melanin screens sunlight passing through skin. • The amount of sunlight passing through skin regulates Vitamin D Production • Which 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

  20. 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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