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Genetics and Inheritance of Human Traits: Understanding the Human Organism

This chapter explores the genetic basis of human traits, including the inheritance of traits through genes and the influence of environmental factors. It discusses the structure of human chromosomes, the formation of gametes, and the principles of genetics described by Mendel. It also examines specific human traits such as blood groups and genetic diseases like Huntington's disease and sickle cell anemia.

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Genetics and Inheritance of Human Traits: Understanding the Human Organism

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  1. Chapter 11: Human Heredity Section 1: “It Runs in the Family”

  2. “It Runs in the Family” • Many of the characteristics of human children are genetically determined • Many human traits are inherited by the action of dominant and recessive genes, although other traits are determined through more complicated gene interactions • Genetics is one of the most important fields in biology

  3. The Human Organism • The study of ourselves begins with human chromosomes • A human diploid cell contains 46 chromosomes arranged in 23 pairs • These 46 chromosomes contain 6 billion nucleotide pairs of DNA • The principles of genetics described by Mendel require that organisms inherit a single copy of each gene from each parent • In humans, the gametes, or reproductive cells, contain a single copy of each gene

  4. The Human Organism • Gametes are formed in the reproductive organs by the process of meiosis • Each egg cell and sperm cell contain 23 chromosomes • During fertilization, sperm and egg unite and a zygote, or fertilized egg, is produced • Of the 46 chromosomes found in a human diploid cell, two are the sex chromosomes • The remaining 44 chromosomes are the autosomes

  5. Human Traits • There are some traits that are strongly influenced by environmental factors • Nutrition and exercise • Although it is important to consider the influence of the environment on the expression of some genes, it must be understood that environmental effects on gene expression are not inherited; genes are • Genes that are denied a proper environment in which to reach full expression in one generation can, in a proper environment, achieve full potential in a later generation

  6. Chapter 11: Human Heredity Section 2: The Inheritance of Human Traits

  7. Human Blood Groups • A gene that has three or more alleles is said to have multiple alleles • Although many alleles may exist, it is important to remember that only two alleles are present in diploid (2N) organism • ABO and Rh blood groups are examples of human traits determined by multiple alleles

  8. ABO Blood Groups • In 1900, the Austrian physician Karl Landsteiner discovered that human blood could be classified into four general types • Landsteiner blood groups • Determined by the presence of absence of specific chemical substances in the blood • Landsteiner discovered that the red blood cells could carry two different antigens, which he called A and B • Molecules that can be recognized by the immune system

  9. ABO Blood Groups • The presence or absence of the A and B antigens produces four possible blood types • A, B, AB, and O • Type A blood – antigen A • Type B blood – antigen B • Type AB blood – antigen A and B • Type O blood – neither antigen

  10. ABO Blood Groups • Especially important in blood transfusions • A transfusion of the wrong type can cause a violent, even fatal, reaction in the body as the immune system responds to an antigen not found on its own cells • People with AB blood can receive blood from any of the four types because they already have both possible antigens on their blood cells • The ABO blood groups are determined by a single gene with three alleles: IA, IB, and i • EXAMPLE • If type B blood is given to a person with type A or type O blood, a reaction will occur against the red blood cells carrying the B antigen

  11. Rh Blood Groups • In addition to the ABO antigens, there is another antigen on the red blood cells, called the Rh antigen • Named after the rhesus monkey in which the antigen was first discovered • People who have the Rh antigen on their red blood cells are said to be Rh positive (Rh+) • People who do not have the Rh antigen on their red blood cells are said to be Rh negative (Rh-) • In blood banks, the ABO and Rh blood groups are often expressed together in symbols such as AB-, or O+

  12. Huntington Disease • Huntington disease, which is produced by a single dominant allele, is an example of a genetic disease • People who have this disease show no symptoms until they are in their thirties or forties, when the gradual damage to their nervous system begins • People who have the dominant allele for Huntington disease have the disease and suffer painful progressive loss of muscle control and mental function until death occurs

  13. Sickle Cell Anemia • In 1904, Doctor James Herrick noticed an unusual ailment afflicting one of his young patients • Had been complaining of weakness and dizzy spells • Open sores on legs • Red blood cells were bent and twisted into shapes that resembled sickles

  14. The Cause of Sickle Cell Anemia • Sickle cell anemia is caused by a change in one of the polypeptides found in hemoglobin • Protein that carries oxygen in red blood cells • When a person who has sickle cell anemia is deprived of oxygen the hemoglobin molecules join together and form fibers • Cause the red blood cells to undergo dramatic changes in shape • More rigid • Become stuck in capillaries • Movement of blood through these vessels is stopped and damage to cells and tissues occur • Serious injury or death may result

  15. The Genetics of Sickle Cell Anemia • The allele for normal hemoglobin (HA) is codominant with the sickle cell allele (HS) • Heterozygous (HAHS) individuals are carriers • ½ of the hemoglobin is normal • Suffer few ill effects of the disorder • Homozygous (HSHS) individuals are sufferers • All hemoglobin molecules are affected by the sickle cell allele • Severely afflicted by the disease

  16. The Molecular Basis of Sickle Cell Anemia • The allele for sickle cell hemoglobin differs from the allele for normal hemoglobin by a single nucleotide • The substitution of one nucleotide in the allele results in the substitution of a different amino acid in the sickle cell hemoglobin protein • Makes hemoglobin less soluble in blood

  17. The Distribution of Sickle Cell Anemia • In the US, people of African ancestry are the most common carriers of the sickle cell trait • In the rest of the world, sickle cell anemia is found in the tropical regions of Africa and Asia • Approximately 10% of Americans of African ancestry and as many as 40% of the population in some parts of Africa carry the trait

  18. The Distribution of Sickle Cell Anemia • People who are heterozygous for sickle cell anemia (HAHS) are partially resistant to malaria, a serious disease that affects red blood cells • Sickle cell hemoglobin is thought to offer this resistance because sickled cells are frequently removed from the circulation and destroyed, killing any malaria parasites with them • People who are homozygous for normal hemoglobin (HAHA) on the other hand, have no resistance to malaria • The incidence of sickle cell anemia parallels the incidence of malaria throughout the tropical areas of the world

  19. Polygenic Traits • Human traits that are controlled by a number of genes are called polygenic traits • Height • Body weight • Skin color

  20. Chapter 11: Human Heredity Section 3: Sex-Linked Inheritance

  21. Sex-Linked Inheritance • Genes that are located on the sex chromosomes of an organism are inherited in a sex-linked pattern • As in many organisms, the sex in humans is determined by the X and Y chromosomes • In females, meiosis produces egg cells that contain one X chromosome and 22 autosomes • In males, meiosis produces sperm cells of which half contain one X chromosome and 22 autosomes • The sex of a person is determined by whether an egg cell is fertilized by an X-carrying sperm or a Y-carrying sperm

  22. The Human XY System • Although meiosis is a precise mechanism that separates the two sex chromosomes of a diploid cell into single chromosomes of haploid gamete cells, errors sometimes do take place • The most common of these errors is nondisjunction • Nondisjunction is the failure of chromosomes to separate properly during one of the stages of meiosis

  23. Nondisjunction Disorders • Roughly 1 birth in 1000 is affected by an abnormality involving nondisjunction of the sex chromosomes • Turner syndrome • Female in appearance but their female sex organs do not develop at puberty and they are sterile • 45X or 45XO • Klinefelter syndrome • Male in appearance, and they, too, are sterile • 47XXY

  24. Nondisjunction Disorders • What can we learn from these abnormalities of the sex chromosome? • An X chromosome is absolutely essential for survival • Sex seems to be determined by the presence or absence of a Y chromosome and not by the number of X chromosomes • The Y chromosome contains a gene that switches on the male pattern of growth during embryological development • If this gene is absent, the embryo follows a female pattern of growth

  25. Sex-Linked Genetic Disorders • Genes that are carried on either the X or the Y chromosome are said to be sex-linked • In humans, the small Y chromosome carries very few genes • The much larger X chromosome contains a number of genes that are vital to proper growth and development • Recall that males have one X chromosome • Thus all X-linked alleles are expressed in males, even if they are recessive • In order for a recessive allele to be expressed in females, there must be two copies of it

  26. Colorblindness • Colorblindness is a recessive disorder in which a person cannot distinguish between certain colors • Most types of colorblindness are caused by sex-linked genes located on the X chromosome • The alleles for colorblindness render people unable to make some of the pigments in the eye necessary for color vision • Most common is red-green colorblindness

  27. Colorblindness • In humans, color vision depends on the varying sensitivity of three groups of specialized nerve cells in the retina of the eye • One group is sensitive to blue light, one to red light, and one to green light • Colors of any given shade excite a specific level of activity from each of the three groups of nerve cells

  28. Colorblindness • Because the gene for color vision is carried on the X chromosome, the dominant allele for normal color vision is represented as XC and the recessive allele for red-green color blindness is represented as Xc • Homozygous (ZCZC) and heterozygous (XCXc) females have normal color vision • A female who is heterozygous for colorblindness is said to be a carrier because she carries the recessive allele but does not express it

  29. Colorblindness • Although she is not colorblind, she is capable of passing on the allele for colorblindness to her offspring • Only homozygous recessive females (XcXc) are colorblind • Because males have only one X chromosome, they are either colorblind (XcY) or have normal color vision (XCY)

  30. Hemophilia • Another recessive allele on the X chromosome produces a disorder called hemophilia, or bleeder’s disease • In hemophilia, the protein antihemophilic factor (AHF) necessary for normal blood clotting is missing • People with hemophilia can bleed to death from minor cuts and may suffer internal bleeding from bumps or bruises • Hemophilia can be treated by injecting AHF isolated from donated blood

  31. Muscular Dystrophy • Muscular dystrophy is an inherited disease that results with the progressive wasting away of skeletal muscle • Children with muscular dystrophy rarely live past early adulthood • The most common form of MD is caused by a defective version of the gene that codes for a muscle protein known as dystrophin • This gene is located on the X chromosome • Researchers are now using molecular techniques to insert healthy copies of the dystrophin gene into muscle cells

  32. Sex-Influenced Traits • Many traits that may seem to be sex-linked, such as male pattern baldness, are actually caused by genes located on autosomes, not on sex chromosomes • Why then is baldness so much more common in men than it is in women? • Male pattern baldness is a sex-influenced trait • A sex-influenced trait is a trait that is caused by a gene whose expression differs in males and females

  33. Chapter 11: Human Heredity Section 4: Diagnosis of Genetic Disorders

  34. Diagnosis of Genetic Disorders • Humans have been aware of genetic disorders throughout history • For years, physicians have longed for a way to detect and treat genetic disorders • Today, for some disorders, detection is as simple as an examination of a person’s chromosomes

  35. A Chromosomal Abnormality – Down Syndrome • In Down syndrome, there is an extra copy of chromosome 21 • Down syndrome results in mental retardation that ranges from mild to severe • It is also characterized by an increased susceptibility to many diseases • In the US, 1 baby in every 800 is born with Down syndrome

  36. Prenatal Diagnosis • Down syndrome and other genetic disorders can now be diagnosed before birth by analyzing cells from the developing embryo • Amniocentesis • Requires the removal of a small amount of fluid from the sac surrounding the embryo • Cells are grown in a lab, treated with a chemical to prevent cell division, and carefully examined • Karyotype is prepared to make certain that the chromosomes are normal • Chorionic villus biopsy • A sample of embryonic cells is removed directly from the membrane surrounding the embryo • More rapid results • Recent studies have linked limb defects in babies to CVB tests done before the tenth week of pregnancy

  37. Ethical Considerations • The emerging ability to identify genetic disorders before birth has already begun to force parents and physicians to face ethical issues that past generations could never have imagined • How should parents react to the news that their child might be born with a serious or fatal genetic disorder? • What factors – medical, economic, social, and ethical – should be considered in such cases, and who should make the decision?

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