Chapter 11: Human Heredity Section 1: “It Runs in the Family” “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 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 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 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 Chapter 11: Human Heredity Section 2: The Inheritance of Human Traits 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 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 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 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 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+ 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 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 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 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 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 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 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 Polygenic Traits • Human traits that are controlled by a number of genes are called polygenic traits – Height – Body weight – Skin color Chapter 11: Human Heredity Section 3: Sex-Linked Inheritance 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 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 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 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 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 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 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 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 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) 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 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 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 Chapter 11: Human Heredity Section 4: Diagnosis of Genetic Disorders 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 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 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 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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