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What makes us human? Human Chromosomes • To analyze chromosomes, cell biologists photograph cells in mitosis, when the chromosomes are fully condensed and easy to see. • The biologists then cut out the chromosomes from the photographs and group them together in homologous pairs. • A picture of chromosomes arranged in this way is known as a karyotype (Homologous pairs) Human Chromosomes • A Human body cell contains 46 chromosomes. • A haploid sperm, carrying just 23 chromosomes, fertilized a haploid egg, also with 23 chromosomes. – 22 autosomes (# 1-22) – 1 sex chromosome (X or Y) • The diploid zygote, or fertilized egg, contained the full complement of 46 chromosomes. Karyotype • These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Human Chromosomes • Two of your 46 chromosomes are known as sex chromosomes, because they determine an individual’s sex. (Chromosomes # 23) • Females have two copies of a large X chromosome. Males have one X and one small Y chromosome. • The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes. (Chromosomes # 1-22) Sex Chromosomes: • All egg cells carry a single X chromosome (23X). • Half of all sperm cells carry an X chromosome (23X) and half carry a Y chromosome (23Y). • This ensures that just about half of the zygotes will be 46XX and half will be 46XY. Human Traits • A pedigree chart, which shows the relationships within a family, can be used to help with this task. • Many human traits are polygenic (controlled by many genes) • Environmental effects on gene expression are not inherited; genes are. Interest Grabber Section 14-1 A Family Tree • To understand how traits are passed on from generation to generation, a pedigree, or a diagram that shows the relationships within a family, is used. In a pedigree, a circle represents a female, and a square represents a male. A filled-in circle or square shows that the individual has the trait being studied. The horizontal line that connects a circle and a square represents a marriage. The vertical line(s) and brackets below that line show the children of that couple. Go to Section: Interest Grabber continued 1. This pedigree shows the inheritance of attached ear lobes. Which parent has attached ear lobes? 2. How many children do the parents have? Which child has attached ear lobes? 3. Which child is married? Does this child’s spouse have attached ear lobes? Do any of this child’s children have attached ear lobes? Go to Section: Section 14-1 Figure 14-3 A Pedigree A circle represents a female. A horizontal line connecting a male and female represents a marriage. A half-shaded circle or square indicates that a person is a carrier of the trait. A completely shaded circle or square indicates that a person expresses the trait. Go to Section: A square represents a male. A vertical line and a bracket connect the parents to their children. A circle or square that is not shaded indicates that a person neither expresses the trait nor is a carrier of the trait. Human Genes • Some of the very first human genes to be identified were those that control blood type. • The Rh blood group is determined by a single gene with two alleles —positive and negative. Rh stands for “rhesus monkey,” the animal in which this factor was discovered. – The positive (Rh+) allele is dominant, so persons who are Rh+/Rh+ or Rh+/Rh− are said to be Rh-positive. – Individuals with two Rh− alleles are Rh-negative. Blood Groups Section 14-1 Phenotype (Blood Type Go to Section: Genotype Antigen on Red Blood Cell Safe Transfusions To From Human Alleles • Many human genes have become known through the study of genetic disorders. • Genetic Disorders can be caused by – recessive alleles – dominant alleles – Codominant alleles • What makes an allele dominant, recessive, or codominant? – It all depends on the nature of a gene’s protein product and its role in the cell. Concept Map Section 14-1 Autosomol Disorders caused by Dominant alleles Codominant alleles include include include Huntington’s disease Sickle cell disease Galactosemia Albinism Cystic fibrosis Go to Section: Recessive alleles Phenylketonuria Tay-Sachs disease Achondroplasia Hypercholesterolemia Section 14-1 Figure 14-8 The Cause of Cystic Fibrosis Chromosome #7 CFTR gene Go to Section: The most common allele that causes cystic fibrosis is missing 3 DNA bases. As a result, the amino acid phenylalanine is missing from the CFTR protein. Normal CFTR is a chloride ion channel in cell membranes. Abnormal CFTR cannot be transported to the cell membrane. The cells in the person’s airways are unable to transport chloride ions. As a result, the airways become clogged with a thick mucus. Sickle Cell Disease • Sickle cell disease is a common genetic disorder found in African Americans. • Sickle cell disease is characterized by the bent and twisted shape of the red blood cells • These sickle-shaped red blood cells are more rigid than normal cells and tend to get stuck in the capillaries, the narrowest blood vessels in the body. • As a result, blood stops moving through these vessels, damaging cells and tissues beyond the blockage. • Sickle cell disease produces physical weakness and damage to the brain, heart, and spleen. In some cases, it may be fatal. Sickle Cell Disease • Hemoglobin is the protein that carries oxygen in the blood. • Mutation: the amino acid valine in place of glutamic acid. • As a result, the abnormal hemoglobin is somewhat less soluble than normal hemoglobin. Blood gets stuck in cappillaries. Why do so many African Americans carry the sickle cell allele? • Most African Americans can trace their ancestry to west central Africa. • Malaria, a serious parasitic disease that infects red blood cells, is common in this region of Africa. • People who are heterozygous for the sickle cell allele are generally healthy and are resistant to malaria. Checkpoint Questions: 1. What are sex chromosomes? What determines whether a person is male or female? 2. Using an example, explain how a small change in a person’s DNA can cause a genetic disorder. 3. How does studying genetic disorders such as PKU help biologists understand normal alleles? 4. What are some problems biologists face in studying human inheritance? 5. If a woman with type O blood and a man with type AB blood have children, what are the children’s possible genotypes? Sex-Linked Genes • Is there a special pattern of inheritance for genes located on the X chromosome or the Y chromosome? • The answer is yes. Because these chromosomes determine sex, genes located on them are said to be sexlinked genes. Sex-Linked Genes • Males have just one X chromosome. Thus, all Xlinked alleles are expressed in males, even if they are recessive. • This means that the recessive phenotype of a sex-linked genetic disorder tends to be much more common among males than among females. • In order for a recessive allele, such as the one for colorblindness, to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. • In addition, because men pass their X chromosomes along to their daughters, sex-linked genes move from fathers to their daughters and may then show up in the sons of those daughters. Expression of X-Linked Alleles: Colorblindness • X-linked alleles are always expressed in males, because males have only one X chromosome. • Males who receive the recessive Xc allele all have colorblindness. • Females, however, will have colorblindness only if they receive two Xc alleles. Sex-Linked Genes: Hemophilia • Hemophilia is another example of a sex-linked disorder. • Two important genes carried on the X chromosome help control blood clotting. • A recessive allele in either of these two genes may produce a disorder called hemophilia • In hemophilia, a protein necessary for normal blood clotting is missing. • About 1 in 10,000 males is born with a form of hemophilia. • People with hemophilia can bleed to death from minor cuts and may suffer internal bleeding from bumps or bruises. • Fortunately, hemophilia can be treated by injections of normal clotting proteins. Sex-Linked Genes: Duchenne Muscular Dystrophy • Duchenne muscular dystrophy is a sex-linked disorder that results in the progressive weakening and loss of skeletal muscle. • People with Duchenne muscular dystrophy rarely live past early adulthood. In the United States, one out of every 3000 males is born with Duchenne muscular dystrophy. • Duchenne muscular dystrophy is caused by a defective version of the gene that codes for a muscle protein. • Researchers in many laboratories are trying to find a way to treat or cure this disorder, possibly by inserting a normal allele into the muscle cells of Duchenne muscular dystrophy patients. X-Chromosome Inactivation • Females have two X chromosomes, but males have only one. • In female cells, one X chromosome is randomly switched off. • If just one X chromosome is enough for cells in males, how does the cell “adjust” to the extra X chromosome in female cells? • That turned-off chromosome forms a dense region in the nucleus known as a Barr body. • Barr bodies are generally not found in males because their single X chromosome is still active. X-Chromosome Inactivation: Cats • In cats, for example, a gene that controls the color of coat spots is located on the X chromosome. • One X chromosome may have an allele for orange spots and the other may have an allele for black spots. • In cells in some parts of the body, one X chromosome is switched off. In other parts of the body, the other X chromosome is switched off. X-Chromosome Inactivation: Cats • As a result, the cat’s fur will have a mixture of orange and black spots, as shown in the figure below. • Male cats, which have just one X chromosome, can have spots of only one color. • By the way, this is one way to tell the sex of a cat. If the cat’s fur has three colors—white with orange and black spots, for example—you can almost be certain that it is female. Chromosomal Disorders • The most common error in meiosis occurs when homologous chromosomes fail to separate. • This is known as nondisjunction, which means “not coming apart.” Nondisjunction can occur either during meiosis I, as shown in the figure below, or in meiosis II, Chromosomal Disorders • Nondisjunction causes gametes to have abnormal numbers of chromosomes. • The result of nondisjunction may be a chromosome disorder such as Down syndrome. Down Syndrome • If two copies of an autosomal chromosome fail to separate during meiosis (nondisjunction) an individual may be born with three copies of a chromosome. • This is known as a trisomy, meaning “three bodies.” The most common form of trisomy involves three copies of chromosome 21 and is called Down syndrome. • In the United States, approximately 1 baby in 800 is born with Down syndrome. • Down syndrome produces mild to severe mental retardation. • It is also characterized by an increased susceptibility to many diseases and a higher frequency of some birth defects. Sex Chromosomal Disorders: • Disorders also occur among the sex chromosomes. • Two of these abnormalities are Turner’s syndrome and Klinefelter’s syndrome. • In females, nondisjunction can lead to Turner’s syndrome. • A female with Turner’s syndrome inherits only one X chromosome (genotype XO). • Women with Turner’s syndrome are sterile because their sex organs do not develop at puberty. Sex Chromosomal Disorders: • In males, nondisjunction causes Klinefelter’s syndrome (genotype XXY). • The extra X chromosome interferes with meiosis and usually prevents these individuals from reproducing. • Cases of Klinefelter’s syndrome have been found in which individuals were XXXY or XXXXY. • There have been no reported instances of babies being born without an X chromosome, indicating that the X chromosome contains genes that are vital for normal development. Checkpoint Questions: 1. Why are sex-linked disorders more common in males than in females? 2. How does nondisjunction cause chromosome number disorders? 3. List at least two examples of human sex-linked disorders. 4. Describe two sex chromosome disorders. 5. Distinguish between sex-linked disorders and sex chromosome disorders.