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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. Chapter 11 Complex Inheritance and Human Heredity 11.3 Chromosomes and Human Heredity Karyotype Studies Karyotype—micrograph in which the pairs of homologous chromosomes are arranged in decreasing size. Images of chromosomes stained during metaphase Chromosomes are arranged in decreasing size to produce a micrograph. Karyotype • These human chromosomes have been cut out of a photograph and arranged to form a karyotype. Chapter 11 Complex Inheritance and Human Heredity 11.3 Chromosomes and Human Heredity Telomeres Telomere caps consist of DNA associated with proteins. Serves a protective function for the structure of the chromosome 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: 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. Draw a pedigree to depict the following family •One couple has a son and a daughter with normal skin pigmentation. •Another couple has one son and two daughters with normal skin pigmentation. •The daughter from the first couple has three children with the son of the second couple. •Their son and one daughter have albinism (OMIM 203100); their other daughter has normal skin pigmentation. 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. 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. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance Recessive Genetic Disorders A recessive trait is expressed when the individual is homozygous recessive for the trait. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance Cystic Fibrosis Affects the mucus-producing glands, digestive enzymes, and sweat glands Chloride ions are not absorbed into the cells of a person with cystic fibrosis but are excreted in the sweat. Without sufficient chloride ions in the cells, a thick mucus is secreted. 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. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance Albinism Caused by altered genes, resulting in the absence of the skin pigment melanin in hair and eyes White hair Very pale skin Pink pupils Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance Tay-Sachs Disease Caused by the absence of the enzymes responsible for breaking down fatty acids called gangliosides Gangliosides accumulate in the brain, inflating brain nerve cells and causing mental deterioration. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance Galactosemia Recessive genetic disorder characterized by the inability of the body to digest galactose. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance Dominant Genetic Disorders Huntington’s disease affects the nervous system. Achondroplasia is a genetic condition that causes small body size and limbs that are comparatively short. Chapter 11 Complex Inheritance and Human Heredity 11.1 Basic Patterns of Human Inheritance 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 Comparing Dominance & Recessiveness DOMINANT: RECESSIVE: • Can appear in either sex because an autosome carries the gene. • Can appear in either sex & can skip generations. • Affected individuals have a homozygous recessive genotype, whereas in heterozygotes (carriers) the wild type allele masks expression of the mutant allele. • Parents of an affected individual are heterozygous or have the trait. • Most occur unexpectedly • Incest increases the risk of having a child with an autosomal recessive trait • If the child has it, then at least one parent has it. • Do not skip generations • If no offspring inherit the trait in one generation, its transmission stops because the offspring can pass on only the recessive form of the gene. Comparing Dominance & Recessiveness • Determining whether an allele is dominant or recessive is critical in medical genetics because it helps predict which individuals are at high risk of inheriting a particular condition (phenotype). 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. Chapter 36 Human Reproduction and Development 36.2 Human Development Before Birth Diagnosis in the Fetus Ultrasound Procedure in which sound waves are bounced off the fetus Determines if the fetus is growing properly Determines the position of the fetus in the uterus Determines the gender of the fetus Chapter 36 Human Reproduction and Development 36.2 Human Development Before Birth Amniocentesis Amniocentesis is performed in the second trimester. Fluid from the amniotic sac is removed and analyzed. Diagnosis of chromosome abnormalities and other defects Chapter 36 Human Reproduction and Development 36.2 Human Development Before Birth Chorionic Villus Sampling Chorionic villus sampling is performed during the first trimester. Cells from the chorion are removed and analyzed by karyotyping. Diagnosis of chromosome abnormalities and other genetic defects Fetal Blood Sampling • Diagnosis of genetic or chromosome abnormality • Checks for fetal blood problems and oxygen levels • Medications can be given to the fetus before birth. 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.