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Name____________________________ Chapter 14 Class __________________ Date __________ The Human Genome Summary 14–1 Human Heredity In order to learn more about humans, biologists often use a karyotype to analyze human chromosomes. A karyotype is a picture of a cell’s chromosomes grouped in homologous pairs. Humans have 46 chromosomes. Two of these, X and Y, are sex chromosomes. Females have two X chromosomes (XX). Males have one X and one Y chromosome (XY). The other 44 chromosomes are known as autosomes. All human egg cells carry a single X chromosome. Sperm cells carry either X or Y chromosomes. Half of all sperm cells carry an X chromosome and half carry a Y chromosome. This ensures that just about half of the zygotes will be female and half will be male. To study human inheritance, biologists use pedigree charts. A pedigree chart shows relationships within a family. The inheritance of a trait can be traced through the family. From this, biologists may determine the genotypes of family members. It is difficult to link an observed human trait with a specific gene. Many human traits are polygenic, or controlled by many genes. The environment also affects some traits. The genes controlling blood type were among the first human genes to be identified. A number of genes are responsible for blood groups, but the two best known are the ABO blood groups and the Rh blood group. • Red blood cells can carry two antigens—A and B. Antigens are molecules the immune system can recognize. A person who has only antigen A has type A blood. A person who has only antigen B has type B blood. A person who has both antigens has type AB blood. A person who does not have either antigen has type O blood. A single gene with three alleles determines the ABO blood types. • Red blood cells can also have the Rh antigen. People with the Rh antigen are Rh positive. Those without it are Rh negative. A single gene with two alleles determines the Rh blood group. There are many human genetic disorders. Some, including PKU and Tay-Sachs disease are caused by recessive alleles. Individuals must inherit a recessive allele from each parent. Other disorders, such as Huntington disease, are caused by a dominant allele. Huntington disease is expressed in any person who has the allele. Still other disorders, such as sickle cell disease, are caused by a codominant allele. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 127 Name____________________________ Class __________________ Date __________ Scientists are starting to learn which changes in the DNA sequence cause certain genetic disorders. Sometimes, a small change in the DNA of a single gene affects the structure of a protein and causes a serious genetic disorder. This is the case with cystic fibrosis and sickle cell disease. 14–2 Human Chromosomes Genes on the X and Y chromosomes are said to be sex-linked. They are inherited in a different pattern than are genes on autosomes. Males have just one X chromosome. Thus all X-linked alleles are expressed in males, even if they are recessive. Some examples of sex-linked disorders are colorblindness and hemophilia. Most of the time, the mechanisms that separate chromosomes in meiosis work well, but sometimes errors happen. The most common error during meiosis is nondisjunction. Nondisjunction is the failure of chromosomes to separate properly during meiosis. If nondisjunction occurs, abnormal numbers of chromosomes may find their way into gametes, and a disorder of chromosome numbers may result. • Down syndrome is an example of autosomal nondisjunction. In this disorder, there is an extra copy of chromosome 21. • Nondisjunction can also occur in sex chromosomes. Turner’s syndrome and Klinefelter’s syndrome are two examples. 14–3 Human Molecular Genetics Biologists use molecular biology techniques to read, analyze, and change the DNA code of human genes. DNA analysis techniques can be used in different ways. • DNA analysis can be used to test parents for recessive alleles that code for genetic disorders. • DNA fingerprinting is a DNA analysis technique that can be used to identify individuals. The Human Genome Project is an ongoing effort to analyze the human DNA sequence. Scientists are using the results of the human genome project to help locate genes on human chromosomes. In addition, information about the human genome may be used to help diagnose and treat disease. Data from the human genome may be used to cure genetic disorders by gene therapy. In gene therapy, an absent or faulty gene is replaced by a normal, working gene. In one method of gene therapy, a normal, working copy of a gene is attached to viral DNA. Virus particles deliver this copy of the gene to human cells. The human cells can then make proteins that correct genetic defects. © Pearson Education, Inc., publishing as Pearson Prentice Hall. 128