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1414-1Human Heredity (In a nutshell) Human Heredity Slide 1 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Chromosomes Human Chromosomes Cell biologists analyze chromosomes by looking at karyotypes. Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs. A picture of chromosomes arranged in this way is known as a karyotype. Copyright Pearson Prentice Hall Slide 2 of 43 End Show 14–1 Human Heredity Human Chromosomes Two of the 46 human chromosomes are known as sex chromosomes, because they determine an individual's sex. • Females have two copies of an X chromosome. • Males have one X chromosome and one Y chromosome. The remaining 44 chromosomes are known as autosomal chromosomes, or autosomes. Slide 3 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Traits Pedigree Charts A pedigree chart shows the relationships within a family. Genetic counselors analyze pedigree charts to infer the genotypes of family members. Slide 4 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity A horizontal line connecting a male and a female represents a marriage. A circle represents a female. Human Traits A square represents A vertical line and a a male. bracket connect the parents to their children. A circle or square that is not shaded indicates that a person does not express the trait. A shaded circle or square indicates that a person expresses the trait. Slide 5 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Genes Blood Group Genes Human blood comes in a variety of genetically determined blood groups. A number of genes are responsible for human blood groups. The best known are the ABO blood groups and the Rh blood groups. Slide 6 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Genes The Rh blood group is determined by a single gene with two alleles—positive and negative. The positive (Rh+) allele is dominant, so individuals who are Rh+/Rh+ or Rh+/Rh are said to be Rhpositive. Individuals with two Rh- alleles are said to be Rhnegative. Slide 7 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Genes ABO blood group (Codominance) Slide 8 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Genes Recessive Alleles The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype. Slide 9 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Genes Dominant Alleles The effects of a dominant allele are expressed even when the recessive allele is present. Slide 10 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Human Genes Codominant Alleles Sickle cell disease is a serious disorder caused by a codominant allele. Sickle cell is found in about 1 out of 500 African Americans. Slide 11 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity From Gene to Molecule From Gene to Molecule How do small changes in DNA cause genetic disorders? Slide 12 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity From Gene to Molecule In both cystic fibrosis and sickle cell disease, a small change in the DNA of a single gene affects the structure of a protein, causing a serious genetic disorder. Slide 13 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity From Gene to Molecule Cystic Fibrosis Cystic fibrosis is caused by a recessive allele. Sufferers of cystic fibrosis produce a thick, heavy mucus that clogs their lungs and breathing passageways. 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. Slide 14 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity From Gene to Molecule 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. Slide 15 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity From Gene to Molecule Sickle Cell Disease Sickle cell disease is a common genetic disorder found in African Americans. It is characterized by the bent and twisted shape of the red blood cells. Slide 16 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity From Gene to Molecule Hemoglobin is the protein in red blood cells that carries oxygen. In the sickle cell allele, just one DNA base is changed. As a result, the abnormal hemoglobin is less soluble than normal hemoglobin. Low oxygen levels cause some red blood cells to become sickle shaped. People who are heterozygous for the sickle cell allele are generally healthy and they are resistant to malaria. There are three phenotypes associated with the sickle cell gene. An individual with both normal and sickle cell alleles has a different phenotype—resistance to malaria—from someone with only normal alleles. Sickle cell alleles are thought to be codominant. Copyright Pearson Prentice Hall Slide 17 of 43 End Show 14–1 Human Heredity From Gene to Molecule Malaria and the Sickle Cell Allele Regions where malaria is common Regions where the sickle cell allele is common Slide 18 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Sex-Linked Genes • The X chromosome and the Y chromosomes determine sex. • Genes located on these chromosomes are called sex-linked genes. • More than 100 sex-linked genetic disorders have now been mapped to the X chromosome. Slide 19 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity X Chromosome • The Y chromosome is much smaller than the X chromosome and appears to contain only a few genes. Duchenne muscular dystrophy • ONE X- chromosome in each female cell is randomly switched off to keep gene dosage correct Colorblindness – Called a Barr body. Melanoma X-inactivation center X-linked severe combined immunodeficiency (SCID) Hemophilia Y Chromosome Testis-determining factor Slide 20 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Why are sex-linked disorders more common in males than in females? Males have just one X chromosome. Thus, all Xlinked alleles are expressed in males, even if they are recessive. X-linked Disorder Major symptoms Colorblindness Inability to see some or all colors due to a lack of photoreceptors Hemophilia Blood clotting deficiency due to defective gene coding for clotting factors Duchenne Muscular Dystrophy Weakening and loss of muscle mass due to a defective gene that codes for muscle protein Slide 21 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity • Chromosomal Disorders • The most common error in meiosis occurs when homologous chromosomes fail to separate. • This is known as Homologous chromosomes fail to separate. Meiosis I: Nondisjunction Meiosis II nondisjunction, which means, “not coming apart.” Slide 22 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity – Down Syndrome • If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome. • Down syndrome involves three copies of chromosome 21. Slide 23 of 43 Copyright Pearson Prentice Hall End Show 14–1 Human Heredity Sex Chromosome Disorders Slide 24 of 43 Copyright Pearson Prentice Hall End Show