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More Wrinkles: Beyond Mendelian Genetics • In codominance, a heterozygote expresses the traits of both alleles. • Example: human blood type © 2013 Pearson Education, Inc. Beyond Dominant and Recessive Alleles Codominance In codominance, both alleles contribute to the phenotype. In certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens are speckled with both black and white feathers. The black and white colors do not blend to form a new color, but appear separately. More Wrinkles: Beyond Mendelian Genetics • Polygenic traits are determined by more than one gene. They tend to show more of a continuum than traits determined by a single gene. • Examples: human eye color, skin color, and height © 2013 Pearson Education, Inc. Beyond Dominant and Recessive Alleles Multiple Alleles Genes that are controlled by more than two alleles are said to have multiple alleles. An individual can’t have more than two alleles. However, more than two possible alleles can exist in a population. A rabbit's coat color is determined by a single gene that has at least four different alleles. Beyond Dominant and Recessive Alleles Different combinations of alleles result in the colors shown here. KEY C= full color; dominant to all other alleles cch = chinchilla; partial defect in pigmentation; dominant to ch and c alleles ch = Himalayan; color in certain parts of the body; dominant to c allele ch hCc ch hc chc h, or AIbino: Chinchilla: Himalayan: cc cCC, chcc, , cor , hor cchCc c Full color: ,cch Cc c = albino; no color; recessive to all other alleles Beyond Dominant and Recessive Alleles Polygenic Traits Traits controlled by two or more genes are said to be polygenic traits. Skin color in humans is a polygenic trait controlled by more than four different genes. LE 14-12 AaBbCc aabbcc 20/64 Fraction of progeny 15/64 6/64 1/64 Aabbcc AaBbcc AaBbCc AaBbCc AABbCc AABBCc AABBCC More Wrinkles: Beyond Mendelian Genetics • Pleiotropy occurs when a single gene affects more than one trait. • Example: sickle cell anemia in humans © 2013 Pearson Education, Inc. More Wrinkles: Beyond Mendelian Genetics • Linked genes are often inherited together. The closer two genes are to each other on a chromosome, the more likely they are to be inherited together. • Example: body color and wing size in fruit flies are linked © 2013 Pearson Education, Inc. More Wrinkles: Beyond Mendelian Genetics • Sex-linked traits are determined by genes found on the X chromosome. Men, who have only one X chromosome, need only one recessive allele to express a recessive sex-linked trait. These traits are more common in males than females. • Examples: red-green color-blindness, hemophilia © 2013 Pearson Education, Inc. The Human Genome • A genome is the total genetic material of an organism. • The Human Genome Project determined the DNA sequence of the entire human genome. • Over 99.9% of the 3.2 billion nucleotide pairs in the human genome are identical in all humans. © 2013 Pearson Education, Inc. The Human Genome • Humans have about 22,000 genes. • Many human genes give rise to RNA transcripts that are processed in different ways. So, one gene can provide the instructions for building multiple proteins. • The function of more than half of our genes is still unknown. © 2013 Pearson Education, Inc. DNA and Chromosomes DNA and Chromosomes In prokaryotic cells, DNA is located in the cytoplasm. Most prokaryotes have a single DNA molecule containing nearly all of the cell’s genetic information. Copyright Pearson Prentice Hall DNA and Chromosomes Chromosome E. Coli Bacterium Bases on the Chromosomes Copyright Pearson Prentice Hall DNA and Chromosomes Many eukaryotes have 1000 times the amount of DNA as prokaryotes. Eukaryotic DNA is located in the cell nucleus inside chromosomes. The number of chromosomes varies widely from one species to the next. Copyright Pearson Prentice Hall DNA and Chromosomes Chromosome Structure Eukaryotic chromosomes contain DNA and protein, tightly packed together to form chromatin. Chromatin consists of DNA tightly coiled around proteins called histones. DNA and histone molecules form nucleosomes. Nucleosomes pack together, forming a thick fiber. Copyright Pearson Prentice Hall DNA and Chromosomes Eukaryotic Chromosome Structure Chromosome Nucleosome DNA double helix Coils Supercoils Histones Copyright Pearson Prentice Hall DNA Replication DNA Replication Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. In most prokaryotes, DNA replication begins at a single point and continues in two directions. Copyright Pearson Prentice Hall DNA Replication In eukaryotic chromosomes, DNA replication occurs at hundreds of places. Replication proceeds in both directions until each chromosome is completely copied. The sites where separation and replication occur are called replication forks. Copyright Pearson Prentice Hall DNA Replication Duplicating DNA Before a cell divides, it duplicates its DNA in a process called replication. Replication ensures that each resulting cell will have a complete set of DNA. Copyright Pearson Prentice Hall DNA Replication During DNA replication, the DNA molecule separates into two strands, then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template for the new strand. Copyright Pearson Prentice Hall DNA Replication New Strand Original strand Nitrogen Bases Growth Growth Replication Fork Replication Fork DNA Polymerase Copyright Pearson Prentice Hall DNA Replication How Replication Occurs DNA replication is carried out by enzymes that “unzip” a molecule of DNA. Hydrogen bonds between base pairs are broken and the two strands of DNA unwind. Copyright Pearson Prentice Hall DNA Replication The principal enzyme involved in DNA replication is DNA polymerase. DNA polymerase joins individual nucleotides to produce a DNA molecule and then “proofreads” each new DNA strand. Copyright Pearson Prentice Hall The Human Genome • Single-nucleotide polymorphisms (SNPs) are locations in the genome where the nucleotide sequence differs among humans. • More than 3 million SNPs are known. • SNPs may help scientists identify genes related to human diseases. © 2013 Pearson Education, Inc. Cancer: Genes Gone Awry • Cancer occurs when cells in the body divide out of control. • Mutations in the genes that control cell division result in cancer. • A mutation in a single gene is not enough to cause cancer—mutations in many key genes are required. © 2013 Pearson Education, Inc. Mutations Mutations are changes in the genetic material. Copyright Pearson Prentice Hall Kinds of Mutations Kinds of Mutations Mutations that produce changes in a single gene are known as gene mutations. Mutations that produce changes in whole chromosomes are known as chromosomal mutations. Copyright Pearson Prentice Hall Kinds of Mutations Gene Mutations Gene mutations involving a change in one or a few nucleotides are known as point mutations because they occur at a single point in the DNA sequence. Point mutations include substitutions, insertions, and deletions. Copyright Pearson Prentice Hall Kinds of Mutations Substitutions usually affect no more than a single amino acid. Copyright Pearson Prentice Hall Kinds of Mutations The effects of insertions or deletions are more dramatic. The addition or deletion of a nucleotide causes a shift in the grouping of codons. Changes like these are called frameshift mutations. Copyright Pearson Prentice Hall Kinds of Mutations In an insertion, an extra base is inserted into a base sequence. Copyright Pearson Prentice Hall Kinds of Mutations In a deletion, the loss of a single base is deleted and the reading frame is shifted. Copyright Pearson Prentice Hall Kinds of Mutations Chromosomal Mutations Chromosomal mutations involve changes in the number or structure of chromosomes. Chromosomal mutations include deletions, duplications, inversions, and translocations. Copyright Pearson Prentice Hall Kinds of Mutations Deletions involve the loss of all or part of a chromosome. Copyright Pearson Prentice Hall Kinds of Mutations Duplications produce extra copies of parts of a chromosome. Copyright Pearson Prentice Hall Kinds of Mutations Inversions reverse the direction of parts of chromosomes. Copyright Pearson Prentice Hall Kinds of Mutations Translocations occurs when part of one chromosome breaks off and attaches to another. Copyright Pearson Prentice Hall Significance of Mutations Significance of Mutations Many mutations have little or no effect on gene expression. Some mutations are the cause of genetic disorders. Polyploidy is the condition in which an organism has extra sets of chromosomes. Copyright Pearson Prentice Hall Cancer: Genes Gone Awry • Over a lifetime, mutations build up until a combination of mutations in a single cell allows uncontrolled cell division. • Further mutations expand the tumor cells' ability to divide and spread. • Cancer is most likely to strike older people, those who have been exposed to mutationcausing agents, and those who have inherited mutations in cancer-related genes. © 2013 Pearson Education, Inc. Cancer: Genes Gone Awry • Genes that have been implicated in cancer: – Proto-oncogenes: When mutated, they become oncogenes that stimulate abnormal cell division. – Tumor-suppressor genes: They prevent cancer by inhibiting cell division. • Metastasis is the ability of tumor cells to spread around the body and give rise to secondary tumors. Cancer is much harder to treat once metastasis has occurred. © 2013 Pearson Education, Inc. Environmental Causes of Cancer • A person's environment is responsible for about 80%–90% of the mutations that result in cancer. • Environmental risk factors: – Smoking – Diet – Radiation – Ultraviolet light – Chemicals – Infection by certain viruses and bacteria © 2013 Pearson Education, Inc. Transgenic Organisms and Cloning • A transgenic organism is one that contains a gene from another species. • Typical process for developing transgenic bacteria © 2013 Pearson Education, Inc. Transgenic Organisms and Cloning • Examples of transgenic organisms: – Bacteria that produce insulin and other important products – Plants that • produce medicines • have resistance to pests, diseases, or herbicides • are drought-resistant or able to grow in salty soils – Animals that produce products: • Sheep with increased wool production • Pork with higher levels of omega-3 fatty acids • Salmon that grow faster © 2013 Pearson Education, Inc. Transgenic Organisms and Cloning • Cloning is the creation of an organism that is genetically identical to one that already exists. • In mammals, cloning is done through the process of nuclear transplantation. • Potential uses of cloning: – A routine part of agriculture – Could generate herds of identical animals with desirable traits – Cloning of endangered species could help increase their numbers – Cloning of deceased pets © 2013 Pearson Education, Inc. DNA Technology – What Could Possibly Go Wrong? • Some bacteria and viruses are a danger to human health or to natural habitats. – How likely is an accidental release? • Potential dangers of genetically modified (GM) plants and animals: – Is the safety of GM food adequately tested? – Should GM foods be labeled? © 2013 Pearson Education, Inc. DNA Technology – What Could Possibly Go Wrong? • Potential dangers of GM plants and animals (continued): – Plants that are toxic to pests also harm nontarget species --for example, Monarch butterflies – May lead to the evolution of resistant "superweeds" that can be controlled only with very toxic chemicals – Contamination of natural habitats or populations by transgenic plants and animals or their genes – Cost of GM seeds and products • Effects on human societies © 2013 Pearson Education, Inc. History of Science: Discovery of the Double Helix • By 1950, scientists knew DNA was the genetic material, but they did not know the structure of DNA. • In 1953, Watson and Crick built a model of DNA that was consistent with available evidence. • Watson and Crick used X-ray photos of DNA taken by Franklin and Wilkins as part of their research. © 2013 Pearson Education, Inc. Technology: Gene Therapy • Many genetic diseases occur when people do not have a working gene for making a key protein. • Gene therapy attempts to introduce DNA for the normal, working gene into a person's cells. • Some large setbacks have occurred in gene therapy, but there are some recent promising developments also. © 2013 Pearson Education, Inc. Science and Society: Genetic Counseling • A pedigree is a family tree that shows which relatives are and are not affected by a particular genetic disease. • Medical tests can determine whether a person is a carrier of a disease allele. • Amniocentesis and chorionic villus sampling can determine whether a fetus has a genetic disease. © 2013 Pearson Education, Inc. Science and Society: DNA Forensics • Forensic scientists use short tandem repeat (STR) analysis to determine whether DNA samples match. • Between 1989 and 2011, DNA evidence exonerated 272 people who were imprisoned for crimes they did not commit. • DNA forensics was used to identify the victims of the 2001 World Trade Center terrorist attacks. • DNA forensics can be used to establish paternity and trace familial relationships. • DNA forensics can be used to identify disease-causing microorganisms or endangered species. • Ethical concerns – DNA contains a wealth of private information about family relationships, susceptibility to diseases, and so on. © 2013 Pearson Education, Inc.