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Chapter 12

Chapter 12. Inheritance Patterns and Human Genetics. Section 1 Chromosomes and Inheritance: Objectives. Distinguish between sex chromosomes and autosomes Explain the role of sex chromosomes in sex determination Describe how an X- or Y-linked gene affects the inheritance of traits

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Chapter 12

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  1. Chapter 12 Inheritance Patterns and Human Genetics

  2. Section 1 Chromosomes and Inheritance: Objectives • Distinguish between sex chromosomes and autosomes • Explain the role of sex chromosomes in sex determination • Describe how an X- or Y-linked gene affects the inheritance of traits • Explain the effect of crossing-over on the inheritance of genes in linkage groups • Distinguish between chromosome mutations and gene mutations

  3. Sex Chromosomes and Autosomes • Recall that genes reside on chromosomes • Sex chromosomes contain genes that determine an organism’s sex • The remaining chromosomes that are not directly involved in determining the sex of an individual are called autosomes

  4. Male and Female Karyotypes

  5. Sex Determination • In mammals, an individual carrying X chromosomes (XX) is female • An individual carrying an X and a Y chromosome (XY) is male • Sex of an offspring is determined by the male

  6. Genetic Research – Early Work • Thomas Hunt Morgan (early 1900s) • Did research with the fruit fly, Drosophila melanogaster • Crossed a white-eyed (recessive) male with a normal red-eyes female • The F1 generation all had red eyes, but when crossed together, the ratio became 3 red-eyed to 1 white-eyed • This shows evidence of crossing-over, but it doesn’t show until the F2 generation (exactly like Mendel’s pea plant pp 175)

  7. Effects of Gene Location • Sex-linked Genes and Traits • Genes found on the X chromosome are X-linked genes • A sex-linked trait is a trait whose allele is located on a sex chromosome • Because males only have one X chromosome, a male who carries a recessive allele on the X chromosome will exhibit the sex-linked trait • Sex-linked characteristics occur mostly in males • EX: colorblindness

  8. Effects of Gene Location • Linked Genes • Pairs of genes that tend to be inherited together are called linked genes • EX: in fruit flies, eye color and wing length are inherited together

  9. Chromosome Mapping • The farther apart two genes are located on a chromosome, the more likely a cross-over will occur • Researchers use ‘recombinant percentages’ to construct chromosome maps showing relative gene positions • Chromosome maps: • Depict the linear sequence of genes on a chromosome • Are constructed using crossing-over data from mating experiments • Are practical with species with only a few chromosomes

  10. Mutations • Effects of a mutation can be helpful, harmful, and/or neutral • Germ-cell mutations occur in gametes and can be passed on to offspring • Somatic-cell mutations occur in body cells and affect only the individual organism • EX: certain skin cancers and leukemia result from somatic-cell mutations. They cannot be passed on to offspring. • Lethal mutations cause death, often before birth

  11. Mutations • Chromosomal mutations are changes in the structure of a chromosome or the loss or gain of an entire chromosome Deletion: loss of a piece of a chromosome due to breakage Inversion: chromosomal segments break off, flips around backward, and reattaches Translocation: piece of one chromosome breaks off and reattaches to a nonhomologous chromosome

  12. Mutations • Nondisjunction occurs when a chromosome fails to separate from its homologue during meiosis. One gamete receives an extra copy of a chromosome and another gamete receives no copies. • This occurs in Down syndrome: a person had 3 copies of chromosome #21 for a total of 47 chromosomes instead of 46 http://img.dailymail.co.uk/i/pix/2007/07_02/gardinerDM1907_468x380.jpg

  13. Mutations • Gene mutations are changes in one or more of the nucleotides in a gene

  14. Homework Section 12.1 Review Pp 240 #1-9

  15. Section 2 Human Genetics: Objectives • Analyze pedigrees to determine how genetic traits and genetic disorders are inherited • Summarize the different patterns of inheritance seen in genetic traits and genetic disorders • Explain the inheritance of ABO blood groups • Compare sex-linked traits with sex-influenced traits • Explain how geneticists can detect and treat genetic disorders

  16. Inheritance of Traits – Pedigrees • Geneticists use pedigrees to trace diseases or traits through families • Pedigrees are diagrams that reveal inheritance patterns of genes Pedigree for Cystic Fibrosis

  17. Genetic Traits and Disorders • A genetic disorder is a disease or disabling condition that has a genetic basis

  18. Genetic Traits and Disorders • Single genes having 2 or more alleles can determine traits, such as blood type or cystic fibrosis • Polygenic Inheritance • Geneticists have learned that most human characteristics are polygenic (they are influenced by several genes) • EX: skin color results from additive effect of 3-6 genes • EX: eye color, height, hair color

  19. Genetic Traits and Disorders • Complex Characters, such as polygenic traits, are influenced by both genes and environment • EX: skin color is influenced by genes and if a person lives in an area with a lot of sun, their skin will naturally by darker • EX: height is influenced by genes and if the person has good nutrition and no disease they should be taller than someone with poor nutrition and disease

  20. Genetic Traits and Disorders • Multiple Allele characters, such as ABO blood groups, are controlled by three or more alleles of a gene • IA, IB, i

  21. Comparing Single Allele, Multiple Allele, and Polygenic Traits Single Allele: Widow’s Peak (W or w) Multiple Allele: Duck Feather Patterns M or MR or md Polygenic Trait: Eye Color A or a -AND- B or b

  22. Comparing Complete, Incomplete, and Co-Dominance • Complete Dominance • Only one trait is seen • Incomplete Dominance • Traits are mixed creating an intermediate • Co-Dominance • Both traits are seen

  23. Genetic Traits and Disorders • X-linked Traits • The gene for colorblindness, an x-linked recessive gene, is found on the X chromosome • Sex-Influenced Traits • A sex-influenced trait, such as pattern baldness, is autosomal and expressed differently in men than in women even if both sexes have the same genotype. • For pattern baldness, the difference is that men have higher levels of testosterone which interacts with the genotype

  24. Sex-Influenced Trait: Goat Beards

  25. Detecting Genetic Disease • Genetic screening examines a person’s genetic makeup and potential risks of passing disorders to offspring • Amniocentesis and chorionic villi sampling help physicians test a fetus for the presence of genetic disorders

  26. Detecting Genetic Disease • Genetic counseling informs screened individuals about problems that might affect their offspring • The process uses a family pedigree to look at family history of disease and can assist parents in deciding whether or not to have children

  27. Treating Genetic Disease • Genetic disorders can be treated in various ways • Treatments include: • Symptom-relieving treatments • Symptom-prevention measures (such as insulin injections for diabetes)

  28. Treating Genetic Disease • In gene therapy, a defective gene is replaced with a copy of a healthy gene • Somatic cell gene therapy alters only body cells • Germ cell gene therapy attempts to alter eggs or sperm

  29. Homework Section 12.2 Review Pp 248 #1-8

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