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Chapter 12 Inheritance Patterns and Human Genetics

Chapter 12 Inheritance Patterns and Human Genetics. 12-1 Chromosomes and Inheritance. Remember…. DNA in chromosomes has instructions for protein synthesis Chromosomes are transmitted from one generation to the next. Sex Determination.

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Chapter 12 Inheritance Patterns and Human Genetics

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  1. Chapter 12Inheritance Patterns and Human Genetics

  2. 12-1 Chromosomes and Inheritance

  3. Remember… • DNA in chromosomes has instructions for protein synthesis • Chromosomes are transmitted from one generation to the next

  4. Sex Determination • 1900s: Thomas Hunt Morgan bred Drosophila melanogaster (fruit flies) • Determined: • Females: XX • Males: XY • “Y” chromosome is smaller and hook-shaped • Autosome: all other chromosomes

  5. Sex chromosomes segregate (form pairs) during meiosis • SO! Each sex cell either has an “X” or a “Y” • Male gametes: can contain “X” OR “Y” • Female gametes: ONLY “X” Why????

  6. Sex Linkage • Morgan also hypothesized that more genes are carried by X chromosome than the Y • X-linked: genes carried on the X chromosome • Y-linked: genes found on the Y chromosome • Sex linkage: presence of gene on sex chromosome

  7. Sex Linked in Drosophila • Normal: Red eyes • Mutation: WHITE eyes • P1 x P1  F1 • Red eye female x White eye male ALL RED EYES • F1 x F1  F2 • Red female x red male  3(red):1(white) • HOWEVER! All white eyes on MALES ONLY

  8. No White Eyed Females • Trait for eye color is carried on X • Red eye female: XR XR • White eye male: Xr Y

  9. Linkage Group • Thousands more genes than chromosomes  many genes on same chromosome • Linkage group: multiple genes on same chromosome • Linked genes tend to be inherited together Why?

  10. Linked Groups in Drosophila • Morgan looked at fly body color and wing size • Gray body, Long wing x Black body, Short wing • BBLL x bbll • If on separate chromosomes: • 9:3:3:1 • If on the same chromosome: • 3:1

  11. So, what actually happened? • Results: • several Gray, long (Ggll) • Several black, short (bbLl) • HOW????

  12. Crossing Over • Exchange of pieces of DNA between homologous chromosomes • Accounts for unexpected phenotypes in the F2 generation of Morgan’s experiment

  13. Chromosome Mapping • Likelihood of genes crossing over depends on distance from each other on chromosome • Farther apart the genes, the more likely that they will cross • If more offspring show the new combination of traits, the farther the genes are on a chromosome

  14. Scientist conduct breeding experiments to determine how frequently genes separate from one another • THUS! They measure distance of genes on chromosomes • THUS! They prepare chromosome maps: diagram used to show linear sequence of gene on chromosome

  15. Alfred H. Sturtevant • Morgan’s student • Used crossing over data to construct chromosome map of Drosophila • Genes that are separated by crossing over 1% of the time are 1 map unitapart

  16. Mutation • MANY Different types: • Chromosomal: Whole chromosome • Genetic: Single nucleotide • Germ-cell mutations: in gametes • Do NOT affect organism • Affect organism’s offspring • Somatic mutations: in body cells • affect organism (skin cancer, leukemia) • NOT passed on to offspring

  17. Chromosome Mutations • Deletion: loss of a piece of chromosome due to chromosomal breakage • All info of that chromosome lost • Inversion: piece of chromosome broken off and reattached upside-down • Translocation: piece of chromosome breaks off and attaches to another, nonhomologous chromosome

  18. Nondisjunction: failure of chromosome to separate from homologue during meiosis • Result: one gamete receives EXTRA COPY of a chromosome

  19. Gene Mutations • Point mutation: single based mutated • Substitution, deletion, insertion • Substitutions: single nucleotide replaced with a different nucleotide ( ATC TTC) * REMEMBER: DNA mRNA  amino acid

  20. Results: 1- base codes for same A.A.  NO CHANGE 2- base codes for different A.A.  Protein change 3- base code for STOP codon  Protein change

  21. Sickle Cell Anemia • Point substitution mutation • Substitutes adenine for thymine • Protein changed: hemoglobin • Result: irregular shaped red blood cells • Circulatory problems: heart, brain, lungs and many other organs and tissues damaged • Widespread in African Americans ( 1 of 500 in US)

  22. In US 1 of every 10 African Americans are heterozygous for sickle cell anemia • Heterozygous: produced both normal and mutated forms of hemoglobin • Usually healthy

  23. Frame Shift Mutations • Deletion: one or more nucleotide deleted from sequence • Insertion: one or more nucleotides inserted • Frame shift mutation: one or more nucleotide deleted or inserted

  24. MUCH more serious effects • Entire rest of sequence mutated • More DNA mutated if deletion/insertion occurs closer to beginning of sequence • More amino acids inaccurately coded for

  25. 12-2 Human Genetics

  26. Quick Look: • 23 pairs of chromosomes • 100,000 genes • Scientists focus on disease causing genes • Easily traced from one generation to the next

  27. Pedigree Analysis • Pedigree: a family record that shows how a trait is inherited over several generations • Study phenotypes of family to see how traits are inherited

  28. Patterns of Inheritance • Certain phenotypes usually repeat in predictable patterns • Carriers: individuals who have one recessive autosomal allele

  29. Genetic Disorders • Diseases or condition that has a genetic basis

  30. Single-allele Traits • Controlled by single allele of a gene • 200+

  31. Huntington’s Disease (HD) *“Autosomal-dominant pattern of inheritance” • Caused by dominant allele on autosome • Symptoms: mild forgetfulness; irritability • Results: loss of muscle control; uncontrollable physical spasms; severe mental illness; death • On-set: 40’s or later *Usually unknown until after individuals have children *THUS! Usually passed on without knowing

  32. Genetic Marker • Short section of DNA that is known to have close association with particular gene • If marker is present, USUALLY gene is present • Sampling can show presence of gene (disorder) • Marker present: 96% chance of HD

  33. Homozygous Recessive Single-allele Traits • Expressed ONLY in homozygous state (aa) • Show autosomal-recessive patt. of inherit. • 250+ • Ex: Cystic fibrosis (CF) and sickle cell anemia

  34. Multiple-allele Traits • Controlled by three or more alleles of the same gene that code for a single trait • Ex: ABO blood groups • Three alleles: IA, IB, i • Codominant: IA, IB

  35. Polygenic Traits • Traits controlled by two or more genes • Show many degrees of variation • Skin color; 3-6 genes • Eye color; • Light blue: very little melanin • Dark brown: a lot of melanin • Many influenced by environment • Height: also influenced by nutrition and disease

  36. X-Linked Traits • Not all are diseases • Ex: Colorblindness, Hemophilia

  37. Sex-Influenced Traits • When presence of female or male hormones influence expression of certain human traits • Ex: Pattern Baldness • Male and female: BB  lose hair • Male: BB’  lose hair due to high testosterone • Female: B’  does NOT lose hair • Genes that code for trait on autosomes NOT gametes!

  38. Due to Nondisjunction • Lack a chromosome OR have an extra • Monosomy: only one copy of a chromosome • 45 instead of 46 • Trisomy: an extra copy of a chromosome • 47 instead of 46

  39. Down Syndrome • AKA Trisomy-21 • Extra chromo. 21 • Severe mental retardation, characteristic facial features, muscle weakness, heart defects, short stature

  40. Nondisjunction in Sex Chromosome • Klinefelters syndrome: XXY • Female appearance; mental retardation; infertile • Turner’s syndrome: XO • Female; do not mature sexually; infertile • Just Y CANNOT survive • X contains info vital to survival

  41. Detecting Human Genetic Disorders • Family history of genetic disorders  Genetic screening before becoming a parent • Examination of genetic makeup • Make karyotype • Test blood for certain proteins • Genetic counseling:medical guidance to inform parents about problems that could affect offspring

  42. Amniocentesis • Physician removes small amount of amniotic fluid from amnion (sac that surrounds fetus) • Fetal cells and proteins analyzed • Karyotype prepared

  43. Chronic villi Sampling • Physician takes sample of chronic villi (tissue that grow between mother’s uterus and placenta) • Produce karyotype • Has same genetic make up as fetus

  44. Sample Taken After Birth • Immediately after birth genetic testing done of fetus • Ex: PKU (phenylketonuria) • Disorder when infant cannot metabolize the amino acid phenylalanine • Excess phenylalanine can cause brain damage • Infants put on diet without phenylalanine

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