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Chromosomes and Human Genetics

Chromosomes and Human Genetics. Starr/Taggart’s Biology: The Unity and Diversity of Life , 9e Chapter 12 Notes edited by: R. LeBlanc, MS (Revised: 1/2012). Spectral Karyotyping. What happens to cell growth and division if gene(s) malfunction?

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Chromosomes and Human Genetics

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  1. Chromosomes and Human Genetics Starr/Taggart’s Biology: The Unity and Diversity of Life, 9e Chapter 12 Notes edited by: R. LeBlanc, MS (Revised: 1/2012)

  2. SpectralKaryotyping • What happens to cell growth and division if gene(s) malfunction? • Orderly cell growth and division are disrupted. • What is the Philadelphia Chromosome? • Chromosome #9; the 1st abnormal chromosome responsible for cancer. • What happen to chromosome #9? • Reciprocal translocation between #9 & #22 causing leukemia, a type of cancer. • How does this effect genes on #9? • The gene at the end of #9 is altered by the added gene from #22 causing an abnormal protein (uncontrolled division of white blood cells). • What is leukemia? (Acute or chronic) • Abnormal stem cells (Leukemias) over-produce white blood cells and push-out RBC’s and platelets. • What are the function of stem cells? • Cells that are unspecialized and others become specialized cells like RBC’s and platelets. • What are white blood cells and what are their function? Platelets? • Part of the bodies defense system. • Used to help blood clotting. 1 2 3 4 5 11 12 6 7 8 9 10 18 13 14 15 16 17 19 20 21 22 Y X

  3. Key Concepts: • Each gene has its own position in sequence on a chromosome • Crossing over allows alleles in sequence to swap places • Allele recombination contributes to variations in phenotypes • The structure of chromosomes may change • Changes in chromosome structure can give rise to genetic abnormalities or disorders

  4. Genes and Their Location • Genes are units of information about heritable traits • Each gene has its own location--a gene locus--on a particular chromosome • Alleles are different molecular forms of a gene • Wild-type is most common form of allele. • Any less common form is mutant allele

  5. How many human genes are there? • Of the nearly 24,000 human genes found in human DNA, more than 4,000 have been patented by private firms and universities, a new study finds.

  6. Types of Chromosomes • Autosomes • Same in both sexes • Human somatic cells have 22 pairs of autosomes • Sex Chromosomes • Chromosomes that determine an individual’s sex (23rd pair) • Humans have 1 pair of sex chromosomes. • Karyotype Analysis (analyze the physical appearance of chromosomes)

  7. Obtain a sample of cells; ex. Red blood Stimulate mitosis with chemical & incubate. Add COLCHICINE to arrest mitosis at metaphase. Draw off medium, add saline solution, and then a fixative. Centrifuge machine Take picture of chromosomes; cut and arrange. Stain cells Human Karyotyping of somatic cells with 22 pairs of autosomes and 1 pair of sex chromosomes. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 XX (or XY)

  8. X X x X Y x Y X X X XX XX XY XY diploid germ cells in female diploid germ cells in male Sex Determination in Humans eggs sperm Meiosis, gamete formation in both female and male: Fertilization: sex chromosome combinations possible in new individual XY XX Fig. 12.4, p. 196

  9. Characteristics of the Y Chromosome • Y Chromosome • The Y chromosome spans about 50 million base pairs (1.5 – 2% of total DNA). • The Y chromosome likely contains between 70 and 300 genes. • Involved in male sexual determination and development. • 9 "disease genes" found on chromosome Y

  10. Characteristics of the X Chromosome • The X chromosome spans about 155 million base pairs (about 5 percent of the total DNA in cells) • The X chromosome likely contains between 900 and 1,200 genes. • 304 "disease genes" are found on chromosome X

  11. Human Embryo and Sex Organ Development • XY Embryo • 8 Weeks old • Gene on Y chromosome governs development of testes (SRY: Sex-determining Region on the Y Chromosome). umbilical cord(lifeline between the embryo and the mother’s tissues) amnion (a protective, fluid- filled sac surrounding and cushioning the embryo)

  12. X-Linked Genes as Clues to Inheritance Patterns. homozygous dominant female recessive male X X X X X X Y Y 1/4 1/4 1/4 1/4 Gametes: • 1900’s Thomas Morgan • Link between sex determination & some non-sexual traits. (ex. Hemophilia; more in males) • White eyed flies appeared genetic mutations. • Recessive sex-linked trait or X-Linked) x All F1 offspring have red eyes Gametes: x 1/2 1/2 1/2 1/2 • Why were there no white eyed flies in the 1st generation? • Using a punnett square show the cross of the 2 parents & ratio of F1 offspring. • Ratio of F2 offspring? F2 generation:

  13. Comparing X-Linked and Sex-Influenced Traits

  14. A a A A A B B B B B C C C C C a a A a a b b b b b c c c c c • Crossing Over and Recombination • One pair of homologous chromosomes in a duplicated state (each has 2 sister chromatids) • 1 Blue & 1 Purple each with 3 different genes • During prophase of meiosis 2 non-sister chromatids exchange segments. • This represents 1 crossover event. • This is the outcome of the crossover. • Genetic recombination between non-sister chromatids.

  15. Chromosomes that can be distinguished physically with a special feature are called cytological markers. Creighton & McClintock’s Experiment Wx Wx Wx wx Wx wx wx Wx wx wx C c c C C c c c C C normal chromosome 9 abnormal chromosome 9 (part of another chromosome attached to it An abnormal event caused a piece of a different chromosome to become attached. C = colored seeds; c = colorless seeds Wx = 2 forms of starch; wx = 1 form of starch. crossing over During meiosis crossing over sometimes occurs. Notice that the mutant chromosome is now associated with c. recombinant chromosomes Fig. 12.8, p. 199

  16. Recombination Patterns and Chromosome Mapping • Experimental crosses with Drosophila (fruit flies) • Crossing over disrupts linkage groups (not rare) Linkage: tendency of genes located on the same chromosome to be transmitted together in heritance. Proved: Certain alleles tend to remain together during meiosis more often than others because they are positioned closer together on the same chromosome.

  17. Linkage Mapping • Drosophila melonogaster Chromosomes X Chromosome What is the percent frequency between complete wings & long wings? 36.1 – 20 = 16.1% (chance of a recombination) 1 Map Unit = 1% frequency of a crossover Probability that crossing over will disruptlinkage genes is proportional to the distance that separates the 2 loci.

  18. Probability of a Crossover A B C D Suppose genes A and B are twice as far apart as two other genes, C and D. What would be the probability that genes A and B would cross over as compared to genes C and D? The probability that crossover will disrupt their linkage is Proportional to the distance that separates the 2 loci; So… AB are TWICE as likely to be disrupted by a crossover vs CD which are HALF as likely to.

  19. Human Genetic Analysis • Pedigrees • Genetic corrections • Polydactyly

  20. Regarding Human Genetic Disorders • Genetic abnormality (a rare uncommon version of a trait, Polydactyly; not life threatening) • Genetic disorder (inherited condition that sooner or later will cause mild to severe medical conditions. • Genetic disease (When workable genes get altered in away that disrupts body functions) • Genetic Examples of Disorders and Abnormalities: • Autosomal recessive/dominant inheritance • X-Linked dominant/recessive inheritance • Changes in chromosome number • Changes in chromosome structure Disease: an illness caused by infectious, dietary, or environmental factors; not by inherited traits.

  21. Genetic Disorder

  22. Some Important Genetic Disorders

  23. Patterns of Autosomal Inheritance • Autosomal Recessive Inheritance • Galactosemia (homozygous recessive) • lacking enzyme to digest lactose • Tay-Sachs • Jewish descent metabolic disorder • Gene #15 mutated; enzyme hexa produced (dissolves nerve and brain cells fatty acids) • Harmful quantities of fatty acid are deposited in the nerve cells of the brain. • Die by age 5 • EXTRA CREDIT (10 PTS): Review the the movie Lorenzo’s Oil and do a report (get form online)

  24. Galactosemia enzyme 1 enzyme 2 enzyme 3 GALACTOSE-1- PHOSOPHATE GALACTOSE-1- PHOSOPHATE LACTOSE GALACTOSE +glucose intermediate in glycolysis • 1 in 100,000 chance of getting this inheritance. • Enzyme 3 is not produced, so high levels of galactose start to accumulate. • Galactose can be detected in urine. • Damages: eyes, liver, and brain • Untreated = death • Treatment: restrict dairy products.

  25. Patterns of Autosomal Inheritance • Autosomal Dominant Inheritance • Progeria (rapid aging) • Huntington disorder • Progressive involuntary movements • Deterioration of the nervous system • Can cause death • Usually noticeable after age 40 causing parents to pass this on without their knowledge

  26. Progeria – Too Young to be Old Fig. 12.16, p. 206

  27. Patterns of X - Linked Inheritance • X-Linked Recessive Inheritance • Hemophilia • 1 in 7000 males • Genes do not code for clotting agent • Duchenne Muscular Dystrophy • X-Linked Dominant Inheritance • Faulty Enamel

  28. Fragile X Syndrome (X-Linked recessive disorder) • Causes a mutant gene • Within this gene a segment of DNA is repeated several times; What kind of mutation is this: • known as Expansion mutation • Can you find the Fragile Site???? Fig. 12.15, p. 205

  29. Inversion Translocation Chromosome Aberrations The physical structure of chromosomes can change resulting in a genetic disorder or abnormality. These can occur spontaneously. Philadelphia Chromosome

  30. Deletion Duplication Chromosome Aberrations • Triggered by: • Viral attacks • Irradiation (ionization radiation) • Chemical assaults or other environmental factors. Note: happens for chromosomes that code for polypeptides of hemoglobin and is not harmful.

  31. Changes in Chromosome Number • Aneuploidy - One extra or one less chromosome • Polyploidy - Three or more of each chromosome • Nondisjunction causes changes in chromosome number (failure to separate during meiosis or mitosis)

  32. Changes in the Number of Autosomes • Down Syndrome - Trisomy 21 • 1 in 1100 in North America • Show mental retardation • 40% have heart defects

  33. Changes in the Number of Sex Chromosomes • Turner Syndrome X0 • One X chromosome; 1 out of 2500 to 10,000 newborn girls. • Non-disjunction in sperm accounts for 75% of cases. • 98% abort early in pregnancy. • Klinefelter’s Syndrome XXY • 1 out of 500-2000 newborns • Non-disjunction; 67% in moms and 33% in fathers • Most infertile • XYY Condition • 1 out of 1000 males • Tall; mildly retarded; most phenotypically normal

  34. Changes in Chromosome Structure • Categories of chromosome aberration • Deletion • Duplication • Inversion • Translocation

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