1 / 39

Chapter 12- Human Genetics

Chapter 12- Human Genetics. Homologous Chromosomes. Autosomes- All other chromosomes except the X and Y chromosome Generally speaking, homologous chromosomes are alike in length, shape and gene sequence

leona
Download Presentation

Chapter 12- Human Genetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Chapter 12- Human Genetics

  2. Homologous Chromosomes • Autosomes- All other chromosomes except the X and Y chromosome • Generally speaking, homologous chromosomes are alike in length, shape and gene sequence • Sex chromosomes= X and Y chromosomes These are physically different but able to synapse during meiosis

  3. Karyotype • Karyotype= number of metaphase chromosomes and their defining characteristics • Cells are cultured in vitro (in glass) • Colchicine is added- blocks formation of microtubule spindles- arrests cells in metaphase

  4. Karyotype • Karyotypes are pictures of paired human chromosomes • Used in identifying chromosome abnormalities

  5. Sex Determination in Humans • XX= female • XY= male • X chromosome carries over 300 genes • Y chromosome carries “male-determining gene” . Expression of this gene leads to formationof testes. Absence of this gene- ovaries automatically form

  6. SRY Gene • SRY= Sex-determining region of the Y chromosome • First 4 weeks of its existence, human embryo is neither male or female • SRY gene regulates many proteins that regulate reactions that are necessary for sex determination

  7. Early Questions About Gene Locations • Sutton- Chromosome Theory of Inheritance= Chromosomes are the units of heredity • Morgan- genes have specific locations on specific chromosomes • Morgan performed experiments using Drosophila melanogaster • Found evidence of the gene for eye color on the X chromosome

  8. Characteristics of Drosophila melanogaster • 1. Easy to maintain and breed • 2. About 0.1 inch long- so tiny that hundreds can be kept in a jar • 3. Have a reproductive cycle of 10-15 days, therefore they can produce many generations of offspring in a matter of weeks

  9. Morgan’s Experiment • In fruit flies- alleles for red eyes (R) are dominant over alleles for white eyes (r). • Morgan hypothesized that the allele for eye color is carried on the X chromosome • P1 generation – Cross a white-eyed male and a homozygous red-eyed female

  10. Sex-linked Traits • Example: red eye color is a dominant trait carried on the X chromosomes in fruit flies; white is recessive • Cross red eyed female with a white eyed male • Get: 2 heterozygous red eyed females and 2 red eyed males

  11. Sex-linked Traits • Now cross the F1: red eyed male with heterozygous red eyed female • You get: • 1 homozygous red eyed female • 1 heterozygous red eyed female • 1 red eyed male • 1 white eyed male

  12. Parental Generation (P1) • F1 generation • F2 generation

  13. Conclusion • The gene for eye color in Drosophila must be carried on the X chromosome • Females have two X chromosomes • Males have only one X chromosome so whatever allele in on the X chromosome for eye color is expressed

  14. Morgan concluded that genes for certain traits are carried on the X chromosomes and that chromosomes and their genes segregate during meiosis • The discovery of sex-linked traits explained why some characteristics caused by recessive genes are appear far more often in males since males have only one X chromosome and females have two X chromosomes

  15. Sex- Linked Trait • 1. Sex- linked trait = a trait that is determined by alleles carried only on an X chromosome • 2.In humans sex linked traits are found on the X chromosome which is much larger than the Y chromosome • 3.In most organisms: • Males are XY • Females are XX

  16. Inheritance of human traits • Sex-linked Traits - determined by X and Y chromosomes (sex chromosomes) • Recessive traits rarely occur in females because they have two X chromosomes, the dominant gene on one X can mask the recessive gene on the second X • Recessive traits in males are expressed because they only have one X

  17. Color vision the gene for color vision is located on the X the recessive gene results in colorblindness (inability to see certain colors) Genotypes: XC XC, XC Xc, Xc Xc, XCY, and XcY Hemophilia a protein needed for normal blood clotting is located on the X the recessive gene results in hemophilia individuals with this disease can bleed to death from a tiny cut - their blood does not clot normally Inheritance of human traits - examples of sex-linked traits

  18. Inheritance of human traits • Sex-Influenced Traits - traits that are generally associated with one sex but is produced by genes carried on autosomes • Example: Baldness • the allele coding for baldness HB is dominant in males and recessive in females • The allele that coding for normal hair HN is dominant in females and recessive in males • HN HN - most likely keep hair male and female • HN HB - male will lose hair, female will keep hair • HB HB - most likely lose hair male and female

  19. Linkage Groups • Linkage groups- the genes located on each type of chromosome • Drosophila has 4 linkage groups • Humans have 23 linkage groups • Crossing over happens among linkage groups

  20. Probability of a crossover is proportional to the distance between genes • The further apart two genes are, the more likely they will be to cross over • Use patterns of crossing over to map genes on the chromosome

  21. Changes in Chromosome Structure • Deletion • Inversion • Translocation • Duplication

  22. Deletions • Loss of a chromosome region • May be loss of one or more genes-almost always causes problems • Caused by irradiaiton, viral attck, chemical action, & other environmetnal factors • Ex: Cat-cry disorder- deletion form chromosome 5

  23. Deletions- cont • Symptoms of cat-cry disorder • abnormally shaped larynx • Infant produces mewing sounds • Mental retardation

  24. Inversion • Segment of DNA detached from chromosome and reattaches in the reverse order • Reversal alters the position and order of the chromosome’s genes • Affects the way the base sequence is read and translated

  25. Translocation • Part of one chromosome exchanges places with the corresponding part of another nonhomologous chromosome • Ex; #8 and #14 translocate- form of cancer results

  26. Duplication • Gene sequences are repeated several to many times • Contained even on normal chromosomes- some DNA duplications are built into the species • EX: hemoglobin in humans and primates- have multiple copies of similar gene sequences

  27. Duplication cont. • 18 of 23 pairs of human chromosomes are nearly identical to corresponding chromosome in chimpanzees and gorillas • Ex: fragile X syndrome-abnormally constricted region on X chromosome

  28. Changes in Chromosome Number • Aneuploidy= one more or one less chromosome • Causes many miscarriages • Polyploidy= inheritance of three or more of each type of chromosome • Ex: 1/2 flowering plants, some insects, fishes & other animals

  29. Polyploidy is lethal for humans • Chromosome number can change during • mitosis • meiosis • fertilization

  30. Changes in Chromosome # cont. • Nondisjunction= one or more pairs of chromosomes fails to separate properly during mitosis or meiosis • Results in: • Trisomy= 3 of one type of chromosome • Monosomy= 1 of one type of chromosome

  31. Changes in Autosome Number • Down’s syndrome= trisomy 21 • Syndrome means a set of symptoms that characterize a disorder • Symptoms- mental impairment, abnormal skeletal development, some have heart defects

  32. Changes in the Number of Sex Chromosomes • Turner’s syndrome= XO • 75% from nondisjunction in dad • 98% of XO zygotes spontaneously abort • Symptoms- short, infertile, no functional ovaries • Some benefit form hormone therapy and corrective surgery

  33. Changes in Sex Chromosome # • Klinefelter Syndrome= XXY • 67% from nondisjunction in mom • Symptoms develop after puberty- taller, sterile or low fertility, smaller testes, sparse facial hair, breast enlargement • Testosterone injections can minimize feminized traits

  34. Changes in Sex Chromosome Number • XYY • Symptoms- taller, may be mildly retarded, basically phenotypically normal • Once thought to be genetically predisposed to be criminals

More Related