Chapter 12- Human Genetics

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

8. 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

9. 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

13. 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

14. 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

15. 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

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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

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

27. 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

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

29. 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

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

31. 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

32. 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

33. 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

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

35. 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

36. 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

37. 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

38. 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

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