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 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. Inheritance of human traits - examples of sex-linked traits
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

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