Post on 23-Dec-2015
transcript
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
Sex chromosomes= X and Y chromosomes These are physically different but able to synapse during meiosis
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
Karyotype
Karyotypes are pictures of paired human chromosomes
Used in identifying chromosome abnormalities
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
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
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
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
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
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
Xr Y
XR XR Xr XRY
XR XR Xr XRY
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
XR Y
XR XR XR XRY
Xr XR Xr XrY
Parental Generation (P1)
F1 generation
F2 generation
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
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
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
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
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
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
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
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
Changes in Chromosome Structure Deletion Inversion Translocation Duplication
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
Deletions- cont
Symptoms of cat-cry disorder– abnormally shaped larynx– Infant produces mewing sounds– Mental retardation
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
Translocation
Part of one chromosome exchanges places with the corresponding part of another nonhomologous chromosome
Ex; #8 and #14 translocate- form of cancer results
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
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
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
Polyploidy is lethal for humans
Chromosome number can change during– mitosis– meiosis– fertilization
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
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
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
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
Changes in Sex Chromosome Number XYY Symptoms- taller, may be mildly
retarded, basically phenotypically normal
Once thought to be genetically predisposed to be criminals