Transcript File

Ch. 15: Chromosomes and Genetics
•
•
•
•
1860’s: Mendel’s genetic experiments
1875: Process of mitosis discovered by cytologists
1890: Process of meiosis discovered by cytologists
1900: Three botanists independently rediscovered
Mendel’s principles of segregation and
independent assortment
• 1902: Cytology and genetics converge as scientists
notice parallels between the behavior of Mendel’s
factors and the behavior of chromosomes.
chromosomal theory of inheritance
 Mendelian factors or genes are located on
chromosomes which segregate and
independently assort.
Thomas Hunt Morgan
Columbia University early 1900’s
selected the fruit fly, Drosophila melanogaster
 easily cultured, short generation time
• Fruit flies have three pairs of autosomes and
one pair of sex chromosomes.
• Wild type – normal or most frequently observed phenotype
• Mutant phenotype – phenotypes due to mutations in the
wild-type gene
 A gene’s symbol is based on the first mutant ( non-wild)
If the mutant is recessive, the first letter is lower case
(e.g. w = white eye allele in Drosophila)
If the mutant is dominant, the first letter is capitalized
 Wild type traits are designated by a superscript +.
Linked Genes
Genes found on the same chromosome
Using recombination frequencies, distances
between linked genes can be determined
Recombination frequencies of 50% indicate unlinked
genes or linked genes located so far apart that crossing
over makes them appear unlinked
X-Y mechanism that determines sex at
fertilization.
 When a sperm cell + ovum = zygote ( XX or
XY)
 A single gene, Sry, on the Y chromosome
leads to normal male development. Sry
probably codes for a protein that regulates
other genes.
Fathers pass X-linked alleles to only and all of their daughters.
 Males receive their X chromosome only from their mothers.
Therefore, fathers cannot pass sex-linked traits to their sons.
Mothers can pass sex-linked alleles to both sons and daughters.
 Females receive two X chromosomes, one from each
parent. Mothers pass on one X chromosome (either
maternal or paternal) to each daughter and son
Some examples of sex-linked traits in humans are color
blindness, muscular dystrophy, and hemophilia.
Sex-linked disorders in humans:
 The human X-chromosome is much larger than the Y.
Thus, there are more X-linked than Y-linked traits.
 Most X-linked genes have no homologous loci on the Y
chromosome.
 Most genes on the Y chromosome not only have no Xcounterparts, but they encode traits found only in
males
(e.g. testis-determining factor).
Nondisjunction – meiotic or mitotic error during
which certain homologous chromosomes or
sister chromatids fail to separate.
• Aneuploidy – a condition where there is an
abnormal number of certain chromosomes.
 An example of this is Down’s syndrome,
which results from trisomy of chromosome
21.
 When an aneuploid zygote divides by
mitosis, it transmits the chromosomal
anomaly to all subsequent embryonic cells.
Chromosome breakage can alter
chromosome structure in four ways:
1. Deletion – complete loss of a fragment of a
chromosome lacking a centromere.
2. Duplication – the lost fragment can join a
homologous chromosome.
3. Translocation – the lost fragment can join
to a nonhomologous chromosome.
4. Inversion – the lost fragment reattaches to
the original chromosome in reverse order.
Crossing-over error can also be a source of
deletions and duplications
Aneuploidy Disorders:
Down syndrome – 1 out of 700 U.S. children
 trisomy 21
 Includes characteristic facial features, short
stature, heart defects, mental retardation,
susceptibility to respiratory infections, and a
proneness to develop leukemia and
Alzheimer’s disease.
 “most” are sexually underdeveloped/sterile
 The incidence of Down syndrome offspring
correlates with maternal age. WHY?
think of how meiosis occurs in females
Sex chromosome aneuploidies in
males:
1. Klinefelter Syndrome
 genotype: XXY
 phenotype: Male sex organs with small
testes; sterile; feminine body and perhaps
breast enlargement; usually of normal
intelligence
• 2. Extra Y
 genotype: XYY
 phenotype: normal male; usually taller
than average;
normal intelligence and fertility
Sex chromosome aneuploidies in
females:
1. Triple-X Syndrome
 genotype: XXX
 phenotype: usually fertile; can show a
normal phenotype
• 2. Turner Syndrome
 genotype: XO
 phenotype: short in stature; secondary
sexual characteristics fail to develop; internal
sex organs do not mature; sterile
Genomic imprinting
causes certain genes to be expressed differently
in the offspring depending upon whether the
alleles were inherited from the ovum or from
the sperm cell.
 Prader-Willi syndrome is cause by a deletion
from the paternal version of chromosome 15.
characterized by mental retardation,
obesity, short stature, and unusually small
hands and feet.
 Angelman syndrome is caused by a deletion
from the maternal version of chromosome 15.
characterized uncontrollable spontaneous
laughter , jerky movements, and various
motor and mental symptoms.
Fragile X syndrome - symptoms more common
when inherited from maternal ovum.
X- Inactivation
Barr body= inactive X in each cell of female
Results in calico cats, sweat gland development