Download Genetics 275 Notes

Genetics 275 Notes-Week 2
Human examples of Epstasis
A/B/O blood type
-antigenicity identity depends on the type of glycoprotein on surface of blood cells
Pathway
U? V? W? X? H ? A or B or i antigen (O blood type)
Bombay phenotype
-h/h
-pseudo O blood group
-pathway from X to H blocked
(pseudo O ) h/h; IAIA x h+ h+; i/i
(true O)
?
h+/h ; IA/i
-all children have blood group A
Analogy to experimental Model System
h+/h ; IA/i x
h+/h ; IA/i
?
9 h+/_ ; IA/i
? A
3 h /_ ; i/i
? O
A
3 h / h ; I /i ? pseudo O
+
1 h / h ; i/i
? pseudo O
Normal Genome
-in general an organism has the specific chromsome complement which comprises its
species specific genome
-these chromosomes are characteristically present as homologous pairs
-chromosome pairs are qualitively different from each other
-the characteristic chromosome number along with their characteristic sizes and shapes
define a karyotype for a species when they are examined under a microscope
[T-19]
Variety of chromosome number in different species
n= # of chromosome pair
What types of normal exception do we find with respect to chromosome #?
1. Normal karotypic differences between the sexes
e.g. X/X vs X/Y in humans
2. Differences within an organism
-germ cells (gametes are haploid vs. somatic cells which are diploid)
-some organs may be polyploid (human liver)
3. Normal differences are a polymorphic feature in some species
e.g. B chromosomes (extra or accessory chromosomes) in corn
4. Geographic intraspecific differences within a species
-variation usually in chromosome shape, size and number which can lead to
incipient speciation
Abnormal/deleterious karyotypes may arise in any organism and involves changes in
chromosome number and or structure of chromosomes
A karyotype also considers size and shape of chromosomes,
1. size
-most species have > 3x size difference between the smallest to the largest
chromosome
2. centromere position
-metacentric
-acrocentric(centromere to one side)
-telocentric(extreme acrocentric)
-most species has 1 centromere
[T-20]
Diagram of human chromosome(acrocentric)
Centromeric indent- measurement of chromsome length comprised by the short arm
[T-21]
Human karyotypes
note: mitogen causes a cell to divide
[T-22]
Characteristic of human metaphase chromosomes
To tell chormosomes apart that are similar in size,
-need landmarks
-have technology to differentially stain the chromosmes(one of these stain
utiltizes differential staining of heterochromatin vs. euchromatin)
The distribution of heterochromatin vs. euchromatin in chromosomes does influence the
staining patterns of chromosomes of any species
e.g. G(Giemsa)-banding differentially stains because of differential coiling and sequence
of underlying DNA
[T-23]
G-banding of human chromosomes
-most genes located on light G-bands which are euchromatin
Aneuploidy
-change in chromosome # in which some chromosomes are missing or some are present >
2x
Polyploidy-chromosome differ by complete set
Most aneuploids differe from normal by having 1 more or 1 less chromosome than usual
i.e. 2n-normal
2n+1 or 2n-1 aneuploidy
4 types of aneuploids
-in all examples below assume normal is 2n
1. Monosomy
2n-1
2. Trisomy
2n+1
3. Nullisomy
2n-n
4. Tetrasomy
2n+2
Most frequent cause of aneuploidy is chromosome non-disjunction
(i.e. failure for homologues to disjoin)
[T-24a]
-nondisjunction at anaphase I results in which all gametes are unbalanced
[T-24b]
-nondisjunction at anaphase II results in which half of the gametes are unbalanced
If nondisjunction occurs in mitotic (somatic) cells, the result is a chromosomal mosaic
individual
-nondisjunction occurs in 1/2000 divisions
Monsomy
-usual consequence is death(only one copy of any particula r gene, lethal heterozygotes
are uncovered)
-sex chromosome aneuploidy is the most commonly found aneuploidy
-consequence also depends on the size and importance of the chromosome and genes on
that chromosome
e.g. Drosophila melanogaster
Wildtype is 2n=8
Monosomy for chromosome II and III is lethal
Monosomy for chromosome IV si viable and fertile in females and males
Monosomy for X is sterile male
In humans, the only viable monosomic is for the X chromosome (sterile female)
Viable Monsomic in Plants
-these are really “artifical” monosomics because the species in which they occur tend to
be polyploid
Various Monsomies
(#1-15 monosomic lines)
-m+/m+ or m+/O
x
“diploid” (actually hexaploid)
-has mutant trait you want to study
-m/m
2 reuslts can occur:
a-all WT progeny m+/m
b-1/2 will be m+/m
-1/2 will be m/O(manifestation of mutant trait)
Cytogenetic behaviour
-homologous pairs form bivalents but the monsomic has no pairing partner
-this leads to a phenomenon called chromosome lag during anaphase
[T-25]
Nullisomy
e.g. wheat can tolerate nullisomy because it is actually a hexaploid
Human Aneuploidy
-aneuploidy in sex chromosome more viable than autosomal aneuploidy (due to X
inactivation in mammals)
Types
YO-lethal
XO-Turner Syndrome
-(45, X), 1/5000 newborn females
-sterile females
Trisomic XXX or Trisomic X syndrome -(47, XXX)
-females look normal
-1/1000
-physically normal, fertile females
-slight tendeny toward mental retardation
47, XXY-Klinefelter syndrome
-1/1000
-tall, sterile males
-tendency toward mental retardation
XYY-Double Y Karyotype
-fertile males
-tends to be tall
-IQ about normal
Note:Variety of mosaic karyotypes can result from nondisjunction after fertilization.