Download Chromosomal Inheritance - Bishop Seabury Academy

Chromosomal Inheritance
1. Explain how the
observations of
cytologists and
geneticists provided the
basis for the
chromosome theory of
inheritance.
TT X tt
2. Describe the
contributions that
Thomas Hunt Morgan,
Walter Sutton, and A.H.
Sturtevant made to
current understanding of
chromosomal
inheritance.
Morgan
P
F1
X
Sex-linkage
F1
F2
X
Sturtevant
Linkage map of
the pea genome
(functional)
1% recombination frequency = 1 centimorgan
Sutton
In 1902, he provided sufficient evidence for the
theory that genes are parts of chromosomes
from direct observations on the behavior of
chromosomes in reduction division. In 1903, he
showed that the chromosomes behave by
random segregation in the sex cells and
recombination in fertilization, exactly as genes do.
3. Explain why Drosophila
melanogaster is a good
experimental organism.
And, they reproduce
prolifically every two
weeks!
4. Define linkage and
explain why linkage
interferes with
independent assortment.
Gene linkage
b = black body
b+ = gray body
vg+ = normal wings
vg = vestigial wings
b+b vg+vg
bb vgvg
X
b+b vg+vg
bb vgvg
b+b vgvg
bb vg+vg
575
575
575
575
b+b vg+vg
bb vgvg
X
b+b vg+vg
bb vgvg
b+b vgvg
bb vg+vg
965
944
206
185
5. Distinguish between
parental and
recombinant
phenotypes.
YyRr x yyrr
yr
YR
Yr
yR
yr
YyRr
Yyrr
yyRr
yyrr
Parental Types
Recombinants
50% frequency of recombination
b+b vg+vg
bb vgvg
X
b+b vg+vg
bb vgvg
b+b vgvg
bb vg+vg
575
575
575
575
b+b vg+vg
bb vgvg
X
b+b vg+vg
bb vgvg
1150
1150
Phenotypes
Genotypes
Expected
results if
genes are
unlinked
Expected
results if
genes are
totally linked
Actual
Results
Black body,
normal wings
bb vg+vg
575
Gray body,
normal wings
b+b vg+vg
575
1150
965
Black body,
vestigial wings
bb vgvg
575
1150
944
Gray body,
vestigial wings
b+b vgvg
575
206
185
Recombination frequency = 391 recombinants X 100 = 17%
2300 total offspring
6. Explain how crossing
over can unlink genes.
7. Map a linear sequence
of genes on a
chromosome using given
recombination
frequencies from
experimental crosses.
Loci
Recombination
Frequency
Approximate Map
Units
b vg
17.0%
18.5
cn b
9.0%
9.0
cn vg
9.5%
9.5
b
cn
17
9
b cn
vg
9.5
vg
b
9
17
cn
9.5
vg
The actual distance of 18.5 map
units between b and vg is
accounted for by the great
distance between the loci and the
potential for double crossovers
that cancel each other out.
8. Explain what
additional information
cytological maps
provide over crossover
maps.
Linkage map of
the pea genome
(functional)
Cytogenetic map
of the human
genome
(structural)
9. Distinguish between a
heterogametic sex and a
homogametic sex.
The heterogametic sex
(gender) produces two kinds of
gametes and determines the sex
of the offspring, the
homogametic sex (gender)
produces one kind of gamete. In
humans, males are the
heterogametic sex, while
females are the homogametic
sex.
10. Describe sex
determination in
humans.
A British research team has
identified a gene, SRY (sexdetermining region of Y), on
the Y chromosome that is
responsible for triggering the
complex series of events that
lead to normal testicular
development, in the absence
of SRY, the gonads develop
into ovaries.
11. Describe the
inheritance of a sexlinked gene such as
color-blindness.
Red-Green Color Blindness
Pedigree Analysis
12. Explain why a
recessive sex-linked
gene is always
expressed in human
males.
13. Explain how an
organism compensates
for the fact that some
individuals have a
double dosage of sexlinked genes while
others have only one.
Mosaicism
14. Distinguish among
nondisjunction,
aneuploidy, and
polyploidy; explain how
these major
chromosomal changes
occur and describe the
consequences.
Nondisjunction
Meiotic Nondisjunction
Mitotic Nondisjunction
Anaphase I or Anaphase
II, One gamete gets 2,
the other gets 0
If it occurs in embryonic
cells, the abnormal
chromosome # may be
passed to a large # of
cells
Anueploidy
(trisomy)
(monosomy)
+
=
+
0
=
An anueploid zygote passes the aberration on to
all subsequent cells. A common example is
trisomy 21, the cause of Down’s syndrome.
Polyploidy
3n = triploidy
(fertilization of an egg that had nondisjunction
of all chromosomes during oogenesis)
4n = tetraploidy
(division of a zygote without cytokinesis)
Polyploidy is common in plants, but rare in
animals. Polyploid animals are usually mosaics,
and more normal in appearance than
anueploids.
15. Distinguish
between trisomy and
triploidy.
16. Distinguish among
deletions, duplications,
translocations, and
inversions.
Deletion
Duplication
Translocation
Insertion
17. Describe the
effects of alterations in
chromosome structure,
and explain the role of
position effects in
altering the phenotype.
Alternations of chromosome structure
can have various effects:
• Homozygous deletions, including a single X in
males are usually fatal
• Duplications and translocations tend to have
deleterious effects
• Even if all genes are present in normal dosages,
reciprocal translocations between nonhomologous
chromosomes can alter the phenotype because of
position effects
18. Describe the type of
chromosomal alterations
implicated in the following
human disorders: Down
syndrome, Klinefelter syndrome,
extra Y, triple-X syndrome,
Turner syndrome, cri du chat
syndrome, and chronic
myelogenous leukemia.
Effects of Alterations
Down’s syndrome
trisomy 21
Kleinfelter
syndrome
Extra Y
XXY, XXXY, XXXXY,
XXXXXY
XYY
Triple X
XXX
Turner syndrome
X0
Cri du chat
Deletion on #5
Chronic
Myelogenous
Leukemia
Translocation
22<>9
19. Define genomic
imprinting and provide
evidence to support this
model.
It usually doesn’t matter whether
a gene is introduced to the next
generation via maternal or
paternal chromosomes, but for a
small number of cases, parental
imprinting affects the expression
of the gene. A good example of
this is the Prader-Willi/Angelman
syndromes.
20. Explain how the
complex expression of a
human genetic disorder,
such as fragile-X
syndrome, can be
influenced by triplet
repeats and genomic
imprinting.
21. Give some
exceptions to the
chromosome theory of
inheritance, and explain
why cytoplasmic genes
are not inherited in a
Mendelian fashion.
The genetic material
found in mitochondria
and plant plastids, does
not follow the pathways
of assortment and
segregation normal to
nuclear chromosomes.