Transcript 1. Describe the contributions that Thomas Hunt

Chapter 15 RQ
1.
2.
3.
4.
5.
What are genes located on a sex
chromosome called?
In fruit flies, what do we call the normal
phenotype for a character?
Genes that tend to be inherited together
are called what?
What is nondisjunction?
How do people have Down Syndrome?
1. Describe the contributions that Thomas
Hunt Morgan, Walter Sutton, and A.H.
Sturtevant made to current understanding of
chromosomal inheritance.
• Morgan  selected Drosophila as organism
- proposed linkage (characteristics are
inherited together)
• Sturtevant  Morgan’s student
- probability of crossing over between
genes is directly proportional to the
distance between them
• Sutton  noticed parallels of Mendel’s
thoughts and the actual behavior of
chromosomes 
2. Explain why Drosophilia melanogaster is a
good experimental organism.
• It is easily cultured in
the lab
• They are prolific
breeders
• They have a short
generation time
• Their 4 pairs of
chromosomes are
easily differentiated
and observed 
3. Define linkage and explain why linkage
interferes with independent assortment.
• Linked genes do not independently assort
• Linked genes  genes that are located on
the same chromosome and that tend to be
inherited together
- move together through meiosis and
fertilization
- F2 generation doesn’t show 9:3:3:1 ratio in
the dihybrid 
4. Distinguish between parental and
recombinant phenotypes.
Parental
• Progeny that have
the same
phenotype as one
or the other of the
parents
Recombinant
• Progeny whose
phenotypes differ
from either parent

5. Explain how crossing over can unlink genes.
• During meiosis,
exchange of parts
between homologous
chromosomes breaks
linkages in parental
chromosomes and
forms recombinants
with new allelic
combinations 
6. Map the linear sequence of genes on a
chromosome using the given recombinant
frequencies from experimental crosses.
Loci
b
vg
Recombinant
Frequency
17.0%
Approximate Map
Units
18.5*
cn b
9.0%
9.0
cn vg
9.5%
9.5
*higher because b & vg are relatively far apart and double
crossovers occur between these loci and cancel each other
out  leading to underestimation of map distance
next slide
#6 continued…
1.
2.
3.
Establish distance between the genes farthest apart
Determine the frequency between the 3rd (cn) and the 1st
(b)
Consider possible placements
•
b-----------------------vg
•
cn---------b
•
cn----------b-----------vg OR
•
Determine recombinant frequency between 3rd (cn) and 2nd
(vg) to eliminate 
b-------------cn---------------vg
 17
9
b----------cn-----------vg
7. Explain what additional information
cytological maps provide over crossover
maps.
• Cytological maps locate genes with
respect to chromosomal features,
such as stained bands that can be
viewed with a microscope 
8. Distinguish between a heterogametic sex
and a homogametic sex.
Heterogametic sex
• The sex that
produces two kinds
of gametes and
determines the sex
of the offspring
• Ex: human male
(sperm – X or Y)
Homogametic sex
• The sex that
produces one kind
of gamete
• Ex: human female
(egg – X only)

9. Describe sex determination in humans.
•
•
•
•
2 chromosomes determine sex – X and Y
Males carry X and Y options
Females carry X only
It is the gene SRY (sex determining region
of Y) that triggers for complex events
leading to the development of testes, etc.
• Default is the development of ovaries 
10. Describe the inheritance of a sex-linked
gene such as color blindness.
• Refers to X-linked traits
• X is larger than Y, so there
are more traits
• Fathers  pass on X to
daughter
- no sex-linked traits to
son
• Mothers  give to both
type of offspring
- sex linked trait is a
recessive allele
- females show trait only is
homozygous 
11. Explain why a recessive sex-linked gene is
always expressed in human males.
• It is said to be ‘hemizygous’ - an
organism having only one copy of a
gene in a diploid organism
• Ex: son (XY)
- on X, the recessive trait for
colorblindness
- on Y, no dominant trait to shadow
recessive 
12. Distinguish among nondisjunction,
aneuploidy, and polyploidy; explain how these
major chromosomal changes occur and
describe the consequences.
• Nondisjunction 
meiotic or mitotic
error during which
certain homologous
chromosomes or
sister chromatids
fail to separate 
Aneuploidy…
• Aneuploidy 
condition of having
an abnormal
number of certain
chromosomes
(result of
nondisjunction)
ex: Down’s
syndrome (trisomy
of #21) 
Polyploidy…
• Polyploidy  a chromosome number that is more
than 2 complete chromosome sets (3 haploid
sets  3n) 
13. Distinguish between trisomy and triploidy.
• Trisomy  an aneuploid cell
that has a chromosome in
triplicate
• Triploidy  polyploid
chromosome number with
3n 
14. Distinguish among deletions, duplications,
translocations, and inversions.
• Deletion  chromosomes which lose a
fragment and lack a centromere
• Duplications  fragments without
centromeres that join a homologous
chromosome
• Translocation  when the fragments join
to a non-homologous chromosome
• Inversions  when the fragments reattach
to the original chromosome but in a reverse
order 
15. Describe the affects of alterations in
chromosome structure, and explain the role
of position effects in altering the phenotype.
• Effects of alterations 
- homozygous deletions, including a single X in
males (usually lethal)
- duplications and translocations tend to have
deleterious effects
- reciprocal translocations and inversions between
non-homologous chromosomes can alter phenotype
• Position effect influence on a gene’s expression
because of its location among neighboring genes 
16. 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.
•
•
•
•
•
Down’s syndrome  trisomy of 21; 1 in 700 children
Klinefelter  XXY , feminine maleness
Extra Y  XYY, normal male, usually taller
Triple X  XXX, usually fertile, normal female
Turner’s  XO, only known viable human monosomy
- short and sterile, secondary sex characteristics don’t
develop (unless estrogen therapy is applied)
• Cri du chat  deletion on chromosome 5, mental retardation
- smaller head, unusual features, cry sounds like a mewing
cat
• Chronic myelogenous leukemia (CML)  part of chromosome
22 switches places with a small fragment from chromosome
9 (translocations) 
17. Define genomic imprinting and provide
evidence to support this model.
• It is the process that induces intrinsic
changes in chromosomes inherited from
males and females; causes certain genes to
be differently expressed in the offspring
depending upon whether the alleles were
inherited from the ovum or the sperm cell
• Same alleles have different effects if
maternal and paternal
• DNA methylation is one mechanism 
Example…
• Deletion of a particular segment of
chromosome 15
– If it came from dad: Prader-Willi
syndrome (mental retardation, obesity,
short, small hands and feet)
– If it came from mom: Angelman
syndrome (uncontrollable laughter,
motor and mental symptoms)
18. Give some exceptions to the chromosome
theory of inheritance, and explain why
cytoplasmic genes are not inherited in a
Mendelian fashion.
Exceptions 
• Extranuclear genes are found in
cytoplasmic organelles like plastids and
mitochondria
• These are not inherited in Mendelian
fashion because they are not distributed
by segregating chromosomes in meiosis 
The End !