Transcript Chapter 15~ The Chromosomal Basis of Inheritance

The following is a map of four genes on a
chromosome:
a) Between which two genes would you
expect the lowest highest frequency of
recombination?
Sex-linked Trait
Problem
 Example:
Eye color in
fruit flies

(red-eyed male) x
(white-eyed female)
XRY
x
 Remember:
X rX r
the Y
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
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Female Carriers
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Pleiotrophy
 Example:
Sickle cell. A mutation in
a gene causes multiple symptoms
Epistasis

In genetics, epistasis
is the phenomenon
where the effects of
one gene are
modified by one or
several other genes,
which are sometimes
called modifier
genes.
Polygenic Inheritance

Polygenic inheritance is when a single trait is
controlled by 2 or more sets of alleles. Most
human traits are polygenically inherited.
Examples would be skin and eye color. This
explains how you can have several different
phenotypes for one trait and how parents can
have offspring with eye color or skin color
different from what they have.
Chapter 15~
The
Chromosomal Basis of
Inheritance
The Chromosomal Theory
Inheritance

Genes have specific
loci on chromosomes
and chromosomes
undergo segregation
and independent
assortment
of
Chromosomal Linkage





Morgan
Drosophilia melanogaster
XX (female) vs. XY (male)
Sex-linkage: genes located on a
sex chromosome
Linked genes: genes located on the
same chromosome that tend to be
inherited together
Classes of chromosomes
autosomal
chromosomes
sex
chromosomes
Discovery of sex linkage
P
F1
true-breeding
red-eye female
X
true-breeding
white-eye male
100%
red eye offspring
Huh!
Sex matters?!
generation
(hybrids)
F2
generation
100%
red-eye female
50% red-eye male
50% white eye male
What’s up with Morgan’s flies?
x
RR
r
R
Rr
x
rr
Rr
r
Rr
Rr
R
R
r
RR
Rr
Rr
rr
Doesn’t work
that way!
R
Rr
Rr
100% red eyes
r
3 red : 1 white
Linked genes

http://bcs.whfreeman.com/thelifewire/content/
chp10/1002s.swf
Genetics of Sex

In humans & other mammals, there are 2 sex
chromosomes: X & Y
–
2 X chromosomes


–
develop as a female: XX
gene redundancy,
like autosomal chromosomes
an X & Y chromosome


X
Y
X
XX
XY
X
XX
XY
develop as a male: XY
no redundancy
50% female : 50% male
Let’s reconsider Morgan’s flies…
x
XR XR
Xr
XR
XR
XR X r
XR X r
x
X rY
Y
XRY
XRY
100% red eyes

XR
BINGO!
Xr
XR Xr
XRY
XR
Y
XR XR
XRY
X R Xr
X rY
100% red females
50% red males; 50% white males
Genetic recombination



Crossing over
Genes that DO
NOT assort independently of
each other
Genetic maps
The further apart
2 genes are, the higher the
probability that a crossover will
occur between them and
therefore the higher the
recombination frequency
Linkage maps
Genetic map
based on recombination
frequencies
Genes on sex chromosomes

Y chromosome
–
few genes other than SRY



sex-determining region
master regulator for maleness
turns on genes for production of male hormones
–

many effects = pleiotropy!
X chromosome
–
other genes/traits beyond sex determination

mutations:
–
hemophilia
– Duchenne muscular dystrophy
– color-blindness
Human sex-linkage




SRY gene: gene on Y chromosome that triggers the
development of testes
Fathers= pass X-linked alleles to all daughters only (but
not to sons)
Mothers= pass X-linked alleles to both sons & daughters
Sex-Linked Disorders: Color-blindness; Duchenne
muscular dystropy (MD); hemophilia
Hemophilia
sex-linked recessive
HhXHXxh HHXHY
XH
female / eggs
male / sperm
XH
XH
Y
XH XH
XHY
XH Xh
Xh
XH
Xh
XH Xh
carrier
XhY
disease
XHY
Y
X-inactivation
 Female mammals inherit 2 X chromosomes

one X becomes inactivated during
embryonic development
 condenses into compact object = Barr body
 which X becomes Barr body is random
 patchwork trait = “mosaic”
patches of black
XH 
XH Xh
tricolor cats
can only be
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female
Xh
patches of orange
Human sex-linkage

X-inactivation: 2nd X chromosome in females
condenses into a Barr body (e.g., tortoiseshell
gene gene in cats)
Errors of Meiosis
Chromosomal Abnormalities
2006-2007
Chromosomal abnormalities

Incorrect number of chromosomes
–
nondisjunction

–
chromosomes don’t separate properly during meiosis
breakage of chromosomes




deletion
duplication
inversion
translocation
Nondisjunction
 Problems with meiotic spindle cause errors in
daughter cells



2n
homologous chromosomes do not separate
properly during Meiosis 1
sister chromatids fail to separate during Meiosis 2
too many or too few chromosomes
n-1
n
n+1
n
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Alteration of chromosome number
error in Meiosis 1
error in Meiosis 2
all with incorrect number
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1/2 with incorrect number
Nondisjunction
 Baby has wrong chromosome number~
aneuploidy

trisomy
 cells have 3 copies of a chromosome

n+1
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monosomy
 cells have only 1 copy of a chromosome
n-1
n
n
trisomy
monosomy
2n+1
2n-1
Human chromosome disorders
 High frequency in humans



most embryos are spontaneously aborted
alterations are too disastrous
developmental problems result from biochemical
imbalance
 imbalance in regulatory molecules?
 hormones?
 transcription factors?
 Certain conditions are tolerated


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upset the balance less = survivable
but characteristic set of symptoms = syndrome
Down syndrome
 Trisomy 21
3 copies of chromosome 21
 1 in 700 children born in U.S.

 Chromosome 21 is the
smallest human chromosome

but still severe effects
 Frequency of Down
syndrome correlates
with the age of the mother
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Sex chromosomes abnormalities
 Human development more tolerant of
wrong numbers in sex chromosome
 But produces a variety of distinct
syndromes in humans




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XXY = Klinefelter’s syndrome male
XXX = Trisomy X female
XYY = Jacob’s syndrome male
XO = Turner syndrome female
Klinefelter’s syndrome
 XXY male
one in every 2000 live births
 have male sex organs, but
are sterile
 feminine characteristics

 some breast development
 lack of facial hair
tall
 normal intelligence

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Klinefelter’s syndrome
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Jacob’s syndrome male
 XYY Males
1 in 1000 live male
births
 extra Y chromosome
 slightly taller than
average
 more active
 normal intelligence, slight learning disabilities
 delayed emotional maturity
 normal sexual development

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Trisomy X
 XXX
1 in every 2000 live births
 produces healthy females

 Why?
 Barr bodies
 all but one X chromosome is inactivated
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Turner syndrome
 Monosomy X or X0
1 in every 5000 births
 varied degree of effects
 webbed neck
 short stature
 sterile

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replication
error of
Changes in chromosome structure
 deletion

 duplication
crossing over

error of
loss of a chromosomal segment
repeat a segment
 inversion

reverses a segment
 translocation

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move segment from one chromosome
to another
Chromosomal errors VI
Deletion
Duplication
Homologous
chromosomes
Inversion
Reciprocal
translocation
Nonhomologous
chromosomes
Genomic imprinting



Def: a parental effect on
gene expression
Identical alleles may have
different effects on offspring,
depending on whether they
arrive in the zygote via the
ovum or via the sperm.
Fragile X syndrome: higher
prevalence of disorder and
retardation in males