Transcript Chromosomal Basis of Inheritance

Chapter 15
The Chromosomal
Basis of
Inheritance
Morgan
Chose
to use fruit flies as a
test organism in genetics.
Allowed the first tracing of
traits to specific
chromosomes.
Fruit Fly
Drosophila
melanogaster
Early test organism for
genetic studies.
Life Cycle
Egg
Larva
Pupa
Adult
Reasons
Small
Cheap
to house and feed
Short generation time
Many offspring
Few chromosomes
Fruit Fly Chromosomes
Morgan Observed:
A
male fly with a mutation for
white eyes.
Morgan crossed
The
white eye male with a
normal red eye female.
The F1 offspring:
All
had red eyes.
This suggests that white eyes
is a genetic _________?
Recessive.
F1 X F1 = F2
Morgan
expected the F2 to have
a 3:1 ratio of red:white
He got this ratio, however, all of
the white eyed flies were MALE.
Therefore, the eye color trait
appeared to be linked to sex.
Morgan discovered:
Sex
linked traits.
Genetic traits whose
expression are dependent on
the sex of the individual.
Sex Linkage in Biology

1.
2.
3.
Several systems are known:
Mammals – XX and XY
Diploid insects – X and XX
Birds – ZZ and ZW
Chromosomal Basis of
Sex in Humans
X
chromosome - medium
sized chromosome with a
large number of traits.
Y chromosome - much
smaller chromosome with
only a few traits.
Human Chromosome
Sex
Males
- XY
Females - XX
Comment - The X and Y
chromosomes are a
homologous pair, but only for
a small region at one tip.
Sex Linkage
Inheritance
of traits on the
sex chromosomes.
X- Linkage (common)
Y- Linkage (rare)
Males
Hemizygous
- 1 copy of X
chromosome.
Show ALL X traits
(dominant or recessive).
More likely to show X
recessive gene problems
than females.
X-linked Disorders
Color
blindness
Duchenne's Muscular
Dystrophy
Hemophilia (types a and b)
Samples of X-linked
patterns:
X-linked Patterns
Trait
is usually passed from a
carrier mother to 1/2 of sons.
Affected father has no
affected children, but passes
the trait on to all daughters
who will be carriers for the
trait.
Comment
Watch
how questions with
sex linkage are phrased:
Chance of children?
Chance of males?
Can Females be
color-blind?
Yes,
if their mother was a
carrier and their father is
affected.
Y-linkage
Hairy
ear pinnae.
Comment - new techniques
have found a number of
Y-linked factors that can be
shown to run in the males of
a family.
Ex: Jewish priests
Sex Limited Traits
Traits
that are only expressed
in one sex.
Ex – prostate glands
Sex Influenced Traits
Traits
whose expression
differs because of the
hormones of the sex.
Ex. – beards, mammary gland
development, baldness
Baldness
– makes the trait
act as a dominant.
No testosterone – makes the trait
act as a recessive.
Males – have gene = bald
Females – must be homozygous
to have thin hair.
Testosterone
Barr Body
Inactive
X chromosome
observed in the nucleus.
Way of determining genetic
sex without doing a
karyotype.
Lyon Hypothesis
Which
X inactivated is
random.
Inactivation happens early in
embryo development by
adding CH3 groups to the
DNA.
Result - body cells are a
mosaic of X types.
Examples
Calico
Cats.
Human examples are known
such as a sweat gland
disorder.
Calico Cats
XB
= black fur
XO = orange fur
Calico is heterozygous, XB XO.
Question?
Why
don’t you find many
calico males?
They must be XB XOY
Linked Genes
Traits
that are located on the
same chromosome.
Result:
Failure of Mendel's Law of
Independent Assortment.
Ratios mimic monohybrid
crosses.
Crossing-Over
Breaks
up linkages and
creates new ones.
Recombinant offspring
formed that doesn't match
the parental types.
If Genes are Linked:
Independent
Assortment of
traits fails.
Linkage may be “strong” or
“weak”.
Linkage Strength
Degree
of strength related to
how close the traits are on
the chromosome.
Weak - farther apart
Strong - closer together
Genetic Maps
Constructed
from crossingover frequencies.
1 map unit = 1%
recombination frequency.
Genetic Maps
Have
been constructed for
many traits in fruit flies,
humans and other
organisms.
Chromosomal
Alterations
Changes
in number.
Changes in structure.
Number Alterations
Aneuploidy
- too many or too
few chromosomes, but not a
whole “set” change.
Polyploidy - changes in
whole “sets” of
chromosomes.
Aneuploidy
Caused
by nondisjunction,
the failure of a pair of
chromosomes to separate
during meiosis.
Comment
Nondisjunction
in Meiosis I
produces 4 abnormal gametes.
Nondisjunction in Meiosis II
produces 2 normal and 2
abnormal gametes.
Types of Aneuoploidy
2N – 1 (very rare)
Trisomy: 2N + 1 (more common)
Monosomy:
Turner Syndrome
2N
- 1 or 45 chromosomes
Genotype: X_ or X0.
Phenotype: female, but very
poor secondary sexual
development.
Characteristics
Short
stature.
Extra skin on neck.
Broad chest.
Usually sterile
Normal mental development
except for some spatial
problems.
Question
Why
are Turner Individuals
usually sterile?
Odd chromosome number.
Two X chromosomes needed
for ovary development.
Other Sex
Chromosome changes
Kleinfelter
Syndrome
Meta female
Supermale
Kleinfelter Syndrome
2N
+ 1 (2N + 2, 2N + 3)
Genotype: XXY (XXXY, XXXXY)
Phenotype: male, but sexual
development may be poor. Often
taller than average, mental
development fine (XXY), usually
sterile.
More X = more mental problems
George Washington
May
have been a Kleinfelter
Syndrome individual.
Much taller than average.
Produced no children.
Meta female
2N
+ 1 or 2N + 2
Genotype: XXX or XXXX
Phenotype: female, but
sexual development poor.
Mental impairment common.
Super male
2N
+ 1 or 2N + 2
Genotype: XYY or XYYY
Phenotype: male, usually
normal, fertile.
Trisomy events
Trisomy
21: Downs Syndrome
Trisomy 13: Patau Syndrome
Both have various physical
and mental changes.
Question?
Why
is trisomy more
common than monosomy?
Fetus can survive an extra
copy of a chromosome,
but being hemizygous is
usually fatal.
Question?
Why
is trisomy 21 more
common in older mothers?
Maternal age increases risk
of nondisjunction.
Polyploid
Triploid=
3N
Tetraploid= 4N
Usually fatal in animals.
Question?
In
plants, even # polyploids
are often fertile, why odd #
polyploids are sterile. Why?
Odd number of
chromosomes can’t be split
during meiosis to make
spores.
Structure Alterations
Deletions
Duplications
Inversions
Translocations
Result
Loss
of genetic information.
Position effects: a gene's
expression is influenced by
its location to other genes.
Cri Du Chat Syndrome
Part
of p arm of #5 missing.
Good survival.
Severe mental retardation.
Small sized heads common.
Parental Imprinting
of Genes
Gene
expression and
inheritance depends on
which parent passed on the
gene.
Usually caused by different
methylations of the DNA.
Example:
Prader-Willi
Syndrome and
Angelman Syndrome
Both lack a small gene region
from chromosome 15.
Male imprint: Prader-Willi
Female imprint: Angelman
Cause:
Imprints
are "erased" in
gamete producing cells and
re-coded by the body
according to its sex.
Why have parental
imprinting?
Method
to detect that TWO
different sets of
chromosomes are in the
zygote.
Summary
Know
about linkage and
crossing-over.
Sex chromosomes and their
pattern of inheritance.
Variations of chromosomes
and inheritance patterns.
Summary
Be
able to work genetics
problems for this chapter.