Transcript Ch15 PowerPoint LN

Chapter 15
Points of Emphasis
Know:
1. all the bold-faced terms
2. How to identify a sex-linked trait
3. The proper letter configuration to use when doing sexlinked crosses.
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Figure 15.2 Morgan’s first mutant
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Sex-Linked Inheritance
Basic idea:
any gene located on the X chromosome (in
mammals, Drosophila and others) or on the
analogous Z chromosome (in birds and other
species) is said to be sex-linked or X-linked.
First sex-linked gene found was in Drosophila and it was the
recessive white-eye mutation.
A cross between white-eyed females (Xw Xw) with wild-type (redeyed) males (X+ Y) produced all male offspring that were whiteeyed like their mother and all the female offspring had red eyes
like their father.
This criss-cross mode of inheritance is characteristic of sex-linked
genes.
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Sex-Linked Inheritance (cont’d)
Why is this crisscross mode characteristic?

The Y chromosome carries no alleles homologous to those
at the white locus on the X chromosome

X chromosome carries some 2000-3000 genes, in most
organisms, the Y contains only several dozen. So the
males carry only one allele for sex-linked traits.

This one allelic condition is called “hemizygous.”
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Sex-Linked Inheritance (cont’d)
Example: Determine the offspring in a cross between a
heterozygous, red eyed female and a white eyed male.
X+ Xw
Xw
Y
x
Xw Y
X+
Xw
X+ Xw
XwXw
Red female
White female
X+ Y
Xw Y
Red male
White male
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Sex-Linked Inheritance (cont’d)
The reciprocal cross, where the sex-linked mutation appears in
the male parent, results in the disappearance of the trait in the F1
and its reappearance only in the the males of the F2. This type of
skip also indicates sex-linked genes.
Example:
Determine the F1and F2 offspring from a P1 cross
between a:
X+ X+
red female
x
Xw Y
white male
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Gametes:
X+
Xw
F1:
X+ Xw red females and X+ Y males
Y
so the white eyed males have disappeared.
F2:
X+
Y
X+
Xw
X+ X+
X+Xw
Red female
red female
X+ Y
Xw Y
Red male
White male
Only the males are showing the mutant trait (white eyes)
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Sex-Linkage Summary
If sex-linked recessive:
 it is usually found more frequently in the male than in the female
of the species
 it fails to appear in females unless it also appeared in the
paternal parent.
 it seldom appears in both father and son, then only if the
maternal parent is heterozygous.
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Sex-Linkage Summary
If sex-linked dominant:
 it is found more frequently in the female than in the male
 it is found in all female offspring of a male that shows the trait
 it fails to be transmitted to any son from a mother that did not
exhibit the trait herself.
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Figure 15.1 The chromosomal basis of Mendel’s laws
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Figure 15.3 Sex-linked inheritance
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Unnumbered Figure (page 272) Drosophila testcross
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Non-Sex linked genes
Linked genes refers to genes that are on the same chromosome
and tend to be inherited together.
Linked genes do not follow Mendel’s Law of Independent
Assortment
Linked genes do not assort independently because they are
located on the same chromosome and tend to move together
through meiosis.
Since the genes are linked, we would expect them to be passed on
to the offspring together and the offspring would not have any
different combinations than the parents.
But, recombination can occur between linked genes.
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Figure 15.4 Evidence for linked genes in Drosophila
Since the observed ratio
of the phenotypes was not
1 : 1 : 1 : 1, Morgan
suspected that some
genes are transmitted
together because they are
on the same chromosome.
The offspring that were
like the parents
demonstrated linked genes
but the recombined
phenotypes suggested
something else- crossing
over- was occurring
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Figure 15.5a Recombination due to crossing over
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Figure 15.5b Recombination due to crossing over
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Recombination Frequencies
Where Are Those Gene’s Located? A Genetic Map

It was hypothesized that the recombination frequencies
reflected the distances between genes on a chromosome.

It was predicted that the farther away two genes were from
each other, the higher the probability they would crossover
and therefore the higher the recomb. frequency.

Since the genes were farther away, there are more places or
points between them where a cross over can occur.

So a linkage map can be constructed from knowing how
often genes cross over.
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Figure 15.6 Using recombination frequencies to construct a genetic map
An Example: 3 genes: body color (b), wing size (vg), eye color
(cinnabar, cn, which is bright red). The recombination frequency
between b and cn is 9% and between cn and vg, 9.5%.
Multiple crossovers
cause the frequency
between b and vg to
not equal the sum of
9 + 9.5%.
One map unit = 1% cross over frequency
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Figure 15.7 A partial genetic map of a Drosophila chromosome
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Figure 15.8 Some chromosomal systems of sex determination
X-O System: grasshoppers have only one
type of sex chromosome. Males have only
one sex chromosome and thus are XO.
Z-W System: birds; the sex determiner is in
the egg, not the sperm and thus the
chromosomes are labeled Z and W; Males
are ZZ and females are ZW
Bees and ants have no sex chromosomes;
female bees develop from fertilized eggs
(2n) and the males are from unfertilized eggs
and are haploid
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Figure 15.9 The transmission of sex-linked recessive traits
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Determination of Sex
Anatomical indications at about 8 weeks
Sex-determining region Y
SRY gene on the Y chromosome is responsible for development of the testes.
SRY codes for a protein called the testis-determining factor or TDF. This
protein controls the expression of many other genes involved in testicular
development and sperm production.
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Fathers pass sex-linked alleles to all daughters but none of sons.
Mothers can pass sex-linked alleles to sons and daughters.
If the sex-linked trait is recessive, the female must be homozygous
but the males will only need one copy and are called hemizygous.
Therefore, more males express the trait/disorder than females.
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X-linked Genes
There are about 1000 X-linked genes.
•
Most code for something other than female anatomical traits.
•
Some of the non-sex determining X-linked genes are responsible for
hemophilia, red-green color blindness, congenital night blindness,
high blood pressure, duchene muscular dystrophy and fragile X
syndrome.
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Y-linked Genes
The Y chromosome is not only much smaller but also has about 26 genes and
gene families.
•
Most of these genes are involved with general cellular activities.
•
9 genes are involved with sperm production.
• When all 9 genes are missing or defective, the result is very low
sperm counts and infertility.
• It is not thought that about 1/3 of infertile couples are unable to
have children as a consequence of the male mate not having the
necessary sperm producing genes on his Y chromosome.
•
SRY gene is responsible for male anatomical traits.
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Sex-linked Disorders in Humans
Duchenne Muscular Dystrophy

weakening of muscles and loss of coordination due to loss of a muscle
protein called dystrophin, coded by a gene on X chromosome.

people live into their early 20s.
Hemophilia

sex-linked recessive

modern treatment is to inject those afflicted with the missing clotting
protein.

females tend to be carriers
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Sex-linked Disorders in Humans cont’d
X Inactivation in Female Mammals

One X chromosome in every cell is inactive so a female has one copy
active, like a male but it is not the same X chromosome in all of her
cells.

Barr bodies: condensed, inactive X chromosome attached to nuclear
membrane.

Occurs randomly in embryonic cells so females are a mosaic of two
types of cells. All mitotic divisions after the inactivation have the same
inactive X chromosome.

Calico or tortoiseshell cats

Methyl groups on cytosine seem to cause the inactivation of one of the
two X chromosomes. Which chromosome is chosen is determined by a
gene making RNA that binds to the X chromosome that will then be
inactive.
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Figure 15.10 X inactivation and the tortoiseshell cat
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Figure 15.10x Calico cat
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Goof-Ups in Chromosomal Structure and Number
1. Aneuploidy: abnormal chromosomal number due to
nondisjunction.

Trisomy: having 3 of that chromosome in a cell

Monosomic: having one less than the expected
chromosomal number.
2. Polyploidy: an extra set of chromosomes

Triploidy (3n) or tetraploidy (4n)

Triploidy can occur if a diploid egg cell is fertilized so
that means that all of the chromosomes went through
nondisjunction.

Tetraploidy: a 2n zygote duplicates all its chromosomes
and then fails to divide giving you a 4n cell that then30
undergoes mitosis.
Goof-Ups in Chromosomal Structure and Number cont’d
3. Alterations in structure
a) Deletion: a piece of a chromosome is lost and therefore
the cell containing that chromosome and all its descendants
will be missing certain genes.
b) Duplication: if this piece that is lost attaches to another
(sister chromatid) then you have a duplication because the
receiving sister chromatid has the chromosomal segment and
then it also gets the lost piece.
c) Inversion: a lost chromosomal fragment reattaches in the
reverse order.
d) Translocation: a lost piece attaches to a nonhomologous
chromosome.
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Figure 15.13 Alterations of chromosome structure
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Figure 15.12 A tetraploid mammal?
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Figure 15.11 Meiotic nondisjunction
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Human Disorders
Down’s Syndrome

1 out of every 700 children born

trisomic 21

altered facial features, short height, heart defects, mental
retardation.

a concern for pregnant mothers over age of 30; fetal testing
for trisomy 21 in the embryo
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Figure 15.14 Down syndrome
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Human Disorders
Klinefelter’s Syndrome

XXY male

one out of every 2000 births

small testes and sterile

may include breast enlargement
Superfemales or metafemales

XXX with 2 Barr bodies

can be normal fertile women to physiological defects
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Figure 15.x2 Klinefelter syndrome
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Human Disorders
Turner’s Syndrome

XO female

short stature, webbing of neck skin, underdeveloped gonads
XYY Males

also misnamed as “tall-aggressive syndrome”

More XYY males have been found among the noninstitutionalized population.

subnormal IQs which may contribute to impulsive behavior.
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Figure 15.x3 XYY karyotype
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Genomic Imprinting
 Definition: a gene is silenced and will have different effects
depending on whether it is in a male or a female regardless of
whether the allele was delivered in a sperm or an egg.
 Methyl groups are added to cytosine nucleotides which prevents
the transcription of a gene. The organism uses the product from
the unaffected allele.
 about 20 mammalian genes subject to imprinting
 Fragile X Syndrome: the tips of an X chromosome appear to
hang onto the rest of the chromosome.

1 out of every 1500 males and one of every 2500 females

mental retardation is the result
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Figure 15.15 Genomic imprinting (Layer 1)
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Figure 15.15 Genomic imprinting (Layer 2)
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Figure 15.15 Genomic imprinting (Layer 3)
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Figure 15.16 Cytoplasmic inheritance in tomato leaves
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Non-mendelian Inheritance (cytoplasmic inheritance)
•
Not all eukaryotic genes are on nuclear DNA
•
Some genes are in the mitochondria and chloroplasts
•
These genes are transmitted to the “daughter” organelles.
•
Mendelian inheritance is not demonstrated here.
•
Example: in plants, genes controlling coloration in the plastids all come
from the egg and not from the pollen. The coloration in the leaves is
therefore due to the maternal plastid genes only.
•
Example: mitochondrial genes in humans (and other mammals) are all
maternal. Mitochondria in the sperm do not enter the egg at the time of
fertilization.
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•
Mitochondrial mutations show up in the proteins making up the electron
transport chain and ATP synthase.
•
Nervous and muscular systems are the most susceptible to energy losses
so mitochondrial diseases show up the most in these body systems.
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Meiosis reduces chromosome number and rearranges genetic
information.
•
Explain how the reduction and rearrangement are
accomplished in meiosis.
•
Several human disorders occur as a result of defects
in the meiotic process. Identify ONE such
chromosomal abnormality; what effects does it have
on the phenotype of people with this disorder?
Describe how this abnormality could result from a
defect in meiosis.
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REDUCTION
•
homologous chromosomes pair, then separate anda move to
opposite poles during first meiotic division.
•
sister chromatids or chromatids separate during the second
meiotic division.
REARRANGEMENT
•
crossing over occurs
•
independent assortment of the tetrads (random alignment)
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CHROMOSOMAL ABNORMALITY
•
Identify ONE chromosomal abnormality
• Down syndrome, XO or Turner; Fragile X
•
Phenotype description
DESCRIBE- name or identify the meiotic event
•
Nondisjunction for Down’s Syndrome
•
Nondisjunction for Turner (sex chromosome)
•
Cri du chat ( deletion in chromosome 5)
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DESCRIBE- description of the meiotic event
•
Nondisjunction for Down’s Syndrome
•
Nondisjunction for Turner (sex chromosome)
•
Cri du chat ( deletion in chromosome 5)
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