Transcript Chapter 15

Chapter 15
The Chromosomal
Basis of Inheritance
Morgan & Strutevant
Sex Chromosomes
Linked Genes
Gene Map
Why you are smarter than Sarah Palin
Human Genetic Disorders
Chromosome theory of inheritance
supported Mendel’s laws
(what are mendel’s laws?)
Chromosome theory of inheritance
supported Mendel’s laws
1. Law of Segregation- pairs of factors
separate during gamete formation
Law of Independent Assortmentseparated pairs of factors sort themselves
into gametes independently of each other.
THOMAS HUNT MORGAN was the first scientist to
associate a specific gene with a specific
chromosome. His experiments provided convincing
evidence that chromosomes are the location of
Mendel’s heritable factors.
Gene stained with fluorescent dye shows the same locus on homologous chromosomes.
Experimental organism =
Fruit fly = Drosophila melanogaster
Discovery of Sex Linkage
(sex experiments w/ flies = creepy)
Cross #1 :
P generation
(pure breeding)
F1 results: all had
red eyes=wild
• Suggests:
• like Mendel’s
CROSS #2 (bro x sis)
F1 x
F1 = F2
F2 results:
• 3 wild type: 1 mutant
• Except that:
– only males are mutant
– all the females are wild
type and
– Males are mutant 1/2
of the time.
Figure 15.3 Sex-linked inheritance
Red is wild type
White is mutant…
F1- all are wild type
F2- all of the females are wild type.
1/2 males are wild type
Discovered Sex chromosomes
The gene for eye color was
Inherited differently among
Male and female flies in the F2.
Difference: x & y chromosomes
Genes located on a sex chromosome
Are called X LINKED.
The number of genes in a cell is far greater
than the number of chromosomes; in fact,
each chromosome has hundreds or
thousands of genes.
Thus, linked genes tend to be inherited
Gene Linkage and Map Units
• Gene linkage was explained by
Thomas Hunt Morgan in 1910.
• When he was examining traits of
the fruit fly• Drosophila melanogaster.
Why study the fruit fly?
Great for research because it was
benign (harmless) unlike MEDFLY
Reproductive cycle: 2 weeks
Small genome: 2n is 8
-so great for genetic research.
Gene Linkage and Map Units
• Genes are said to be
LINKED when they
• exist on the same
• and they are inherited
The genes of chromosome 9
are “LINKED”.
Gene Linkage and Map Units
• When genes are linked,
the expected phenotype
ratios during breeding
experiments deviate from
the Mendelian ratios of:
1) F1 xF1 dihybrid cross
Ex (AaBb x AaBb)
F2 = 9:3:3:1
More importantly:
2) F1 dihybrid test cross
Ex (AaBb x aabb)
F2 = 1:1:1:1
Unnumbered Figure (page 272) Drosophila testcross
For Example:
• Parents who were pure for two
traits (homozygous) were crossed
(AABB x aabb).
• The F1 generation produced
individuals that were
heterozygous for both traits.
• An F1 individual is test-crossed
with a homozygous recessive
individual. (AaBb x aabb)
• If both genes were located on
different chromosomes the
expected phenotypic ratio
should be 1:1:1:1.
• The actual ratios suggested that
the genes DID NOT assort
independently. Most of the
phenotypes matched the P1
generation parents.
• Recombinant phenotypes (those different from
either P1 gen parents) were the result of crossing
over instead of independent assortment.
The recombination frequency can be calculated by
dividing the total number of recombinants
by the total number of offspring.
Figure 15.5b Recombination due to crossing over
• One of Morgan’s students, Alfred Sturtevant came up with a
method for constructing a genetic map, showing the
position of genes on a chromosome.
• An important observation the recombination frequencies
reflect the distances between genes.
• Therefore, genes farther apart have a greater chance of
being separated by crossing over.
• As the distance between genes increases, so does their
recombination frequency.
• The distance between genes are expressed in “map units”
where one map unit = 1% recombination frequency.
• Map units are called centimorgans, in honor of Morgan.
Morgan’s work:
Evidence of
“linked genes”
in drosophila.
Evidence of the
basis of inheritance.
“Where does a lot of that earmark
money end up anyway? […] You've
heard about some of these pet
Projects… they really don't make a
whole lot of sense and sometimes
these dollars go to projects that
have little or nothing to do with
the public good.
Things like fruit fly research in
Paris, France.
I kid you not.”
Palin "fruit fly research"
Next- human genetic disorders.
Chapters 14 & 15:
Genetic Diseases & Disorders
What is the difference between
an infectious disease and a
genetic one???
• Infectious illnesses/ diseases caused by
microorganisms: virus, bacteria, protists, fungi,
tiny animals (worms) that harm cells.
• They can be transmitted from one host to
another… contagious.
• Genetic is inherited. Defect in the DNA code.
• Thousands of genetic disorders are caused
by recessive genes (mutations in DNA).
• Some are mild and some are deadly.
• Most of these alleles code for a malformed
protein or for creating no protein at all.
• NOT CONTAGIOUS, but can be
transmitted to offspring.
Inheritance Patterns of
Genetic Disorders:
Autosomal Recessive gg = disease
Autosomal Dominant Gg = disease
Autosomal Codominant
Sex Linked Recessive XgXg XgY = disease
Sex Linked Dominant XGXg XGY = disease
ANEUPLOIDY of autosomes (too many/few) disease
ANEUPLOIDY of sex chromosomes= “
CHROMOSOME alteration during crossing over
Examples of recessively inherited
disorders (autosomal recessive)
1) Cystic fibrosis: most lethal disease in the US
Caused by defective chloride membrane channels.
Leads to thick mucus building up in the lungs, digestive tract…
low weight, susceptible to respiratory infections.
2) Tay-Sachs: caused by a dysfunctional enzyme
that no longer breaks down fats.
Leads to fatty build-up in the brain and around nerves.
Accumulation of the lipids in brain cells causes progressive
nervous system dysfunction and is usually fatal by age four.
3) Phenylketonuria (PKU):
inability to properly break
down the amino acid
phenylalanine. Untreated
causes mental retardation.
Managed with restricted
diet low in phenylalanine.
Sickle Cell: caused by a single amino
acid substitution in hemoglobin. Red
blood cells of individuals with this
defect are unable to effectively
transport oxygen throughout the
(Pleiotropic effect on multiple organs)
DOMINANT disorders:
• Huntington’s disease: is a
degenerative disease of the
nervous system, the allele
expresses itself later in life
so although it is caused by
a single allele and is lethal,
it may already be passed
on to the next generation.
• Achondroplasia (dwarfism):
Characterized by problems
with bone growth.
Also… polydactyl
Sex Linked Disorders & Patterns of Inheritance
NO heterozygote among males because the gene is on the X chromosome…
males are XY. Heterozygous females are “carriers”.
Sex linked recessive: Males more susceptible to disease.
1. ALD (adreno leuko dystrophy)- sex linked recessive
2. Red-Green Color Deficiency- sex linked recessive
3. Hemophilia- sex linked recessive
Sex linked dominant: Females more susceptible to disease.
1.Duchenne’s Muscular Dystrophy- sex linked DOMINANT
In humans, such X linked inheritance
Is designated XCXC XCXc XCXC XcY XCY
Ishihara color blindness test
Red Green Color Blindness
is much more prevalent
among males.
• Hemophilia- inability to code for all factors required to
form normal blood clots. Surface wounds ok.
• Duchenne’s Muscular Dystrophy- Absence of an
essential muscle protein. Results in deteriorating muscles
and loss of coordination.
(sex linked dominant- so females are more likely to show
the disorder than other x-linked ones)
Large hemorrhage… surface wounds are slow to heal but not
Fatal- it is the internal bleeding and tissue wounds that are a problem
Hemo (blood) Philia (family)
History's most famous carrier of the gene for hemophilia was Victoria (1819-1901),
Queen of England and grandmother to most of the royalty in Europe. In 1853, Queen
Victoria gave birth to her eighth child, Leopold, Duke of Albany, who had hemophilia
and died at the age of 31 from internal bleeding after a fall.
Two of Queen Victoria's four daughters, Alice (b. 1843) and Beatrice (b. 1857), also
carried the gene for hemophilia and subsequently transmitted the disease to three of
Victoria's grandsons and to six of her great-grandsons.
Alice's daughter Alexandra also was a carrier of hemophilia, and she transmitted the
disease to her son Alexis (b. 1904), whose father was Czar Nicholas 11 (1868—
1918) of Russia. Alexis is perhaps the most famous of the European royals with
hemophilia. Alexis was the heir to his father's throne and his medical condition
caused much anxiety in the royal household. Historians are still discussing the role
Alexis's condition played in the Russian revolution of 1918.
• If an organism is born
with an abnormal number
of chromosomes it is
called aneuploidy.
• It is caused by
nondisjunction of
chromosomes during
• The chromosomes fail to
separate and one gamete
receives both copies and
the other gamete
receives none.
Down syndrome or Trisomy 21 is caused by
having three copies of the 21st chromosome.
Sex Chromosome disorders:
X inactivation: during embryonic development in females one
X chromosome randomly condenses into an inactive mass
(called a Barr body) within each cell… so each cell has only
one active X chromosome.
The result: most of the alleles on the X chromosome are
expressed individually.
X-inactivation, is an epigenetic change that results in a
different phenotype but is not a change at the genotypic level.
This can give rise to mild symptoms in female ‘carriers’ of Xlinked genetic disorders.
Reversed in the female germline, so that all oocytes contain an
active x chromosome
•In cats: this leads to tortoiseshell coloration because in
Some cells one x is inactive and in others, the other is.
•In humans: females will express recessive disease alleles
more frequently… EX: faulty sweat glands in some areas.
Figure 15.10x Calico cat
2) Nondisjunction of
a)XXY individuals:
• Male sex organs
• Small, sterile testes
• Female body
including some
breast development
2) Nondisjunction of SEX CHROMOSOMES
a)XXY individuals: Klinefelter syndrome
• Male sex organs
• Small, sterile testes
• Female body characteristics, including some
breast development
b) XYY individuals: no associated syndrome
• Taller than average
c) XXX individuals: super females (trisomy x) taller
than average, slightly lower intelligence
d) XO individuals: Turner syndrome
• Sterile
• Sex organs do not mature
3) Altering Chromosome Structure =
usually occurs during cell division/ chromosome replication.
a) Deletion: removing a segment of chromosome
ex. Cri du chat syndrome
b) Duplication: segments on a chromosome are repeated
c) Inversion: sections of the chromosome are reversed
d) Translocation: a segment of one chromosome is broken off and
reattached on another non-homologous chromosome.
Genetic phenomenon by which certain genes are
expressed in a “parent-of-origin-specific ”
Inheritance process independent of classical
Mendelian Inheritance.
Involves “methylation” or silencing of genes as
well as histone activation of others.
What is Prader-Willi syndrome?
• (PWS) is the most common known genetic cause of life-threatening
obesity in children.
• abnormality on the 15th chromosome inherited from the father.
• It occurs in males and females equally and in all races.
• Prevalence estimates have ranged from 1:8,000 to 1:25,000 with the most
likely figure being 1:15,000.
• PWS typically causes low muscle tone, short stature if not treated with
growth hormone, incomplete sexual development, and a chronic feeling
of hunger that, coupled with a metabolism that utilizes drastically fewer
calories than normal, can lead to excessive eating and life-threatening
• Children with PWS have sweet and loving personalities, but this phase is
also characterized by increased appetite, weight control issues, and motor
development delays along with some behavior problems and unique
medical issues.
• Genetic cause:
Abnormal maternal
chromosome 15
• Prader
Figure 15.x1 Translocation
Figure 15.12 A tetraploid mammal?
Independent assortment of
chromosomes and crossing over
produce genetic recombinants