Ch 15: Chromosomal Inheritance
2016
Chapter 15: Chromosomal Inheritance
From Topic 3.1
Understandings:
• A gene is a heritable factor that consists of a length of DNA and
influences a specific characteristic.
• A gene occupies a specific position on a chromosome.
From Topic 3.2
Understandings:
• In a eukaryote species there are different chromosomes that carry
different genes.
Applications and skills:
• Application: Use of karyograms to deduce sex and diagnose Down
syndrome in humans.
• Application: Non-disjunction can cause Down syndrome and other
chromosome abnormalities.
• Application: Studies showing age of parents influences chances of
nondisjunction.
From Topic 3.4
Understandings:
• Some genetic diseases are sex-linked. The pattern of inheritance is
different with sex-linked genes due to their location on sex
chromosomes.
• Many genetic diseases have been identified in humans but most
are very rare.
Applications and skills:
• Application: Red-green colour blindness and hemophilia as
examples of sexlinked inheritance.
Guidance:
• Alleles carried on X chromosomes should be shown as superscript
letters on an upper case X, such as Xh.
Aim 8: Social implications of diagnosis of mutations, including the
effects on the family and stigmatization.
From Topic 10.2
Essential idea: Genes may be linked or unlinked and are inherited
accordingly.
Nature of science: Looking for patterns, trends and discrepancies—
Mendel used observations of the natural world to find and explain
patterns and trends. Since then, scientists have looked for
discrepancies and asked questions based on further observations to
show exceptions to the rules. For example, Morgan discovered nonMendelian ratios in his experiments with Drosophila (3.1).
Understandings:
• Gene loci are said to be linked if on the same chromosome.
• Unlinked genes segregate independently as a result of meiosis.
Applications and skills:
• Application: Morgan’s discovery of non-Mendelian ratios in
Drosophila.
• Skill: Identification of recombinants in crosses involving two linked
genes.
Guidance:
• Alleles are usually shown side by side in dihybrid crosses, for
example, TtBb. In representing crosses involving linkage, it is more
common to show them as vertical pairs, for example:
• This format will be used in examination papers, or students will be
given sufficient information to allow them to deduce which alleles are
linked.
Aim 8: Ethical issues arise in the prevention of the inheritance of
genetic disorders.
Thomas Hunt Morgan Fly Experiment
• Performed fruit fly
experiments that first
associated a specific gene with
a specific chromosome.
• Studied Drosophila
Melanogaster (fruit fly)
• Easy to culture
• Breed extremely fast
• Short generation time
• Four pair of chromosomes
that are easily observed
under the microscope.
Fruit Fly Genetics Overview
• Three pair of autosomes
and one pair of sex
chromosomes.
• Females have two X’s
• Males have an X and Y
• Wild Type (most
common in the
population)
• Mutant phenotypes
(different from the
wild-type allele)
Sex Linkage
• After an entire year of breeding,
Morgan discovered a white eyed
male fly.
• w+ = red eyes
• w = white eyes
W= stands for the mutant
phenotype
+ = if it’s wild type
Sex Linked Traits
• Sex Linked Traits – traits that
are linked to either the X or Y
chromosome. Usually the X
chromosome… Why?
• Eye color in fruit flies was
linked to the X chromosome.
Predict the phenotypes of
the F1 generation?
-What should the female offspring look like?
-What should the male offspring look like?
Predict the phenotypes of
the F2 generation?
-What should the female offspring look like?
-What should the male offspring look like?
Role of Independent Assortment in Sex Linkage
• Genes on the same
chromosome tend to assort
together and don’t assort
independently.
• Genes on different
chromosomes show a 50%
recombination frequency.
– This means that ~50% of the
offspring are recombinants
(different from the parental
phenotypes).
http://www.sumanasinc.com/webcontent/animations/content/independentas
sortment.html
If 2n= 8, then during I can form
2n => 24 =16 Different gametes…
this is because of which stage
of meiosis?
Linked Genes
• Linked Genes: genes that are
on the same chromosome tend
to be inherited together
• Since they are on the same
chromosome, they will not
show a 9:3:3:1 ratio from
Mendel.
If flower color and pollen shape is 100%
linked, then you would expect this for
phenotype ratio.
But you didn’t, you observed this.
WHY???
Intro to Chi-Square here.
Linked Genes
• Recombination of Linked Genes
– Results of Crossing Over.
http://bcs.whfreeman.com/thelifewire/content/chp10/1002002.html
What do you think the F1 generation was like?
What do you think the test cross produced?
If body color and wing size independently assort, then expect …
Actual Results…2,300 offspring produced, but …
25% 25% 25% 25%
41% 41% 8% 8%
Recombination Frequency
• If wing type and body color where on different
chromosomes (unlinked), they would assort independently
and show the predicted phenotypes 1:1:1:1.
• If the genes were completely linked, expected results from
the test cross would be a 1:1 phenotypic ratio of parental
types only.
Recombination Frequency Continued
• Morgan’s testcross did not produce results consistent with
linked or unlinked. There was a high percentage of parental
phenotypes, which suggested linkage between two genes.
• Chi-squared gave reason to doubt independent
assortment…1:1:1:1
• Chi-squared gave reason to doubt 100% dependent
assortment 1:1
• Morgan discovered that there must be a mechanism for
exchanging parts of chromosomes, which was later found
out to be Crossing Over.
Sex Chromosomes
• Sex Chromosomes vary with
organisms.
• Heterogametic- ie XY
• Homogametic - ie XX
• Human
• XX =
• XY =
SRY Gene
• What determines male development?
• SRY (Sex-determining Region of Y) gene on the y
chromosome triggers a series of events that make testes
develop where the ovaries normally do.
X-linked Examples
• X linked:
• Color blindness: recessive
trait in which there is an
inability or decreased ability
in seeing a color.
XR = normal
Xr = color blindness
• Hemophilia: rare “bleeding”
disease in which the blood
doesn’t clot normally. It is
recessive trait.
XH = normal
Xh = hemophilia
Errors in Chromosomal Inheritance
• Alteration of chromosome number
• Aneuploidy: abnormal number of a specific
chromosome
•If there is three, it is said to be trisomic.
•If it is missing one it is said to be monosomic.
•If there is an error early in development all others will have same
problem.
http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.swf
• Polyploidy: having more than two complete
chromosome sets (3n or 4n instead of 2n)
• All due to non-disjunction
• Either in meiosis one or two
• What would be the result in mitosis?
Examples of Aneuploidy
•
•
•
•
•
•
•
Down Syndrome: Trisomy 21
Patau Syndrome: Trisomy 13
Edwards Syndrome: Trisomy 18
Kleinfelter Syndrome: XXY (Not so normal male)
Extra Y: XYY (Normal Male)
Triple X: XXX (Normal Female)
Turners: XO (Not so normal female)
n+1
Egg
cell
Sperm
cell
n (normal)
Zygote
2n + 1
Trisonomy 21
•Trisomy 21, is a genetic syndrome caused by a triplication of
chromosome 21.
•It occurs in about 1/800 to 1/1000 live births
•95% of the time, Down syndrome is caused by maternal nondisjunction of chromosome 21
Down Syndrome Karyogram
Trisonomy 13
• Patau Syndrome Trisomy 13
• Least common of the live-born
trisomy disorders, with an
incidence of 1 in 10,000
newborns
• 75% of trisomy 13 cases are due
to maternal nondisjunction
• heart defects, small or poorly
developed eyes, spinal cord or
brain abnormalities, cleft lip or
palate, extra toes or fingers, and
decreased muscle tone
• More than 90% of Trisomy 13
newborns die before reaching
one year of age
Aneuploidy in Sex Chromsomes
Polyploidy
• A chromosome number that has more than two complete set
of chromosomes.
• Triploidy
• Tetraploidy
• Polyploidy is important in plants
- For example, strawberries are octoploid (has 8 sets of
chromosomes (instead of 2) for each of their 7
chromosomes. Some species of strawberries are decaploid.
• Rare in animals, some mosaics occur but very uncommon.
http://kisdwebs.katyisd.org/campuses/MRHS/teacherweb/hallk/Teacher%20Documents/AP%20Biology%20Materials/Evolution/Speciation%20by%20Changes
%20in%20Ploidy/25_A02s.swf