1. Ch 15: Chromosomal Inheritance
2016

2. 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
• 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
• 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.
• 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 non-Mendelian 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.

3. 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.

4. 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)

5. 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

6. 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?

7. 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).
If 2n= 8, then during I can form
2n => 24 =16 Different gametes…
this is because of which stage
of meiosis?

8. 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.

9. Linked Genes Recombination of Linked Genes Results of Crossing Over.
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 … 25% 25% % 25%
Actual Results…2,300 offspring produced, but … % 41% 8% 8%

10. 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.

11. 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.

12. Sex Chromosomes Sex Chromosomes vary with organisms.
Heterogametic- ie XY
Homogametic - ie XX
Human
XX =
XY =

13. 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.

14. 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

15. 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.
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?

16. 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
Zygote 2n + 1
n (normal)

17. 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 non-disjunction of chromosome 21

18. Down Syndrome Karyogram

19. 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
From MedicinePlus NIH
Trisomy 13 occurs in about 1 out of every 10,000 newborns. Most cases are not passed down through families (inherited). Instead, the events that lead to trisomy 13 occur in either the sperm or the egg that forms the fetus.
Symptoms
Cleft lip or palate
Clenched hands (with outer fingers on top of the inner fingers)
Close-set eyes -- eyes may actually fuse together into one
Decreased muscle tone
Extra fingers or toes (polydactyly)
Hernias: umbilical hernia, inguinal hernia
Hole, split, or cleft in the iris (coloboma)
Low-set ears
Intellectual disability, severe
Scalp defects (missing skin)
Seizures
Single palmar crease
Skeletal (limb) abnormalities
Small eyes
Small head (microcephaly)
Small lower jaw (micrognathia)
Undescended testicle (cryptorchidism)
Exams and Tests
The infant may have a single umbilical artery at birth. There are often signs of congenital heart disease, such as:
Abnormal placement of the heart toward the right side of the chest instead of the left
Atrial septal defect
Patent ductus arteriosus
Ventricular septal defect
Gastrointestinal x-rays or ultrasound may show rotation of the internal organs.
MRI or CT scans of the head may reveal a problem with the structure of the brain. The problem is called holoprosencephaly. It is the joining together of the two sides of the brain.
Chromosome studies show trisomy 13, trisomy 13 mosaicism, or partial trisomy.
Treatment
There is no specific treatment for trisomy 13. Treatment varies from child to child and depends on the specific symptoms.
Support Groups
Support Organization for Trisomy 18, 13 and Related Disorders (SOFT) --
Hope For Trisomy 13 and
Outlook (Prognosis)
More than 90% of children with trisomy 13 die in the first year.
Possible Complications
Complications begin almost immediately. Most infants with trisomy 13 have congenital heart disease.
Complications may include:
Breathing difficulty or lack of breathing (apnea)
Deafness
Feeding problems
Heart failure
Vision problems

20. Aneuploidy in Sex Chromsomes

21. 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.