Ch 15: Chromosomal Inheritance

2016

From Topic 3.1Understandings:• 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 3.2Understandings:• 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 3.4Understandings:• 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.

Chapter 15: Chromosomal InheritanceFrom Topic 10.2Essential 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.

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 – 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?

Sex Linked Traits

• 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/independentassortment.html If 2n= 8, then during I can form

2n => 24 =16 Different gametes…this is because of which stage of meiosis?

Role of Independent Assortment in Sex Linkage

• 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 … 25% 25% 25% 25% Actual Results…2,300 offspring produced, but … 41% 41% 8% 8%

Linked Genes

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?

• 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)

Examples of Aneuploidy

Spermcell

n + 1

n (normal)

Zygote2n + 1

Eggcell

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

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

• 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

Polyploidy