Download sl revision notes on theoretical genetics

Essential Biology 4.3 Theoretical Genetics
Due Date:
Student Name:
Blog resource: http://tinyurl.com/26unw7c
Click4Biology: http://tinyurl.com/62qztrs
Highlight all objective 1 command terms in yellow
Highlight all objective 2 and 3 command terms in green – these will be part of the discussions in class.
1. Define the following:
Genotype
Gene expression
Phenotype
Dominant allele
Recessive allele
Codominant alleles
Homozygous
Heterozygous
Carrier
Mixed phenotype
Autosome
Sex chromosome
Gene locus
Monohybrid cross
Test cross
2. Outline the conventions for notation of genotypes, using one example of each:
Dominant/recessive alleles
Codominant alleles
Sex-linked traits
1. The allele for tall plants is dominant over the allele for dwarf plants.
a. State the possible genotypes of a tall plant.
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b. Explain how a test cross could be used to determine the genotype of a tall plant.
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3. Mendel is known as the father of genetics for his extensive experimental work with peas and
different types of crosses.
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a. Complete the punnet grid below to show the outcome of the monohybrid cross that results
in peas of different colours.
b. Complete the punnet grid below to show the possible outcomes of a cross between two
members of the F1 generation. Describe all genotypes produced.
4. Human ABO blood types follow a codominant inheritance pattern.
a. Describe what is meant by “some genes have multiple alleles.”
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Essential Biology 4.3 Theoretical Genetics
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b. Complete the table below to show how blood type is inherited.
alleles
i
IA
IB
i
IA
IB
c. Highlight the genotype and phenotype which is an example of
codominance.
5. Complete this pedigree chart to show the inheritance of blood types in this
family.
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Essential Biology 4.3 Theoretical Genetics
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6. Sickle cell is another example of codominance.
a. State the genotypes and phenotypes of these individuals.
HbAHbA
HbAHbs
HbsHbs
genotype
phenotype
b. Predict the phenotype ratios of offspring in the following crosses. Show all your working,
and set it out as expected. Take care with notation.
i.
Carrier mother with affected father
ii.
Affected father with unaffected mother.
iii.
Carrier mother with carrier father.
c. Explain how the prevalence of sickle cell in regions of Africa is an example of natural
selection in action.
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A
B
C
D
E
d. The pedigree chart below shows a family affected by sickle cell:
i.
Deduce the genotype of each individual with a letter.
F
G
H
$
#
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Essential Biology 4.3 Theoretical Genetics
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ii.
Calculate the likelihood of any further children produced by E and her # having
sickle cell anemia.
iii.
Male $ is healthy but of unknown genotype. Calculate the likelihood of any children
produced with female D having sickle cell anemia. Show all working.
7. Some traits are autosomal whereas others are carried on sex chromosomes.
a. Distinguish between autosomes and sex chromosomes.
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b. Annotate the diagram to distinguish between the X and Y chromosomes.
c. Outline the role of the SRY gene on the Y chromosome.
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d. Outline how non-disjunction can lead to gender-related chromosome abnormalities.
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Essential Biology 4.3 Theoretical Genetics
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8. Some inherited disorders are associated with gender.
a. Define sex-linkage.
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b. State two examples of sex-linked genetic disorders.
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c. Explain why sex-linked disorders are more common in males than females.
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d. Explain why human females can be homozygous or heterozygous for sex-linked genes,
where males cannot.
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e. The allele for colour blindness (n) is recessive to the allele for normal vision (N). This gene is
carried in a non-homologous region on the X chromosome. Complete the table below to
show the genotypes and phenotypes of individuals with regard to colour blindness.
Female
Male
XN XN
Normal
Affected
Not possible! Why?
Carrier
f.
In the space below, complete a punnet grid to show a cross between a normal male and a
carrier female. What is the expected ratio of phenotypes?
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Essential Biology 4.3 Theoretical Genetics
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9. Hemophilia is a blood-clotting disorder that is also recessive and sex-linked.
a. State the normal function of the gene associated with hemophilia.
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b. Describe the effects and symptoms of hemophilia.
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c. Use the pedigree chart to deduce the possible genotype(s) of the named individuals.
Leopold
Helen
Alice
Mary
Rubert
Bob
Britney
d. Outline one form of genetic engineering used to help patients with hemophilia.
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e. Suggest reasons why the frequency of some disease-related alleles might be increasing in
the population.
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Essential Biology 4.3 Theoretical Genetics
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Works Cited
1. Allott, Andrew. IB Study Guide: Biology for the IB Diploma. s.l. : Oxford University Press, 2007. 978-019-915143-1.
2. Mindorff, D and Allott, A. Biology Course Companion. Oxford : Oxford University Press, 2007. 978099151240.
3. Clegg, CJ. Biology for the IB Diploma. London : Hodder Murray, 2007. 978-0340926529.
4. Campbell N., Reece J., Taylor M., Simon. E. Biology Concepts and Connections. San Fransisco :
Pearson Benjamin Cummings, 2006. 0-8053-7160-5.
5. Taylor, Stephen. Science Video Resources. [Online] Wordpress, 2010.
http://sciencevideos.wordpress.com.
6. Burrell, John. Click4Biology. [Online] 2010. http://click4biology.info/.
7. IBO. Biology Subject Guide. [Online] 2007. http://xmltwo.ibo.org/publications/migrated/productionapp2.ibo.org/publication/7/part/2/chapter/1.html.
SL REVISION NOTES ON THEORETICAL GENETICS
Two major findings of Mendel:
Law of Segregation: each organism contains two alleles for each trait, and the alleles segregate
during the formation of gametes. Each gamete then contains only one allele for each
trait. When fertilization occurs, the new organism has two alleles for each trait, one from
each parent. Traits retain their individuality—not blended.
Law of Independent Assortment: members of an allelic pair segregate independently from
members of another allelic pair.
Alleles: alternative forms of a gene
Dominant: an allele that expresses itself and masks the effects of the allele(s) for the trait.
recessive: an allele that does not express itself in the phenotype when it is matched with a
dominant allele.
Genotype refers to an individual’s alleles for a gene
Phenotype refers to an individual’s appearance
homozygous: two identical alleles for a trait
homozygous dominant—having two dominant alleles:AA
homozygous recessive—having two recessive alleles: aa
heterozygous—having one dominant and one recessive allele, Aa
Monohybrid Cross: genetic cross that tracks the inheritance of a single character
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Law of Segregation: Mendel found that reproduction between two heterozygous
monohybrid individuals (Aa) resulted in both dominant and recessive phenotypes among
offspring, even though both parents’ phenotypes expressed the dominant phenotype. The
phenotypic ratio among the offspring was 3:1. Three offspring had the dominant
phenotype for every one that had the recessive phenotype.
Punnet square: used to determine all possible combinations and their probabilities.
1.
2.
3.
4.
5.
6.
Assign a symbol for each allele (will depend which allele is dominant).
Determine the genotype of each parent.
Determine the two possible kinds of gametes each parent can make.
Determine the gene combinations.
Determine the phenotypes of each potential offspring.
Calculate the genotypic and phenotypic ratios.
Incomplete dominance: when the heterozygous condition is intermediate in phenotype to the
two homozygous conditions. (Example: Red snapdragons crossed with white
snapdragons produce pink F1’s). Discuss why this is not blending.
Codominance: both alleles are expressed by the heterozygote.
IA allele codes for an enzyme that puts A carbohydrate on surface of blood cells.
IB allele codes for an enzyme that puts B carbohydrate on surface of blood cells.
i allele codes for neither carbohydrate (recessive to IA and IB alleles).
Multiple alleles (not always just two alleles): Four possible phenotypes for blood groups
that are determined by combinations of three different alleles of one gene.
HL EXTRA
Dihybrid Cross: genetic cross that tracks the inheritance of two characters simultaneously.
Law of independent assortment: Mendel found that members of an allelic pair
segregate independently from members of another allelic pair. (Use example
when two heterozygotes are crossed with each other to demonstrate that alleles
from one locus segregate independently from those at a second locus.)
Probability can be used to solve complex Mendelian genetics problems.
Polygenic Inheritance: when a number of different pairs of alleles at several loci are important for
expression of a trait. Such traits are typically quantitative in nature, not qualitative as seen in
dominance/recessive relationships. Skin Color (AABBCC = very dark skin; aabbcc = very light skin).
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Environment: Genetics are not the only factor that affects the phenotype. The environment can
have a dramatic effect.
For example, pigmentation (freckles or skin color) increases in the presence of sunlight.
(Some medications—for malaria—prevent tanning, so chemical environment can also
affect genetics.).
SOME GOOD GENETICS QUESTIONS
Long wing in fruit flies is
dominant to short wing.
Homozygous long-winged flies
were crossed with short-winged
flies. The F1 were allowed to
mate amongst themselves and
then the F2 counted. There
were 1149 long-winged flies
and 379 short-winged flies.
What was the genotype and
phenotype of the F1 flies and
explain the numbers in the F2.
Being aggressive is the dominant
characteristic of the Belliger Ant and
gentle is the recessive. Two aggressive
ants were crossed and in the F1 there
were three times more aggressive ants
than gentle ants. Explain this.
Mendelian Genetics Practice Problems
For the problems listed below, you are to solve the type of inheritance: simple dominance, codominance, sex-linked gene, multiple alleles, etc. and explain the rationale of your choice.
1. Mice I
o
a) cross 1: red-eyed mouse X white-eyed mouse
gives F1: all red-eyed
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cross 2: red-eyed F1 X red-eyed F1
gives F2:
36 red-eyed
13 white-eyed
o
b) cross 1: long-eared mouse X short-eared mouse
gives F1:
12 long-eared
10 short-eared
cross 2: long-eared F1 X long-eared F1
gives F2:
36 long-eared
13 short-eared
2. Flowers
cross 1: blue-flowered plant X white-flowered plant
gives F1: all pale-blue-flowered
cross 2: pale-blue F1 X pale-blue F1
gives F2:
27 blue
49 pale-blue
24 white
3. Blood Type
o a) cross 1: person, type A blood X person with type B
gives F1: all type AB blood
cross 2: type AB F1 X type AB F1
gives F2:
2 type A
4 type AB
1 type B
o
b) cross 1: type A blood X type B
gives F1:
2 type A blood
3 type AB blood
1 type B blood
2 type O blood
4. Mice II
cross 1: tail-less mouse X normal mouse
gives F1:
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10 tail-less
9 normal
cross 2: tail-less F1 X tail-less F1
gives F2:
10 normal
21 tail-less
9 dead
5. Flies
cross 1: red-eyed female X red-eyed male
gives F1:
50 red-eyed female
25 red-eyed male
25 white-eyed male
cross 2: white-eyed male F1 X red-eyed female F1
52 crosses give:
30 red male
33 red female
48 crosses give:
22 red male
24 red female
21 white male
23 white female
6. Pedigrees
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You can check your answers on the Solutions Page.
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