1. Genes may be linked or unlinked and are inherited accordingly.
INHERITANCE
Genes may be linked or unlinked and are inherited accordingly.
AHL Topic 10.2
IB Biology
Miss Werba

2. TOPIC 10 – GENETICS and EVOLUTION
10.1
MEIOSIS
10.2
INHERITANCE
10.3
GENE POOLS and SPECIATION
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3. THINGS TO COVER U.1 U.2 U.3 U.4 U.5 A.1 A.2 Statement Guidance
Gene loci are said to be linked if on the same chromosome.
U.2
Unlinked genes segregate independently as a result of meiosis.
U.3
Variation can be discrete or continuous.
U.4
The phenotypes of polygenic characteristics tend to show continuous variation.
U.5
Chi-squared tests are used to determine whether the difference between an observed and expected frequency distribution is statistically significant.
A.1
Morgan’s discovery of non-Mendelian ratios in Drosophila.
A.2
Completion and analysis of Punnett squares for dihybrid traits
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:
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4. THINGS TO COVER A.3 S.1 S.2 S.3 Statement Guidance NOS 3.2
Polygenic traits such as human height may also be influenced by environmental factors.
S.1
Calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
S.2
Identification of recombinants in crosses involving two linked genes.
S.3
Use of a chi-squared test on data from dihybrid crosses.
NOS 3.2
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.
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5. UNLINKED vs LINKED GENES
Unlinked Genes:
Genes that occur on separate chromosomes
Follow Mendel’s Law of Independent Assortment - inheritance of alleles is random
Linked genes:
Genes that occur on the same chromosome
Likely to be inherited together
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6. DIHYBRID CROSSES
A.2
S.1
Application: completion and analysis of Punnett squares for dihybrid traits
Skill: calculation of the predicted genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes
The basic principles applied to solving monohybrid problems are also used for dihybrid problems.
The only difference is that 2 traits are considered at the same time, rather than one.
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7. DIHYBRID CROSSES Assign letter symbols for alleles
Determine the parental genotypes
Work out the possible allele combinations for each parent
Draw up the Punnett square & complete.
Determine genotype ratio – by counting the different genotypes
Determine the phenotype ratio – by determining the possible phenotypes from the genotypes
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8. DIHYBRID CROSSES Example:
Two traits will be considered in Drosphilia flies:
Eye colour
Wing length
These traits are coded for by unlinked genes.
Assume that:
Red eyes is dominant over pink eyes
Long wings are dominant over short wings
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9. DIHYBRID CROSSES Example:
A fly with pink eyes and short wings was crossed with a pure breeding one that had red eyes and long wings.
Determine the genotype and phenotype ratios in the F1 and F2 generations.
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10. DIHYBRID CROSSES Example: Let: R = red eyes , r = pink eyes
L = long wings, l = short wings
Parents are:
pink eyes and short wings :
possible gametes:
red eyes and long wings:
rrll rl
RRLL RL
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11. DIHYBRID CROSSES Example: rl RL RrLl F1 Genotype ratio:
F1 Phenotype ratio: rl RL
RrLl
100% RrLl
100% red eyes, long wings
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12. DIHYBRID CROSSES Example: RrLl x RrLl
If F1 generation are left to interbreed:
Possible gametes formed are: RL Rl rL rl
RrLl x RrLl
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13. DIHYBRID CROSSES Example: F2 Genotype ratio: RRLL; RRll; rrLL; rrll
RrLl; rrLl; Rrll; RrLL; RRLl
F2 Phenotype ratio: 9 red eyes, long wings
3 red eyes, short wings
3 pink eyes, long wings
1 pink eyes, short wings RL Rl rL rl
RRLL
RRLl
RrLL
RrLl
RRll
Rrll
rrLL
rrLl
rrll
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14. DIHYBRID CROSSES Example: In this cross the parents were:
red eyes, long wings and
pink eyes, short wings.
The recombinants are:
red eyes, short wings
pink eyes, long wings
The recombinant ratios observed are:
9 red eyes, long wings (same as parent genotype)
3 red eyes, short wings (recombinant)
3 pink eyes, long wings (recombinant)
1 pink eyes, short wings (same as parent genotype)
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15. DIHYBRID CROSSES
A.2
S.1
When two pure breeding individuals are crossed, the F1 will all have the same genotype and phenotype.
They will all be heterozygous.
If allowed to interbreed, the F2 will have a ratio of:
9 : 3 : 3 : 1
This is the same ratio that Gregor Mendel found over 300 years ago in his studies of pea plants.
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16. DIHYBRID CROSSES RrLl x RrLl  9:3:3:1 RrLl x RrLL  3:2
A.2
S.1
Common expected ratios of dihybrid crosses:
RrLl x RrLl  9:3:3:1
RrLl x RrLL  3:2
RrLl x Rrll  4:3:1
RRll x rrLL  all RrLl
RRLL x rrll  all RrLl
Rrll x rrLl  1:1:1:1
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17. Looking for patterns, trends and discrepancies
MORGAN
NOS 3.2
Looking for patterns, trends and discrepancies
Mendel used observations to find and explain patterns and trends in the natural world. Morgan discovered non-Mendelian ratios in his Drosophila experiments.
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18. MORGAN’S EXPERIMENT
A.1
Thomas Hunt Morgan discovered non-Mendelian ratios in his experiments with Drosophila
Morgan bred a white-eyed male mutant with red eyed female flies:
F1 – all had red eyes (consistent with Mendel)
F2 – had a small number (~25%) with white eyes
This is also consistent with Mendalian theory
However, all the white-eyed flies were male – which is inconsistent with Mendalian theory
It suggested that the two traits were linked.
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19. MORGAN’S EXPERIMENT The gene for white eyes is located on
The X chromosome!
They are sex-linked.
Proof?
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20. LINKED GENES
U.1
S.2
Linked genes are pairs or groups of genes which are carried on the same chromosome and are inherited together.
Mendel’s law of independent assortment is not always true for genes on the same chromosome.
It is still possible for recombination to occur between linkage groups, but it is far less likely.
Gene linkage can occur on the autosomes and sex chromosomes.
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21. LINKED GENES
U.1
S.2
The distance between two gene loci is measured as a recombination fraction (θ).
It is measured in CentiMorgans (cM) – equivalent to a 1% chance of recombination.
If two loci are not linked, then θ = 0.5
indicating that the genes separate together in 50% of all meioses
If two loci are linked, then θ = 0.1
indicating that the genes separate together in 90% of all meioses and crossing over occurs rarely (only 10%)
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22. LINKED GENES TtBb T and B are linked T and B not linked
U.1
S.2 T t B b
T and B not linked T t B b
T and B are linked
Alleles in linkages groups are shown as vertical pairs:
This means that T is linked to B (on same chromosome) and t is linked to b (on same chromosome).
Alleles are normally shown side-by-side in dihybrid crosses
TtBb
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23. LINKED GENES
U.1
S.2
Linked genes can only be broken apart by crossing over and gene recombination during prophase I of meiosis
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24. INHERITANCE
Q1.
A test cross was conducted on a fly heterozygous for eye colour and body colour:
The results are shown below:
151 wild-type (normal colour eyes & normal body colour)
8 purple eyes and normal body colour
10 normal eyes and black body colour
131 purple eyes and black body colour
What is a test cross?
Explain how these F1 offspring support the idea that genes P and B are linked.
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25. INHERITANCE Q1. Draw a chromosome picture for the heterozygous parent.
Which F1 phenotypes represent:
parental-type offspring?
recombinant offspring?
Calculate the percentage crossing over or recombination fraction (θ).
If a recombination fraction of 1% is equivalent to one map unit (used to map chromosomes), calculate the map distance between genes P and B.
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26. INHERITANCE Q2. Two genes are linked as shown here:
The genes are far apart such that crossing-over between the alleles occurs occasionally.
Which statement is true of the gametes?
All of the gametes will be Em and eM.
There will be equal numbers of EM, eM, Em and em.
There will be approximately equal numbers of EM and eM gametes.
There will be more Em gametes than em gametes.
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27. INHERITANCE
Q3. Predict the genotypes of the offspring (and their expected frequency) for the following mating:
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28. INHERITANCE
Q4.
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29. INHERITANCE
Q5. In some maize plants the seed is enclosed in a green sheath called a tunica. The allele (T) for this is dominant to the allele (t) for normal, unenclosed seeds. The endosperm of the seed can be starchy (allele E) or sugary (allele e). The genes for these two characteristics are linked. The table below shows the outcome of crosses between a plant heterozygous for both characteristics and one that is homozygous recessive for both characteristics.
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30. INHERITANCE
Q5.
State the genotype of the heterozygous parent using the correct notation. [1]
Identify which individuals are recombinants in this cross. [1]
Explain what has occurred to cause these results. [2]
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31. CHI-SQUARED ANALYSIS
A.2
Chi squared analysis can be used to compare the predicted and actual outcomes of genetic crosses.
Example:
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32. CHI-SQUARED ANALYSIS Examples:
Corn genetics chi square analysis (answer key)
Chi-square in biology: Testing for a dihybrid ratio
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33. POLYGENIC INHERITANCE
U.3
A single characteristic controlled by multiple genes
Gives rise to continuous variation in a phenotype.
eg.
skin colour in humans
seed colour in wheat
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34. POLYGENIC INHERITANCE
U.3
A.3
Other examples:
susceptibility to heart disease, cancer, mental illness
the Autism spectrum
obesity
eye colour
height
It is suspected that multiple gene interactions and environmental factors play a role in inheritance here.
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35. SKIN COLOUR Human skin colour is controlled by 3 or 4 genes
depends on the melanin concentration of the skin (a black pigment)
each gene has alleles which promote melanin production and alleles which do not.
Thus there is a wide range of phenotypes possible
Range from:
all alleles promoting melanin production
all alleles impeding melanin production
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36. SKIN COLOUR And again, it is also influenced by environmental factors
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37. SKIN COLOUR Watch this TED talk
Nina Jablonski is an anthropologist and author of Skin: A Natural History, a close look at human skin’s many remarkable traits.
Nina Jablonski says that differing skin colors are simply our bodies' adaptation to varied climates and levels of UV exposure. Charles Darwin disagreed with this theory, but she explains, that's because he did not have access to NASA.
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38. SKIN COLOUR Watch this TED talk
Advantages of dark skin in hot climates?
UV protection
Protection against skin damage and cancer
Dark skinned people in cold climates / low-sunlit areas may need vitamin D supplements
Advantages of pale skin in cold climates?
Increased vitamin D production in low-sunlight conditions
Increased immune protection
Pale skinned people in hot climates / high-sunlit areas need sunscreen!
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39. INHERITANCE Q6. What is polygenic inheritance?
A character that is controlled by two or more genes
A character that is controlled by more than two copies of a gene
Inheriting more than two alleles of a gene
Inheriting a linked group of genes
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40. INHERITANCE Q7. Define the term polygenic inheritance. [1]
Explain, using a named example, how polygenic inheritance gives rise to continuous variation. [2]
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