Download Variations from Mendel`s original Crosses

Variations from Mendel’s original Crosses - PPT
•Muliple alleles
•Polygeneic inheritance
•Linked dihybrid crosses
Linked to part 2 – Multiple alleles and polygenic inheritance provide further variability within a trait

Give examples of characteristics determined by multiple alleles in an organism other than humans

Compare the inheritance of the ABO and Rhesus blood group

Define what is meant by polygenic inheritance and describe one example of polygenic inheritance in humans or another
organism
Linked to part 3 – Studies of offspring reflect the inheritance of genes on different chromosomes and genes on the same
chromosomes

Describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations

Predict the difference in inheritance patterns if two genes are linked

Explain how cross-breeding experiments can identify the relative position of linked genes

Discuss the role of chromosomes mapping in identifying relationships between species
•Mendel’s original pea experiments studied the segregation of ___ different characteristics.
•He looked at each of the 7 characteristics separately – he could do this because they were all on ____________
chromosomes. In other words they were not ___________ together on the same chromosome. If they were on
the same chromosome they could not _______________ independently.
•Single genes that are linked to the ___ chromosome - such as haemophilia and __________ blindness - showed
a variation to the usual Mendelian ratios because not all individuals have two X chromosomes. (see Beyond
Mendel ppt in Blueprint of Life)
•Another variation we have already studied is co-dominance – such as ______ coat colour in cattle. (see Beyond
Mendel ppt in Blueprint of Life)
•Let us consider a few more variations from the standard…
PART 1 - Multiple alleles
•With Mendel’s original crosses there were only ever ______ alleles of the one gene to choose from; short or
_______, angular or __________, etc.
•Sometimes there are _________ than two alleles to inherit even though we can still only inherit a _______ of 2
alleles (but 1 if the allele is linked to the X or Y chromosome).
•More than two choices of alleles (and there can sometimes be _____________ – we won’t be doing any of
those!!!) are known as ____________ ____________.
•Examples in humans are the ________ blood groups (3 alleles) and hair or eye colour.
•Let’s have a look at the ABO alleles where allele A and B are co-dominant when inherited together while both are
dominant if inherited with O.
Draw in an example of a cross:
•Non-human examples occur in genes for coat patterns in many animals such as in mice.
•In mice the following genotypes match the following phenotypes:
•CyCa = Yellow coat
•CbCa = Black coat
•CaCa = Agouti (mixed) coat
•CbCy – Black coat
•Therefore black is the most _______________ allele, the yellow allele is dominant over ______________ and
the agouti allele is recessive to both black and yellow for the coat colour gene.
Draw in an example of a cross:
PART 2 – Polygenic Inheritance
•______________ inheritance occurs when there is more than ____ gene involved in a particular phenotypic trait.
•Each _________ involved can also have ____________ alleles.
•Examples in humans include ________, skin pigmentation, weight, cleft palate, neural tube defects,
__________________, the Rhesus factor and, most ______________ characteristics.
•As there are ____________ genes involved with polygenic inheritance it means there are several genes
influencing the _____________, and because of this the number of possible phenotypes tends to be _______.
The phenotypic characteristics ________________ from one to another and can appear _______________.
•Polygenic inheritance usually shows what is called ______________ _________________, that is a trait that
covers one end of a scale to another and can be shown with a _______ shaped bar graph.
•When only one gene influences a phenotypic trait, such as, yellow or green seed, or A/B blood group, the
phenotype variation generally tends to be ____________________. That is, it is either one character or
_______________ and ____________________ in between.
•This graph shows ________________ variation of the ______________ skin pigmentation.
•Each of the 3 genes influencing skin pigmentation has two alleles, 1 for light skin and 1 for dark skin. The two
alleles show _______________________ dominance patterns and show a _______________ effect.
•There are 64 different combinations of the 2 alleles of the 3 genes.
•An extremely pale skinned person receives 3 pale skin ___________ from mum and 3 ______ skin alleles from
dad.
•A dark skinned person receives a total of 4 or 5 dark skin alleles from mum and dad.
A non-human example are chicken combs:
List the phenotypes and the corresponding genotypes.
PART 3a – Dihybrid Crosses
Let’s consider a cross between a pure breeding plant with round and yellow seed with one that is pure breeding
with an angular/wrinkled and green seed.
F1 Parents genotype:
F1 Parents phenotype:
F1 Gametes:
F2 Parents genotype:
F2 Parents genotype:
F2 Gametes:
F2
Non-linked genes
•Located on __________________ chromosomes.
•Sort ______________________ at meiosis.
•Crossing over _________________ alter the way the gene is segregated into _____________________.
•Allow typical Mendelian ratio of _______________ in the F2 generation of a _________________ cross.
Linked genes
•If two genes are linked they are ____________________ together on a particular chromosome.
•If genes are _________________ linked, there is ________ chance of crossing over occurring between them at
meiosis and ________________________ the way they segregate at meiosis into gametes.
•If genes are _______ closely linked then there is a ________________ chance they may end upon different
chromosomes after crossing over and therefore segregate differently.
Section 3b – Linked Dihybrid Crosses
With no crossing over
F1 Parents pure breeding:
F2 Parents genotype:
F2 Gametes:
•In this case, if genes are linked closely together on a chromosome and do not get the chance to cross over at
meiosis, then we should never see both a dominant trait and recessive trait together in the same organism.
•We will only see both dominant, or both recessive traits in a 3:1 ratio and not the expected 9:3:3:1. They are
therefore not sorting independently.
•If the 2 genes are very closely linked, the ratio is the same as the monohybrid cross ratio in the F2 generation.
With crossing over
F1 Parents pure breeding
F2 Parents genotype:
F2 Gametes:
F2
If the genes are not closely linked then there is a high chance they will undergo crossing over. If this occurs then
they segregate as if they were independent and show the typical 9:3:3:1 ratio in the F2 generation.
Linked or non-linked?
•Say we conducted some crosses with sweet pea plants. We crossed a pure breeding plant with long pollen
grains and purple flowers (both dominant) with a pure breeding plant with round pollen grains and red flowers.
•The F1 generation would give 100% long pollen & purple plants whether or not the two genes were linked.
•If we counted 1,600 plants in the F2 generation we could expect the following results…
•The actual results do not match with any of the expected results. What can we make of this?
•We can tell that the two genes of this dihybrid cross are ______________ because the actual numbers counted
do not match the 9:3:3:1 ratio for unlinked genes.
•The actual results counted for each trait are ____ _______________ those for ‘linked and not crossed over’ and
those for ‘linked and crossed over’. What does this mean?
•It means that ______________ of the plants have undergone recombination (crossing over) of the long/round
pollen and red/purple genes and some ______________.
•If the actual results are _______________ to the ‘linked and not crossed over’ results, then we can say that they
are so close there has been little chance of ___________ ___________ and therefore the genes must be closely
linked (close together).
•If the actual results are closer to the ‘linked and crossed over’ results then we can predict that the genes are
quite a way apart on the chromosome and have a _____________ chance of crossing over.
Calculating linkage
•Linkage can be calculated by working out the amount of plants that have undergone _______________ (crossing
over) compared to the total number of plants. This can be worked out as a percentage.
•If there is a _____________ percentage of crossing over then the genes are ______ closely linked, and if there
is a small percentage of recombination then the genes are ______________ linked.
•Let’s have a look at our example. _____ (78 + 78) plants showed recombination from a total of 1,600. Therefore,
156/1,600 = 0.0975 x 100 = ____________ % recombination.
•A cross that is carried out in order to calculate recombination is a __________ cross.
•Remember that recombination is random so the recombination % is not going to be __________ every time.
•By working out the recombination % of a few linked genes, geneticists can identify their ______________
position, or loci, on a chromosome. This is known as a chromosome _______.
•Say we have 3 genes (ABC) linked on a particular chromosome and we want to know which ones are more
closely linked.
•After conducting 3 sets of test crosses we observe the following results:
Cross AB
Results
Cross BC
AB
20
BC
26
AC
17
Ab
2
Bc
1
Ac
1
aB
2
bC
1
aC
1
ab
6
bc
5
ac
7
Tot. 30
Results
Tot. 33
Cross AC
Results
Tot. 26
Calculate the recombinations here:
Draw a map of these genes on a chromosome:
Uses of linkage
•Linkage is used to identify the ___________ of __________ along a chromosome.
•It is often difficult to locate actual _____________ due to the randomness of _________________, which can
change (albeit slightly) all the time.
•Scientists have tried using recombination maps to compare __________. The more closely related the species,
the more _______________ their chromosome maps should be.
•This has created problems for taxonomists who may find that the new genetic maps do not concur with
information already available. How do they make a decision whether or not to change the current classification of
certain organisms? If they do decide to make the change, how do they notify the scientific community? How much
would it cost to make the changes?
•Scientists first working on the Human ___________ project (to map the ___________ of genes on the whole
_______________) used linkage maps, but these maps could not identify the exact position of genes they soon
became redundant.