There is more than one type of dominance when it comes to inheritance
There are three types
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Complete dominance
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Incomplete dominance
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Co-dominance
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Co-dominance is an example of a relationship between 2 alleles where both alleles are expressed in their complete form. The resulting phenotype is generally a blend of both of the characteristics that the 2 alleles code for. Both proteins produced and do not mix/blend like they do for incomplete dominance.
There is also more to alleles than what you learned in Level 1, there can also be
Multiple alleles and lethal alleles
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Multiple alleles
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Lethal alleles
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Is when there is a mutation on an allele, causing the production of a nonfunctional protein that will affect the organism's survival. Some lethal alleles are fully dominant, meaning if just one is present in the genotype (eg. in a heterozygote) then it can kill the individual. Some lethal alleles require two copies to be lethal (homozygous dominant). Some lethal alleles are found on the recessive allele and there is no detectable effect in the heterozygote. |
Manx cat example
ML is a mutated allele that results in disrupted spine growth. Two of these alleles will cause the individual with them to die (2 lethal alleles). However, if a cat is carrying one of these lethal alleles and one normal (M) allele (MML), incomplete dominance allows the positive effect of the normal allele to mix with the negative effect of the ML allele, resulting in some disruption to spine growth but only enough to produce no tail. The cat stays alive. Two M alleles results in a normal cat.
ML is a mutated allele that results in disrupted spine growth. Two of these alleles will cause the individual with them to die (2 lethal alleles). However, if a cat is carrying one of these lethal alleles and one normal (M) allele (MML), incomplete dominance allows the positive effect of the normal allele to mix with the negative effect of the ML allele, resulting in some disruption to spine growth but only enough to produce no tail. The cat stays alive. Two M alleles results in a normal cat.
All the above examples involved looking at one gene at a time (monohybrid cross). Below we look at two genes, making the following crosses
Dihybrid
For example, crossing two pea plants and observing the inheritance of:
Dihybrid crosses look at the inheritance of these two traits at the same time. So from crossing a wrinkled green pea and a round yellow pea, what is the probability that wrinkled yellow peas would be produced etc. |
Let's cross two heterozygous round yellow pea plants and look at the offspring.
First of all we need to work out all the possible gametes each parent could produce!
First of all we need to work out all the possible gametes each parent could produce!
So a heterozygous round yellow pea would have the genotype
RrYy
and gametes are haploid, so they contain half the DNA compared to the parents
each gamete will contain one allele for each gene (one of each letter) The possible gametes are
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Genotype ratio = 1RRYY:2RRYy:2RrYY:4RrYy:1RRyy:2Rryy:2rrYy:1rrYY:1rryy
now group them based on phenotypes to get your phenotype ratio
Round and yellow |
Round and green |
Wrinkled and yellow |
Wrinkled and green |
RRYY x 1 RRYy x 2 RrYY x 2 RrYy x 4 |
RRyy x 1 Rryy x 2 |
rrYy x 2 rrYY x 1 |
rryy x 1 |
1 + 2 + 2 + 4 = 9 |
1 + 2 = 3 |
2 + 1 = 3 |
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9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green
9:3:3:1
(remember this magic number)
(remember this magic number)
But if you were to look at a round yellow pea, how would you know its genotype?
Are they round RR or round Rr? And are they yellow YY or yellow Yy?
Their genotype could be RRYY, RRYy, RrYY, RrYy - we don't know!
To find out, you'd need to do a
Are they round RR or round Rr? And are they yellow YY or yellow Yy?
Their genotype could be RRYY, RRYy, RrYY, RrYy - we don't know!
To find out, you'd need to do a
test cross
cross the unknown genotype with a homozygous recessive individual for both genes - so in this case, a wrinkled green pea - we know to be wrinkled and green their genotype must be rryy
Scenarios...
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Sometimes alleles are inherited together, as a couple. For example, red hair and pasty skin. We call this
linkage, or linked genes
What unlinked genes look like
(this is what you've always known) These are genes that are found on different homologous pairs. For example, we could say that the gene for pea colour is on chromosome 4 (red), and the gene for pea shape is on chromosome 11 (blue). They are on different homologous chromosomes therefore are not linked. This means they are unlikely to be inherited together.
Genes that are found on complete opposite ends of a chromosome can said to be not linked if they are likely to be swapped during crossing over. |
What linked genes look like
(this may take a while to grasp) These are genes that are found on the same chromosome. For example, we could say that pea colour and shape genes are both found on chromosome 4 (red). This means that the allele for pea colour and shape, due to being on the same chromosome, are going to be inherited together (as each parent gives one of each chromosome to their offspring - that one chromosome contains that parent's contribution of alleles for those genes).
Linked genes are more likely to stay together during meiosis and as a result, appear in gametes together. |
Here's two genes and their alleles for the below examples
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Someone who is heterozygous for both genes would be
QqRr
and a quick runner
QqRr
and a quick runner
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Linked genes and meiosis
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Unlinked genes and meiosis
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This individual is going to undergo meiosis to make gametes (haploid - contains one of each letter). To see the effect of linkage, we must look at the gametes and/or the offspring produced if fertilization occurs.
Remember, chromosomes must replicate before
any type of cell division. Both chromosomes make an identical copy of themselves and the copies stay attached until Meiosis II when the spindle splits them.
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Next, all of the steps of meiosis occur (you should draw them out!), forming 4 haploid gametes, each containing one allele for each gene (one of each letter)
This picture represents the final step of meiosis - these chromosomes will be segregated into gametes.
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Chromosomes undergo segregation to form haploid gametes
Notice how if you inherit Q you are stuck with R, and if you inherit q you are stuck with r? There is no way to get any other combinations, unless crossing over occurs. For the most part, this individual is only able to produce gametes that are QR and qr. This means she cannot give her offspring Qr or qR - this creates less possibilities and thus less variation in offspring genotypes.
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The only way that variation can be increased in the gametes for linked genes is if crossing over occurs during Meiosis I. This doesn't happen every time so these allele combinations will be less common compared to the others.
Therefore if crossing over does occur, the following gametes are possible (notice how there are more options)
This individual is going to undergo meiosis to make gametes (haploid - contains one of each letter). To see the effect of linkage, we can compare these gametes to that of linked genes. We will see that unlinked genes can form more allele combinations in the gametes.
Remember, chromosomes must replicate before
any type of cell division. All 4 chromosomes make an identical copy of themselves and the copies stay attached until Meiosis II when the spindle splits them.
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Next, all of the steps of meiosis occur (you should draw them out!), forming 4 haploid gametes, each containing one allele for each gene (one of each letter
This picture represents the final step of meiosis - these chromosomes will be segregated into gametes.
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Chromosomes undergo segregation to form haploid gametes
This individual is heterozygous for being a quick runner (QqRr) and, unlike the example on the previous tab, they are able to produce four different gametes (QR, Qr, qR, qr) each time meiosis occurs (not just if crossing over occurred) - this creates more possibilities and thus more variation in offspring genotypes.
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To identify linked genes you can look for a ratio you don't expect.
If the above two heterozygotes were crossed (mated), you would expect a 9:3:3:1 ratio. However, if the genes are linked, you may find a ratio more like 12:1:1:3.
If dominant alleles are inherited together then the dominant phenotype will be super common and the 'mixture' phenotypes will be less common (as they can only get expressed if crossing over occurs prior to the formation of the gametes from one or both parents).
Basically, if you get a ratio that is not what you'd expect then you can investigate the idea of linked genes.
If the above two heterozygotes were crossed (mated), you would expect a 9:3:3:1 ratio. However, if the genes are linked, you may find a ratio more like 12:1:1:3.
If dominant alleles are inherited together then the dominant phenotype will be super common and the 'mixture' phenotypes will be less common (as they can only get expressed if crossing over occurs prior to the formation of the gametes from one or both parents).
Basically, if you get a ratio that is not what you'd expect then you can investigate the idea of linked genes.