All genes on a single chromosome form a linkage group
Genes carried on sex chromosomes are sex-linked
The other 22 pairs of chromosomes are called autosomes
Without crossing over, all genes remain together during meiosis and pass into the gametes and they do not segregate in accordance with Mendel's Law of Independent Assortment
When on different chromosomes, there are 4 combinations, AB Ab aB ab but if the genes are on the same chromosome, it can only be AB or ab in the Kerboodle example without crossing over
This leads to certain characteristics being more likely to happen if another characteristic is expressed. For instance, if A is expressed it is likely that B is also going to be expressed
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Epistasis
Epistasis is the interaction of genes suppressing or masking the effect of other genes
Mice coat colour example
Gene A controls the distribution of melanin (black pigment) and therefore decided if they are banded or not
The dominant A of this allele leads to bands of black hairs
The recessive a of this allele produces uniform black hairs when with another 'a'
Gene B controls the colour of the coat by determining the expression of gene A
The dominant B of this allele leads to the production of melanin
The recessive b of this allele leads to no pigment being produced and so it will be white no matter what (as long as it is bb)
An AABB mouse is crossed with an aabb mouse. All of the offspring will be AaBb agouti mice. These mice are all crossed to produce a ratio of 9:4:3 of agouti: albino: black
This is because A is affected by B. If bb is present than the A gene will have no impact and the mouse will be white no matter what
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Epistasis continued
Other forms of epistasis
genes determining the enzymes in a chemical pathway
e.g. if a plant produces a red pigment its petals using the following biochemical pathway
starting molecule - (enzyme A) - intermediate molecule - (enzyme B) - red pigment
The production of the two enzymes are coded for by genes A and B with the dominant alleles coding for functional enzymes whilst recessive alleles code for non-functional enzymes
If the alleles for either gene are both recessive e.g. aa or bb then the red pigment cannot be produced whether the other gene was dominant or not
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