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non-mendelian inheritance

Non-Me­ndelian inheri­tance

1) Multiple alleles: more than two phenotypes due to more than two alleles being present
2) Incomplete dominance: both phenotypes may be dominant, resulting in an interm­ediate phenotype.
3) Codomi­nance: both dominant traits are present in offspring, resulting in both alleles being expressed together.
3) Codomi­nance: both dominant traits are present in offspring, resulting in both alleles being expressed together.
4) Pleiot­ropy: mutations have more than one phenotypic effect.
5) Epistasis: intera­ction of more than one gene masks the phenotypic effect of one of them.

Sex linkage

This is because the recessive allele on the X chromosome will show in the phenotype in males, as they have XY, so there is no dominant allele to override it (whereas females have XX, so will always have the dominant allele).
X chromo­somes – 1,000 to 2,000 genes Y chromo­somes – 70 genes
Males and females carry a different dose of X, so males have dosage compen­sation – they produce double the amount of gene product (upreg­ulation of X in males)


Genes are physically linked when they are on the same chromo­some.
Genes on separate chromo­somes are never linked
Genes that are farther away from each other are more likely to be separated during a process called homologous recomb­ina­tion, and are practi­cally unlinked

Breaking linkage

Two parental chromo­somes, two recomb­inant chromo­somes
Crossing over always occurs and is regulated
The probab­ility of crossing over between two genes depends on their distance from each other on the chromo­some.
Double crossing over undoes the effect of one crossing over, therefore, the parental and recomb­inant genes stay the same.
Double crossing overs close to each other are rare, due to crossing over interf­erence: one crossing over reduces the possib­ility of a second.

Genetic mapping

The percentage of recomb­inant offspring (recom­bin­ation frequency) = distance between genes on a chromosome
1% frequency = 1 map unit (centi­morgan, cM)

How to plot a gene map

1. the lowest frequency is the double crossing over
2. the highest is the parental genes
3. find the pairs of the other crossing over/s
4. to determine the gene in the middle, look at which allele has swapped between the parental gene and the double cross.
5. to calculate the distance between alleles, remember that it is the recomb­ination frequency between one cross and the recomb­ination frequency of another cross. This is calculated by adding together the frequency of both alleles in the pair, and the alleles in the double cross.

Linkage group

Linkage can only be detected when recomb­ination frequency is less than 50%
When genes are far apart on the chromo­some, crossing over will always occur, so genes appear unlinked.
There is a difference between ‘physi­cally linked’ and ‘genet­ically linked’ – if asked how many linkage groups there are in an organism with x chromo­somes, you cannot tell.
Genetic mapping can only establish linkage groups.


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