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Condition in which the phenotypic effects of a gene’s allele are fully and simultaneously expressed in the hetereozygote. For example, human Sickle cell haemoglobin in humans -> heterozygotes distinguishable


Multiple alleles

More than two alleles at a locus. ABO blood group system in humans. 3 alleles - A,B, O that make antigens further complicated by existence of both complete and co-dominance in the same system with AB dominant to O which can give rise to odd ratios.


Sex-linked inheritance

Gene carried on X chromosome. Generally, for recessives, alleles are transmitted mainly through females but manifest themselves mainly in males. Lesch-Nyhan syndrome : fatal error in purine metabolism leads to self-mutliation and death.



A section of chromosome is removed. In Cri-du-Chat in humans tip of chromosome 5 lost.



A section of chromosome is doubled up. When chromsomes bearing tandem duplications pair up during meiosis, there is very precise parking of particular DNA sequences; but duplications can cause mispairing. This slippage means that there is an intrinsic tendency for the size of tandem duplication to increase or decrease. Huntington’s disease in humans -> over 23 CAG repeats.



A section of chromosome has been rotated and replaced in position. Inversions are crossover suppressors; prevent crossover between inverted and normal segments of chromosome. This can cause an excess or deficiency of certain genes. If small, these inversions produce viable gametes and abnormal children. If large, the gametes may be lethal.



Two non-homologous chromosomes exchange parts. Often associated with cancers -> chronic myeloid leukaemia (9-22 translocation)



An error in cell division in which homologous chromosomes fail to separate and migrate to posit poles; responsible for trisomy and monosomy. Down’s syndrome is a common example of trisomy on chromosome 21. Many physical, behavioural and physiological changes.



Changes in number of chromosome sets; usually lethal in humans; odd numbers of chromosomes are associated with infertility as cannot pair during meiosis. Adenocarcinoma cancerous cells have twice as much DNA as expected.



Polyploid condition resulting from the duplication of one diploid set of chromosomes.



Polyploid condition formed by the union of 2 or more distinct chromosome sets (i.e. different species) with a subsequent doubling of chromosome number. Associated with the origin of wheats and many other crops, Einkorn hybridised with another species to produce Emmer wheat. Further hybridisation resulted in modern hexaploidy wheat.


Multiple crossovers

More than 2 recombination events between two loci. Whenever there’s an even number of crossovers between two loci, it will restore the parental genotype. When there’s an odd number of crossovers between two loci, it will produce a recombinant genotype.


Crossover interference

The formation of a chiasmata influences the probability of a formation of separate chiasmata nearby -> physically difficult for 2 to form close together. Certain parts of the chromosome are more likely to recombine than others. Neurospora show positive interference, the closer the two loci are together, the more likely of observing a crossover event.



The proportion of total variation in the population that’s due to genetic variation. Depends on genes and the environment. Achillea; select plants from different altitudes. Different genotypes have different fitness grown in different environments with different rates of reaction.


Patrilineal inheritance

The Y chromosome; differential regions recombine with X where recombination takes place. Many palindromes where Y can recombine with itself. Many genes have been lost to the X or decayed due to mutation. The remaining genes tend to code for male traits. e.g. SRY active early in development makes males male.


Matrilineal inheritance

Genes in cytoplasm or organelles, passed down only through female parent. Maternally inherited bacteria - Wolbachia passed on through female cytoplasm. Stops male development, makes males infertile and kills adult males.


Genetic drift + bottlenecks

Random change in small populations ; strength of effect depends on population size and what the allele frequencies are but both affect sampling variance. In humans, isolated populations in Finland led to a disease pattern with Tristan (retinoblastoma) as heterozygosity increasingly decreases.


Positive assortive mating

Non-random mating. Rhagoletis pomonella - hawthorn pest that moved to apple. Host choice and emergence time difference mean that apple race mates with apple and hawthorn with hawthorn.


Negative assortive mating

Avoid mating with themselves. Tay-sachs disease used to be extremely prevalent in populations of Ashkenazi Jews due to small population size but effectively eliminated by Rabbi Eckstein.