What are the exceptions to Mendel's laws?

Question

**What are the exceptions to the $-$** - Law of dominance - Law of independent assortment - Law of segregation**My knowledge:** *Exception of law of dominance:* - Incomplete dominance In incomplete dominance when a red snapdragon flowered plant is crossed with a white flowered plant an intermediate phenotype appears in the F1 hybrid instead of a parental phenotype. - Codominance Unlike dominance in codominance when a A ($I^AI^A$) blood group individual mates with B ($I^BI^B$) blood group individual the offsprings have blood group AB ($I^AI^B$) instead of A or B.*Exception of law of segregation:* - Nondysjunction During meiosis homologous chromosomes/sister chromatids and hence genes may move to a common gamete violating law of segregation.*Exception of law of independent assortment:* - Linkage When genes are present on the same chromosome they tend to remain together and enter into the same gamete. This is the reason behind deviation of dihybrid test cross ratio from 1:1:1:1 and occurrence of parental combination in high frequencies.*Am I correct? Are there any more exceptions to these laws?*

Answer 1

Nice work! It seems like you're really into it, so in case you want to read Mendel's actual paper (which, interestingly, doesn't ever come out and say "yo, here are my 3 laws":

http://www.esp.org/foundations/genetics/classical/gm-65.pdf

The answer to your question depends somewhat on whether you want a violation to each of the 'laws' as stated, or whether you're asking if these are all the phenomena that throw off Mendel's predictions. I'm going to answer the implied question of 'what else can mess up ratios' and leave you to explore.

*X-linkage: many animals have 'asymmetric' chromosomes, such as the X and Y in humans, with females being XX and males XY. The Y is a tiny little thing with little genetic information on it, so while females get a full genetic copy of X information from each parent, males get such genes only from the mother and nothing from dad. The technical term for this state is 'hemizygous'

*Lethality: In Manx cats, there is a dominant gene that when heterozygous causes short tails... and when homozygous, the embryos die.

*Genes involved in related functions. Mendel 'declared' himself out of this one by choosing to study only features that were distinct and unrelated. If two genes perform the same function, a mutation in either is harmless, but in both leaves the organism without function, so only genotype aabb dies. This is a subset of a broader class of situations where genes function in the same process; look up 'epistasis', which can yield phenotypic ratios for processes in which two genes are involved of 9:7, 15:1, etc. (always a result of taking a 9:3:3:1 ration and combining two or more classes)

*Maternal inheritance: In animals, mother makes the egg which is generally HUGE; the sperm generally contributes JUST DNA. Any products that are essential to the early functioning of the egg, therefore, are the mother's gift (and thus reflect the mother's genotype--NOT the embryo's). To learn more, look up "drosophila development" and "maternal effect" as keywords

*segregation distortion/meiotic drive: This one is a really cool 'short circuit' of natural selection. There exist 'killer alleles' that (for example) produce a poison and resistance to that poison... but they do this as SPERM. So a male of genotype Aa* (where a* has an allele that produces an anti-sperm poison, but is also resistant to that poison) makes both A and a* sperm... but the a* sperm kill all the A sperm, so the male only passes the a* allele, despite being heterozygous.

*All manner of imprinting. Imprinting is 'marking' of genetic information (DNA) in ways that don't change the nucleotide sequence, but CAN alter whether protein products are made. In some cases, DNA inherited from the mother is 'marked' differently than DNA inherited from the father. Look up "igfr2 gene imprinting" for more

I'm sure there are many others that I'm not remembering and more still that I haven't learned about!