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?

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    $\begingroup$ One of the biggest exceptions is that these laws are not applicable to haploid organisms. $\endgroup$ – WYSIWYG Jul 15 '16 at 13:04
  • $\begingroup$ Recombination and linkage are different things or same ??? $\endgroup$ – anupma rathore May 29 '17 at 18:28

You are right but the list is certainly not exhaustive. Here are a few more concepts you may want to consider.

Exception of law of dominance:

  • epistasis

    Allelic effect at a given locus depend on the variants at other loci

  • environment dependence and specific case of frequency dependence

    The concept of dominance can be used for any quantitative trait. Typically, when the quantitative trait of interest is fitness, then the trait often depend on the environment. A typical and interesting case is when the fitness depends on the frequency of alleles in the population. Depending of the frequency of the allele at a given locus, the relationship might be a relationship of dominance, additivity or dominance for the other allele.

Exception of law of segregation:

  • meiotic drive

    from wiki: "Meiotic drive is a type of intragenomic conflict, whereby one or more loci within a genome will affect a manipulation of the meiotic process in such a way as to favor the transmission of one or more alleles over another, regardless of its phenotypic expression. More simply, meiotic drive is when one copy of a gene is passed on to offspring more than the expected 50% of the time."

Exception of law of Independent assortment:

  • epistasis for fitness

    Imagine a case where a given combination of allele is lethal at very young age, then you would never see these two alleles together in an individual. It is per se, not a case of exception to independent assortment but it gives the feeling of such exception. A case more extreme when a given combination of alleles prevents fecundation could however be considered as an exception to independent assortment.

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    $\begingroup$ Another e.g worth including Sex-linked genes, they do not follow mendelian law of Dominance. $\endgroup$ – Tyto alba Apr 4 '17 at 8:36
  • 1
    $\begingroup$ Also Sex-limited traits [where a functional autosomal gene (allele) is completely slient in one sex] and sex-influenced traits [where a functional allele changes behaviour, dominant/recessive behaviour with sex of the bearer]. $\endgroup$ – Tyto alba Apr 26 '17 at 8:06

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