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Mendelian Genetics

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Beyond Mendelian Genetics- Extensions and modifications - While alleles are transmitted from parent to offspring according to Mendelian principles, they sometimesfail to display the clear-cut dominant-recessive relationship observed by Mendel.- In many cases, contrasts to Mendelian genetics, two or more genes are known to influence the phenotype ofa single characteristic.- Another exception to Mendelian inheritance is the presence of genes on sex chromosomes, where malesonly receive a single copy of genes on that chromosome.- Phenotypes are the combined result of both genetics and the environment within which genes areexpressed.- The result of the various exceptions to Mendelian principles is the occurrence of phenotypes that differfrom those resulting from mono-, di-, and tri-hybrid crosses.- Extranuclear inheritance, resulting from the expression of DNA found in mitochondria and chloroplasts canmodify Mendelian inheritance patterns. These genes are transmitted through the female gamete.Mendelian Genetics postulates the following-#1: Unit factors come in pairs. #2: Unit factors have either a dominant or recessive form. #3: Unit factors segregate/ separate during gamete formation. #4: Unit factors assort independently from one another. In relation to chromosomes:#1: Chromosomes come in pairs.#2: GENES have either a dominant or recessive form.#3: Chromosomes segregate/ separate during gamete formation.#4: Chromosomes assort independently from one another.Mendel’s postulates for OTHER inheritance patterns do NOT hold true in all respectsThese both hold TRUE for other types of inheritance.• #3: Unit factors segregate/ separate during gamete formation.• #4: Multiple unit factors assort independently from one another.These postulates DO NOT.• #1: Unit factors come in pairs. • #2: Unit factors have either a dominant or recessive form.1Therefore, Was Mendel lucky! Traits he chose in the pea plant showed up very clearly…One allele was dominant over another, so phenotypes were easy to recognize.But sometimes phenotypes are not very obvious…NB: If gene expression does not follow dominance/recessive situation (i.e. there is some interaction) or if more thanone pair of genes influence the expression of single trait, the classic 3:1 or 9:3:3:1 F2 segregation ratios may bemodified.Types of interaction: Allelic interaction (two alleles at one locus influence the outcome of a single trait) Gene interaction (when two or more different genes influence the outcome of a single trait).  Allelic genes: Genes located in the identical locus of homologous chromosomesAllelic interaction- Results in incomplete dominance, co-dominance or overdominance- Allelic interaction normally modify 3:1 genetic ratio to either 2:1 for lethal gene effects or 1:2:1 forincomplete dominance effects.Incomplete dominance• Cross between parents with contrasting traits may produce offspring with intermediatephenotypes.• Occurs when the phenotype is controlled by a single gene with two alleles, neither of which isdominant.Example: Snapdragon flowers come in many colours.• If you cross a pure breeding red snapdragon (RR) flowered plants with a white pure breeding snapdragon(rr) flowered plants • You get plant with PINK flowers (Rr)!• Is this blending?• Check F2 generation ratios• Notice that the phenotypic and genotypic ratio are the same• Usually use R and R’ or A and A’ because neither is dominant.Genetic basis of incomplete dominance: allele contributes different amounts of functional protein; total amountdetermines phenotypes.b) Lethal gene effectEssential genes are those that are absolutely required for survival􀂄 Absence of their protein product leads to a lethal phenotype􀂄 It is estimated that about 1/3 of all genes are essential for survival􀂄 Non-essential genes are those not absolutely required for survival2- Lethal gene: is that which produces a harmful protein product capable of killing the organism that inherits itespecially under homozygous condition.- Lethal genes can be recessive in which case it causes death only when in homozygous recessive conditionor- It can be dominant in which case it causes death even in heterozygous condition.- These alleles are typically the result of mutations in essential genesExample: coat colour in mice.- Normal wild-type mice have coats with a dark overall pigmentation (Agouti) controlled by allele A/A.- A mutation in the allele result in yellow coat colour; AY - If a yellow mouse is mated to a homozygous wild type mouse, a 1:1 ratio of yellow to wild-type mouse isobserved in the progeny.Phenotype: Yellow mice Normal miceGenotype: AYA × AAF1 Offspring AYA AYA AA AANote: 1:1 ratio: Yellow Mice 2: 2 normal (wild type)This observation suggests (1) That a single gene with two alleles determines these phenotypic alternatives, (2) That the yellow mouse was heterozygous for these alleles, and (3) That the allele for yellow is dominant to an allele for normal color. However, if two yellow mice are crossed with each other, the result is always as follows: Note:-1. The 2:1 phenotypic ratio is a departure from the expectations for a monohybrid self-cross. 2. Because no cross of yellow × yellow ever produced all yellow progeny, as there would be if either parent were ahomozygote, it appears that it is impossible to obtain homozygous yellow mice.Genetic explanationThe allele AY for yellow is dominant to the wild-type allele A for coat colour, but AY acts as a recessive lethalallele with respect to a character that we call viability. Thus, a mouse with the homozygous genotype AY/AY dies before birth and is not observed among the progeny. All surviving yellow mice must be heterozygous AY/A, so a cross between yellow mice will always yield thefollowing results: 3The AY allele is pleiotropic. It is entirely possible, however, that both effects of the AY pleiotropic alleles result fromthe same basic cause, which promotes yellowness of coat in a single dose and death in a double dose.Pleiotropic gene: Gene that affect more than one trait. Pleiotropy may be the result of a single gene product (protein)giving rise to multiple functional characteristics.Examples: recessive lethal genes, Sickle cell syndrome.OverdominanceThis refer to the inheritance where heterozygotes have traits that are beneficial than homozygotes. The F1 phenotypemay exceed that of


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