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Modifications of Mendelian Ratios.docx

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University of Waterloo
BIOL 239
Heidi Engelhardt

Modifications of Mendelian Ratios Dominance is not always complete  Incomplete dominance : The F1 hybrid resembles neither purebred parent (often an intermediate phenotype) o Genotype and phenotype ratio is 1:2:1 o Phenotype ratio is equal to the genotype ratio o Snapdragons  Among snapdragons there is a gene which produces flower pigment (a protein).  The allele Ar produces the wild type red pigment and the allele Aw produces white (lack of red pigment).  When red (ArAr) plants are crossed with white (AwAw) plants the resulting F1 are pink (ArAw).  only one “dose” of red pigment is produced (from Ar ), resulting in pink offspring that do not resemble either of their parents.  Note* it is proper to designate incompletely dominant or co-dominant alleles as both uppercase  Codominance : Alternative traits are both visible in the F1 hybrid o Shows same genotype and phenotype ratio as incomplete dominance: 1:2:1 o Neither allele is dominate or recessive to the other o Multiple alleles of a single gene o Codominance: ABO Blood Groups  The gene controlling the cell surface sugars responsible for the ‘ABO’ blood grouping system is a multiple allele system in which IA and IB are codominant to each other but they are both dominant to the i allele.  Allele IA attaches a sugar different to that of allele IB and allele I does not attach the sugar at all.  Heterozygotes IAIB have both types of sugars on their cell surface.  Phenotype (blood type) Possible genotype A B AB O Codominance is described by phenotype  IA = makes enzyme A to add A sugar  IB = makes enzyme B to add B sugar  i = no enzymes - no sugars added  IAIB = both enzymes to add both A + B sugars  single gene (I) with altered alleles (IA + IB + i )  IA + IB are codominant to each other and dominant to i  Polymorphic gene as the 3 alleles are commonly found  Variations on complete dominance are consistent with Mendel’s law of segregation o The type of dominance relationship exhibited by different alleles does not affect the way they are transmitted. Rather, it is a reflection of the way in which the proteins they encode act in the cell. A gene may have more than two alleles  Multiple alleles exist for most genes o e.g., blood type : A (IA), B (IB), O (i) o e.g., trait of lentil seed coat pattern: spotted, dotted, clear (no pattern), marble-1 or marble-2 o e.g., drosophila eye colours white, red, or eosin o e.g., mouse coat colour black, black and tan, or agouti o e.g., rabbit coat colour agouti, albino, chinchilla, Himalayan  Reciprocal crosses can be conducted between purebreeding lines representing all phenotypes, to establish the dominance relationships between all possible pairs of alleles.  This reveals a dominance series, in which alleles are listed in order from dominant to recessive. o A dominance series: Four alleles for coat pattern o Crosses using homozygotes to establish dominance series o Many mutant alleles found for coat colour  Genotype Phenotype c+_ Wild type c ch Himalayan cc Albino  albino killed by predation in wild  in nature c+ has a frequency of > 99%  c+ is a monomorphic allele gene o Do not assume the wild-type allele is always dominant! o New alleles arise through mutation  Chance alterations in the genetic material arise spontaneously in nature Allele frequency  The percentage of the total number of copies of a gene in a population represented by a particular allele o Wild-type allele : greater than 1 % o Mutant allele : less than 1 % o Monomorphic : gene with only one wild-type allele o Polymorphic : gene with more than one wildtype allele  One gene may contribute to several visible characteristics o Pleiotropy – Multiple phenotypic effects caused by a single gene  Sickle cell syndrome: an example of pleiotropy o A variation on pleiotropy: Some alleles may cause lethality  Some alleles may result not only in a visible phenotype, but also affect viability  The assignment of dominant and recessive pertains only to the phenotype being regarded  Recessive lethal alleles - yellow coat o Allele for yellow coat is dominant for coat colour, but recessive for lethality (recessive lethal allele) o Two heterozygotes for A+ and yellow (AY) produce only yellow and agouti offspring in 2:1 ratio  Manx cats - spinal development o Homozygous fetus dies in utero o All Manx cats are heterozygous for the Manx allele o Manx allele is dominant for taillessness and recessive for lethality (recessive lethal allele)  Both are examples of 1 allele affecting 2 different phenotypes (pleiotropy)  just because its dominant for one phenotype, does not mean it has to
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