variations in mendelian ratios
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Variations in Mendelian Ratios. Explanations for not getting what you would expect…. Exceptions to Mendel’s Principles. Linkage Incomplete Dominance Co-Dominance Multiple Alleles Sex-Linked Genes Polygenic Epistasis Pleitropy. Linkage. - PowerPoint PPT PresentationTRANSCRIPT
Variations in Mendelian Ratios
Explanations for not getting what you would expect…
Exceptions to Mendel’s Exceptions to Mendel’s PrinciplesPrinciples
• Linkage• Incomplete Dominance• Co-Dominance• Multiple Alleles• Sex-Linked Genes• Polygenic• Epistasis• Pleitropy
LinkageLinkage
• The characters Mendel examined happened to be on separate chromosomes - that is why he observed independent assortment.
• If, however, the genes are on the same chromosomes, they will be inherited together.
Linkage (con’t)Linkage (con’t)
• For example, consider the following parental nuclei. Both father and mother have a pair of chromosomes with alleles for two different genes:
Linkage (con’t, still…)Linkage (con’t, still…)
• If we look at this with a Punnett square what is going to happen in the next generation:
Linkage (con’t, still…again)Linkage (con’t, still…again)
• There are fewer genotype combinations than in the usual cross involving two alleles.
Incomplete and CodominanceIncomplete and Codominance
• In incomplete and co-dominance, neither of the two alleles is dominant over the other.
• The result is a heterozygote with a different phenotype from the homozygous dominant.
Incomplete DominanceIncomplete Dominance
• the effect of the two alleles is blended
Co-DominanceCo-Dominance• both alleles are expressed independently and are uniquely
recognizable• In white clover, one form of homozygous plant exhibits leaves with a
chevron pattern; the alternate homozygous form exhibits leaves with a large light-colored area.
• Heterozygous plants exhibit both the chevron and light colored area on their leaves.
Multiple AllelesMultiple Alleles
• Sometimes more than two alleles are present at a locus for a gene.
• An example is blood groups in humans.
Multiple Alleles (con’t)Multiple Alleles (con’t)
• Blood type, for example, is determined by three alleles. – Ia codominant– Ib codominant– i recessive
Bloodtype
Alleles IIaa IIbb ii
IIaa IaIa IaIb Iai
IIbb IaIb IbIb Ibi
ii Iai Ibi ii
Sex-Linked GenesSex-Linked Genes
• Alleles for a gene on the X chromosome (but absent on the Y) are said to be sex-linked.
Sex DeterminationSex Determination
• If two X chromosomes (XX) are present in the embryo, it generally develops into a female.
• If one X and Y (XY) are present in the embryo, it generally develops into a male.
• Males determine the sex of the offspring as they can create either an X sperm or a Y sperm.
• If an X sperm fertilizes an egg, the embryo becomes a female.
• Females can only create X eggs.
Back to Sex-LinkageBack to Sex-Linkage
• This means that males may inherit just one allele for a characteristic and that allele will be expressed, whether it is dominate or recessive.
• The three genotypes that are present in the female (AA, Aa, aa) are the usual ones.
• In males there are two new genotypes (A and a).• Males are said to be hemizygous for sex-linked
traits because they only have one X chromosome.
Example Sex-LinkageExample Sex-Linkage
• Genotypes are expressed with their respective sex chromosome; the Y has no alleles.
Sample Cross (Sex Linkage)Sample Cross (Sex Linkage)
Interactions Between GenesInteractions Between Genes
• Independence of gene inheritance does not necessarily mean that genes act independently.
• Because genes effect the phenotype via biochemical reactions, their effects depend on the chemical and physical environment created, in part, by other genes. (Understanding this will come later… hopefully!)
• Therefore, the phenotypic effect of one gene depends on one or more other genes.
PolygenyPolygeny
• When two or more independent genes have a similar and additive effects on the same characteristic, it is called polygenic inheritance.
• Height and skin color in humans, for example, are polygenic.
EpistasisEpistasis
• When the presence of one allele of a gene determines which allele of another gene is expressed.
• Coat, eye, and nose color in Labs is a great example
Coat ColourCoat Colour
(Phenotype)(Phenotype)
AlleleAllele GenotypesGenotypes
BlackBlack B BBEE
BBEe
BbEE
BbEe
ChocolateChocolate b bbEE
bbEe
YellowYellow e BBee
Bbee
bbee
• The e allele is epistatic.
• When present, it prevents the B or b allele from being expressed and the labs are all yellow.
PleiotropyPleiotropy
• A gene that effects more than one characteristic is pleiotropic.
• Sickle Cell is an example.