Variations in Mendelian Ratios Explanations for not getting what you would expect…
Jan 20, 2016
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.