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.