Exceptions to Mendel

Exceptions to Mendel

Many Genes Have Multiple Alleles

·  A population might have more than two alleles for a given gene.

·  In labrador retriever, coat color is determined by one gene with four different alleles. Five different colors result from the combinations of these alleles.

·  Even if more than two alleles exist in a population, any given individual can have no more than two of them: one from the mother and one from the father.

Genes That Interact

·  More than one pair of genes may interact to produce the phenotype.

A.  Epistasis: a gene at one locus alters the phenotypic expression of a gene at a second locus.

B.  The homozygous recessive condition masks the effect of a dominant allele at another locus.

C.  Albino animals inherit allelic pair (aa) preventing production of melanin, expression of eye, hair, color.

Labrador Retriever Genetics

·  Black is dominant to chocolate B or b

·  Yellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e

Phenotype / Possible Genotypes
/ BBEE
BbEE
BBEe
BbEe
/ bbEE
bbEe
/ BBee
Bbee
bbee

Task: Determine the number of chocolate labs produced from a black female and a yellow male (BbEe x bbee)

/ See if you can figure out how the multiple alleles act together to determine the coat color of a mouse.
There are three varieties of coat color: black, brown, and white are controlled by four different alleles (similar to the labradors)
This is an example of epistasis!
Page 263 in text!

Polygenic Inheritance

1.  Polygenic inheritance occurs when a trait is controlled by several allelic pairs at different loci.

2.  Allelic pairs at different loci on a chromosome or on different chromosomes all control one trait.

3.  Gene alleles can be contributing or non-contributing.

4.  Contributing alleles have an addictive effect, resulting in quantitative variations.

5.  Examples include seed color in wheat and skin color and height in humans.

6.  Polygenic traits are subject to environmental effects that cause intermediate phenotypes; so they produce continuous variations whose frequency distribution forms a normal (bell-shaped) curve. (page 259 in text)

Codominance

·  Both alleles can be expressed

·  For example, red cows crossed with white will generate roan cows. Roan refers to cows that have red coats with white blotches.

·  This phenotype might seem to support the blending theory. (The blending theory predicts pink F1 progeny.)

·  The F2 progeny, however, demonstrate Mendelian genetics. When the F1 roan individuals self-fertilize, the F2 progeny have a phenotypic ratio of 1 red:2 roan:1 white.

·  This mode of inheritance is called incomplete dominance.

·  The phenotypic outcomes for cow color and incomplete dominance in general can be explained biochemically.

·  One allele of the gene codes for an enzyme that functions in the production of the red color. The other allele codes for the gene to make white color. If both alleles are present, both are expressed, resulting in a cow that has some red and some white.

·  MendelÕs laws are not compromised here, he just happened to find in peas examples of complete dominance only.

Incomplete Dominance

·  In some cases, an intermediate phenotype is shown

·  Neither allele is dominant

·  In snapdragons, flower color can be red, pink, or white. The heterozygous condition results in pink flowers (or an intermediate trait)

·  A white snapdragon crossed with a red snapdragon produces all pink offspring

·  Two pinks crossed together produce 1/4 white, 2/4 pink, and 1/4 red

·  When dealing with incomplete dominance and codominance it does not matter what letter you use, as long as the heterozygous condition always denotes the intermediate trait. In the diagram R is used, but you could also use W or even P. Ww = pink, Pp = pink if these letters are used.

Polygenic Traits

·  Polygenes mediate quantitative inheritance

·  Individual heritable characters are often found to be controlled by groups of several genes, called polygenes.

·  Each allele intensifies or diminishes the phenotype.

·  Variation is continuous or quantitative (Òadding upÓ) - also called quantitative inheritance

Pleiotropy

·  Some single alleles have more than one distinguishable phenotypic effect - This is called pleiotropy.

·  An example is the coloration pattern and crossed eyes of Siamese cats, which are both caused by the same allele. These unrelated characters are caused by the same protein produced by the same allele.

·  Another example is the gene that causes pigment color in rats. White rats also have very sensitive eyes and often become blind.

Sex Linked Genes

·  Some genes are located on the X chromosome. Females receive two alleles for these genes, but males only receive one.

·  In humans, hemophilia and color blindness are sex-linked traits.

·  When doing a punnet square, use large X's and Y's to denote male and female, use superscript letters to designate the alleles.

·  If the parent is a male, the genotype is automatically known. A colorblind male has to be b, since he only has one allele and colorblindness is recessive. A normal male must then be B

·  Females can be heterozygous for the colorblindness trait - they are called carriers. A female can be BB - normal, Bb - carrier, or bb - colorblind

·  The following shows a cross between a normal man and a woman who is a carrier.

Task: Determine the probability of a woman with hemophilia having children with hemophilia assuming she marries a normal man.

Lethal Genes

·  Some genes are lethal when both alleles are present. Lethality can occur before or after birth

·  An example is the "creeper" allele in chickens, which causes the legs to be short and stunted.

·  Creeper is a dominant gene, heteroyzous chickens display the creeper phenotype

·  If two creeper chickens are crossed, one would expect to have (from mendelian genetics) 3/4 of the offspring to be creeper and 1/4 to be normal

·  Instead the ratio obtained is 2/3 creeper and 1/3 normal.

·  This occurs because homozygous creeper chickens die.

Blood Types - Multiple Alleles and Codominance

·  In humans, there are four blood types (phenotypes): A, B, AB, and O

·  Blood type is controlled by three alleles. A, B, O

·  O is recessive, two O alleles must be present for the person to have type O blood

·  A and B are codominant. If a person receives an A allele and a B allele, their blood type is type AB

·  Crosses involving blood type often use an I to denote the alleles - see chart.

·  When doing blood type crosses, you will need to know whether at type A or B person is heterozygous or homozygous. Type O's are automatically OO and type AB is automatically AB. Crosses are performed the same as any other.

·  The blood type determines what antibodies are located within the blood. Type A blood has type B antibodies. If type B blood is put into their bodies, their immune system reacts as if it were a foreign invader, the antibodies clump the blood - can cause death.

·  Type AB blood has no antibodies, any blood can be donated to them - they are called the "universal acceptors"

·  Type O blood has no surface markers on it, antibodies in the blood do not react to type O blood, they are called the "universal donors"