Non Mendelian Patterns of Inheritance

TABUK UNIVERSITY

Faculty of Applied Medical Sciences

Department of Laboratory Technologies

AY: 1433-1434/2012-2013 Lecture 4

Mendel Inheritance: How do alleles interact with each other?

• Dominant allele: An allele that expresses its phenotype in the heterozygous state.

• Recessive allele: An allele whose phenotypic effect or

trait is not expressed in a heterozygout state.

Dominant vs. Recessive= interaction

Dominant/Recessive interactions

• The dominant allele will “cover up” the recessive allele.

• Application: A girl inherits: a long eyelash allele from her father and a short eyelash allele from her mother.

• The short eyelash allele is dominant and the long eyelash allele is recessive.

What is the length of the girl’s eyelashes?

Her eyelashes are short

Some alleles are neither dominant nor recessive

Some exceptions to Simple Mendelian Genetics= Non Mendelian interactions

Beyond dominant and recessive alleles

Is dominance the only type of inheritance patterns?

Interaction 1: Incomplete Dominance

When the 2 different alleles are present, the phenotype will look

like a BLENDING of the two.

Neither allele is dominant and heterozygous individuals have an

intermediate phenotype between two parents

For height of the Sapping Evergreen Tree: the interaction of genes show incomplete dominance.

Application: A tall tree is crossed with a short tree, what will the height of the new tree be?

Medium height

Another question, a short tree is crossed with another short tree, what will the height of the new tree be?

Short height

F1 generation

F2 generation

RR

All RW

WW 1 : 2 : 1

Gametes

Gametes

R

W

R W

RR RW

RW WW

Flower Color in Four’O clocks:

Incomplete dominance in carnations

Red flowers - 2 alleles to make a red pigment

White flowers – 2 alleles; can’t make red pigment

Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

Interaction 2: Codominance

• Neither allele can mask the other and both are fully expressed

in the heterozygous offspring and not in an “intermediate” form.

• Example: red flowers that are crossed with blue flowers that

yield red and blue flowers.

A Capital Letter is used for first allele (R) and a Capital Letter with apostrophe (R’) for second allele. Given the Genotype RR’.

• Application: The genes for fur design of Panthers are codominant. The 2 possible designs for fur are spotted and striped.

A spotted panther is crossed with a striped panther. What will the fur design of the baby panther be?

Spotted with stripes

Application: in cattle, the heterozygous (Rr) determines roan coat color (mixed red(RR) and white(rr) hairs). When two roan cattle are crossed, the phenotypes ratio is 1 red:2 roan:1 white.

Which of the following crosses could produce the highest percentage of roan cattle?

• A) roan x roan • B) red x white • C) white x roan • D) red x roan • E) All of the above crosses would give the same

percentage of roan.

Codominance– Ex. ABO blood group– both alleles expressed, IAIB = type AB blood

IA allele expresses A antigen on the surface of red cell IB allele expresses B antigen on the surface of red cellIndividuals with AB (IA IB ) blood group express both A and B antigens on their red cell surface.

• 1. type A = IAIA or IAi• 2. type B = IBIB or IBi• 3. type AB = IAIB

• 4. type O = ii

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Codominance Application

• Example: homozygous Type B (IBIB) x

heterozygous Type A (IAi)

IAIB IBi

IAIB IBi

1/2 = IAIB

1/2 = IBi

IB

IA i

IB

copyright cmassengale

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Codominance Application

IAIB

ii

Parents:genotypes = IAi and IBiphenotypes = A and B

IB

IA i

i

If a boy has a blood type O and his sister has blood type AB,

• what are the genotypes and phenotypes of their parents?

• boy - type O (ii) X girl - type AB (IAIB)

Interaction 3: Multiple Alleles

A gene may have more than 2 alleles for a trait in a population.

Often, no single allele is dominant; each allele has its own effect.

Multiple alleles (can occur in complete and incomplete

dominance and in codominance)

Example: ABO blood system

ABO blood systemFour phenotypes (A,B,AB,O) and three alleles (A, B, and i)each individual only inherits two alleles: one from each parent The gene encodes an enzyme that adds sugar on the surface of red blood

cells. This gene, designated I, has three common alleles: IB, its enzyme adds

galactose; IA, its enzyme adds galactosamine; and i, which codes for a protein that does not add a sugar.

If a type A receives a transfusion of type B blood, the immune system recognizes that the type B blood cells possess a “foreign” antigen (galactose) and attacks the donated blood cells, causing the cells to clump, or agglutinate.

This also happens if the donated blood is type AB.

However, if the donated blood is type O, no immune attack will occur, as there are no galactose antigens on the surfaces of blood cells produced by the type O donor.

In general, any individual’s immune system will tolerate a transfusion of type O blood.

ABO blood types

Both clump

clump

clump

No clump

Interaction 4: Pleiotropy• Single gene that causes multiple phenotypic traits

(ex. sickle-cell disease) In sickle cell anemia, a defect in the oxygen

carrying hemoglobin molecule causes anemia, heart failure, kidney failure, enlargement of the spleen....

Interaction 5: Polygenic Inheritance

2 or more genes combine their effects to produce single phenotypic trait

Example: skin and eye color, obesity, recurrent aborption and heart disease

Polygenetic Inheritance

• Indicated by quanitative variation usually

• This occurs when there is an additive effect from two or more genes.

• Pigmentation in humans is controlled by at least three (3) separately inherited genes.

Interaction 6: Epistasis

• It’s a type of polygenic inheritance where the alleles at one gene locus can hide or prevent the expression of alleles at a second gene locus.

• Example: In Labrador retrievers (dog) one gene (E,e) affects one other of coat color (B,b) by controlling

the amount of the pigment eumelanin in the fur.

eeNo dark pigment in fur

eebb eeB_

Yellow fur Yellow fur

E_Dark pigment in fur

E_bb E_B_

Brown fur Black fur

Epistasis

A dominant allele (B) produces a black coat while the recessive allele (b) produces

a brown coat

However, a second gene controls whether any eumelanin is deposited in the fur.

The homozygous recessive dogs (ee) will have yellow fur

The degree to which an allele is expressed may depend on the environment factors.

Some alleles are heat-sensitive, for example: such alleles are more sensitive to temperature than other alleles.

The arctic foxes make fur pigment only when the weather is warm.

Arctic fox in winter in summer

Interaction 7: Environmental Effects on gene expression

Temperature Effects on Phenotype

• In Himalayan rabbits, Melanin is produced in cooler areas of body.

• an enzyme of melanin production is heat-sensitive (Homozygous genotype).

pH of the soil will change the color of hydrangea flowers from blue to pink

The effect of environment of phenotype

Environmental Effects

brown eye color requires phenylalanine from diet (alimentation) to produce melanin, the eye pigment

Alleles implicate with survival of individuals carrying this genes those carrying homozygous allele die.

Ex. Creeper hen have vestigial wings and legs.If two creeper hen are crossed, there is a ratio of 2 creepers : 1 normal hen among offsprings (not 3 : 1).Creeper allele (C) is dominant in its effect on wings and legs length, but recessive on its effect on viability.CC lethal, die before hatchingCc creeper hencc normal hen, survive after hatching on

Interaction 8: Lethal alleles

Non- Mendelian Inheritance

Mitochondria is organell with its own DNA (is not interfeared by DNA genom). During fertilization sperm penetrate its head into the egg & leave the tail containing mitochondria outside.

zygote carry only mitochondria from egg (maternal) every individual mitochondrial DNA is inherited down the maternal line : from mother to children, and so on.Sex linked inheritance

Alleles (genes) which occupy loci within sex chromo –somes, though express traits of non sex-determinersare said to be “sex linked”.

Does these examples of inheritance patterns argue that Mendel was wrong?

No, Genetics is not as simple as Gregor Mendel concluded, (one gene, one trait).

We know now that there is a range of dominance and that genes can work together and interact.

• Incomplete dominance• Codominance• Multiple alleles• Polygenic traits• Epistasis

• Pleiotropy• Environmental effects on

gene expression• Sex linkage