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Useful GeneticsProfessor Rosie Redfield

The University of British Columbia

The UBC crest is © UBC

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All about dominanceOutline:• Definition of dominance• other usage of ‘dominance’• Dominance in context• Why mutant alleles are often recessive

to their wild-type counterparts• Why Mendel always saw dominance• Considering more than 2 alleles

Lecture 3G:??

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Lecture 3G:??

Learning Objectives:• Decide whether a pair of alleles exhibits

dominance• Explain why dominance occurs• Interpet dominance as part of a spectrum

of allele interactions• Predict phenotypes affected by more than

two alleles

Suggested reading:• Open Genetics, Ch. 3.3.• Wikipedia entry on Dominance

All about dominance

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Dominance defined

Consider two alleles that give different phenotypes when homozygous. If a heterozygous individual has the same phenotype as one of the homozygotes, geneticists say that allele is ‘dominant’ to the other.

Dominance describes a relationship between two alleles, not a property of a single allele.

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If

CYCY CYCG CGCG

homozygote heterozygote homozygote phenotype = phenotype ≠ phenotype

we say that CY is dominant to CG.

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If the heterozygote has an in-between phenotype:

PTPT PTPS PSPS

homozygote heterozygote homozygotephenotype ≠ phenotype ≠ phenotype

(avoid ‘semi-dominance’ or ‘incomplete dominance’)

We say this is a blended or ‘additive’ phenotype

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If the heterozygote has both phenotypes:

SySy SySo SoSo

homozygote heterozygote homozygotephenotype ≠ phenotype ≠ phenotype

(avoid ‘co-dominance’) We say that both phenotypes are seen

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Complete dominance

Equal blending

Both phenotypes

How pairs of alleles affect phenotypes

Textbook alleles

Real alleles

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Equal blending

Both phenotypes

How pairs of alleles affect phenotypes

Textbook alleles

Real alleles

+/- allele pairs(1 functional, 1 nonfunctional)

Complete dominance

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Why are defective alleles typically recessive to functional alleles?

Because, for many functions, half the usual amount of protein is enough.(‘Haplo-sufficiency’ is the norm.)

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“Some characters... did not permit of a sharp and certain separation, since the difference was of a ‘more or less’ nature, which is difficult to define. Such features were not suitable for the individual experiments, which were restricted to characters that appear clearly and decisively on the plants.”

Mendel wrote:

Until recently, this remained true for both genetics textbooks and researchers.

Ein Theil der angeführten Merkmale lässt jedoch eine sichere und scharfe Trennung nicht zu, indem der Unterschied auf einem oft schwierig zu bestimmenden "mehr oder weniger" beruht. Solche Merkmale waren für die Einzel-Versuche nicht verwendbar, diese konnten sich nur auf Charactere beschränken, die an den Pflanzen deutlich und entschieden hervortreten.”

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E.g.. homologous genes control coat colour in many mammals (including us).

3 common alleles of the B (black) gene BB = black coat bb = dark brown coat blbl = light brown coat

The dominance relationships of these alleles form an ‘allelic series’: B > b > bl

(Relationships between alleles of many other genes are not this simple.)

Many genes have 3 or more alleles. Each pair has their own dominance relationship.

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ay > aw > at > a.

Coat colour in mammals

B > b > bl

I R BB or Bb or Bb'

I R bb or bb'(b–) I R

b'b'

Compare the black cat, chocolate brown cat, and cinnamon brown cat...

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3 common alleles of the B (black) gene control coat colour in most mammals: BB = black coat bb = dark brown coat blbl = light brown coat

They have the following dominance relationships: B > b > bl

What is the coat colour of a Bb’ dog?

Black

Dark brown

Light brown

More information is needed

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• Explicitly defined dominance

• Considered confusions arising from incorrect usage of ‘dominance’

• Put dominance on a solid biochemical foundation

• Put dominance in its full genetic context

• Considered multiple alleles

What we’ve done

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Lecture 3H:How genes are named, and how we use the names

Coming up...

Good names Bad names

LR SR R 1

LP SP P 2