This video is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported license. Useful Genetics Professor Rosie Redfield The University of British Columbia The UBC crest is © UBC 1

All about dominance Outline: 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: ?? 2

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 3 Suggested reading: Open Genetics, Ch Wikipedia entry on Dominance All about dominance

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. 4

If C Y C Y C Y C G C G C G homozygote heterozygote homozygote phenotype = phenotype phenotype we say that C Y is dominant to C G. 5

If the heterozygote has an in-between phenotype: P T P T P T P S P S P S homozygote heterozygote homozygote phenotype phenotype phenotype (avoid semi-dominance or incomplete dominance) We say this is a blended or additive phenotype 6

If the heterozygote has both phenotypes: S y S y S y S o S o S o homozygote heterozygote homozygote phenotype phenotype phenotype (avoid co-dominance) We say that both phenotypes are seen 7

Complete dominance Equal blending Both phenotypes How pairs of alleles affect phenotypes Textbook alleles Real alleles 8

Equal blending Both phenotypes How pairs of alleles affect phenotypes Textbook alleles Real alleles +/- allele pairs (1 functional, 1 nonfunctional) Complete dominance 9

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.) 10

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. 11

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 b l b l = light brown coat The dominance relationships of these alleles form an allelic series: B > b > b l (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. 12

a y > a w > a t > a. Coat colour in mammals Compare the black cat, chocolate brown cat, and cinnamon brown cat... 13

3 common alleles of the B (black) gene control coat colour in most mammals: BB = black coat bb = dark brown coat b l b l = light brown coat They have the following dominance relationships: B > b > b l What is the coat colour of a Bb dog? Black Dark brown Light brown More information is needed 14

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 15 What weve done

Lecture 3H: How genes are named, and how we use the names Coming up Good names Bad names L R S R R 1 L P S P P 2