BMS2042 Lecture Notes - Lecture 3: Haemophilia A, Rna Splicing, Zygosity
30 May 2018
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Week 2. Extensions to Mendelian Inheritance II,
Mutation and gene function, Linkage recombination
and genetic mapping
EXTENSIONS TO MENDELIAN INHERITANCE II
• Gene interactions:
o When two genes affect different traits, dihybrid cross gives an F2 9:3:3:1 phenotypic
ratio however most traits are controlled by two or more genes
-> need to look at gene interactions
o Two genes controlling a single trait gives rise to novel phenotypes
eg. skin colouration in corn snakes, when cross together pure breeding orange and black
F1 are all camouflage (wildtype)
F2: 9 camouflage: 3 orange: 3 black: 1 albino
o Double homozygous recessive mutant produces a novel phenotype and often occurs
when genes act in parallel pathways
• Complementary gene action:
o 9:7 F2 ratio
o Need a dominant allele for each gene to produce the trait called complementary gene
action (or duplicate recessive epistasis)
o Eg. colour in sweet peas
If either cc or pp will be white. Need a dominant allele for both genes to get purple
(C and P dominant to c and p)
find more resources at oneclass.com
find more resources at oneclass.com
• Epistasis:
o Recessive epistasis: F2 9:3:4 ratio
o Phenotype produced by a mutant allele of one gene (epistatic allele) blocks or masks the
phenotype produced by alleles of another gene
For recessive epistasis, only masks phenotypic effects when homozygous
o Usually occurs because genes act in the same pathway for trait
o No new phenotypes are produced but fewer than 4 phenotypes are expected
o Eg. coat colour in Labradors
ee is epistatic to B gene (e blocks b, when genotype ee, E gene is non-functional hence
stop at yellow and no further colouration is possible) Need both functioning genes to get
black
o Eg. when C and P are dominant to c and p, need a dominant allele for each gene to
produce the trait, cc or pp will be colourless
find more resources at oneclass.com
find more resources at oneclass.com
o Eg.Bombay blood group phenotype where you get an O type offspring
-no substance h produced -> a’t stik to lood, it’s like you hae o atiges -> O
blood type phenotype
o Can also see dominant epistasis: 12:3:1
• Suppression:
o F2 13:3 ratio
o Suppressor allele: allele of one gene that suppresses the effect of a mutation of another
gene resulting in a wildtype
o May or may not have their own phenotype
o Can be dominant or recessive
o Produce modified dihybrid ratios
o Eg. p+ = wiltype, p = recessive mutant, recessive s allele is at another locus and
suppresses effect of p
-> pp:ss is wildtype
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Mutation and gene function, linkage recombination and genetic mapping. Extensions to mendelian inheritance ii: gene interactions, when two genes affect different traits, dihybrid cross gives an f2 9:3:3:1 phenotypic ratio however most traits are controlled by two or more genes. > need to look at gene interactions: two genes controlling a single trait gives rise to novel phenotypes eg. skin colouration in corn snakes, when cross together pure breeding orange and black. If either cc or pp will be white. > pp:ss is wildtype: duplicate genes, 15:1 dihybrid ratio, either a or b is required, eg. fruit shape i(cid:374) shepherd"s purse (cid:894)(cid:449)eed(cid:895) > dominant allele of either = triangular shape. Mutations within the coding sequence: may or may not affect aa sequence. Will be passed from cell to cell during cell division. Progenitor of a population of mutant cells called a clone of cells. Only dominant mutations will affect the phenotype.
