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Summary Monohybrid crosses
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Detailed notes on Monohybrid crosses
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BLGY1232 Monohybrid crosses, test crosses and back crossesNaming genes (Nomenclature)
Gene names are always written in italics
Dominant alleles are always written first
Dominant alleles are written in capitals
Recessive alleles are written in lower case
Wildtype alleles are often denoted by “+” the use of “+” for a dominant allele is
OK if you are analyzing the segregation of alleles at a single locus not all wildtype
alleles are dominant
When considering a single gene just use letters
Loci can be referred to by 3 letters that are an abbreviation of the gene name
The yellow mouse
Wild type is recessive to the yellow coat colour
Homozygous Y/Y mice are never found as yellow is a lethal gene so zygotes with 2
copies of Y do not survive
Yellow is a pleiotropic gene as has multiply phenotypic consequences
Aberrant segregation; All yellow mice must be heterozygous as if they were
homozygous they would die before being born so do not appear in the progeny
therefore then 2 yellow mice are crossed there is a 2:1 yellow to wildtype
Analyzing genetic crosses
Represent a cross with a Punnett square
If you start with SS and ss all offspring with have dominant phenotype
For F2 if you let them reproduce; ¼ SS, ½ Ss, ¼ ss
When you get 2:1 you are looking at one gene with heterozygous alleles
Represent crosses using branch diagrams
F1 tells you which phenotype is dominant
F2 segregation ratio tells you how many loci involved
By selfing we can determine the genotypes of F2 as if they are homozygous they will
breed true and if they are heterozygous they will give you a 3:1
Test cross – a cross between an individual of unknown genotype and an individual of
unknown genotype and an individual that is true breeding for the recessive trait
Back cross – a cross between an individual and one of its parents
Test crosses enable us to distinguish between phenotypically identical but
genotypically different individuals – It is between a plant with the dominant
phenotype and the homozygous recessive parent; if the plant was homozygous all
progeny will have the dominant phenotype, is it was heterozygous there will be a 1:1
of dominant phenotype to recessive phenotype
If blending inheritance occurs all F1 are the same but the parental phenotypes re-
emerge in F2 generation where the progeny segregate in the ration 1AA:2Aa:1aa
co-dominance/incomplete dominance
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