Embark on a nuanced exploration of genetic principles with these focused notes crafted for Year 1 students in the Introduction to Genetics (BIOL0003) module at University College London. Specifically delving into the modifications to Mendel's laws, this document offers a concise examination of Mend...
Modifications to Mendel’s Second Law
Deviations from Mendel’s Second Law – Gene Interaction
Mendel’s second law
o Law of independent assortment
During gamete formation – the segregation of alleles of one allelic pair is independent of the
segregation of the alleles of another allelic pair
o Dihybrid cross
P1 = SSYY x WWGG
F1 = all SWYG
All smooth yellow heterozygous
F2 = 9:3:3:1 ratio
o Inheritance of shape and colour is independent – giving the 9:3:3:1 ratio
o Mouse genome
Agouti mouse
Brown-black coat
o Strand has yellow band
Black mouse
Homozygous recessive aa for non-agouti
o Removes yellow band
Brown mouse
Pigment mutation – makes brown mouse bb
Cross agouti and brown mouse
AABB = agouti
aabb = brown
P1 = AABB x aabb
F1 = all AaBb = all agouti
F2 = 9:3:3:1 ratio of agouti: cinnamon: black: brown
o
Cinnamon = brown hair with yellow band
Albino mouse
, Modifications to Mendel’s Second Law
C locus – mouse can make no pigment
Cross black and albino mouse
BBCC = black
o B = dominant pigment
o C = ability to make pigment
bbcc = albino
o b = recessive pigment
o c = inability to make pigment
P1 = BBCC x bbcc
F1 = All BbCc = all black
F2 = genotype 9:3:3:1
o Phenotype 9:3:4 ratio of black: brown: albino
Last two classes have the same
phenotype – as they have the epistatic
albino allele at the C locus that masks
the phenotype at the B locus
Epistatic interaction
Expression of one gene locus hides the expression of another
o Mouse coat colour genes in horses
o Albino tiger
o Humans
Secretor – phenotype that has two parts
Secretor red
Non-secretor blue
Secretor phenotype = you express your ABO blood groups in your body fluids
Non-secretor phenotype = you don’t express your ABO blood groups in your body fluids
o Haemoglobins in pregnancy
Before birth – fetus has a different haemoglobin mix
As haemoglobin has to be better at absorbing oxygen
HbF (fetal) may persist – protects against sickle cell symptoms
o Purple and white foxgloves – complementation
Purple is dominant to white
P1 – white line 1 x white line 2
Two distinct ways of plant being white
o Making purple pigment – takes a number of steps
If any one of the steps are blocked = purple pigment not produced
o 2 lines with mutant allele at two different loci – plants are both white but for
different reasons
White line 1 = w1w1W2W2
White line 2 = W1W1w2w2
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