Introduction to Genetics (BIOL0003) Notes - Modifications to Mendel

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