Genetic Diagrams• To understand how to be able to construct genetic diagrams
BB bb
B B b b
Bb Bb Bb Bb
Wednesday, April 19, 2023
Genetic Diagrams• Genetic diagrams show the possible genes for offspring
• Alleles are different versions of the same gene, and most of the time there are two copies for each gene (one from each parent)
• If they’re different alleles one might be ‘expressed’ by the organism (dominant allele)
• In genetic diagrams letters are used to represent genes – dominant alleles are always shown with CAPITAL LETTERS, whilst recessive alleles are shown with lower case
Genetic Diagrams – Example• Hamster can be either normal or crazy – in this example
normal is dominant (B) and crazy is recessive (b)
• Hamster normal gene – B• Hamster crazy gene – b
• A crazy hamster (recessive characteristic) must have both alleles as recessive – bb
• A normal hamster (dominant characteristic) can have two combinations: either both dominant – BB, or one dominant and one recessive – Bb
Hamster Cross Example• Crossing a thoroughbred crazy hamster (bb) and a
thoroughbred normal hamster (BB)
B B
b Bb Bb
b Bb Bb
Normal hamster
Cra
zy
ham
ster
Hamster Cross Example• Crossing a thoroughbred crazy hamster (bb) and a thoroughbred
normal hamster (BB) (both homozygous) – all offspring normal
Parents Normal Crazy
Parent’s alleles
Gamete’s alleles
Possible combinations
BB bb
B B b b
Bb Bb Bb Bb
Hamster Cross Example• If two of the heterozygous offspring were then crossed (Bb x
Bb) then we would have 3x normal (75%) and 1x crazy (25%)
Parents Normal Normal
Parent’s alleles
Gamete’s alleles
Possible combinations
Bb Bb
B b B b
BB Bb Bb bb
Genetic Diagrams – Question• Complete the genetic cross to show the possible
combination of gametes: -▫ Female produces two X gametes (XX)▫ Male produces one X gamete (X) and one Y gamete (Y)
X Y
X XX XY
X XX XY
Male
Fem
ale
Sex Cross Example• Probability of male / female offspring – 50:50
Parents Male Female
Parent’s alleles
Gamete’s alleles
Possible combinations
XY XX
X Y X X
XX XX XY XY
Genetic Diagrams – Question• Eye colour can be brown and blue (simplistic) – in this
example brown is dominant (B) and blue is recessive (b)
• Brown gene – B• Blue gene – b
• An individual with blue eyes (recessive characteristic) must have both alleles as recessive – bb
• An individual with brown eyes (dominant characteristic) can have two combinations: either both dominant – BB, or one dominant and one recessive – Bb
Eye Colour – Homozygous• Crossing a homozygous blue-eyed person (bb) and a
homozygous brown-eyed person (BB)
B B
b Bb Bb
b Bb Bb
Brown-eyed - homozygous
Blu
e-e
yed
-
hom
ozy
gou
s
Eye Colour• Crossing a homozygous blue-eyed person (bb) and a homozygous
brown-eyed person (BB) – all offspring brown-eyed
Parents Brown-eyed Blue-eyed
Parent’s alleles
Gamete’s alleles
Possible combinations
BB bb
B B b b
Bb Bb Bb Bb
Eye Colour – Heterozygous• Crossing two heterozygous brown-eyed individuals (Bb)
B b
B BB Bb
b Bb bb
Brown-eyed - heterozygous
Bro
wn
-eye
d -
h
ete
rozy
gou
s
Eye Colour• Crossing two heterozygous brown-eyed individuals (Bb) –
3x brown-eyed (75%) and 1x blue-eyed (25%)
Parents Brown-eyed Brown-eyed
Parent’s alleles
Gamete’s alleles
Possible combinations
Bb Bb
B b B b
BB Bb Bb bb
Eye Colour – Hetero & Homozygous• Crossing one heterozygous brown-eyed individual (Bb) and
one homozygous blue-eyed individual (bb)
b b
B Bb Bb
b bb bb
Blue-eyed – homozygous
Bro
wn
-eye
d -
h
ete
rozy
gou
s
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