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Genetic Diagrams
Noadswood Science, 2012
Tuesday, May 23, 2017
Genetic Diagrams
• To understand how to be able to construct genetic diagrams
BB
bb
B
B
b
b
Bb
Bb
Bb
Bb
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)
Crazy hamster
Normal hamster
B
B
b
Bb
Bb
b
Bb
Bb
Hamster Cross Example
• Crossing a thoroughbred crazy hamster (bb) and a thoroughbred
normal hamster (BB) (both homozygous) – all offspring normal
Parents
Parent’s alleles
Normal
Crazy
BB
bb
Gamete’s alleles
B
B
b
b
Possible combinations
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
Bb
Parent’s alleles
Gamete’s alleles
Possible combinations
Normal
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)
Female
Male
X
Y
X
XX
XY
X
XX
XY
Sex Cross Example
• Probability of male / female offspring – 50:50
Parents
Parent’s alleles
Gamete’s alleles
Possible combinations
Male
Female
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)
Blue-eyed homozygous
Brown-eyed - homozygous
B
B
b
Bb
Bb
b
Bb
Bb
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
BB
bb
Parent’s alleles
Gamete’s alleles
B
B
b
b
Possible combinations
Bb
Bb
Bb
Bb
Eye Colour – Heterozygous
• Crossing two heterozygous brown-eyed individuals (Bb)
Brown-eyed heterozygous
Brown-eyed - heterozygous
B
b
B
BB
Bb
b
Bb
bb
Eye Colour
• Crossing two heterozygous brown-eyed individuals (Bb) – 3x
brown-eyed (75%) and 1x blue-eyed (25%)
Parents
Brown-eyed
Brown-eyed
Bb
Bb
Parent’s alleles
Gamete’s alleles
Possible combinations
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)
Brown-eyed heterozygous
Blue-eyed – homozygous
b
b
B
Bb
Bb
b
bb
bb
Eye Colour
• Crossing one heterozygous brown-eyed individual (Bb) and one
homozygous blue-eyed individual (bb) – 50:50
Parents
Brown-eyed
Blue-eyed
Bb
Parent’s alleles
bb
Gamete’s alleles
B
b
b
b
Possible combinations
Bb
Bb
bb
bb