Genetic Diagrams

Noadswood Science, 2012

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

Eye Colour• Crossing one heterozygous brown-eyed individual (Bb) and

one homozygous blue-eyed individual (bb) – 50:50

Parents Brown-eyed Blue-eyed

Parent’s alleles

Gamete’s alleles

Possible combinations

Bb bb

B b b b

Bb Bb bb bb