Genetic Diagrams Noadswood Science, 2012. Genetic Diagrams To understand how to be able to construct...

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