Genetics: a ‘how to’ guide
Doing sums step by step
Step 1: the data
Write down the data schematically
Leave ‘unknown’ open
Phenotypes and genotypes are important
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long . .
F2: 631 Long . .217 Short . .
Step 2: fill in what you know
Fill in the alleles you know from the data Like: Long is dominant, so all long flies have at
least one T
Leave open what you don’t know right away
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long . .
F2: 631 Long . .217 Short . .
Step 2: fill in what you know
Fill in the alleles you know from the data Like: Long is dominant, so all long flies have at
least one T
Leave open what you don’t know right away
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T .
F2: 631 Long T .217 Short t t
Step 3: going back and forth
Use one generation to conclude about the former or the next generation Like: short (tt) can only pass on short (t)
Leave open what remains unknown
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T .
F2: 631 Long T .217 Short t t
Step 3: going back and forth
Use one generation to conclude about the former or the next generation Like: short (tt) can only pass on short (t)
Leave open what remains unknown
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T .
F2: 631 Long T .217 Short t t
Step 3: going back and forth
Use one generation to conclude about the former or the next generation Like: short (tt) can only pass on short (t)
Leave open what remains unknown
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T t
F2: 631 Long T .217 Short t t
Step 3: going back and forth
Use one generation to conclude about the former or the next generation Like: short (tt) can only pass on short (t)
Leave open what remains unknown
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T t
F2: 631 Long T .217 Short t t
T t
T TT Tt
t Tt tt
Step 3: going back and forth
Use one generation to conclude about the former or the next generation Like: short (tt) can only pass on short (t)
Leave open what remains unknown
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T t
F2: 631 Long 3 TT / Tt217 Short 1 t t
T t
T TT Tt
t Tt tt
Step 4: more conclusions
P: ....... X ……. phenotypes . . . . genotypes
F1: 57 Long T t
F2: 631 Long 3 TT / Tt217 Short 1 t t
Use one generation to conclude about the former or the next generation Like: only Long (Tt) in F 1 means P1 gives T and P2
gives t
Leave open what remains unknown
T t
T TT Tt
t Tt tt
Step 4: more conclusions
P: ....... X ……. phenotypes TT t t genotypes
F1: 57 Long T t
F2: 631 Long 3 TT / Tt217 Short 1 t t
Use one generation to conclude about the former or the next generation Like: only Long (Tt) in F 1 means P1 gives T and P2
gives t
Leave open what remains unknown
T t
T TT Tt
t Tt tt
Step 4: more conclusions
P: ....... X ……. phenotypes TT t t genotypes
F1: 57 Long T t
F2: 631 Long 3 TT / Tt217 Short 1 t t
Try and finish by filling in the rest Like: TT is Long and tt is short
T t
T TT Tt
t Tt tt
Step 4: more conclusions
P: Long X Short phenotypes TT t t genotypes
F1: 57 Long T t
F2: 631 Long 3 TT / Tt217 Short 1 t t
Try and finish by filling in the rest Like: TT is Long and tt is short
T t
T TT Tt
t Tt tt
Step 5: check your work
P: Long X Short phenotypes TT t t genotypes
F1: 57 Long T t
F2: 631 Long 3 TT / Tt217 Short 1 t t
Check the genotypes and ratio’s TT x tt always leads to Tt
Check the phenotypes against the genotypes Only tt is short winged
T t
T TT Tt
t Tt tt