Dihybrid Cross

A cross between two true-breeding parents that possess different forms (alleles) of two genes

true-breeding plant with

round yellow seeds

true-breeding plant with wrinkled green seedsX

Non linked genes

R

r

Y

yR

R

r

r

Y

Y

y

y

Genotype = RrYy

HOMOLOGOUS PAIRS DNA REPLICATION

Non linked genes

R

R

r

r

Y

Y

y

y

R

R

r

r

y

y

Y

Y

RY

ry

Ry

rY

Meiosis 1 Gametes

R = allele for round

r = allele for wrinkled

Y = allele for yellow

y = allele for green

Original cross RRYY X rryy

gametes All RY All ry

F1 All RrYy

Second cross RrYy RrYyx

gametes RY Ry rY ry

F1 Self-fertilised

RY Ry rY ry

RY Ry rY ry

RY

Ry

rY

ry rRyY

RrYY RrYyRRYY RRYy

rRYY

RRyY RRyy Rryy

rRyy

rRYy

RryY

rryy

rrYyrrYY

rryY

F2

(phenotypic ratio)

9 round yellow

3 round green

3 wrinkled yellow

1 wrinkled green

9:3:3:1 ratio

Recombination

• In a dihybrid cross two of the F2 phenotypes resemble the original parents

• Two display new combinations• The process by which new

combinations of parental characteristics arise is called recombination

• The individuals possessing them are called recombinants

RRYY rryy

RRyy rrYY

Original parents

Recombinants

Mendel’s second law

• The principle of independent assortment

During gamete formation, the alleles of a gene segregate into different gametes independently of the segregation of the two alleles of another gene

Dihybrid cross 2• Let R = Red let r = white• Let S = straight let s =

curly RrSs x RrSs • Possible gametes• RS,Rs,rS,rs x

RS,Rs,rS,rs• 9/16 red straight• 3/16 red curly• 3/16 white straight• 1/16 white curly• 800 offspring• red straight = 450• red curly = 150• white straight = 150• white curly = 50

X RS Rs rS rs

RS

Rs

rS

rs

RRSS

RRSs

RrSS

RrSs rrss

RRSs RrSS RrSs

RRss RrSs Rrss

RrSs rrSS rrSs

Rrss rrSs

Dihybrid cross 3• Let H = hairless, h = hairy• Let T = tall, t = dwarf HhTt x HhTt• Possible gametes• HT,Ht,hT,ht x

HT,Ht,hT,ht• 9/16 hairless tall• 3/16 hairless dwarf• 3/16 hairy tall• 1/16 hairy dwarf• 1280 offspring• Hairless tall = 720• Hairless dwarf = 240• Hairy tall = 240• Hairy dwarf = 80

X HT Ht hT ht

HT

Ht

hT

ht

HHTT

HHTt

HhTT

HhTt hhtt

HHTt HhTT HhTt

HHtt HhTt Hhtt

HhTt hhTT hhTt

Hhtt hhTt

Dihybrid cross 4• Let P = purple, p = cut• Let N = normal, n = twisted

PpNn x PpNn• Possible gametes• PN,Pn,pN,pn x

PN,Pn,pN,pn • 6400 offspring• Homozygous (4/16) =

1600• Purple (12/16) = 4800• Cut (12/16) = 1600• Twisted(4/16) = 1600

X PN Pn pN pn

PN

Pn

pN

pn

PPNN

PPNn

PpNN

PpNn ppnn

PPNn PpNN PpNn

PPnn PpNn Ppnn

PpNnppNN

ppNn

Ppnn ppNn

Dihybrid Cross 4

• Let R = Red let r = white• Let S = straight let s = curly• rrSS x RRss• Gametes• rS x Rs• F1 • 100% RrSs • 100% Red straight

X rS

Rs RrSs

Dihybrid Test cross 1

• Let H = hairless, h = hairy

• Let T = tall, t = dwarf

HHTT x hhtt• Gametes• HT x ht• F1 100% HhTt • F1 100% hairless tall

X ht

HT HhTt

Dihybrid test cross 2

• Let P = purple, p = cut• Let N = normal, n = twisted• PpNn x ppnn• PN,Pn,pN,pn x pn• 1000 offspring• in 1:1:1:1ratio• Purple normal (¼) = 250• Purple twisted (¼) = 250• Cut normal (¼) = 250• Cut twisted (¼) = 250

X pn

PN PpNn

Pn Ppnn

pN ppNn

pn ppnn

Linked genes 1

R Y

r y

HOMOLOGOUS PAIR

Genotype = RrYy

DNA REPLICATION

R Y

R Y

r y

r y

Linked genes 2

Meiosis 1

R Y

R Y

r y

r y

Gametes

RY

ry

Large numbers of parental gametes

Crossing over

X

X

Ry

rY

Small numbers of recombinant gametes

Linked genes 3

H t

h T

HOMOLOGOUS PAIR

Genotype = HhTt

DNA REPLICATION

H t

H t

h T

h T

Crossing over

Frequency of crossing over

Chiasmata can occur at any point along a chromosome

More crossing over (recombination) occurs between two distantly located genes than two that are close together

Only cross over 1 would break the link between A and B or a and b

Any one of crossovers 2,3,4 and 5 would break the link between genes B/b and C/c

Linked genes 4• Let R = Red let r = white• Let S = straight let s = curly• The genes are linked R and S ,

r and s• RrSs x RrSs • Possible gametes• RS , rs x RS , rs• 6400 offspring• ¾ Red straight = 4800 • ¼ white curly = 1600• Small numbers of red curly

and white straight by crossing over

RS rs

RS RRSS RrSs

rs RrSs rrss

Linked genes test cross• Let H = hairless, h = hairy• Let T = tall, t = dwarf• Genes T and H and t and h are

linked on the same chromosome HhTt x hhtt• Possible gametes• HT , ht x ht• 1000 offspring• in 1:1 ratio• 50% hairless tall = 500 ( 460)• 50% hairy dwarf = 500 ( 445)• Small number of recombinants by

crossing over• Hairless dwarf ( 45)• Hairy dwarf (50)

X ht

HT HhTt

ht hhtt

Linked genes 5

Meiosis 1

H t

H t

h T

h T

Gametes

Ht

hT

Large numbers of parental gametes

Crossing over

X

X

HT

ht

Small numbers of recombinant gametes

Linked genes 6• Let P = purple, p = cut• Let N = normal, n = twisted• P and n and p and N are on

the same chromosome• PpNn x ppnn• Possible gametes• Pn , pN x pn• 1280 offspring• 50% purple twisted = 610• 50% cut normal = 600• Small numbers of recombinant

phenotypes purple normal (33) and cut twisted (37) by crossing over

X pn

Pn Ppnn

pN ppNn

Gametes and Ratios Linked genes Non linked genes

RrYy (R+Y) RY, ry RY,Ry,rY,ry

PpTt (P+T) PT, pt PT,Pt,pT,pt

HhSs (H +s) Hs, hS HS,Hs,hS.hs

RrYy X RrYy 3 : 1 9:3:3:1

RrYy X rryy 1:1 1:1:1:1

HhSs X hhss 1:1 1:1:1:1

PpTt X PpTt 3:1 9:3:3:1

Genetics Strategy• Monohybrid• One characteristic (two alleles, two phenotypes)• e.g. Eye colour- Red eyes and White eyes• Dominance – Rr x Rr 3 red : 1 white Rr x rr 1:1 ratio• Co dominance – two alleles, three phenotypes RR (red) RW (pink) and WW (white)• Multiple alleles – more than two alleles e.g. ABO blood groups• Sex linkage – show X and Y chromosome XRY x XRXr 1:1:1:1 ratio

Genetics Strategy• Dihybrid• Two characteristics, four phenotypes• Colour and Size• A = red, a = green S = short, s = long • Non linkage• AaBb x AaBb 4 phenotypes 9:3:3:1• AaBb x aabb 4 phenotypes 1:1:1:1• Linkage• AaBb x AaBb 2 phenotypes 3:1• AaBb x aabb 2 phenotypes 1:1

Red short

Red long

Green short

Green long

Red short (A+B)

Green long (a+b)

0R

Red long (A+b)

Green short (a+B)