What is a Dihybrid Cross?

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What is a Dihybrid Cross?

• Aim To show the outcome of a genetic cross
  involving two characteristics

• A dihybrid cross is one which involves two
  characteristics.
• The genes for each feature is normally present on
  a separate pair of homologous chromosomes and are
  said to be unlinked.

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Carrying out a Dihybrid Cross
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The classic dihybrid cross example

• Round seeds are dominant to wrinkled
• Yellow seeds are dominant to green
• Mendel crossed true breeding pea plants with
  round, yellow seeds with true breeding plants
  with wrinkled, green seeds.
• What are the possible f2 generation phenotypes
  and ratio of these phenotypes?

• Identify the traits being crossed (give allele)
• Identify the genotype for both parents
• Identify the gametes for each parent (use FOIL)
• Complete punnett square for F1 generation (if
  needed)
• Identify the genotype and phenotype ratio for F1
• Read the question back answer it if you cant
  move on to step 6
• Identify gametes for both F1s to be crossed (use
  FOIL)
• Complete punnett square for F2 generation
• Identify the genotype and phenotype ratio for F2
• Read the question back answer it!

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F2 generation results
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• 1600 offspring resulted from this cross. Predict
  the numbers of each possible Phenotype

The actual numbers of each phenotype were as
follows
Yellow / Round 920 Yellow / Wrinkled
294 Green / Round 302 Green / Wrinkled 84
Explain why predicted and actual numbers are not
identical
Explanation - Fusion of gametes is random
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• Yellow seeds (Y) are dominant to green (y)
• Round reeds (R) are dominant to wrinkled (r)
• ? cross 2 true breeding plants
• RRYY x rryy

• ? F1 genotypes All RrYy
• ? F1 phenotypes Yellow Round
• ? Next the F1 were crossed
• RrYy x RrYy
• Possible F1 gametes RY, Ry, ry, rY
• Punnet square for F2 generation results

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More on dihybrid crosses

• ? RECOMBINATION
• In a dihybrid cross two of the F2 phenotypes
  resemble the original parents and 2 display new
  combinations of the characteristics
• The process by which new combinations of parental
  characteristics arise is RECOMBINATION

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RECOMBINATION

• Individuals with the same phenotype as the
  parents are known as PARENTAL types
• Individuals with new combinations are
  RECOMBINANTS

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For example

• Round, yellow x wrinkled, green
• F1 all round yellow
• F2 9 round,yellow3round,green
• 3wrinkled,yellow1wrinkled,green

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linkage
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Linkage

• Can you

• State what is meant by linked genes.
• Explain the consequences of genes being linked.
• Explain the effect crossing over has on linked
• genes.
• State the effect that crossing over has on
  distantly located genes.
• Describe what cross-over values (COV) are.

1st task Under your drosophila cross from
yesterday leave space to copy in AND LEARN
table 14.2
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Linkage

• The normal dihybrid cross we have looked at,
  results in a 9331 ratio.
• In the early 1900s it was noticed that this ratio
  did not always occur.
• In some F2 generations the parental phenotypes
  appeared more frequently than expected.
• Sometimes recombinant phenotypes did not appear
  at all.

Why do you think this is?
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Linked Genes

• To find out if genes are still linked after
  meiosis, a testcross (backcross) can be
  completed.
• If the backcross has the ratio 1111, it means
  that the linked genes have been separated due to
  crossing over.
• However, if the testcross produces a ratio of
  11, crossing over did not occur and more of the
  parental genotypes will appear in the F2
  generation.

Fig 14.9
Fig 14.8
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Frequency of recombination

• Crossing over at the chiasma can occur along any
  point of the chromosome
• The closer 2 linked genes are on a chromosome the
  less likely it is that a chiasma will occur
  between them and split them up they are more
  likely to be inherited together.
• The further away 2 linked genes are on a
  chromosome, the more likely a chiasma will occur
  to split them up - they are less likely to be
  inherited together.

LEAVE SPACE TO COPY IN Fig. 14.10
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COV

• The distance between a pair of linked genes is
  indicated by the number of F2 recombinants that
  results from a testcross.
• This is called the recombination frequency or
  CROSSOVER VALUE (COV).

Number of F2 recombinants
COV

X 100
Total number of F2offspring
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Chromosome maps

• The COV can be used to create a chromosome map,
  where the order and position of the genes (loci)
  can be identified.
• The greater the COV, the longer the distance
  between the 2 genes, and the shorter the COV, the
  shorter the distance between the 2 genes.
• So if you carry out many test crosses on linked
  genes and calculate the COV, you can tell if the
  genes are close together or far apart (ie you can
  work out their loci)

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Chromosome maps

• A recombinant frequency (COV) of 1 represents
  one unit of measure on the genetic map.
• If genes A/a and B/b show a recombination
  frequency of 9, then they will be
• 9 units apart on the genetic map.

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Constructing a Gene Map

• To construct a genetic map for a particular
  chromosome, a line is drawn and one gene is then
  placed on the map. (this is the reference point)
• The other genes are then placed on the map by
  positioning them at the correct number of units
  from the initial gene from each other.

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Constructing a Gene Map

• Example

Genes Recombinant Frequency ()
A/a x B/b 5
A/a x C/c 13
A/a x D/d 3
B/b x C/c 18
B/b x D/d 8
C/c x D/d 10
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Constructing a Gene Map

• Identify the two most distantly linked genes.
• B/b C/c- 18 units apart

C/c
B/b
18 units
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Constructing a Gene Map

• Identify the gene that is closest to the one on
  the left (B/b)
• B/b A/a- 5 units apart

C/c
A/a
B/b
18 units
5 units
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Constructing a Gene Map

• Now place the remaining gene (D/d) on the map.
• B/b D/d- 8 units apart
• D/d C/c -10 units apart
• It is equally correct to draw the genes in the
  reverse order.

C/c
B/b
A/a
D/d
5 units
3 units
18 units
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Constructing a Gene Map

• Now try this yourself.

Genes COV ()
T/t x L/l 12
T/t x B/b 4
T/t x S/s 21
L/l x B/b 16
L/l x S/s 9
B/b x S/s 25
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Constructing a Gene Map
S/s
L/l
B/b
T/t
4 units
12 units
25 units