Monohybrid crosses

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Monohybrid crosses
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By the end of this lesson

• You should be able to

• Know what a monohybrid cross
• Know how to draw out a monohybrid cross
• Predict the proportions of phenotypes of
• the F2 offspring in a monohybrid cross
• Know why there is a difference between
• observed and predicted ratios in monohybrid
• crosses

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F1 and F2 generations

• When 2 parents are crossed they produce
  offspring.
• We refer to these offspring as the First Filial
  generation (F1)
• When we do genetics experiments, we often take
  individuals from the F1 generation, and cross
  them
• Their offspring are called the Second Filial
  generation (F2 )

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X
Parents
F1 generation
Then we cross two members of the F1 generation
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X
F1 generation
F2 generation
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Monohybrid Crosses

• In standard grade we have to work out theoretical
  crosses.
• When we do this we only consider one
  characteristic at a time
• This is called a monohybrid cross

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An example

• The gene for coat colour in mice has 2 alleles
• Brown which is dominant
• White which is recessive
• What GENOTYPE would a HOMOZYGOUS brown mouse
  have?
• BB
• What GENOTYPE would a HOMOZYGOUS white mouse
  have?
• bb

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Brown mouse
White mouse
Parents phenotype
X
BB
bb
Parents genotype
All b
All B
Gametes
F1 genotype
All Bb
F1 phenotype
All heterozygous brown
Now we cross 2 individuals from the F1 generation
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Brown mouse
Brown mouse
F1 phenotype
X
Bb
Bb
F1 genotype
B or b
B or b
Gametes
F2 genotype
Aargghh, how do we work this out????? Use a
punnett square
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Punnett square
Gametes from one parent
B b
B BB (Brown) Bb (Brown)
b Bb (Brown) bb (White)
Gametes from other parent
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Results!

• So all the possible F2 generation genotypes are
• BB (Brown)
• Bb (Brown)
• Bb (Brown)
• bb (white)
• The F2 phenotypic ratio would be
• 3 brown 1 white
• The F2 genotypic ratio would be
• 1 BB 2 Bb 1 bb
• 121

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Does this happen in nature?

• It is important to remember that when we are
  working out ratios, that it is only probable as
  fertilisation is a random process, and the true
  ratio is a matter of chance.

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Work

• Insert and complete the Genetics- Monohybrid
  Cross sheet from your diagram pack.

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Work

• Calculate the F2 generation phenotypic and
  genotypic ratios of a cross between these flies-
  homozygous straight- winged and curved-winged
  fruit fly
• Straight wing is dominant

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Work

• Once you are confident that you understand how to
  write out a monohybrid cross, ask your teacher
  for a question sheet.
• Remember to set our your answers in the correct
  format, as you have been shown!

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By the end of this lesson

• Can you do the following?

• Do you know what a monohybrid cross is?
• Can you draw out a monohybrid cross?
• Can you predict the proportions of
• phenotypes of the F2 offspring in a
• monohybrid cross?
• Explain why there could be a difference
• between the observed and predicted ratio