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Genetic Barley
We supply:
Living materials
Preserved materials
Catalogue Codes:
Background
S 2.1 (200 seeds)
There is a gene controlling pigment production in barley that has
the alternate alleles:
A: Pigment produced - green. (dominant)
a: No pigment - albino (or white). (recessive)
where pigment production is dominant to albino.
The seed provided has been obtained as a result of crossing plants heterozygous for the gene concerned.
For example:
Aa x Aa
Skeletons
Genotype of seed:
Microscopes
Anatomical Models
When the seeds germinate, plants that are AA will be green, Aa plants will also be green, and aa plants
will be albino. So when the seeds provided are germinated, we would expect the plants obtained to
approximate a 3 green : 1white ratio. White (albino) plants lack chlorophyll and will die as young
seedlings because they are unable to photosynthesise.
Germination:
Seeds can be readily germinated by spreading them out over cotton wool soaked with water in a petri
dish. The cotton wool must be kept moist and not allowed to dry out. Seedlings can be assessed as either
containing or not containing chlorophyll when they have reached a height of 4 to 5 cm. This takes 4 to 5
days at 22 - 24C, and 7 to 8 days at lower temperatures. The germination rate is approx. 70-80%, with the
number of seeds per packet having been adjusted to allow for this factor.
Question that you may consider for your students. (brief answers in brackets)
Assuming that the only information given to the student is that the seeds are the product of the cross Aa x
Aa.
1. What colour plants do you expect the seeds to give rise to? (Green and White.)
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2.
Culture media
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Stains and Indicators
Seeds
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Chemistry Models
AA: 2Aa : aa
If a large number of seedlings are obtained, what would you expect the approximate colour
ratio to be? (About one quarter of the seedlings to be white, the remainder (3/4) will be green.)
Environmental
3.
What is the genotype of the white seedlings? (aa)
4.
Why do the white seedlings fail to survive? (They lack chlorophyll and are unable to
Forensic Equipment
photosynthesise.)
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5.
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Given that the seeds are the products of fertilisation between heterozygotes, what are the
possible genotypes of the green plants? (AA or Aa)
6.
How would you determine the actual genotype of a green barley plant? (Allow the plant to
mature and self fertilise. Collect the seed from the plant and allow them to germinate. If the offspring
comprise some white as well as green plants then the parent plant must be heterozygous, that is Aa.
If the offspring comprise all green plants then the parent plant must have been homozygous for
green, that is AA.)
Ref: S 2.1
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26/3/09