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Genetics questions


Genetics questions

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A genetic cross was made between pure-breeding snapdragon plants with red flowers and pure-breeding snapdragon plants with white flowers. The cross produced F1 offspring that had only pink flowers. When the F1plants were self-pollinated, the resulting F2 generation had some red, some white and some pink flowers.


(i) Identify the relationship between the red and white alleles for flower colour. [1] (ii) Deduce the genotype of the F1plants. [1]

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(iii) Construct a Punnett grid to show the cross between two F1plants. [2] (iv) Deduce the proportion of the different phenotypes of the F2 offspring. [1]


(i) Identify the relationship between the red and white alleles for flower colour. [1] First thing to ask is what colour are the flowers in the F1 generation

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They are pink so is there enough evidence to say one colour is Dominant and one recessive? So what is the alternative?


Answer

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codominance / incomplete/partial dominance


(ii) Deduce the genotype of the F1plants. [1] Do we have a Dominant and recessive? They are Codominant so both alleles are represented by upper case letters e.g:

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Red = CR CR Pink = CR CW

White = CW CW


(iii) Construct a Punnett grid to show the cross between two F1plants. [2]

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CR

CW

CR

C RC R C R C W

CW

CRCW CWCW


(iv) Deduce the proportion of the different phenotypes of the F2 offspring. [1] 1 red:2 pink:1 white or

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25%red :50 %pink : 25% white

CR

CW

CR

C RC R C RC W

CW

CRCW CWCW


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Problem 2


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In some maize plants the seed is enclosed in a green sheath called a tunica. The allele (T) for this is dominant to the allele (t) for normal, unenclosed seeds. The endosperm of the seed can be starchy (allele E) or sugary (allele e). The genes for these two characteristics are linked. The table shows the outcome of crosses between a plant heterozygous for both characteristics and one that is homozygous recessive for both characteristics.


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Phenotype

Number

Tunica present, starchy

326

Unenclosed seeds, starchy

111

Tunica present, sugary

118

Unenclosed seeds, sugary

295


(i) State the genotype of the heterozygous parent using the correct notation.[1] (ii) Identify which individuals are recombinants in this cross. [1]

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(iii) Explain what has occurred to cause these results [2]


(i) State the genotype of the heterozygous parent using the correct notation.[1] First thing to consider is what does heterozygous mean Has both Dominant and recessive alleles

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Second thing to consider is that this is a linked trait The allele for each trait is paired together


Correct notation shows the linkage Dominant linked alleles T

E

t

e

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Recessive linked alleles


(ii) Identify which individuals are recombinants in this cross. [1] First what combinations do you get from linked dominance and recessive?

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Tunica present, starchy and Unenclosed seeds, sugary


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unenclosed seeds, starchy and tunica present, sugary must be the recombinants T

e

t

e

AND

t

E

t

e


(iii) Explain what has occurred to cause these results [2]

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crossing over; between non-sister chromatids (in prophase I) this results in exchange of alleles so some gametes are T e or t E


Genetics questions