A biologist collects seeds from a pale yellow sunflower in his back yard. He gro
ID: 58207 • Letter: A
Question
A biologist collects seeds from a pale yellow sunflower in his back yard. He grows four, to adulthood, in a greenhouse, and bags the flowers so that they are forced to self-fertilize. Asking questions about whether his flower is cross pollinated, he grows all the seeds from each of these four sunflowers and records the results. His results are as follows. All flowers except some of the progeny from the fourth seed have normal petals: Seed number one. F1 has pale yellow flowers. F2 Progeny are all pale yellow. Seed number two. F1 has pale yellow flowers. F2 progeny are 73 pale yellow flowers and 27 red flowers. Seed number three. F1 has dark yellow flowers. F2 progeny are 77 dark yellow flowers and 23 pale yellow flowers. Seed number four. F1 has dark yellow flowers and normal petals. F2 have 59 dark yellow flowers with normal petals, 21 dark yellow flowers with curled petals, 17 pale yellow flowers with normal petals, and 3 pale yellow flowers with curled petals.
a) Based upon your intuition for the crosses above, how many alleles determine flower color? What is the dominance relationship among them?
b) Regarding flower color and petal shape, how many loci determine this? Diagram the crosses for seed number 4 and show a genetic hypothesis for his results.
Explanation / Answer
a)
There are 3 alleles which determine flower colour. Colour dark yellow and pale yellow show co-dominance, in which allele for dark yellow is co-dominant over pale yellow allele. Red colour is recessive allele to pale and dark yellow.
This can be understood like this
Allele for pale yellow :IA
Allele for dark yellow : IB
Allele for red : Ii
On looking at phenotypes, expression of phenotypes follows
dark yellow seed > pale yellow > red
Pale yellow will be expressed only when both allele are IA IA
Dark yellow will be expressed only when alleles on homologous chromosomes are either IA IB or IB B
Red will be expressed only when alleles on homologous chromosomes are Ii Ii
b)
We know alleles are different phenotypes of same gene. So, for flower colour there will be one gene i.e. one loci. Other phenotype is petal shape. Thus making total two loci.
Petal shape has only two phenotypes, so definetly there will be one dominant and other recessive.
Lets denote dominant allele for petal shape by X and its recessive by x
So, for seed 4
F1 genotype : IA IB Xx (because yellow and dark / red and normal petal / curled petal are expressed in F2)
F2: by punnet square
IA X
IA x
IB X
IB x
IA X
IA X IA X
IA X IA x
IA X IB X
IA X IB x
IA x
IA x IA X
IA x IA x
IA x IB X
IA x IB x
IB X
IB X IA X
IB X IA x
IB X IB X
IB X IB x
IB x
IB x IA X
IB x IA x
IB x IB X
IB x IB x
Pale yellow normal petal : 3 / 16 = 18.7 %
Pale yellow curled : 1 / 16 = 6.25 %
Dark yellow normal : 10 / 16 = 62.5 %
Dark yellow curled : 2 / 16 = 12.5 %
You can see there is not much difference in expected no. genotypes compared to observed genotypes. Thus, holding our hypothesis true i.e. involves co-dominance for flower color and dominance for petal shape.
IA X
IA x
IB X
IB x
IA X
IA X IA X
IA X IA x
IA X IB X
IA X IB x
IA x
IA x IA X
IA x IA x
IA x IB X
IA x IB x
IB X
IB X IA X
IB X IA x
IB X IB X
IB X IB x
IB x
IB x IA X
IB x IA x
IB x IB X
IB x IB x
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