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In maize plants, two gene loci on different chromosomes have the following alleles - VCE - SSCE Biology - Question 23 - 2007 - Paper 1
Question 23
In maize plants, two gene loci on different chromosomes have the following alleles. Gene locus one Gene locus two R : green stems D : normal height r : red stem... show full transcript
Worked Solution & Example Answer:In maize plants, two gene loci on different chromosomes have the following alleles - VCE - SSCE Biology - Question 23 - 2007 - Paper 1
Step 1
C. $ rac{3}{4}$ of the green stem, dwarf offspring will be heterozygous at gene locus one.
Answer
To determine the proportion of green stem, dwarf offspring that are heterozygous at gene locus one, we analyze the phenotypic ratios expected from the cross Rr Dd x Rr Dd.
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Determine Allele Frequencies: Given that R is dominant over r and D is dominant over d, the phenotypic ratio for a dihybrid cross typically follows a 9:3:3:1 ratio.
- Green stem (R_) and normal height (D_): 9 parts.
- Green stem (R_) and dwarf (dd): 3 parts.
- Red stem (rr) and normal height (D_): 3 parts.
- Red stem (rr) and dwarf (dd): 1 part.
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Identify Dwarf Offspring: The dwarf phenotype (dd) includes both red and green stems:
- Green stem (Rr dd) will be rac{3}{4} the total number of dwarf plants.
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Calculate Heterozygosity: Among the green stem offspring, half will be heterozygous (Rr) at the gene locus for stem color. Therefore, the ratio of the dwarf, green stem plants that are heterozygous at gene locus one is rac{3}{4} of the dwarf phenotype.
In conclusion, approximately rac{3}{4} of the green stem, dwarf offspring will indeed be heterozygous at gene locus one.