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Genetics - Test Cross — Biology Questions & Practice

Practise 40 Genetics - Test Cross questions from Biology for competitive exams such as UPSC, Banking, SSC and Railway. Read the worked questions below, check every answer with an explanation, and track what you get wrong.

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Practice questions on Genetics - Test Cross — with answers

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1. In pea plants, round seeds (R) is dominant over wrinkled seeds (r). An individual showing the round seeds phenotype is test-crossed with a homozygous recessive individual (rr). ALL of the offspring show the round seeds phenotype. What is the genotype of the original individual?

AHomozygous dominant (RR)
BHeterozygous (Rr)
CHomozygous recessive (rr)
DCannot be determined from this cross

Show answer

Answer: A

If a test cross (with rr) produces only round seeds-phenotype offspring, the tested individual must be homozygous dominant (RR), since a heterozygote would produce a 1:1 ratio including some wrinkled seeds offspring.

2. In pea plants, round seeds (R) is dominant over wrinkled seeds (r). An individual showing the round seeds phenotype is test-crossed with a homozygous recessive individual (rr). The offspring show approximately a 1:1 ratio of round seeds to wrinkled seeds phenotypes. What is the genotype of the original individual?

AHeterozygous (Rr)
BHomozygous dominant (RR)
CHomozygous recessive (rr)
DCannot be determined from this cross

Show answer

Answer: A

A 1:1 ratio of round seeds to wrinkled seeds offspring from a test cross indicates the tested individual is heterozygous (Rr), since half its gametes carry the recessive allele.

3. In pea plants, yellow seeds (Y) is dominant over green seeds (y). An individual showing the yellow seeds phenotype is test-crossed with a homozygous recessive individual (yy). ALL of the offspring show the yellow seeds phenotype. What is the genotype of the original individual?

AHomozygous dominant (YY)
BHeterozygous (Yy)
CHomozygous recessive (yy)
DCannot be determined from this cross

Show answer

Answer: A

If a test cross (with yy) produces only yellow seeds-phenotype offspring, the tested individual must be homozygous dominant (YY), since a heterozygote would produce a 1:1 ratio including some green seeds offspring.

4. In pea plants, yellow seeds (Y) is dominant over green seeds (y). An individual showing the yellow seeds phenotype is test-crossed with a homozygous recessive individual (yy). The offspring show approximately a 1:1 ratio of yellow seeds to green seeds phenotypes. What is the genotype of the original individual?

AHeterozygous (Yy)
BHomozygous dominant (YY)
CHomozygous recessive (yy)
DCannot be determined from this cross

Show answer

Answer: A

A 1:1 ratio of yellow seeds to green seeds offspring from a test cross indicates the tested individual is heterozygous (Yy), since half its gametes carry the recessive allele.

5. In pea plants, purple flowers (P) is dominant over white flowers (p). An individual showing the purple flowers phenotype is test-crossed with a homozygous recessive individual (pp). ALL of the offspring show the purple flowers phenotype. What is the genotype of the original individual?

AHomozygous dominant (PP)
BHeterozygous (Pp)
CHomozygous recessive (pp)
DCannot be determined from this cross

Show answer

Answer: A

If a test cross (with pp) produces only purple flowers-phenotype offspring, the tested individual must be homozygous dominant (PP), since a heterozygote would produce a 1:1 ratio including some white flowers offspring.

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Genetics - Test Cross — Biology Questions & Practice

Practice questions on Genetics - Test Cross — with answers

1. In pea plants, round seeds (R) is dominant over wrinkled seeds (r). An individual showing the round seeds phenotype is test-crossed with a homozygous recessive individual (rr). ALL of the offspring show the round seeds phenotype. What is the genotype of the original individual?

3. In pea plants, yellow seeds (Y) is dominant over green seeds (y). An individual showing the yellow seeds phenotype is test-crossed with a homozygous recessive individual (yy). ALL of the offspring show the yellow seeds phenotype. What is the genotype of the original individual?

5. In pea plants, purple flowers (P) is dominant over white flowers (p). An individual showing the purple flowers phenotype is test-crossed with a homozygous recessive individual (pp). ALL of the offspring show the purple flowers phenotype. What is the genotype of the original individual?

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