In tomato plants (*Solanum lycopersicum*), purple stem (P) is dominant over green stem (p)… — JAMB UTME

In tomato plants (*Solanum lycopersicum*), purple stem ( $P$ ) is dominant over green stem ( $p$ ), and cut leaf ( $C$ ) is dominant over potato leaf ( $c$ ). A plant heterozygous for both traits ( $PpCc$ ) is crossed with a homozygous recessive plant ( $ppcc$ ). What proportion of the offspring is expected to have purple stems and potato leaves?

1/4 (25%)Answer
B
3/16 (18.75%)
C
9/16 (56.25%)
D
1/16 (6.25%)

Answer

1/4 (25%)

In a dihybrid test cross between a double heterozygote (

PpCc

) and a double homozygous recessive individual (

ppcc

), the heterozygous parent produces four gamete types (

PC

Pc

pC

pc

) in equal proportions (

1/4

each). When combined with the recessive parent's

pc

gamete, four phenotypic classes result with an expected frequency of

1/4

(

25\%

) each. Therefore, the proportion of offspring showing purple stems and potato leaves (

Ppcc

) is

1/4

Step-by-Step Solution

Identify parental genotypes and determine gametes produced

The heterozygous parent (

PpCc

) produces four types of gametes in equal proportions:

PC

Pc

pC

, and

pc

(

1/4

each). The homozygous recessive parent (

ppcc

) produces only one type of gamete:

pc

(

100\%

Mendel's Law of Independent Assortment states that alleles for different traits segregate independently during gamete formation.

Determine offspring genotypes and phenotypes

Combining gametes yields four offspring genotypes:

PpCc

(purple stem, cut leaf),

Ppcc

(purple stem, potato leaf),

ppCc

(green stem, cut leaf), and

ppcc

(green stem, potato leaf), each occurring at a frequency of 1/4.

A test cross matches each distinct gamete from the heterozygous parent with a recessive allele pair from the tester parent.

Calculate the expected proportion for the targeted phenotype

The proportion of offspring displaying purple stems (

P_

) and potato leaves (

cc

), represented by genotype

Ppcc

, is 1/4 or 25%.

Exactly one of the four equally probable phenotypic outcomes exhibits the dominant stem color and recessive leaf form.

Key Concept

Dihybrid Test Cross Ratio

Estimated Time:1m 30s

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