Question

In pea plants, long stems are dominant to short stems, purple flowers are dominant to white, and round peas are dominant to wrinkled. Each trait is determined by a single, different gene. A plant that is heterozygous at all three loci is self-crossed, and 2,048 progeny are examined. How many of these plants would you expect to be long stemmed with purple flowers, producing wrinkled peas?

Answer 1

Assuming the genes are not linked nor interacting with each other, the expected number of long stemmed plants with purple flowers, producing wrinkled peas, L-P-rr, is 288.

Available data

• long stems are dominant to short stems
• purple flowers are dominant to white,
• round peas are dominant to wrinkled

• Each trait is determined by a single, different gene.

• A heter0zyg0us plant is self-crossed
• F1) 2,048 descendants

Let us name te alleles as follows

• L ⇒ Long stem
• l ⇒ Short stem
• P ⇒ Purple flowers
• p ⇒ white flowers
• R ⇒ Round peas
• r ⇒ wrinkled peas

Cross) heter0zyg0us plant is self-crossed

Parental) LlPpRr

Gametes) LPR, LPr, LpR, Lpr, lPR, lPr, lpR, lpr

Assuming that these genes are not linked nor interacting with each other, to make the calcs easier we will make crosses considering each gene separately.

• Stems

Parental) Ll  x  Ll

F1) 1/4 LL, 2/4 Ll, 1/4 ll

     3/4 long stem, 1/4 short stem

• Flower color

Parental) Pp  x  Pp

F1) 1/4 PP, 2/4 Pp, 1/4 pp

     3/4 purple flowers, 1/4 white flowers

• Peas

Parental) Rr  x  Rr

F1) 1/4 RR, 2/4 Rr, 1/4 rr

     3/4 round peas, 1/4 wrinkled peas

Now, we will get the proportion of individuals with long stems, purple flowers, producing wrinkled peas, L-P-rr

Note: The symbol - is representing either a dominant or a recessive allele.

From the crosses, we know that

• 3/4 are long stemmed,
• 3/4 have purple flowers, and
• 1/4 have wrinkled peas.

To get the proportion of individuals expressing all these traits together, we just need to multiply these values.

3/4 L- x 3/4 P- x 1/4 rr = 9/64 L-P-rr

Knowing that  the total number of plants is 2,048 and that there are 64 possible gametes combinations from this cross, we will use the proportion 9/64 to get the number of plants with these traits.

64 ------------- 2,048 plants

9 -------------- X = (9 x 2048) / 64 = 288 plants.

We expect to find 288 long stemmed plants with purple flowers, producing wrinkled peas.

You will learn more about trihybrid crosses at

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