genetics

1. Trihybrid cross
BAKHTIARI_HOSSEIN
Figure 3-2 Copyright © 2006 Pearson Prentice Hall, Inc.

2. Trihybrid cross
The mating of two individuals,
organisms, or strains that have
different gene pairs that determine
three specific traits or in which three
particular characteristics or gene loci
are being followed.

3. Example:
23 = 8 * 8 = 64
MEANING:
The fork line method can be used by
figuring the occurrence of each gene or
set of genes to be found in the gamete,
and then multiply them together. This
can be multiplied by figuring each
trait or combination of traits separate
for both the male and female, or each
gene individually regardless of sex.

4. P1: WWGG * ww gg
F1: Ww Gg * Ww Gg
F2: Ww * Ww Gg * Gg
WW Ww Ww ww GG Gg Gg gg
Phenotypes:
¾W ¾ G = 9/16 WG
¼ g = 3/16 Wg
¼ w ¾ G = 3/16 wG
¼ g = 1/16 wg
Phenotypic ratio:
9:3:3:1

5. Genotypes:
¼ GG = 1/16 WW GG
¼ WW 2/
4 Gg = 2/16 WW Gg
¼ gg = 1/16 WW gg
¼ GG = 3/16 Ww GG
2/4 Ww 2
/4 Gg = 4/16 Ww Gg
¼ g = 2/16 Ww gg
¼ GG = 1/16 ww GG
¼ ww 2
/4 Gg = 2/16 ww Gg
¼ gg = 1/16 ww gg
genotypic ratio: 1:2:1:2:4:2:1:2:1

6. Trihybrid Crosses
• Trihybrid or three-factor cross
• More complex by “easily” calculated
following principles of segregation,
independent assortment and probability
• Punnett square has 64 boxes…
• Demonstrates that Mendel’s principles apply
to inheritance of multiple traits

7. .

8. Forked-line Method
• Also called branch diagram