Name:
11.1B
Date:
Period:
Two-Trait Crosses
Two factor crosses involve analyzing two traits at the same time, such as pea pod color and pod
shape. Predicting the outcome of two-factor crosses require basically the same procedure as
that for crosses involving one trait. Keep in mind that in two-factor crosses the genes
controlling the two different traits are located on nonhomologous chromosomes. During
meiosis, nonhomologous chromosomes assort independently. This means that each of the
chromosomes of any pair of homologous chromosomes has an equal probability of ending up in
a gamete with either chromosome from any other pair of homologous chromosomes. The
genes that are located on nonhomologous chromosomes also assort idependently.
Because of independent assortment, a plant that is heterozygous for two traits (genotype
example AaBb) will produce equal numbers of four types of gametes – AG, Ab, aB, and ab.
What does all that mean!?!?
Let’s look at some examples. Two-Trait Punnett square steps:
a) assign letter symbols
b) show the P cross
c) draw a Punnett square and plot the
gametes from each parent
d) list the phenotypic ratios of the offspring as a fraction
(we’re not going to list genotypes for two-traits)
We’re crossing two hybrids for two traits, pea pod color and shape. In this species of peas,
Yellow is dominant over Green and Round is dominant over Wrinkled.
a) Y – yellow y - green
R - round
r – wrinkled
b) RrYy x RrYy
c)
d)
Yellow Round 9/16
Yellow Wrinkled
Green Round 3/16
Green Wrinkled
3/16
1/16
I wonder if that ratio means anything, looks
kind of familiar…
1. If RRYY was crossed with rryy: (R = round, r = wrinkled, Y = yellow, y = green)
a. What alleles would be in the gametes from parent RRYY? _________________
b. What alleles would be in the gametes from parent rryy? _________________
c. When the two types of gametes fuse, what is the genotype of the offspring?
________
d. What is the phenotype of the offspring?
______________________________________
2. RrYy was crossed with RrYy.
a. List the 4 possible gametes from parent RrYy:
_________________________________
b. What fraction of their offspring will be:
Round & yellow _________________
round & green _________________
Wrinkled & yellow _________________ wrinkled & green __________________
3. In horses, black coats (B) and walking gaits (W) are dominant to white coats (b) and pacing
gaits (w).
a. If the male horse is homozygous for both dominant traits, what is his genotype?
______
b. What alleles would be found in the horse’s sperm? _________________
c. The female horse is homozygous for both recessive traits, what is her genotype?
_____
d. What alleles are found in the horse’s eggs? _________________
e. Construct a Punnett square chart to show the possible offspring from the mating of
these two horse types.
4. Now examine the results of mating two horses from the F1 generation.
a. What is the genotype of the male horse? _________________
b. What alleles would be contained in his gametes?
______________________________
c. What is the genotype of the female horse? _________________
d. What alleles would be contained in her gametes?
______________________________
e. Construct a Punnett square chart to show the possible offspring from the mating of
these two horse types.
f. List the genotypic fractions of the possible offspring in the F2 generation.
___ BBWW
___BbWW
___bbWW
___BBWw
___BbWw
___bbWw
___BBww
___Bbww
___bbww
g. List the phenotypic fractions of the possible offspring in the F2 generation.
___ black walkers
___ black pacers
___ white walkers
___ white pacers
h. Conclusion: Which horse type(s) would be most likely to occur during the F2
generation?
____________________________________________________________
i.
Why would this be helpful to horse breeders?