Question

dihybrid cross step - by - step instructions
name avianna p per date oct 6th 350 block
you already know how use a punnett square to solve a problem such as: \in pea plants, tall (t) is dominant over short (t). if a plant that is heterozygous for height is crossed with one that is short, what is the probability that an offspring will be short?\
suppose, instead, you are given a problem looking at two traits instead of one: \in pea plants, tall (t) is dominant over short (t), and purple flowers (c) is dominant over white (c). if one parent is homozygous dominant for height and heterozygous for flower color, and the other is heterozygous for height and has white flowers, what is the probability that an offspring will be tall with white flowers?\
this is called a \dihybrid cross\ problem. following is a step - by - step procedure for solving this type of problem.
step 1: determine the parents genotypes
the first thing you need to be able to do is write out the genotype, for both traits, of each parent. you list both genotypes, one after the other. for example, the parents from above problem would be written as ttcc and ttcc. sometimes you are just told the phenotype, such as \short\ or \white flowers\. recessive phenotypes are always homozygous recessive genotypes. for example, a plant that has white flowers must have the \cc\ genotype.
practice: write out the genotypes of the following parents:
a homozygous dominant for height and has white flowers
b heterozygous for height and heterozygous for flower color
c short and homozygous dominant for flower color
d heterozygous for height and homozygous dominant for flower color
e homozygous dominant for height and heterozygous for flower color
f heterozygous for height and has white flowers
g homozygous dominant for height and homozygous dominant for flower color
h short and heterozygous for flower color
i short and has white flowers
step 2: determine the gametes for each parent
once you have the parents genotypes, you need to fill out the gamete part of the punnett square. each gamete will have one allele of each trait.
to determine the gametes that a parent will produce, you combine the first allele of the first gene with the first allele of the second gene, then the first allele of the first gene with the second allele of the second gene, then the second allele of the first gene with the first allele of the second gene, then the second allele of the first gene with the second allele of the second gene
practice: determine what gametes will be produced by the following parents:

genotype	gametes
ttcc	tt tc tc tc
ttcc	tc tc tc tc
ttcc	tc tc tc tc
ttcc	tt tc tc tc
ttcc	tc tc tc tc
ttcc	tc tc tc tc

step 3: complete the punnett square
put the gametes from one parent in the four boxes along the top, and put the gametes from the other parent in the four boxes along the sides. then complete the genotypes of the offspring (the 16 boxes). follow the format shown, both widows peak alleles, then both earlobe alleles. dominant before recessive (e.g. aa not aa).
practice: complete the example:

	tc	tc	tc	tc
tc	ttcc	ttcc	ttcc	ttcc
tc	ttcc	ttcc	ttcc	ttcc
tc	ttcc	ttcc	ttcc	ttcc

step 4: determine the probabilities of the phenotypes
suppose the results of a cross are as shown at left. what is the probability that an offspring will be tall and have purple flowers? to answer this, count all the offspring that are both tall and have purple flowers, divide that number by the total (16) and multiply by 100. round to the nearest whole number.
practice: determine the following probabilities:
a. what is the probability that an offspring will be tall and have purple flowers? _/16 = 38% (37.5 rounded to 38)
b. what is the probability that an offspring will be tall and have white flowers?
c. what is the probability that an offspring will be short and have purple flowers?
practice:
now try this complete problem: cross a short plant that is heterozygous for flower color with a plant that is heterozygous for height and has white flowers.

1. what is the probability of an offspring being tall with purple flowers?
2. what is the probability of an offspring being tall with white flowers?
3. what is the probability of an offspring being short with purple flowers?
4. what is the probability of an offspring being short with white flowers?

Explanation:

Step1: Determine Parent Genotypes

Let's assume the cross is between a short plant that is heterozygous for flower - color (ttCc) and a plant that is heterozygous for height and has white flowers (Ttcc).

Step2: Determine Gametes

For ttCc, the gametes are tC and tc. For Ttcc, the gametes are Tc and tc.

Step3: Create Punnett Square

	tC	tc
tc	ttCc	ttcc

Step4: Calculate Probabilities

1. Probability of tall with purple flowers (T - C -): There is 1 TtCc out of 4 offspring, so the probability is $\frac{1}{4}=25\%$.
2. Probability of tall with white flowers (T - cc): There is 1 Ttcc out of 4 offspring, so the probability is $\frac{1}{4}=25\%$.
3. Probability of short with purple flowers (ttC -): There is 1 ttCc out of 4 offspring, so the probability is $\frac{1}{4}=25\%$.
4. Probability of short with white flowers (ttcc): There is 1 ttcc out of 4 offspring, so the probability is $\frac{1}{4}=25\%$.

Answer:

1. 25%
2. 25%
3. 25%
4. 25%