Working genetic problems involving two traits at the same time are referred to as dihybrid problems. Example RrLl, rrLl, RRLL, etc. where the "R" represent flower color in roses and the "L" represents stem length in roses. Read the pages indicated by D-4 and study behavioral objectives 17 and 18. The method of solving these problems is similar to that of working monohybrid problems. If the parents genotype are given than a Punnett square can be set up to determine the genotype and phenotype the offspring. Since there are two sets of genes the number of genetically different types of gametes to be placed in the columns and rows increase. How many genetically different number of gametes can be produced by an individual with the genotype of RrLl?
number of different types of gametes = 2n
; where n = the number of
heterozygous genes involved in the
cross
There are two sets of heterozygous genes; therefore, 2n = 22 = 4
List these four different
types of gametes. RL, Rl, rL, rl
1. How many genetically different number of gametes can be produced
by an individual with the genotype of RRLl?
Identify these gametes.
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Using the Punnett square show the genotypes and phenotypes of the offspring
which may be formed by crossing the two parents above.
RrLl x
RRLl
The first step is to set up the Punnett square with the corrupt haploid
genotypes for sperm and egg gametes in cells along the top and down the
left side.
| ///////// | RL | Rl | rL | rl |
| RL | ||||
| Rl |
The second step is to fill in the diploid genotypes of all possible offspring.
| ///////// | RL | Rl | rL | rl |
| RL | RRLL | RRLl | RrLL | RrLl |
| Rl | RRLl | RRll | RrLl | Rrll |
The third step is to determine and count the various types of phenotypes.
The phenotype ratio would be 6 red and long to 2 red and short.
The genotype ratio would be 1 RRLL : 2 RRLl : 1 RRll : 1 RrLL : 2 RrLl
: 1 Rrll
Note: the column and row headings represent haploid cell and the cells
inside represent diploid cells.
RrLL = diploid
RL = haploid
In the following problems assume that in certain plants yellow (YY or
Yy) is dominant over green (yy) and disk-shaped (DD or Dd) is dominant
over sphere-shaped (dd).
Give the possible phenotype ratio of the F1 for the following
crosses (1-7):
2. YyDd x YyDd
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In certain plants yellow (YY or Yy) is dominant over green (yy) and
disk-shaped (DD or Dd) is dominant over sphere-shaped (dd).
3. YYdd x yyDD
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In certain plants yellow (YY or Yy) is dominant over green (yy) and
disk-shaped (DD or Dd) is dominant over sphere-shaped (dd).
4. YyDd x yydd
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In certain plants yellow (YY or Yy) is dominant over green (yy) and
disk-shaped (DD or Dd) is dominant over sphere-shaped (dd).
5. YyDd x Yydd
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In certain plants yellow (YY or Yy) is dominant over green (yy)
and disk-shaped (DD or Dd) is dominant over sphere-shaped (dd).
6. YyDd x YyDD
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In certain plants yellow (YY or Yy) is dominant over green (yy) and
disk-shaped (DD or Dd) is dominant over sphere-shaped (dd).
7. YyDD x yydd
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Let us review some terms and concepts.
Consider the following genotype: YyDd
8. How many genes are represented in this genotype?
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9. How many alleles are represented in this genotype?
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10 Would this represent a diploid or a haploid cell?
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As mentioned above there are two types of problems which may be asked to solve. The problems above involve giving the genotypes of the parents and having you to determine the genotypes and phenotypes of the offspring. These problems can be determined by using the Punnett square.
The second type of problem is where information concerning the offspring is given and you are asked to determine the genotypes of the parents. You can not use the Punnett square method to solve these problems.
Example: In the following problems assume that in certain plants yellow
(YY or Yy) is dominant over green (yy) and disk-shaped (DD or Dd) is dominant
over sphere-shaped (dd).
A yellow disk-shaped fruit plant crossed with a green sphere-shaped
plant gives all yellow disk-shaped F1. State the genotypes of the parents.
The method used for this type of problem is as follows:
Draw four lines and label two as P1 and two as P2. Each line represents a single set of genes for the parents. Each parent has two sets of genes because this is a dihybrid problem.
P1
_____ _____
X P2 _____
______
In this case the phenotypes of the parents are given. List which
alleles must be present because of their description. A yellow disk-shaped
fruit plant crossed with a green sphere-shaped plant gives all yellow
disk-shaped F1. State the genotypes of the parents.
Since parent one is yellow and disk, it must have at least one large
"Y" and one large "D". Parent two is green and sphere, it must have
two small "y" and two small "d". See below.
Y
D
yy dd
P1
_____ _____
X P2 _____
______
List genotype of the F1 offspring as much as possible.
"all offspring are yellow and disk" =
Y__ D___
Remember all offspring must receive one allele from each parent. Since
all offspring are yellow than parent one must be "YY". It parent one was
"Yy" some of the offspring would be "yy". Since all offspring are disk
than parent one must be "DD". If parent one was "Dd" than one of the offspring
would be "dd".
YY DD
yy dd
The answer is : P1
_____ _____
X P2 _____
______
Using the method above solve the following problems.
11. If a yellow sphere-shaped fruit plant is crossed with a green disk-shaped plant, give the genotypes of the parents when the F1 ratio is as follows:
1 yellow disk-shaped
1 yellow sphere-shaped
1 green disk-shaped
1 green sphere
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12. If a yellow disk-shaped fruit plant is crossed with a green disk-shaped plant produce the following phenotype ratio, what are the genotypes of the parents?
3 yellow disk-shaped
1 yellow sphere-shaped
3 green disk-shaped
1 green sphere YyDD x yydd
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13. Give the genotypes of the parents when both parents are yellow disk-shaped and produce the following offspring:
10 green disk-shaped
5 green sphere-shaped
29 yellow disk-shaped
11 yellow sphere-shaped
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14. Give the genotypes of the parents when the following offspring are produced.
22 green disk-shaped
23 green sphere-shaped
25 yellow disk-shaped
21 yellow sphere-shaped
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Assume that in Drosophila, gray body (GG or Gg) and straight wings
(SS or Ss) are dominant over ebony (black body (gg) and curved wings (ss).
15. If a gray body curved-winged male is mated with an ebony body straight
-winged female, state the possible genotypes of each parent.
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Assume that in Drosophila, gray body (GG or Gg) and straight wings (SS or Ss) are dominant over ebony (black body (gg) and curved wings (ss).
16. In the problem above, if one parent was homozygous dominant
for both phenotypes and the other parent was homozygous recessive for both
phenotypes, give the F1 and F2 offspring genotypes.
F1
F2
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Assume that in Drosophila, gray body (GG or Gg) and straight wings (SS or Ss) are dominant over ebony (black body (gg) and curved wings (ss).
17. State the genotypes of the Drosophila parents if a gray body straight-winged and ebony body straight-winged should produce the following:
19 ebony body straight-winged
5 ebony body curved-winged
20 gray body straight winged
7 gray body curved-winged
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Assume that in Drosophila, gray body (GG or Gg) and straight wings (SS or Ss) are dominant over ebony (black body (gg) and curved wings (ss).
18. What would be the genotypes of the parents in a different cross
if three fourths of the F1 were gray body straight-winged and one fourth
were ebony body and straight winged?
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Assume that in Drosophila, gray body (GG or Gg) and straight wings (SS or Ss) are dominant over ebony (black body (gg) and curved wings (ss).
19. In another cross what would the genotypes of both parents be if
50% of the F1 were ebony curved-winged and 50% were
ebony and straight winged?
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This is the end of lesson three. In the next lesson you will
learn various version for monohybrid and dihybrid problems. You work the
problems the same as demonstrated in the last two lessons; however, the
genotypes will have a different effect on the phenotype. Click here to
go back to the home page and lesson four Variations of Mendelian Genetics.
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Last modified October 22, 1997.