Download genetics problems

Name _______________________________
Period _________
Pre-AP Biology
GENETICS PROBLEMS
In the following problems, you will review the basics of solving genetics problems. Before you begin,
review the following basic concepts:
1. These problems deal with Mendelian genetic traits. Mendelian genetic traits are determined by a
single gene locus with exactly two alleles. One allele is dominant and the other allele is
recessive.
2. Dominant alleles are represented by capital letters. Recessive alleles are represented by lowercase letters. The first letter of the dominant trait is usually used to represent the alleles.
3. Organisms can be described by their genotypes or phenotypes.
a. Genotype = actual alleles
i. heterozygous = alleles are different (Aa)
ii. homozygous dominant = alleles are both dominant (AA)
iii. homozygous recessive = alleles are both recessive (aa)
b. Phenotype = what the organism looks like
c. Homozygous dominant and heterozygous individuals express the dominant phenotype.
d. Homozygous recessive individuals express the recessive phenotype.
4. Organisms are diploid and have 2 alleles at each gene locus. Each trait is represented by two
letters: BB, Bb, bb. This is the organisms genotype for that trait.
5. Gametes are haploid and have 1 allele at each gene locus. Each trait is represented by one letter:
B, b. This is the gamete’s genotype for that trait.
6. To determine the gametes that an organism can produce, take one allele (letter) from each gene
locus (trait) for each gamete. Use the shortcut – FOIL (first, outside, inside, last).
Example:
BB can produce gametes with genotype B
Bb can produce gametes with genotype B or b
AaBB can produce gametes with genotype AB, aB
AaBb can produce gametes with genotype AB, Ab, aB, ab
7. The initial cross in a problem is called the parental generation (P). This results in the F1
generation. The F2 generation comes from crossing two F1 offspring.
8. Each offspring will receive one allele (letter) from each parent. It is helpful to use a Punnett
square to determine the genotypes of each offspring. Example – if the P generation is Aa, the
offspring (F1) genotype possibilities are AA, Aa, aa:
A
a
A
AA
Aa
a
Aa
aa
Name _______________________________
Period _________
Pre-AP Biology
PUNNETT SQUARE PRACTICE
1. BB x bb
2. Bb x bb
MONOHYBRID CROSSES
1. In tomatoes, red fruits are dominant over yellow fruits.
a. What letter would you choose to represent the red and yellow alleles?
RED ALLELE = _____________
YELLOW ALLELE = _____________
2. Using your answers from #1, determine the genotypes of these organisms (remember, 2
letters for each trait).
a. homozygous red-fruited plant =
_______________
b. heterozygous plant =
_______________
c. yellow-fruited plant =
_______________
d. What is the phenotype of plant B?
_______________
3. If you cross a homozygous red-fruited plant with a yellow-fruited plant, what are the
genotypes of the F1 generation?
Name _______________________________
Period _________
Pre-AP Biology
4. What are the phenotypes of the F1 generation?
5. What will be the genotype of the F2 generation (cross two F1 individuals)?
Monohybrid crosses involving two heterozygotes (Aa x Aa) always produce the same
PHENOTYPE and GENOTYPE RATIOS:


PHENOTYPE RATIO = 3:1
(3 dominant : 1 recessive)
GENOTYPE RATIO = 1:2:1
(1 homozygous dominant : 2 heterozgotes : 1 homozygous recessive)
6. In humans, the gene for farsightedness is inherited as a dominant allele. Normal vision is
recessive. What is the probability that children will have normal vision if a normal man
marries a woman who is farsighted and had a normal father?
7. Silky feathers in fowl is recessive to normal feathers. If 100 birds were raised from a
cross between two heterozygotes, how many would you expect to be normal? Silky?
DIHYBRID CROSSES
Dihybrid crosses examine two alleles on different chromosomes.
1. In peas, a single gene codes for stem length and another single gene codes for seed shape.
Each gene has two alleles, one dominant and one recessive. For stem length, tall plants are
dominant over short plants. For seed shape, smooth peas are dominant over wrinkled peas.
A. Choose letters to represent each gene and its alleles:
Seed shape:
smooth = ___________
Stem length: tall =
___________
wrinkled = ___________
short =
___________
Name _______________________________
Period _________
Pre-AP Biology
B. What are all of the possible gametes if you cross two individuals that are
heterozygous for both traits?
C. Fill in the punnett square to figure out the possible offspring from a cross between
two individuals that are heterozygous for both traits:
2. In fruit flies, the genes for wing length and body hair each have two alleles. Long wings
and a hairless body are dominant to alleles for short wings and a hairy body.
a. Determine the genotypes and phenotypes of the offspring from the cross between
a short-winged, hairy male and a homozygous dominant female.
# of long-winged, hairless
____________
# of longed-winged, hairy
____________
# of short-winged, hairless
____________
# of short-winged, hairy
____________
Name _______________________________
Period _________
Pre-AP Biology
b. Determine the genotypes and phenotypes of the F2 generation (cross between two
F1 individuals).
# of long-winged, hairless
____________
# of longed-winged, hairy
____________
# of short-winged, hairless
____________
# of short-winged, hairy
____________
NOTE – Dihybrid crosses involving heterozygotes (AaBb x AaBb) always result in a 9:3:3:1
phenotype ratio. (9 dominant for both : 3 dominant for one trait, recessive for the other : 3
dominant for one trait, recessive for the other : 1 recessive for both).
3. In snerds, matted-hair is dominant to frizzled-hair and buck-toothed is dominant to snaggletoothed.
a. A frizzled-hair, snaggle-tooth snerd is mated to a homozygous dominant matted-hair,
buck-toothed snerd. What is/are the genotypes and phenotypes of the F1?
# of matted-hair, buck-toothed ____________
# of matted-hair, snaggle-tooted ____________
# of frizzled-hair, buck-toothed ____________
# of frizzled-hair, snaggle-toothed __________
b. Without doing any calculations, what will be the phenotype ratio of the F2?