Guided Practice
Learning Objectives
In this guided practice lesson, you will apply your knowledge of Mendelian genetics and DNA replication to solve problems with step-by-step guidance.
- Solve Punnett square problems for monohybrid and dihybrid crosses
- Calculate genotypic and phenotypic ratios
- Trace DNA replication through the steps
- Apply genetics vocabulary to solve problems
Worked Examples
Example 1: Monohybrid Cross
Problem: In pea plants, tall (T) is dominant over short (t). Cross a heterozygous tall plant (Tt) with a short plant (tt). What are the expected phenotypic ratios of the offspring?
Solution:
- Identify parent genotypes: Tt x tt
- Set up Punnett square:
t t T Tt Tt t tt tt - Count offspring genotypes: 2 Tt, 2 tt
- Determine phenotypes: Tt = tall, tt = short
- Answer: 50% tall (2/4), 50% short (2/4) = 1:1 ratio
Example 2: DNA Replication Steps
Problem: Describe the role of helicase in DNA replication.
Solution:
- Helicase is an enzyme that works at the replication fork
- It unwinds and separates the double helix by breaking hydrogen bonds between complementary base pairs
- This creates two single strands that serve as templates
- The area where helicase works is called the replication fork
Practice Quiz
Test your understanding with these 10 questions. Click on each question to reveal the answer.
1. In guinea pigs, black fur (B) is dominant over white fur (b). If two heterozygous black guinea pigs are crossed, what percentage of offspring will have white fur?
Answer: 25%
Explanation: Bb x Bb produces: BB (25%), Bb (50%), bb (25%). Only bb (25%) will have white fur since black is dominant.
2. What is the difference between genotype and phenotype?
Answer: Genotype is the genetic makeup (the alleles an organism has, like Bb or TT), while phenotype is the physical expression of those genes (what the organism looks like, such as black fur or tall height).
3. During DNA replication, what enzyme joins Okazaki fragments on the lagging strand?
Answer: DNA ligase
Explanation: DNA ligase seals the gaps between Okazaki fragments by creating phosphodiester bonds between the sugar-phosphate backbones.
4. A plant has the genotype RrYy. How many different types of gametes can this plant produce?
Answer: 4 different gametes: RY, Ry, rY, ry
Explanation: Using the FOIL method or independent assortment, each gamete gets one allele from each gene, creating 4 combinations.
5. If the template strand of DNA reads 3'-TACGGA-5', what will the complementary strand read?
Answer: 5'-ATGCCT-3'
Explanation: DNA bases pair A-T and G-C. The new strand runs antiparallel (opposite direction) to the template.
6. In a dihybrid cross (AaBb x AaBb), what fraction of offspring will be homozygous for both traits?
Answer: 4/16 or 1/4
Explanation: Homozygous for both means AABB, AAbb, aaBB, or aabb. Each has a probability of 1/16, so total = 4/16 = 1/4.
7. Why is DNA replication called "semi-conservative"?
Answer: Because each new DNA molecule consists of one original (conserved) strand and one newly synthesized strand. Half of the original molecule is "conserved" in each daughter molecule.
8. A red flower (RR) is crossed with a white flower (rr). If the offspring are all pink (Rr), what type of inheritance is this?
Answer: Incomplete dominance
Explanation: In incomplete dominance, the heterozygote shows a blended or intermediate phenotype between the two homozygotes.
9. What is the role of primase in DNA replication?
Answer: Primase synthesizes short RNA primers that provide a 3'-OH group for DNA polymerase to begin adding nucleotides. DNA polymerase cannot start synthesis without a primer.
10. In humans, hemophilia is an X-linked recessive disorder. If a carrier female (X^H X^h) has children with a normal male (X^H Y), what is the probability of having an affected son?
Answer: 25% of all children, or 50% of sons
Explanation: Punnett square shows: X^H X^H (normal girl), X^H X^h (carrier girl), X^H Y (normal boy), X^h Y (affected boy). 1/4 of all children or 1/2 of the boys will be affected.
Next Steps
- Review any questions you found challenging
- Continue to the Lab Analysis lesson to apply these concepts to experimental data
- Practice more Punnett square problems