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Genetics and Inheritance Study Guide – Step-by-Step Guidance

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Q1a. What would be the genotype of a person with type B blood who has a parent with type O blood?

Background

Topic: Mendelian Genetics – Multiple Alleles & Codominance

This question tests your understanding of how the ABO blood group system is inherited, specifically how codominance and multiple alleles work together to determine blood type.

Key Terms and Concepts:

  • Alleles: , , and (where $I^A$ and $I^B$ are codominant, and $i$ is recessive)

  • Genotype: The combination of alleles an individual has (e.g., )

  • Phenotype: The observable trait (e.g., blood type B)

  • Type O blood: Genotype

  • Type B blood: Possible genotypes or

Step-by-Step Guidance

  1. Recall that a parent with type O blood must have the genotype (since is recessive and both alleles must be $i$ to express type O).

  2. Consider the possible genotypes for type B blood: or .

  3. Since one parent can only pass on the allele, the child must have inherited $i$ from the type O parent.

  4. Combine this information to determine which genotype for type B blood is possible in this scenario.

Try solving on your own before revealing the answer!

Final Answer:

The person must have inherited the allele from the type O parent and the allele from the other parent, resulting in the genotype .

Q1b. Describe the phenotype of someone whose genotype is . Don’t just list what their blood type would be—describe what the surface of their red blood cells would look like.

Background

Topic: Codominance in Blood Types

This question is about how codominant alleles are both expressed in the phenotype, specifically in the context of the ABO blood group system.

Key Terms and Concepts:

  • Codominance: Both alleles are fully expressed in the phenotype.

  • Antigens: Proteins on the surface of red blood cells that determine blood type.

  • genotype: Both type A and type B antigens are produced.

Step-by-Step Guidance

  1. Recall that the allele codes for the A antigen and the allele codes for the B antigen.

  2. Since both alleles are codominant, both antigens will be expressed on the surface of the red blood cells.

  3. Think about what this means for the appearance of the red blood cell surface: both types of antigens will be present.

Try describing the phenotype in your own words before revealing the answer!

Final Answer:

The red blood cells would have both type A and type B antigens on their surface, reflecting the codominant expression of both alleles.

Q2a. Draw a Punnett square depicting a cross between a GgSs plant and a ggSs plant.

Background

Topic: Dihybrid Crosses

This question tests your ability to set up and analyze a dihybrid cross, which involves two genes (leaf color and leaf pattern) each with two alleles.

Key Terms and Concepts:

  • Dihybrid cross: A cross between individuals heterozygous for two traits.

  • Punnett square: A tool to predict the genotypes and phenotypes of offspring.

  • Alleles: G/g for leaf color, S/s for leaf pattern.

  • Gametes: Possible combinations of alleles each parent can contribute.

Step-by-Step Guidance

  1. List all possible gametes for each parent:

    • GgSs parent: can produce GS, Gs, gS, gs gametes.

    • ggSs parent: can produce gS, gs gametes (since both alleles for color are g).

  2. Set up a Punnett square with the GgSs parent's gametes on one side and the ggSs parent's gametes on the other.

  3. Fill in the Punnett square by combining the alleles from each parent for each box.

  4. Identify the genotypes in each box and note any patterns or repetitions.

Try drawing and filling in the Punnett square before checking the answer!

Final Answer:

The completed Punnett square will show all possible genotype combinations for the offspring. For example, you should see genotypes like GgSS, GgSs, ggSS, ggSs, Ggss, ggss, etc.

Note: Some rows/columns may be identical and can be simplified.

Q2b. What is the phenotype ratio of the offspring from this cross?

Background

Topic: Dihybrid Crosses – Phenotypic Ratios

This question asks you to interpret the results of your Punnett square to determine the ratio of observable traits in the offspring.

Key Terms and Concepts:

  • Phenotype: Observable traits (e.g., green striped, green solid, yellow striped, yellow solid).

  • Genotype to phenotype mapping: Use the key provided (GG/Gg = green, gg = yellow; SS/Ss = striped, ss = solid).

Step-by-Step Guidance

  1. From your Punnett square, count the number of each genotype that corresponds to each phenotype.

  2. Group the genotypes by phenotype (e.g., all green striped, all green solid, etc.).

  3. Write the ratio of each phenotype based on your counts.

Try calculating the phenotype ratio before revealing the answer!

Final Answer:

The phenotype ratio is 3 Green Striped : 1 Green Solid : 3 Yellow Striped : 1 Yellow Solid.

This ratio comes from grouping the genotypes according to the dominant and recessive alleles for each trait.

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