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

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

Background

Topic: Multiple Alleles & Codominance (ABO Blood Groups)

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

Key Terms and Concepts:

• Alleles: $I^A$ (Type A antigen), $I^B$ (Type B antigen), $i$ (no antigen)

• Codominance: $I^A$ and $I^B$ are codominant; $i$ is recessive to both.

• Type B blood: Can be $I^B I^B$ or $I^B i$

• Type O blood: Must be $ii$

Step-by-Step Guidance

1. Recall that a person with type O blood has the genotype $ii$ (homozygous recessive).

2. For a child to have type B blood, their genotype could be either $I^B I^B$ or $I^B i$.

3. Since one parent is type O ($ii$), they can only pass on the $i$ allele to their child.

4. Therefore, the child must have inherited the $I^B$ allele from the other parent and the $i$ allele from the type O parent.

Try solving on your own before revealing the answer!

Final Answer: $I^B i$

The genotype must be $I^B i$ because the type O parent can only contribute an $i$ allele, and the child must have one $I^B$ to express type B blood.

Q1b. Describe the phenotype of someone whose genotype is $I^A I^B$. 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 ABO Blood Groups

This question is testing your understanding of how codominant alleles are both expressed in the phenotype, specifically in the context of blood cell antigens.

Key Terms and Concepts:

• Codominance: Both $I^A$ and $I^B$ alleles are expressed equally.

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

Step-by-Step Guidance

1. Recognize that the genotype $I^A I^B$ means the individual has one allele for type A antigen and one for type B antigen.

2. Because of codominance, both antigens will be produced and displayed on the surface of the red blood cells.

3. Think about what this means for the cell surface: both A and B antigens will be present, not just one or the other.

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. This means both types of antigenic proteins are present, 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 that are heterozygous for two traits.

• Alleles: G/g for leaf color (green/yellow), S/s for leaf pattern (striped/solid).

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

Step-by-Step Guidance

1. Determine the possible gametes each parent can produce:

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

   • ggSs parent: can produce gS, gs

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. List the genotypes in each box, but do not calculate the phenotype ratios yet.

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

Final Answer:

The Punnett square will show combinations such as GgSS, GgSs, ggSS, ggSs, GgSs, Ggss, ggSs, ggss. Each box represents a possible genotype for the offspring.

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

Background

Topic: Dihybrid Crosses and Phenotypic Ratios

This question tests your ability to interpret the results of a dihybrid cross and determine the expected phenotypic ratio among the offspring.

Key Terms and Concepts:

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

• Genotype to phenotype mapping: GG or Gg = green, gg = yellow; SS or Ss = striped, ss = solid.

Step-by-Step Guidance

1. From your Punnett square, identify the genotype of each offspring.

2. Determine the phenotype for each genotype using the allele definitions provided.

3. Count the number of each phenotype (e.g., green striped, green solid, yellow striped, yellow solid).

4. Express the ratio of each phenotype in the offspring.

Try calculating the phenotype ratio before revealing the answer!

Final Answer:

3 Green Striped : 1 Green Solid : 3 Yellow Striped : 1 Yellow Solid

This ratio reflects the expected distribution of phenotypes based on the genotypes produced in the Punnett square.