Genetics — Mendelian inheritance and monohybrid crosses (KCSE Biology Form 4)

TL;DR

Mendelian inheritance explains how traits are passed from parents to offspring through genes. Monohybrid crosses specifically look at the inheritance of a single trait. Understanding dominant and recessive alleles is key to predicting offspring characteristics.

1. The Mental Model

Think of inheritance like a recipe. You get two copies of each ingredient (genes) – one from each parent. These copies (alleles) determine what the final dish (your trait) will be, with some ingredients being stronger (dominant) than others (recessive).

2. The Core Material

What is Genetics?

Genetics is the study of heredity, which is how characteristics are passed from parents to their offspring. It's why you might have your mum's eyes or your dad's hair colour.

Key Terms You Need to Know

• Gene: A basic unit of heredity that carries information for a specific trait. Think of it as a specific instruction for one feature.
• Allele: Different forms of a gene. For example, for the gene that determines flower colour, there might be an allele for red and an allele for white.
• Dominant Allele: An allele that always expresses its trait when present. We usually represent dominant alleles with a capital letter (e.g., 'T' for tall).
• Recessive Allele: An allele that only expresses its trait when two copies are present (i.e., no dominant allele is there). We represent recessive alleles with a lowercase letter (e.g., 't' for short).
• Genotype: The genetic makeup of an organism – the actual combination of alleles it has (e.g., TT, Tt, tt).
• Phenotype: The observable physical characteristic of an organism, determined by its genotype (e.g., tall, short).
• Homozygous: Having two identical alleles for a particular gene (e.g., TT or tt).
• Heterozygous: Having two different alleles for a particular gene (e.g., Tt).
• Parental Generation (P generation): The first set of parents crossed in a genetic experiment.
• First Filial Generation (F1 generation): The offspring resulting from the cross of the P generation.
• Second Filial Generation (F2 generation): The offspring resulting from the self-pollination or interbreeding of the F1 generation.

Gregor Mendel and His Laws

Gregor Mendel, an Austrian monk, is considered the "Father of Genetics." He conducted experiments with pea plants and discovered fundamental principles of heredity. His work led to two main laws:

1. Law of Segregation: During the formation of gametes (sex cells like sperm or egg), the two alleles for a heritable character separate (segregate) from each other, so that each gamete carries only one allele.
2. Law of Independent Assortment: Alleles for different genes assort independently of each other during gamete formation. (We'll focus more on segregation for monohybrid crosses).

Monohybrid Crosses

A monohybrid cross is a genetic cross between two individuals that differ in only one specific trait. It helps us predict the genotypes and phenotypes of offspring.

How to Perform a Monohybrid Cross using a Punnett Square:

1. Identify the parents' genotypes: What alleles do they have for the trait?
2. Determine the gametes: Based on the Law of Segregation, what alleles can each parent pass on?
3. Draw a Punnett Square: This is a grid that helps visualise all possible combinations of alleles from the parents.
4. Fill in the square: Combine the alleles from the top and side to show the possible genotypes of the offspring.
5. Determine genotypic and phenotypic ratios: Count the different genotypes and phenotypes to get their ratios.

Here's a flowchart to help you visualise the steps:

graph TD
    A[Start: Identify Parental Genotypes] --> B{Determine Gametes for Parent 1};
    B --> C{Determine Gametes for Parent 2};
    C --> D[Draw Punnett Square];
    D --> E[Fill in Punnett Square with Offspring Genotypes];
    E --> F[Count Genotypes and Phenotypes];
    F --> G[Calculate Genotypic and Phenotypic Ratios];
    G --> H[End: Predict Offspring Characteristics];

Example: Tall vs. Short Pea Plants

Let's say 'T' represents the dominant allele for tallness and 't' represents the recessive allele for shortness.

Cross 1: Homozygous Tall (TT) x Homozygous Short (tt)

1. Parental Genotypes: TT (Tall) x tt (Short)
2. Gametes:
   • TT parent produces only 'T' gametes.
   • tt parent produces only 't' gametes.
3. Punnett Square:

4. Offspring Genotypes: All Tt
5. Offspring Phenotypes: All Tall

Cross 2: F1 Generation Self-Cross (Tt x Tt)

1. Parental Genotypes: Tt (Tall) x Tt (Tall)
2. Gametes:
   • Tt parent produces 'T' and 't' gametes (50% each).
   • Tt parent produces 'T' and 't' gametes (50% each).
3. Punnett Square:

4. Offspring Genotypes:
   • 1 TT
   • 2 Tt
   • 1 tt
   • Genotypic Ratio: 1 TT : 2 Tt : 1 tt
5. Offspring Phenotypes:
   • 3 Tall (TT, Tt, Tt)
   • 1 Short (tt)
   • Phenotypic Ratio: 3 Tall : 1 Short

3. Worked Example

Problem: In guinea pigs, black fur (B) is dominant over white fur (b). If you cross a heterozygous black guinea pig with a white guinea pig, what are the expected genotypic and phenotypic ratios of their offspring?

Solution:

1. Identify Parental Genotypes:

   • Heterozygous black guinea pig: Since black (B) is dominant and it's heterozygous, its genotype must be Bb.
   • White guinea pig: Since white (b) is recessive, for it to show white fur, its genotype must be bb.

2. Determine Gametes:

   • Parent 1 (Bb) can produce gametes: B and b.
   • Parent 2 (bb) can produce gametes: b and b.

3. Draw Punnett Square:

4. Fill in Punnett Square (Offspring Genotypes):

   • Bb
   • bb
   • Bb
   • bb

5. Determine Genotypic and Phenotypic Ratios:

   • Genotypes:
     • Bb: 2
     • bb: 2
     • Genotypic Ratio: 2 Bb : 2 bb, which simplifies to 1 Bb : 1 bb
   • Phenotypes:
     • Black fur (Bb): 2
     • White fur (bb): 2
     • Phenotypic Ratio: 2 Black : 2 White, which simplifies to 1 Black : 1 White

Answer: The expected genotypic ratio is 1 Bb : 1 bb, and the expected phenotypic ratio is 1 Black : 1 White.

4. Key Takeaways

• Genes carry information for traits, and alleles are different versions of those genes.
• Dominant alleles mask recessive alleles when present together.
• Genotype is the genetic code (e.g., Tt), while phenotype is the observable trait (e.g., tall).
• Punnett squares are a visual tool to predict offspring genotypes and phenotypes.
• Mendel's Law of Segregation explains how alleles separate into gametes.

Common Mistakes to Avoid

• Confusing genotype and phenotype: Remember, genotype is the letters (Bb), phenotype is what you see (Black fur).
• Incorrectly assigning dominant/recessive alleles: Always use capital letters for dominant and lowercase for recessive.
• Forgetting to simplify ratios: Always reduce your genotypic and phenotypic ratios to their simplest form (e.g., 2:2 becomes 1:1).
• Not determining gametes correctly: Make sure each parent's gametes reflect the segregation of their alleles.
• Mixing up homozygous and heterozygous: Homozygous means two same alleles (BB or bb), heterozygous means two different alleles (Bb).

5. Now Try It

A pure-breeding red flower (RR) is crossed with a pure-breeding white flower (rr). Red (R) is dominant over white (r).

1. What are the genotypes and phenotypes of the F1 generation?
2. If two F1 generation plants are crossed, what are the expected genotypic and phenotypic ratios of the F2 generation?

What success looks like: You should be able to clearly show the Punnett squares for both crosses and state the correct genotypic and phenotypic ratios for both the F1 and F2 generations. For the F1, you should find all offspring are heterozygous red. For the F2, you should find a 1 RR : 2 Rr : 1 rr genotypic ratio and a 3 Red : 1 White phenotypic ratio.