B3: Reproduction and Inheritance

Inheritance - Monohybrid Cross and Pedigrees

How traits are passed from parents to offspring

Patterns of Inheritance

Predicting offspring traits using genetics

Key Genetic Terms

Understanding alleles, genotypes, and phenotypes

Alleles

Different versions of a gene. Represented by letters: Capital = Dominant (A), lowercase = recessive (a)

Dominant vs Recessive

Dominant: Only one copy needed to show trait. Recessive: Two copies needed to show trait

Genotype

The genetic makeup (alleles). Examples: AA (homozygous dominant), Aa (heterozygous), aa (homozygous recessive)

Phenotype

The observable characteristic. What you can see or measure (e.g., brown eyes, tall height)

Monohybrid Cross

Tracking inheritance of a single trait

A monohybrid cross examines the inheritance of one characteristic controlled by one gene. We use Punnett squares to predict offspring genotypes and phenotypes.

Example: Pea Plant Height

T allele = Tall (dominant)
t allele = Short (recessive)
TT or Tt → Tall phenotype
tt → Short phenotype (only when both recessive)

Typical Ratios

Tt × Tt (heterozygous parents):

• Genotype ratio: 1 TT : 2 Tt : 1 tt

• Phenotype ratio: 3 Tall : 1 Short

Interactive Punnett Square Builder

Step through monohybrid crosses

Pea Height

Dominant: T (Tall)

Recessive: t (Short)

Parent 1

Parent 2

Punnett Square

Tall

Short

Genotype Ratio

1 TT : 2 Tt : 1 tt

Phenotype Ratio

3 Tall : 1 Short

Other Inheritance Patterns

Beyond simple dominance

Codominance

Both alleles are equally dominant - both expressed in phenotype

Example: ABO blood groups. A and B alleles are codominant. AB blood type shows both A and B antigens

Sex-Linked Inheritance

Genes located on sex chromosomes (usually X chromosome)

Example: Red-green color blindness. Males (XY) more affected because they only have one X chromosome

Pedigree Analysis

Family tree showing inheritance of trait across generations

• Squares = males, Circles = females

• Filled shapes = affected individuals

• Horizontal line = parents, Vertical line = children

Worked Example

Predicting offspring with probabilities

Question: Two parents are heterozygous for brown eyes (Bb). Brown (B) is dominant over blue (b). Predict offspring eye colors with probabilities.

Step 1: Identify parent genotypes

Parent 1: Bb (heterozygous)

Parent 2: Bb (heterozygous)

Step 2: Determine possible gametes

Each parent can produce: B or b

Step 3: Create Punnett Square

Step 4: Calculate ratios and probabilities

Genotype Ratio:

1 BB : 2 Bb : 1 bb

Phenotype Ratio:

3 Brown eyes : 1 Blue eyes

Probabilities:

• 75% chance (3/4) of brown eyes

• 25% chance (1/4) of blue eyes

• 25% chance of homozygous dominant (BB), 50% heterozygous (Bb), 25% homozygous recessive (bb)

Key Terms Flashcards

Click to reveal definitions

Term

Allele

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Inheritance Quiz

Question 1 of 8

What is the genotype of an individual with two identical alleles?