Chapter 12: Mendel & Inheritance

This chapter delves into Gregor Mendel's groundbreaking experiments with pea plants, which established the foundational principles of inheritance through the concepts of dominant and recessive traits.

Review Questions:

Q1: Mendel performed hybridizations by transferring pollen from the _______ of the male plant to the female ova.
A: a. anther
Q2: Which is one of the seven characteristics that Mendel observed in pea plants?
A: b. seed texture
Q3: What F1 offspring would you expect from crossing green-seeded and yellow-seeded true-breeding parents (yellow dominant)?
A: b. 100 percent yellow seeds
Q4: If 650 inflated-pod plants are found in the F2 generation, how many constricted-pod plants would you expect (3:1 ratio)?
A: b. 165
Q5: What ratio of offspring would result from a cross between a white-eyed male and a heterozygous red-eyed female?
A: 1 red-eyed female : 1 white-eyed female : 1 red-eyed male : 1 white-eyed male (i.e., 1:1:1:1)
Q6: What are the possible genotypes and phenotypes for a cross between PpYY and ppYy?
A: Genotypes: PpYy, PpYY, ppYy, ppYY
Phenotypes: Purple/yellow and white/yellow
Punnett square size: 4 squares (2 × 2)
Q7: A true-breeding violet terminal × white axial flower cross — what is expected in the F2?
A: d. 75% violet flowers in an axial position
Q8: The observable traits expressed by an organism are described as its ________.
A: a. phenotype
Q9: A recessive trait will be observed in individuals that are ________ for that trait.
A: c. homozygous
Q10: If black and white mice mate and all offspring are gray, what inheritance pattern is this?
A: d. incomplete dominance
Q11: ABO blood groups are an example of:
A: d. multiple alleles and codominance
Q12: In a mating between two heterozygous carriers of a recessive lethal allele, what genotypic ratio is expected?
A: c. 1:2:0 (homozygous recessives die)
Q13: If the allele encoding polydactyly is dominant, why do most people have five fingers?
A: d. The polydactyl allele is very rare in the human population.
Q14: A farmer breeds speckled chickens from black and white parents. What offspring will appear after breeding speckled x speckled?
A: c. 50% speckled, 25% black, 25% white
Q15: In a dihybrid cross (AaBb), what is the expected gamete ratio for F1 (AB, Ab, aB, ab)?
A: a. 1:1:1:1
Q16: The forked line and probability methods use which probability rule?
A: b. product rule
Q17: How many different genotypes and phenotypes are expected in a trihybrid cross (all heterozygous parents)?
A: d. 27 genotypes; 8 phenotypes
Q18: Labrador retriever fur color (controlled by E and B alleles) is an example of what inheritance pattern?
A: a. epistasis
Q19: Which scenario does not follow the Law of Independent Assortment?
A: d. Men are more likely to experience hemophilia than women.
:
🌱 Mendel and the Foundations of Genetics
🧪 Why Mendel’s Work Was Revolutionary
- Selected pea plants for their distinct traits and ability to self- or cross-fertilize.
- Used true-breeding strains and quantitative methods over large sample sizes.
- Introduced the concept of discrete units of heredity (genes), which contrasted the blending theory.
🌼 Mendel’s Experimental Method
Three Stages:
1. True-breeding strains with clear traits (P generation)
2. Cross-fertilization → F1 generation (all show dominant trait)
3. Self-fertilization of F1 → F2 generation (3:1 phenotypic ratio)
Key Results:
- F1: all dominant trait
- F2: 3 dominant : 1 recessive → Actually 1:2:1 genotype (homo dom : hetero : homo rec)
🧬 Mendel’s Principles
1. Law of Segregation
- Alleles separate during gamete formation (meiosis).
- Each gamete receives one allele.
2. Law of Independent Assortment
- Alleles for different genes assort independently during gamete formation.
- Explains dihybrid 9:3:3:1 ratio.
📊 Probability in Genetics
- Product Rule (AND): Multiply probabilities of independent events.
- Sum Rule (OR): Add probabilities of mutually exclusive events.
🧪 Test Cross
- Cross a dominant phenotype (unknown genotype) with homozygous recessive.
- Resulting offspring ratios reveal genotype.
🧾 Vocabulary Recap
- Allele: Alternative version of a gene
- Homozygous / Heterozygous
- Genotype: Genetic makeup
- Phenotype: Physical expression
🧬 Extensions to Mendelian Genetics
🔸 Incomplete Dominance
- Heterozygote shows intermediate phenotype (e.g., red + white = pink)
🔸 Codominance
- Both alleles expressed (e.g., AB blood type)
🔸 Multiple Alleles
- More than two alleles exist in population (e.g., IA, IB, i for blood types)
🔸 Polygenic Inheritance
- Trait controlled by multiple genes (e.g., height, skin color)
🔸 Pleiotropy
- One gene influences multiple traits (e.g., sickle cell, cystic fibrosis)
🔸 Environmental Influence
- Trait expression depends on external factors (e.g., temperature-sensitive pigment in Siamese cats)
🔸 Epistasis
- One gene masks the expression of another (e.g., albinism gene masking coat color)
🧬 Dihybrid Cross (Two Traits)
- Classic 9:3:3:1 phenotypic ratio (e.g., RrYy x RrYy)
- Demonstrates independent assortment
🔍 Human Traits and Pedigree Analysis
- Dominant disorders: Appear every generation (e.g., Huntington's)
- Recessive disorders: May skip generations (e.g., albinism)
- Use pedigrees to trace inheritance in families.
♀♂ Sex-linked Traits (Mostly X-linked)
- Males more affected (only one X)
- Females can be carriers
- Examples: Color blindness, hemophilia
⚠ Lethal Alleles
- Recessive lethal: Only lethal in homozygous state (2:1 ratio in offspring)
- Dominant lethal: Lethal even in heterozygous state (e.g., Huntington’s)

What genotype ratio do you expect from a monohybrid cross of two heterozygotes (Aa x Aa)?
✎ Answer: 1 AA : 2 Aa : 1 aa
What is the probability of an offspring being homozygous recessive from an Aa x Aa cross?
✎ Answer: 1/4
What’s the probability of producing a plant with yellow AND round seeds in a dihybrid cross (YyRr x YyRr)?
✎ Answer: 9/16 (3/4 yellow × 3/4 round)
If two carriers of a recessive lethal allele mate, what is the expected genotypic ratio among surviving offspring?
✎ Answer: 2:1 (heterozygous : homozygous dominant)
How does the product rule apply in a trihybrid cross when calculating the probability of a specific genotype like aabbcc?
✎ Answer: Multiply the chance of each independent event → (1/4) × (1/4) × (1/4) = 1/64

🧬 Non-Mendelian Inheritance

What is codominance? Give an example.
✎ Answer: Both alleles are fully expressed in the heterozygote; example: AB blood type
What is the phenotype ratio for incomplete dominance when two heterozygotes are crossed (e.g., pink flowers)?
✎ Answer: 1 red : 2 pink : 1 white
What pattern of inheritance is observed when one gene masks another, such as pigment production overriding coat color in mice?
✎ Answer: Epistasis
How many phenotypes are expected in the F2 generation of a dihybrid cross?
✎ Answer: 4 phenotypes (9:3:3:1 ratio)

🧬 X-linked Traits & Sex-linked Inheritance

Why are X-linked recessive traits more common in males?
✎ Answer: Males have only one X chromosome, so a single recessive allele will be expressed
If a carrier female for color blindness (XᴺXⁿ) has children with a normal male (XᴺY), what is the probability a son will be colorblind?
✎ Answer: 50%
What is hemizygous, and why are males considered hemizygous for X-linked traits?
✎ Answer: Hemizygous = having only one allele for a gene; males have only one X chromosome

👨‍👩‍👧‍👦 Pedigree Analysis

In a pedigree, if two unaffected parents have an affected child, what is the most likely inheritance pattern?
✎ Answer: Autosomal recessive
If a trait appears in every generation and affects both males and females equally, what pattern is this?
✎ Answer: Autosomal dominant
If a trait only affects males and is passed from carrier mothers, what pattern is this?
✎ Answer: X-linked recessive