Patterns of Inheritance: Mendelian Genetics, Probability, and Pedigree Analysis

Patterns of Inheritance

Introduction

This chapter explores the foundational principles of inheritance as established by Gregor Mendel, the application of probability to genetic crosses, and the analysis of inheritance patterns using pedigrees. Understanding these concepts is essential for solving genetics problems and interpreting how traits are passed from one generation to the next.

Mendel and Pea Plants

Mendel’s Unique Approach

• Controlled Crosses: Mendel used pea plants (Pisum sativum) and controlled their mating, allowing him to track inheritance patterns over generations.

• Quantitative Analysis: He counted offspring and analyzed ratios, which was unique for his time.

Example Character and Traits

• Character: Flower color

• Traits: Purple flowers, white flowers

Mendel’s Experiments and Conclusions

• True-breeding: Plants that, when self-fertilized, produce offspring identical to themselves.

• P-generation: Parental generation; true-breeding individuals for contrasting traits.

• F1 generation: First filial generation; all offspring showed only the dominant trait.

• F2 generation: Second filial generation; both traits reappeared in a 3:1 ratio (dominant:recessive).

• Surprise in F2: The recessive trait reappeared, disproving blending inheritance.

• Mendel’s Explanation: Traits are determined by discrete units (genes) that segregate during gamete formation.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so each gamete carries only one allele.

Traits and Alleles

Key Definitions

• Alleles: Alternative forms of a gene found at the same locus on homologous chromosomes.

• Purple allele: Produces purple pigment in flowers.

• White allele: Produces no pigment, resulting in white flowers.

• Locus: The specific location of a gene on a chromosome.

• Diploids: Have two alleles per character (one from each parent).

• Haploids: Have one allele per character (e.g., gametes).

Allele Segregation During Meiosis

• During meiosis, homologous chromosomes (and thus alleles) separate, ensuring each gamete receives only one allele for each gene.

Solving Heredity Problems: Monohybrid Crosses

Monohybrid Crosses

Monohybrid crosses involve a single character (e.g., flower color). The following steps are used to solve these problems:

1. Write allele and genotype keys

2. Determine genotypes of the parents

3. Determine gametes produced by each parent

4. Show fertilization using a Punnett square

Key Terms

• Genotype: The genetic makeup (e.g., PP, Pp, pp).

• Phenotype: The observable trait (e.g., purple or white flowers).

Practice Problem 1: Parental Cross

• Cross: White (pp) × Homozygous purple (PP)

• Genotype ratio: 0 PP : 1 Pp : 0 pp

• Phenotype ratio: 100% purple : 0% white

Practice Problem 2: F1 Cross

• Cross: Heterozygous (Pp) × Heterozygous (Pp)

• Genotype ratio: 1 PP : 2 Pp : 1 pp

• Phenotype ratio: 3 purple : 1 white

Practice Problem 3: Test Cross

• Test cross: Crossing an individual with a dominant phenotype (unknown genotype) with a homozygous recessive individual to determine genotype.

• Application: If all offspring are dominant, parent is homozygous dominant; if 1:1 ratio, parent is heterozygous.

Dihybrid Crosses

Introduction

Dihybrid crosses track two characters simultaneously (e.g., seed color and shape). They demonstrate the Law of Independent Assortment.

• Genotype AABB: Represents two characters, both homozygous dominant.

• Genotype AB: Represents a gamete (haploid).

• Genotype AA: Not a gamete; gametes have one allele per gene.

• FOIL technique: Used to determine all possible gametes from a parent with genotype AaBb (gametes: AB, Ab, aB, ab).

Dihybrid Practice Problem 1: Parental Cross

• Cross: YYRR × yyrr

• All offspring: YyRr (heterozygous for both traits)

Dihybrid Practice Problem 2: F1 Cross

• Cross: YyRr × YyRr

• Phenotype ratio: 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled

Independent Assortment

• Associated with: Metaphase I of meiosis

• Number of characters: Two or more

• Gamete types from double heterozygote: Four (e.g., YR, Yr, yR, yr)

• F2 phenotype ratio (both parents hybrid): 9:3:3:1

Dihybrid Practice Problem 3 & 4: Probability

• Probability rules: Used to calculate the likelihood of specific genotypes or phenotypes in offspring.

• Example: Probability of yyRR from YyRr × YyRR = Probability of yy × Probability of RR

Pedigree Analysis

Introduction

Pedigrees are diagrams that show the inheritance of traits across generations, useful for studying human genetics and identifying inheritance patterns.

• Top row: Generation I

• Affected individuals: Usually shaded

Pedigree Practice Problem 1: Tay Sachs Disease

• Pattern: Recessive inheritance (affected individuals can have unaffected parents)

• Genotype of affected individual in generation I: Homozygous recessive (e.g., tt)

• Genotype of female in generation II with children: Likely heterozygous if she has both affected and unaffected offspring

Pedigree Practice Problem 2: Achondroplasia Dwarfism

• Pattern: Dominant inheritance (affected individuals in every generation)

• Genotypes:

  • Individual 1: Heterozygous dominant (Dd)

  • Individual 2: Heterozygous dominant (Dd)

  • Individual 3: Homozygous recessive (dd)

Pedigree Summary

• Dominant trait: Appears in every generation; affected individuals have at least one affected parent.

• Recessive trait: Can skip generations; affected individuals can have unaffected parents.

Beyond Mendel: Extensions of Mendelian Genetics

Vocabulary	Description	Example
Multiple alleles	More than two alleles exist for a gene in a population	ABO blood groups
Incomplete dominance	Heterozygote phenotype is intermediate between the two homozygotes	Red × white snapdragons produce pink offspring
Codominance	Both alleles are fully expressed in the heterozygote	AB blood type
Polygenic inheritance	Multiple genes affect a single trait	Human skin color
Pleiotropy	One gene affects multiple traits	Sickle cell disease
Nature vs Nurture	Both genetic and environmental factors influence traits	Height, intelligence

Additional info: The above table summarizes key extensions to Mendelian genetics, highlighting the complexity of inheritance beyond simple dominant-recessive relationships.