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

Patterns of Inheritance

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical inheritance, describing how traits are passed from parents to offspring through discrete units called genes. Gregor Mendel's experiments with pea plants established the basic principles of heredity, including the concepts of dominant and recessive alleles, segregation, and independent assortment.

Mendel and Pea Plants

Unique Aspects of Mendel’s Studies

• Controlled Crosses: Mendel used true-breeding pea plants and controlled pollination to study inheritance patterns.

• Quantitative Analysis: He counted offspring and analyzed ratios, allowing him to deduce patterns of inheritance.

Example Character and Traits

• Character: Flower color

• Traits: Purple flowers, white flowers

Mendel’s Experiments and Conclusions

• True-breeding: Plants that produce offspring of the same variety when self-pollinated.

• P-generation: Parental generation, true-breeding for a trait (e.g., PP x pp).

• F1 generation: First filial generation, all heterozygous (e.g., Pp), showing the dominant trait.

• F2 generation: Second filial generation, produced by selfing F1 individuals.

• Surprise in F2: Reappearance of the recessive trait in a 3:1 ratio (dominant:recessive).

• Explanation: Traits are determined by pairs of alleles that segregate during gamete formation.

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

Traits and Alleles

Definitions and Key Concepts

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

• Purple allele: Produces purple pigment in flowers.

• White allele: Nonfunctional enzyme, no pigment produced.

• 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 per gene.

Solving Heredity Problems: Monohybrid Crosses

Monohybrid Crosses

Monohybrid crosses involve a single character with two alleles. The following steps are used to solve genetics 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) x Homozygous purple (PP)

• Genotype ratio: 100% Pp

• Phenotype ratio: 100% purple

Practice Problem 2: F1 Cross

• Cross: Heterozygous (Pp) x 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; if 1:1 ratio, parent is heterozygous.

Dihybrid Crosses and Independent Assortment

Dihybrid Crosses

Dihybrid crosses track two characters simultaneously (e.g., seed color and shape). Each parent can produce multiple types of gametes due to 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 (AB, Ab, aB, ab).

Practice Problem: Dihybrid Cross (YYRR x yyrr)

• All offspring: YyRr (heterozygous for both traits)

• Phenotype: All yellow and round

Practice Problem: F1 Dihybrid Cross (YyRr x YyRr)

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

Independent Assortment

• Meiosis phase: Metaphase I is associated with independent assortment.

• Number of gamete types: For two heterozygous genes, four types of gametes are formed.

• F2 phenotype ratio: 9:3:3:1 for two heterozygous parents.

Rules of Probability

• Used to predict the likelihood of specific genotypes or phenotypes in offspring, especially in complex crosses.

• Product rule: Probability of independent events occurring together is the product of their individual probabilities.

Pedigree Analysis

Understanding Pedigrees

• Pedigrees: Diagrams that show inheritance patterns of traits across generations in families.

• Generations: Top row is generation I, next is II, and so on.

• Affected individuals: Usually shaded symbols.

Pedigree Practice Problem 1: Tay Sachs Disease

• Pattern: Recessive inheritance (affected individuals are often born to 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 appear in every generation).

• Genotypes:

  • Individual 1: Heterozygous dominant (Dd)

  • Individual 2: Heterozygous dominant (Dd)

  • Individual 3: Homozygous recessive (dd)

Determining Inheritance Patterns from Pedigrees

• Dominant traits: Appear in every generation; affected individuals have at least one affected parent.

• Recessive traits: May skip generations; affected individuals can have unaffected parents.

Beyond Mendel: Non-Mendelian Inheritance

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 homozygotes	Red x white snapdragons produce pink flowers
Codominance	Both alleles are fully expressed in heterozygotes	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

Summary

• Mendelian genetics explains basic inheritance patterns using laws of segregation and independent assortment.

• Monohybrid and dihybrid crosses, Punnett squares, and probability rules are essential tools for predicting offspring genotypes and phenotypes.

• Pedigree analysis helps determine inheritance patterns in families.

• Non-Mendelian inheritance includes multiple alleles, incomplete dominance, codominance, polygenic traits, and pleiotropy.