Patterns of Inheritance: Mendelian Genetics and Beyond

Patterns of Inheritance

Introduction to Mendel’s Experiments

Gregor Mendel’s experiments with pea plants laid the foundation for modern genetics. He used model organisms to study inheritance patterns, focusing on observable traits called characters and their specific forms called traits.

• Model Organism: An organism suitable for genetic studies due to ease of breeding and observable traits.

• Character: A heritable feature (e.g., flower color).

• Trait: A variant of a character (e.g., purple or white flowers).

• Example: Mendel used pea plants for their distinct traits and ease of control.

Why Pea Plants?

• Easy to grow and control pollination.

• Produce many offspring quickly.

• Exhibit many observable traits.

• Traits appear in distinct, easily distinguishable forms.

Genotype vs. Phenotype

Genetic traits are determined by alleles, which can be dominant or recessive. The genotype is the genetic makeup, while the phenotype is the observable trait.

• Allele: Different versions of a gene.

• Dominant allele: Expressed if present ($Y$ for yellow peas).

• Recessive allele: Masked by dominant allele ($y$ for green peas).

• Genotype: Combination of alleles (e.g., $YY$, $Yy$, $yy$).

• Phenotype: Observable trait (e.g., yellow or green peas).

Genotype	Phenotype
YY	Yellow
Yy	Yellow
yy	Green

Punnett Squares

Punnett squares are tools for predicting the genotypes and phenotypes of offspring from parental crosses.

• Rows and columns represent parental gametes.

• Each box shows a possible genotype of offspring.

• Each outcome is independent (Mendel’s Law of Independent Assortment).

Mendel’s Laws

Law of Segregation

During gamete formation, alleles for a gene separate so each gamete carries only one allele.

• Explains why offspring inherit one allele from each parent.

Law of Independent Assortment

Alleles of different genes assort independently during gamete formation, leading to genetic variation.

• Observed in dihybrid crosses.

Monohybrid and Dihybrid Crosses

Monohybrid Cross

Cross between parents differing in one trait.

• Genotype ratio: $1:2:1$

• Phenotype ratio: $3:1$

Dihybrid Cross

Cross between parents differing in two traits.

• Phenotype ratio: $9:3:3:1$

Genotype	Phenotype
RRYY	Round Yellow
RrYy	Round Yellow
rryy	Wrinkled Green
etc.	etc.

Probability in Genetics

Genetic probabilities can be calculated using the Rule of Multiplication (AND rule) and Rule of Addition (OR rule).

• Rule of Multiplication: Probability of independent events occurring together is the product of their probabilities.

• Rule of Addition: Probability of either event occurring is the sum of their probabilities.

Non-Mendelian Genetics

Incomplete Dominance

Heterozygotes show an intermediate phenotype (e.g., red and white flowers produce pink).

Codominance

Both alleles are fully expressed (e.g., AB blood type).

Genotype	Surface Molecules	Phenotype
IAIA or IAi	A only	Type A
IBIB or IBi	B only	Type B
IAIB	A and B	Type AB
ii	None	Type O

Epistasis

One gene affects the expression of another gene (e.g., H protein in blood type).

Polygenic Inheritance

Multiple genes affect a single trait (e.g., human height).

Pleiotropy

One gene affects multiple traits (e.g., Marfan syndrome).

Multifactorial Characters

Traits influenced by both genetic and environmental factors (e.g., hydrangea flower color affected by soil pH).

Pedigrees and Inheritance Patterns

Pedigrees are charts depicting family relationships and inheritance of traits over generations.

• Circles = females, squares = males.

• Shaded = affected, unshaded = unaffected.

Autosomal Inheritance

• Autosomal dominant: Only one allele needed for disorder.

• Autosomal recessive: Two alleles needed for disorder.

Sex-Linked Inheritance

• X-linked: Genes on X chromosome; males more likely to express recessive disorders.

• Y-linked: Genes on Y chromosome; only males affected.

X-Inactivation

In females, one X chromosome is randomly inactivated in each cell, leading to mosaic expression (e.g., calico cats).

Summary Table: Key Mendelian Concepts

Concept	Definition	Example
Law of Segregation	Alleles separate during gamete formation	Yellow/green peas
Law of Independent Assortment	Genes assort independently	Dihybrid cross
Incomplete Dominance	Intermediate phenotype	Pink flowers
Codominance	Both alleles expressed	AB blood type
Epistasis	One gene affects another	Blood type H protein
Polygenic	Many genes, one trait	Height
Pleiotropy	One gene, many traits	Marfan syndrome

Additional info: These notes cover the essential concepts of Mendelian genetics, including inheritance patterns, probability, and extensions to non-Mendelian genetics, suitable for General Biology students preparing for exams.