Patterns of Inheritance: Mendelian Genetics and Beyond

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Patterns of Inheritance

Introduction to Genetics

Genetics is the scientific study of heredity, focusing on how traits are transmitted from one generation to the next. The field has ancient roots, but modern genetics began with the experiments of Gregor Mendel in the 19th century. Understanding inheritance patterns is crucial for interpreting genetic information, predicting trait transmission, and addressing genetic disorders.

Heredity: The transmission of traits from parents to offspring.
Genetics: The branch of biology that studies heredity and variation in organisms.
Character: A heritable feature that varies among individuals (e.g., flower color).
Trait: A variant of a character (e.g., purple or white flowers).

Mendel’s Laws

Early Theories of Inheritance

Pangenesis (Hippocrates): Proposed that particles from all parts of the body are collected in the gametes. This is incorrect because changes in somatic cells do not affect gametes.
Blending Hypothesis: Suggested that parental traits mix in offspring. This is rejected because traits can reappear after skipping generations.

Mendel’s Experiments

Used Pisum sativum (garden pea) to study inheritance.
Crossed true-breeding plants (homozygous) to observe trait transmission over generations.
Defined generations as:
- P generation: Parental generation (true-breeding).
- F1 generation: First filial generation (hybrids).
- F2 generation: Second filial generation (offspring of F1 self-cross).

Mendel’s Law of Segregation

This law describes the inheritance of a single character:

Each organism carries two alleles for each gene, one from each parent.
Alleles segregate during gamete formation, so each gamete carries only one allele for each gene.
Homozygous: Two identical alleles (e.g., PP or pp).
Heterozygous: Two different alleles (e.g., Pp).
Dominant allele: Determines phenotype in heterozygotes.
Recessive allele: Masked in heterozygotes.

Punnett Square: A diagram used to predict the outcome of a genetic cross.

Example: Crossing two heterozygous plants (Pp x Pp) yields a 3:1 phenotypic ratio in the F2 generation.

Homologous Chromosomes and Genes

Homologous chromosomes carry alleles for the same genes at corresponding loci.
During meiosis, homologous chromosomes separate, providing the physical basis for the law of segregation.

Mendel’s Law of Independent Assortment

Alleles of different genes assort independently during gamete formation.
Demonstrated by dihybrid crosses (e.g., RrYy x RrYy), resulting in a 9:3:3:1 phenotypic ratio.
Applies only to genes on different chromosomes or far apart on the same chromosome.

Probability in Genetics

Rule of Multiplication: Probability of two independent events both occurring is the product of their individual probabilities.
Rule of Addition: Probability of an event that can occur in multiple ways is the sum of the probabilities for each way.

Example: Probability of offspring with genotype AABBCC from AaBbCc x AaBbCc cross:

Pedigree Analysis

Pedigrees are family trees used to track inheritance patterns in humans.
Can help determine genotypes and predict probabilities of inherited disorders.

Autosomal Disorders

Some human disorders are inherited as dominant or recessive traits controlled by a single gene.

Disorder	Inheritance	Symptoms
Cystic fibrosis	Autosomal recessive	Thick mucus, lung infections
Sickle-cell disease	Autosomal recessive	Anemia, organ damage
Achondroplasia	Autosomal dominant	Dwarfism
Huntington’s disease	Autosomal dominant	Neurodegeneration

Genetic Testing and Screening

Techniques include carrier screening, fetal testing (amniocentesis, chorionic villus sampling), fetal imaging, and newborn screening.
These methods can inform reproductive decisions but may raise ethical concerns.

Variations on Mendel’s Laws

Incomplete Dominance

Heterozygotes have an intermediate phenotype between the two parental varieties.
Example: Red (RR) x White (rr) snapdragons produce Pink (Rr) offspring.

Multiple Alleles and Codominance

Some genes have more than two alleles in the population.
ABO blood group: Three alleles (IA, IB, i) produce four phenotypes (A, B, AB, O).
IA and IB are codominant; both are expressed in type AB individuals.

Genotype	Phenotype (Blood Type)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

Pleiotropy

A single gene affects multiple phenotypic traits.
Example: Sickle-cell disease affects hemoglobin structure, red blood cell shape, and resistance to malaria.

Polygenic Inheritance

Multiple genes contribute additively to a single phenotypic character.
Example: Human height and skin color are influenced by several genes.

Environmental Effects

Phenotype can be influenced by both genetic and environmental factors (e.g., nutrition, sunlight).
Mendel minimized environmental variation in his experiments by using controlled conditions.

The Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

Genes are located on chromosomes at specific loci.
Chromosomes undergo segregation and independent assortment during meiosis, explaining Mendel’s laws.

Linked Genes and Crossing Over

Genes located close together on the same chromosome (linked genes) tend to be inherited together.
Crossing over during meiosis can separate linked genes, producing recombinant offspring.
Recombination frequency: Percentage of recombinant offspring, used to map gene loci (linkage maps).

Sex Chromosomes and Sex-Linked Genes

Sex Determination

In mammals, sex is determined by the presence of X and Y chromosomes (XX = female, XY = male).
Other species may use different systems or environmental factors (e.g., temperature in reptiles).

Sex-Linked Inheritance

Genes located on sex chromosomes (especially X) show unique inheritance patterns.
X-linked recessive disorders (e.g., hemophilia, color blindness) are more common in males, who have only one X chromosome.

Y Chromosome and Human Evolution

The Y chromosome is passed from father to son with little recombination, making it useful for tracing paternal ancestry.
Studies have linked certain Y chromosome lineages to historical figures (e.g., Genghis Khan).

Summary Table: Key Inheritance Patterns

Pattern	Description	Example
Complete dominance	One allele masks the other	Purple vs. white pea flowers
Incomplete dominance	Heterozygote is intermediate	Pink snapdragons
Codominance	Both alleles expressed	AB blood type
Pleiotropy	One gene, multiple traits	Sickle-cell disease
Polygenic inheritance	Many genes, one trait	Skin color, height
Sex-linked	Gene on sex chromosome	Hemophilia

Key Terms and Concepts

Genotype: Genetic makeup of an organism (e.g., PP, Pp, pp).
Phenotype: Observable traits (e.g., purple or white flowers).
Testcross: Cross between an individual of unknown genotype and a homozygous recessive individual to determine genotype.
Carrier: Heterozygous individual who carries a recessive allele but does not express the trait.
Linkage map: Diagram showing relative positions of genes on a chromosome based on recombination frequencies.

Practice and Further Study

Work through practice problems involving Punnett squares, pedigree analysis, and probability calculations.
Review the inheritance patterns of specific genetic disorders.
Explore online resources and videos for additional explanations and examples.