Patterns of Inheritance: Mendelian Genetics and Human Genetic Disorders

Study Guide - Smart Notes

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

Patterns of Inheritance

What Is the Physical Basis of Inheritance?

Inheritance is the process by which traits are passed from parents to offspring. The physical basis of inheritance lies in genes, which are segments of DNA located on chromosomes.

Genes: Units of heredity that encode information needed to produce proteins, cells, and entire organisms.
Chromosomes: Structures carrying genes; humans have 23 pairs.
Locus: The specific location of a gene on a chromosome.
Alleles: Alternative forms of a gene found at the same locus.

Example: The gene for flower color in pea plants may have alleles for purple or white flowers.

How Were Principles of Inheritance Discovered?

Gregor Mendel, through experiments with pea plants, established the basic principles of inheritance. He used controlled crosses and statistical analysis to deduce how traits are inherited.

Mendel's Method: Used true-breeding plants, controlled pollination, and tracked traits over generations.
Key Steps: Choosing organisms with easily distinguishable traits, ensuring true-breeding, and applying quantitative analysis.

Example: Mendel's pea plant experiments revealed predictable patterns of inheritance for flower color and seed shape.

How Are Single Traits Inherited?

Single-gene inheritance involves traits determined by one gene with two alleles. Mendel's monohybrid crosses demonstrated the segregation and independent assortment of alleles.

Dominant Allele: Expressed when present (e.g., purple flower color).
Recessive Allele: Masked by dominant allele (e.g., white flower color).
Genotype: The genetic makeup (e.g., PP, Pp, pp).
Phenotype: The observable trait (e.g., purple or white flowers).

Example: In a cross between true-breeding purple (PP) and white (pp) flowers, all F1 offspring are purple (Pp), but F2 generation shows a 3:1 ratio of purple to white.

Mendel's Laws Explaining Inheritance

Law of Segregation: Each organism carries two alleles for each gene, which segregate during gamete formation.
Law of Independent Assortment: Genes for different traits are inherited independently if located on different chromosomes.

Example: Dihybrid crosses (e.g., seed color and shape) produce a 9:3:3:1 phenotypic ratio in F2 generation.

Do Mendelian Rules Apply to All Traits?

Not all traits follow simple Mendelian inheritance. Some traits are influenced by multiple alleles, incomplete dominance, codominance, or multiple genes.

Incomplete Dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents).
Codominance: Both alleles are fully expressed (e.g., human blood type AB).
Multiple Alleles: More than two alleles exist for a gene (e.g., ABO blood group).

HTML Table: Human ABO Blood Types

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

How Are Multiple Traits Inherited?

Polygenic inheritance involves traits controlled by two or more genes, often resulting in continuous variation (e.g., skin color, height).

Pleiotropy: One gene affects multiple traits.
Environmental Influence: Phenotype can be affected by environmental factors (e.g., temperature, nutrition).

Example: Fur color in Himalayan rabbits is influenced by temperature.

How Are Genes Located on the Same Chromosome Inherited?

Genes located close together on the same chromosome tend to be inherited together, a phenomenon known as genetic linkage. Crossing over during meiosis can separate linked genes.

Genetic Recombination: Exchange of genetic material between homologous chromosomes.
Linkage Maps: Diagrams showing the relative positions of genes on a chromosome.

How Are Sex and Sex-Linked Traits Inherited?

Sex determination in humans is controlled by the X and Y chromosomes. Sex-linked traits are associated with genes located on sex chromosomes, often the X chromosome.

Sex-Linked Traits: Traits controlled by genes on the X or Y chromosome (e.g., color blindness, hemophilia).
Inheritance Patterns: Males are more likely to express X-linked recessive traits because they have only one X chromosome.

Example: Hemophilia is an X-linked recessive disorder more common in males.

How Are Human Genetic Disorders Inherited?

Genetic disorders can be caused by mutations in single genes, chromosomal abnormalities, or multifactorial inheritance. Pedigree analysis helps trace inheritance patterns in families.

Autosomal Recessive Disorders: Require two copies of the mutant allele (e.g., cystic fibrosis, sickle-cell anemia).
Autosomal Dominant Disorders: Only one copy of the mutant allele is needed (e.g., Huntington disease).
X-Linked Disorders: Caused by mutations in genes on the X chromosome (e.g., hemophilia).

HTML Table: Examples of Human Genetic Disorders

Disorder	Inheritance Pattern	Key Features
Cystic Fibrosis	Autosomal Recessive	Thick mucus in lungs, digestive problems
Sickle-Cell Anemia	Autosomal Recessive	Abnormal hemoglobin, anemia
Huntington Disease	Autosomal Dominant	Neurodegeneration, late onset
Hemophilia	X-Linked Recessive	Blood clotting problems

Key Terms and Concepts

Genotype: The genetic constitution of an organism.
Phenotype: The observable characteristics of an organism.
Homozygous: Having two identical alleles for a gene.
Heterozygous: Having two different alleles for a gene.
Punnett Square: Diagram used to predict the outcome of a genetic cross.

Important Equations

Monohybrid Cross Ratio:
Dihybrid Cross Ratio:

Additional info: These notes expand on Mendelian genetics, including exceptions to simple inheritance, and provide context for human genetic disorders relevant to college-level General Biology.