Patterns of Inheritance
Understanding how traits are passed from parents to children helps us make sense of genetic conditions. Here’s a guide to the main types of inheritance:
1. Autosomal Dominant
One altered gene from either parent is enough to cause the trait or disorder.
Examples: Huntington’s disease, Marfan syndrome
2. Autosomal Recessive
Two altered copies (one from each parent) are needed to be affected.
Examples: Cystic fibrosis, Tay-Sachs disease
3. X-linked Inheritance
Genes located on the X chromosome. Males are more often affected by recessive forms.
Examples: Hemophilia, Duchenne muscular dystrophy
4. Y-linked Inheritance
Passed from father to son through the Y chromosome. Only affects males.
Example: Certain types of male infertility
5. Codominance
Both alleles are fully expressed in the trait.
Example: AB blood type
6. Incomplete Dominance
The trait is a blend of both alleles.
Example: Pink snapdragons (red × white)
Non-Mendelian Inheritance
Some traits follow more complex patterns:
1. Polygenic Inheritance
Multiple genes work together to influence a trait, often creating wide variation.
Examples: Height, skin color
2. Multiple Alleles
Some genes have more than two possible versions.
Example: ABO blood group system
3. Mitochondrial Inheritance
DNA in mitochondria comes only from the mother. Often affects energy-demanding organs.
Example: Mitochondrial myopathy
4. Genomic Imprinting
The effect of a gene depends on which parent it came from.
Examples: Prader-Willi syndrome, Angelman syndrome
5. Epigenetics
Changes in how genes are expressed without altering DNA code. Can be inherited.
- DNA methylation
- Histone modification
- Non-coding RNA
6. Gene Expression
Regulation of when and how genes are transcribed into proteins — the bridge between genotype and phenotype.
Genetic Variation & Mutation
Genetic differences make each person unique — but can also cause genetic disorders.
1. Sources of Variation
- Independent assortment of chromosomes
- Crossing over during meiosis
- Random fertilization
2. Mutations
Changes in the DNA sequence. Types include:
- Point mutations – single letter changes
- Insertions/deletions – extra or missing DNA
- Chromosomal mutations – large structural changes