Genetic Inheritance Patterns Lecture 23

Lecture 23: The Patterns of Genetic Inheritance-1

• The lecture covers patterns of genetic inheritance, specifically pedigrees.
• A pedigree is a chart illustrating the genetic history of a family across several generations.
• Pedigrees are used to determine how traits are transmitted (including disease causes).
• Pedigrees predict the probability of offspring inheriting a trait.
• Pedigrees help identify whether a trait is dominant or recessive, and whether the gene is autosomal or X-linked.
• Pedigree construction involves symbols for males, females, mating, offspring (birth order), twins (monozygotic/dizygotic), pregnancies (affected/unaffected), marriage (consanguineous/normal), deceased individuals, and unknown sex.
• The proband (propositus) is the first person with a disease who seeks medical attention.

Types of Inheritance

• Mendelian inheritance patterns include: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, Y-linked.
• Non-Mendelian inheritance includes: imprinting, mitochondrial, multifactorial, sporadic, contiguous gene syndromes.

Complicating Factors

• Complicating factors influencing inheritance include: non-penetrance, new mutations, adult-onset conditions, consanguinity, and gene interactions.

Pedigree Evaluation

• Evaluate transmission patterns (vertical/horizontal).
• Analyze the ratio of affected males to females.
• Determine if the disease/gene is passed through unaffected individuals, and male to male transmissions.
• Calculate the percentage of affected children.

Vocabulary

• Character: heritable feature
• Trait: each variant for a character
• Monohybrid cross: inheritance of a single character
• P generation: parental generation
• F1 generation: first filial generation (offspring of P generation)
• F2 generation: second filial generation (offspring of F1 generation)
• Punnett square: illustrates how affected parents transmit normal/disease genes to offspring
• Allele: alternate version of a gene
• Dominant allele: expressed in heterozygotes
• Recessive allele: not expressed in heterozygotes
• Homozygote: pair of identical alleles
• Homozygous dominant: BB
• Homozygous recessive: bb
• Heterozygote: two different alleles (Bb)
• Genotype: genetic makeup
• Phenotype: appearance of an organism

Autosomal Dominant Inheritance

• One copy of a dominant allele expresses a trait.
• The gene is located on one of the 22 autosomes.
• Affected individuals typically have an affected parent.
• Both sexes are equally likely to be affected.
• Roughly 50% of offspring from an affected individual are also affected.
• Examples include Huntington's disease, achondroplastic dysplasia, and neurofibromatosis.
• This type of inheritance displays a vertical pattern of affected generations.

Autosomal Recessive Inheritance

• Two copies of a recessive allele are needed for the trait to be expressed.
• Both parents are carriers or heterozygous.
• Affected offspring may have unaffected parents.
• Trait can skip generations.
• Males and females are equally affected.

X-linked Recessive Inheritance

• Gene is located on the X chromosome.
• Men are more affected than women.