pedigree

Following Patterns of Inheritance in Humans

Pedigree Chart

  • Definition: A pedigree chart is a graphic flowchart presentation of a family tree that shows patterns of inheritance for a single gene using symbols. It provides a way to analyze how a trait ‘runs in the family’.

  • Structure:

    • Roman Numerals: Symbolize generations in the pedigree chart.

    • Arabic Numbers: Symbolize individuals within a given generation.

    • Birth Order: Within each group of offspring, individuals are drawn left to right, from first-born to last-born.

    • Symbols Used:

    • Male: Represented by a square.

    • Female: Represented by a circle.

    • Mating: A horizontal line between male and female.

    • Identical Twins: Merged symbols (two circles or squares connected).

    • Non-Identical Twins: Separate symbols.

    • Affected Individuals: Shaded or marked symbols.

    • Deceased Individuals: Symbol crossed out.

    • Sex Unknown: Shaded or unfilled symbols.

Autosomal Inheritance

  • Definition: Autosomal inheritance refers to the inheritance of traits whose genes are found on the autosomes (chromosomes #1-22 in humans).

  • Significance: Responsible for many inherited genetic disorders.

  • Types of Autosomal Disorders:

    • Autosomal Dominant Disorders: The disease-causing allele is dominant. Thus, the presence of one abnormal gene will result in the expression of the trait.

    • Autosomal Recessive Disorders: The disease-causing allele is recessive. Here, two copies of the abnormal gene are needed for the trait to be expressed.

Autosomal Diseases in Humans

  • List of Common Autosomal Disorders:

    • Huntington's Disease: A neurodegenerative disorder caused by a dominant allele.

    • Cystic Fibrosis: A condition affecting the respiratory and digestive systems, caused by a recessive allele.

    • Sickle Cell Anemia: A blood disorder resulting from a mutation in the hemoglobin gene, causing red blood cells to become misshapen.

    • Marfan Syndrome: A disorder affecting connective tissue, caused by a dominant allele.

    • Retinoblastoma: A type of eye cancer that occurs due to a mutation in a tumor suppressor gene.

    • Tay-Sachs Disease: A fatal genetic disorder caused by a recessive allele affecting the nervous system.

  • Illustration of Sickle Cell Anemia:

    • Normal red blood cell: Contains normal hemoglobin.

    • Mutated hemoglobin: Forms strands that cause the cells to take on a sickle shape, leading to various health issues.

Pedigree Charts & Autosomal Inheritance

  • Significance of Analysis: Analyzing a pedigree chart helps to determine whether the inheritance pattern for a trait is autosomal dominant or recessive.

Autosomal Dominant Inheritance

  • Genotype of Affected Individuals: Affected individuals typically have dominant genotypes, which can be either homozygous dominant (AA) or heterozygous (Aa).

  • Inheritance Probability: If one parent has the dominant allele and the other has a normal gene, there is a 50% chance for each child to inherit the abnormal gene and hence the dominant trait. Each child has a 50:50 chance of inheriting the disorder.

  • Example 1: Affected child can be from two affected parents where one child is unaffected:

    • Parent 1 genotype: Aa

    • Parent 2 genotype: Aa

    • Possible offspring genotypes: aa (unaffected), Aa (affected).

  • Example 2: Two affected individuals (I-1 and I-2) having unaffected children (II-4 and II-5), showcasing the possibility of unaffected offspring from affected parents:

    • Offspring genotypes: could include aa (unaffected), Aa (affected), Aa (also affected).

Autosomal Recessive Inheritance

  • Genotype of Affected Individuals: Affected individuals have recessive genotypes (aa).

  • Carriers: Individuals who are heterozygous (Aa) are referred to as carriers. These individuals do not express the trait but can pass the recessive allele to offspring.

  • Probability of Recessive Disorders: Two unaffected parents can have an affected child 25% of the time if both parents are carriers (heterozygotes).

  • Example 1: Two unaffected parents can have an affected child if both parents are carriers (Aa x Aa):

    • Punnett Square:

    • Parents: Aa x Aa

    • Offspring can be: 25% aa (affected), 50% Aa (carriers), 25% AA (unaffected).

  • Example 2: Unaffected parents (III-1 and III-2) can have affected children (IV-1 and IV-2). The recessive trait can skip generations:

    • Possible combinations could show that both parents are carriers (Aa).

    • Their children genotypes include combinations such as aa (affected) and Aa (carriers).