Introduction to Pedigree Analysis

Introduction to Pedigree Analysis

• Many disorders are genetic, similar to traits like dimples.
• Neurofibromatosis affects skin and nervous tissues, occurring in 1 out of 4,000 births. Symptoms include pea-sized bumps; severe cases involve brain and spinal tumors.
• Surgery can remove tumors, but there's no complete cure for neurofibromatosis.
• Studying human genetic disorders is challenging due to human reproduction nature.
• This chapter explains how to understand the inheritance of traits in human families.

Learning Objectives

• Understand the importance of pedigrees in studying inheritance in humans.
• Construct and analyze pedigrees involving different genetic conditions in humans.

Warm Up - Genetic Disorders Activity

• There are cases when a newly born child does not manifest a genetic disorder.
• Techniques exist to determine if a child or embryo possesses a genetic disorder during gestation.
• The activity focuses on genetic disorders through a simulation of genetic counseling.
• A class is divided into groups, assigned a genetic disorder and a designated video to watch.
• Genetic disorders include: Cystic fibrosis, Achondroplasia, Phenylketonuria, and Sickle-cell disease
• After watching the videos, the group prepares a three-minute skit about a couple affected with the disorder

Analyzing Inheritance: Peas vs. Humans

• Gregor Mendel's plant breeding experiments enhanced the understanding of biological trait inheritance.
• Studying pea traits is easier because mating can be controlled.
• Assign desired characteristics of parents in a cross is possible with peas.
• Peas have relatively shorter generation time, and can produce many offspring.
• Analyzing inheritance in humans is more difficult and complex.
• Controlled matings aren't possible for ethical reasons in humans.
• The generation time is relatively longer in humans with only one or few offspring per pregnancy.
• Studying inheritance in humans relies on family records, often incomplete.

Using Pedigrees

• Pedigree analysis is a useful approach to study the inheritance of traits in humans.
• A pedigree is a graphical representation of how a trait is inherited in a family.
• It's an effective method to determine the mode of inheritance of a trait.
• Pedigrees also assess the risk of recurrence of genetic traits/disorders.
• Challenges in studying human inheritance involve problems in information collection. Accurate pedigrees require knowledge of the number of children, sex, and occurrence of traits among relatives.
• Genetic counseling provides advice to families with or at risk of genetic conditions.
• A genetic counselor elaborates on genetic, psychological, and other implications of the condition.
• Accurate genetic counseling relies on the family's correct recollection of trait inheritance among relatives.

Constructing Pedigrees: Symbols and Notations

• Squares represent male individuals; circles represent females.
• Shaded squares or circles denote affected individuals; unshaded shapes indicate unaffected individuals.
• A slash indicates a deceased individual.
• A proband is the individual being studied/observed, who first sought a genetic counselor.
• Half-shaded shapes indicate carriers for a genetic condition (heterozygous).
• The carrier status may not always be shown in a pedigree.
• Diamond symbols indicate unknown sex when the family is unsure about the number of children of a relative.
• A horizontal line between a couple denotes mating.
• A vertical line connects parents to their offspring.
• A double horizontal line represents consanguinity (genetically related individuals mating).
• Branching from a common point denotes twins.
• Connecting members of a twin represents monozygotic (identical) twins.
• Otherwise, twins are considered dizygotic (fraternal).
• Roman numerals denote generations, and individuals are numbered consecutively from left to right.

Analyzing Pedigrees

• Identifying whether a trait is dominant or recessive is crucial.
• It determines the chances of trait recurrence in the family.
• Traits analyzed in the lesson, like those observed by Mendel in peas, are autosomal.
• Genes for these traits are found in autosomes (non-sex chromosomes).
• Other traits are sex-linked, with genes on sex chromosomes; discussed later in the unit.
• In pedigree analysis, the specific trait/characteristic and individual carrier status are usually not provided in the problem to encourage analysis.

Autosomal Dominant Traits

• Dominant traits are relatively easier to identify because individuals with the dominant allele express the phenotype.
• An affected individual must have at least one affected parent.
• Autosomal dominant traits tend not to skip generations.
• Achondroplasia, a form of dwarfism, is an example of an autosomal dominant trait.
• A dominant mutation in a bone development gene results in short stature and impaired cartilage conversion.

Autosomal Recessive Traits

• Identifying autosomal recessive traits during health consultations can be more challenging.
• Affected children usually have two unaffected parents, indicating that both parents are carriers.
• Recessive traits are more likely to appear in consanguineous mating, especially if traits are rare in populations.
• A single unaffected individual may have affected children.
• Cystic fibrosis is an autosomal recessive genetic condition resulting from a mutation in a gene coding for chloride ion channels in cell membranes.
• Consequences of the disorder include thick buildup of mucus that clogs the airways and ducts in the body.
• Sickle cell disease is another autosomal recessive condition, resulting from a mutation in the hemoglobin gene, and causes an alteration in the shape of erythrocytes.
• The sickle-shaped red blood cells die prematurely and can become lodged in blood vessels.

Explore pedigree analysis to understand inheritance patterns in human families, focusing on genetic disorders like neurofibromatosis and learn to construct and analyze pedigrees involving various genetic conditions.