Pedigree Charts and Genetic Traits

• Pedigrees are tools used to determine and show how a genetic trait is passed through a family across generations.
• Pedigree genetics helps determine if a person carries a specific trait or to find the inheritance pattern.

Pedigree Basics

• A pedigree is a visual representation, similar to a family tree, showing a specific trait (phenotype) in individuals and their family members.
• Biological females are represented by circles, and biological males by squares.
• Horizontal lines connect parents, while vertical lines depict parent-child relationships.
• Pedigrees may indicate if family members are alive or deceased.
• Genetic counselors develop pedigree analyses to help with condition concerns and risk assessments.
• The first person seeking medical help for a condition in their family is called the proband.
• Counselors examine family health history to identify individuals with the condition or potential carriers.
• A carrier has one copy of the gene for a genetic condition but does not develop the condition due to another gene.
• Carriers can pass on the condition to their children.

Modes of Inheritance

• Genetic conditions are inherited in different patterns.
• Gregor Mendel's experiments with pea plants revealed rules of inheritance applicable to other species, including humans.
• Traits can be dominant or recessive.
• Dominant traits mask recessive traits when present.
• Individuals inherit two gene copies, one from each parent.
• Homozygous individuals inherit two identical gene copies.
• Heterozygous individuals inherit one dominant and one recessive gene.
• Heterozygous individuals can be carriers for certain genetic conditions.

Autosomal Dominant Traits

• Autosomal dominant traits occur when someone has a single copy of the dominant gene (genes not related to sex chromosomes).
• Huntington's disease is an autosomal dominant condition affecting neurons, impacting movement, memory, and personality.
• People develop Huntington's if they inherit the gene from one parent
• Homozygous dominant and heterozygous individuals will develop the disease.
• Individuals need two copies of the recessive gene to avoid developing the condition.
• A person with Huntington's having children with someone without the gene gives their children a 50% chance of inheriting the disease (assuming the affected parent is heterozygous).
• If both parents have the disease, their children have a 75% chance of inheriting it.
• If neither parent has the disease, their children will not have the disease.
• There is no carrier status for autosomal dominant traits.

Autosomal Recessive Traits

• Autosomal recessive traits occur when an individual has two copies of the recessive gene, with no dominant gene present.
• Individuals can display an autosomal recessive trait even if their parents don't, if the parents are heterozygous.
• Heterozygous parents do not show the trait but are carriers for the recessive gene.
• Cystic fibrosis, Tay-Sachs, and sickle cell anemia are examples of autosomal recessive traits.
• Prenatal genetic testing often screens for these conditions to identify carrier parents.
• If both parents have cystic fibrosis, their children will also have it.
• If one parent has cystic fibrosis and the other is not a carrier, none of their children will have cystic fibrosis.
• If one parent is a carrier and the other has cystic fibrosis, their children have a 50% chance of having cystic fibrosis.
• If both parents are carriers, their children have a 25% chance of being unaffected and non-carriers, a 50% chance of being carriers, and a 25% chance of having cystic fibrosis.

Sex-Linked Traits

• Autosomal traits focus non-sex chromosomes
• X and Y chromosomes determine biological sex.
• Individuals receive an X chromosome from their mother and either an X or Y chromosome from their father.
• Sex-linked traits are found on the X chromosome.
• Color-blindness and hemophilia are common sex-linked conditions.
• Biological males are more likely to be affected by sex-linked recessive conditions because they only have one X chromosome.
• Biological females need two copies of the gene to be affected and can be carriers if heterozygous.
• Males cannot be carriers of sex-linked traits.
• Sex-linked traits can exhibit criss-cross inheritance, where a condition switches between parents and their male or female children.
• A mother with a sex-linked condition will always have a son with the condition.
• A color-blind father's daughters can be either color-blind or carriers depending on the gene they get from their mother.
• A father's X chromosome genes do not affect his sons since they inherit his Y chromosome.

Pedigree Analysis Details

• Pedigrees offer insight into a condition's inheritance pattern.
• Pedigrees show if parents, grandparents, siblings, or children have the same condition.
• They include multiple generations of family members if health information is available.
• The oldest generation is at the top, labeled as the first generation with Roman numeral I.
• Each subsequent generation is labeled with the next Roman numeral.
• Roman numerals label each individual for easy identification.
• The proband will be indicated in the diagram.
• Individuals with the trait are shown as filled-in symbols.
• Known carriers are shown with half-filled symbols.
• Unaffected individuals have outlined symbols.
• Square symbols represent Males, circle symbols represent females
• The ratio of males and females affected can give an indication whether the trait is sex linked

Reading a Pedigree

• If two unaffected individuals have unaffected children, it shows an autosomal dominant trait.
• One affected parent typically has a 50% chance of having affected children (in autosomal dominant traits).
• Two affected parents will have all affected children (in autosomal dominant traits), but the severity can vary
• There is no carrier status in autosomal dominant traits; at least one parent must have the trait for offspring to inherit it.
• If two affected individuals have children, they will always be affected by an autosomal recessive trait.
• Unaffected individuals can have affected or unaffected children, depending on whether they are carriers (in autosomal recessive traits).

Identifying Carriers

• Pedigrees can help identify carriers of sex-linked and autosomal recessive traits.
• In both cases, the recessive trait can be masked by a second gene, leaving the individual unaffected but able to pass it on.
• If unaffected parents have affected offspring, it indicates an autosomal recessive trait.
• A father needs to have the trait for a daughter to inherit it, with autosomal recessive traits.
• Carriers can be hidden in generations but reappear if a grandchild has the trait, or earlier in the pedigree if a parent has the trait.
• If an affected parent and an unaffected parent have affected children, the unaffected parent is a carrier (in autosomal recessive traits).
• For sex-linked traits, only females can be carriers, with their second X chromosome masking the recessive trait.
• If there are affected male children, unaffected female children, and unaffected parents, it is likely a sex-linked trait, and the mother is a carrier.
• A sex-linked trait is especially likely if the mother's father has the trait.

Explore how pedigree charts are visual tools used to trace genetic traits through families. Understand their role in identifying carriers and inheritance patterns. Learn about the symbols and how genetic counselors use them.