Bio - pedigrees
Genetic Inheritance Basics
Symbols in Pedigrees:
Males are consistently represented by squares.
Females are consistently represented by circles.
Parental couples are connected by a horizontal line, signifying a mating or marriage.
Offspring (progeny) extend downwards from the parental line, each connected to a horizontal sibship line. This line then connects to individuals, indicating their birth order or sibling relationship.
Example: If two parents have three children, regardless of their biological capacity for more, we only depict the three children observed in the family tree.
Understanding Shading in Pedigrees
Shading Significance:
Fully shaded symbols (filled squares or circles) unequivocally indicate an individual who is affected by the specific trait or disease being studied. This means the individual expresses the phenotype.
Example: A fully shaded circle or square in a pedigree chart immediately signifies an individual expressing the trait or disease.
Half-shaded symbols are often used to indicate carriers of a recessive allele; these individuals are typically unaffected phenotypically but possess one copy of the disease allele (heterozygous).
Autosomal Recessive Inheritance
Definition:
A genetic trait or disease where the responsible allele is located on an autosome (any non-sex chromosome, chromosomes 1-22 in humans) and is recessive. This means two copies of the recessive allele are required for the trait to be expressed.
Allele Representation:
By convention, the lowercase 'a' (e.g., ) represents the disease-causing recessive allele.
The uppercase 'A' (e.g., ) represents the normal or dominant allele.
Key Points:
Males and females are statistically equally affected by autosomal recessive traits since the gene is on an autosome, not a sex chromosome.
The disease often appears to