Biology
Class Notes
Lesson 25 Genetic Disorders and Inheritance
Objective: 3.2.2, 3.2.3
Genetics: The study of traits and patterns of inheritance for the purposes of classification and medical
advancement.
• Involves the use of models and diagrams to document and study heredity:
Ex: Punnet Squares and Pedigree Charts
Inheritance of Traits
Chromosomes are divided into two groups:
 Autosomes: Determine characteristics that do not include gender.
 Sex chromosomes: Determine gender and carry genes for some characteristics.
Sex-linked traits
 Genes for these traits are carried on one or more sex chromosomes.
 Recessive sex-linked traits usually appear only in males.
-Males are XY, if a recessive trait is carried on the X there is no matching gene on the Y to
be dominant.
-Females are XX, they have a second X that may carry a dominant allele for the trait.
Ex: Color blindness is a recessive sex-linked trait.
 Females must get two recessive genes (one on both X’s) to be color blind.
 Males only need one recessive gene to be color blind.
Pedigrees
• A family record that shows which members inherit a specific trait over generations
• Used to study the transmission of sex-linked traits
Constructing Pedigrees
Autosomal Dominant Inheritance
1. The presence of a trait in every generation indicates dominance.
2. Roughly equal numbers of affected males and females indicates autosomal inheritance.
X-Linked Dominant Inheritance
1. The presence of the trait in every generation indicates dominance.
2. The presence of the trait in all daughters of affected males indicates X-linked and dominant inheritance.
(i.e. the father only has one X, the trait is dominant, and he passes that one X to all his daughters).
X-linked Recessive Inheritance
1. When a trait skips a generation, recessive inheritance is indicated.
2. If the trait is found in more males than females, X-linked inheritance is indicated. In other words, boys
only have one X. If they get the affected X from mom, there is no gene on the Y to override it with a
dominant allele.
Problem Set: Pedigree Worksheet
Key Words:
Autosomal Trait
Inherited disease
Pedigree