Genetics: Inheritance Patterns and Pedigree Analysis
Autosomal Recessive Traits:
When both parents are heterozygous, approximately 1/4 of the offspring will be affected.
Appears more frequently among children of consanguineous marriages.
Autosomal Dominant Traits:
Appears equally in both sexes with equal frequency.
Both sexes transmit the trait to their offspring.
Does not skip generations, meaning affected offspring must have an affected parent unless a new mutation occurs.
When one parent is affected and heterozygous and the other parent is unaffected, approximately half of the offspring will be affected.
Unaffected parents do not transmit the trait.
X-linked Recessive Traits:
Typically, more males than females are affected.
Affected sons are born to unaffected mothers, indicating the trait can skip generations.
Approximately half of a carrier mother's sons are affected.
Never passed from father to son.
All daughters of affected fathers are carriers.
X-linked Dominant Traits:
Both males and females are usually affected, often with more females affected than males.
Does not skip generations.
Affected sons must have an affected mother.
Affected daughters must have an affected mother or an affected father.
Affected fathers pass the trait on to all of their daughters.
Affected mothers, if heterozygous, will pass the trait on to half their sons or half of their daughters.
Y-linked Traits:
Only males are affected.
Passed from father to all sons.
Does not skip generations.
Pedigree Analysis Practice:
Pedigree A:
Considered autosomal because both sexes are affected.
Initial attempt with recessive (little 'a' little 'a' for parents) didn't work as affected parents would pass it on.
Concluded as autosomal dominant (Big A little a, Big A little a for parents) because their offspring could be affected (little a little a) and this pattern is consistent with unaffected parents producing affected offspring only if a dominant trait with carriers.
Pedigree B:
Identified as sex-linked.
Concluded as Y-linked due to transmission exclusively from father to son.
Pedigree C:
Identified as autosomal because both a male and a female are affected.
Concluded as autosomal dominant.
Pedigree D:
Only one affected person (female).
Considered autosomal recessive (Big A little a for unaffected parents, little a little a for affected daughter).
Ruled out X-linked recessive because the father is unaffected.
Ruled out X-linked dominant because both parents are unaffected.
Pedigree E:
Males and females equally affected, suggesting autosomal or X-linked.
Attempted autosomal recessive: Possible if affected individuals are little a little a, and their unaffected parents are Big A little a.
Attempted X-linked: Possible if affected individual is XaY and unaffected mother is X+Xa. Also, an affected female could be XaX_a.
Multiple possibilities: Autosomal dominant, autosomal recessive, or X-linked dominant.
Pedigree F:
Could be X-linked recessive, X-linked dominant, autosomal recessive, or autosomal dominant.
Illustrates that small pedigrees can have multiple possible modes of inheritance.
Pedigree G:
Most likely autosomal.
Has to be recessive because it skipped several generations.
Pedigree H:
Recessive because it's skipping generations.
Could be autosomal or sex-linked recessive.
Pedigree I:
Most likely autosomal dominant because the trait does not skip generations.
Every generation with an affected parent has affected offspring.
Every generation without an affected parent does not have affected offspring.
Autosomal recessive is also a possibility, but dominant is more likely given the pattern.
Lab Instructions - Pedigree Analysis:
Purpose: To trace traits in families, record expression, and illustrate relationships and transmission over generations.
Pedigree Diagram Conventions:
Roman numerals (I, II, III…) indicate each generation.
Arabic numbers (1, 2, 3…) indicate members within a generation.
Females are represented by circles (\text{O}).
Males are represented by squares (\square).
Individuals demonstrating the trait are shaded black.
Task 1: Determine the mode of transmission (dominant/recessive, autosomal/sex-linked).
Task 2: Analyze specific pedigrees (two and three) for dominant/recessive and autosomal/sex-linked likelihood, then determine genotypes of specific individuals.
Task 3 (Questions 4, 5, 6): Assume the trait is autosomal and rare, meaning only individuals from the initial family lineage (e.g., family 1 and 2) will have it; those who marry into the family (e.g., individual 3) or from other lineages will not.
Submission: Only the two lab report pages need to be turned in.
Upcoming Exam and Logistics:
Next exam is scheduled for Wednesday, October 8th (one week from the class day).
Chapter 7 will be covered this week, followed by a review on Monday before the exam.
The previous lab can be submitted today (in a box for on-campus students), tomorrow, or Wednesday.