Biology 225:  Genetics Lecture 9

 

 

Outline:         I.          Introduction

II.             Pedigree Symbols

III.           Pedigree Analysis

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I.              Introduction

 

 

 

 

 

 

 

 

 

II.            Pedigree Symbols

 

III.         Pedigree Analysis

 

A.   Rare AutosomalRecessive Traits

 

 

1.     Sample Pedigree  (Albinism)

2.     Characteristics of Autosomal Recessive Inheritance

 

a.  Two unaffected parents (Aa X Aa) may have affected offspring (aa)

 

 

b.   Two affected parents (aa X aa) have only affected offspring

 

 

c.     Members of both sexes have an equal probability of inheriting the allele from a parent of either sex

 

 

d.    In the case of rare traits, most unaffected individuals (that marry into the family) are not heterozygotes (“carriers”); most will be homozygous for the wildtype allele

 

 

e.     The trait often skips generations

 

 

f.      Some consanguinity may be present within the family

B.    Rare Autosomal Dominant

 

 

 

1.     Sample Pedigree

2.     Characteristics of Autosomal Dominant Inheritance

 

a.     Two unaffected parents cannot have affected children (we will discuss some exceptions to this later)

 

 

 

b.    Two affected (A_ X A_) parents can have unaffected offspring as well as affected offspring.

 

 

 

c.     The trait occurs in every generation (again, we will discuss exceptions to this later)

 

 

 

d.    If the trait is rare, most affected individuals will be heterozygous

 

 

 

e.     Offspring of both sexes are equally likely to be affected, and the mother or father may transmit the trait to offspring of either sex.

 

 

 

 

C.   X-linked recessive

 

 

1.     Sample Pedigree

 

2.     Characteristics of X-linked Recessive Inheritance

 

a.     Usually (if the trait is rare), none of the offspring of an affected male (XhY) are affected, but all of his daughters will be carriers of the mutant allele (XHXh)

 

 

b.    These carrier daughters will transmit the mutant allele to 50% of their sons (statistically), and these sons will exhibit the trait.

 

 

 

c.     None of the sons of an affected male will inherit the allele from his father

 

 

 

d.    An affected woman (XhXh) will transmit the mutant allele to all of her sons (who will be affected), and all of her daughters will become carriers (XHXh), assuming their father does not also have the trait.

 

 

 

e.     More males than females in the pedigree will express the trait.

 

D.   X-linked Dominant

 

1.     Sample pedigree

 

 

 

 

2.     Characteristics of X-linked Dominant Inheritance

 

 

a.     Affected males (XDY) would be expected to have no affected sons and no normal daughters

 

 

b.    Both sons and daughters of an affected, heterozygous female have a 50% chance of being affected

 

 

 

c.     Affected homozygous females transmit the trait to all of their offspring.

 

E.    Y-linked (= Holandric)

 

1.     Sample Pedigree

 

 

 

2.     Characteristics of Y-linked Inheritance

 

a.     The trait should be transmitted directly from affected males to all their sons, but to none of their daughters

 

 

 

 

 

 

b.    No females should exhibit the trait