Using Pedigrees to interpret Mode of Inheritance

Objectives

• Use a pedigree to interpret the mode of inheritance the given trait is with 90% accuracy.

Copyright Pearson Prentice Hall

11.2 Pedigrees (It’s in your genes)

Pedigree ChartsA pedigree chart shows the relationships within a family similar to a family tree or blood line of animals.

Genetic counselors analyze pedigree charts to infer the genotypes of family members.

Reading a Pedigree

A circle

represents

a female.

A horizontal line

connecting a male and

a female represents a

marriage.

A shaded

circle or

square

indicates that

a person

expresses the

trait.

A square

represents

a male.

A vertical line and a

bracket connect the

parents to their

children.

A circle or square

that is not

shaded indicates

that a person

does not express

the trait.

I

II

III

Roman

numerals

represent the

generation #

Interpreting a Pedigree

• The main question asked when looking at a pedigree is what is the mode of inheritance.

• The first thing to do is to decide whether the trait of interest is dominant or recessive?

• The second step is to decide whether the trait is Autosomal or Sex Linked (aka X-linked)

Interpreting a Pedigree

• Determine whether the disorder is dominant or recessive.

• If the disorder is dominant, one of the parents must have the disorder.

• Usually found in every generation

Interpreting a Pedigree

• If the disorder is recessive, neither parent has to have the disorder because they can be heterozygous.

• Usually (but not always) skips generations

Fast Track to Pedigree Question

1. The recessive trait will be anyone with a phenotype that is different than both of their parents

2. It is autosomal if any girls with the recessive phenotype have a father OR a son with the dominant phenotype:

If X-linked dominate affected fathers must pass the trait on to all their daughters

Typically in X-linked recessive only males affected

Other Cheats to interpreting a pedigree

• Determine if the pedigree chart shows an autosomal or X-linked disease.

• If most of the males in the pedigree are affected, then the disorder is most likely X-linked

• If it is a 50/50 ratio between men and women the disorder most likely autosomal.

Pedigree PracticeWhat is the mode of inheritance?

X-linked Dominant- every gen. & father to all daughters

Pedigree PracticeWhat is the mode of inheritance?

X-linked recessive -Only males are affected and

sons do not share the phenotype of their father

Pedigree PracticeWhat is the mode of inheritance?

Autosomal Recessive- II-3 girl’s father has dominant

phenotype

Pedigree PracticeWhat is the mode of inheritance?

Autosomal Recessive- II- 6 girl’s father has dominant

phenotype and she had dominant sons

Pedigree PracticeWhat is the mode of inheritance?

Autosomal Dom.- every gen. & in both male & female

Need more information?

• The following slides show the more complete way of assessing pedigrees and interpreting the mode of inheritance.

Autosomal Dominant

• Appears in both sexes with equal frequency

• Both sexes transmit the trait to their offspring

• Does not skip generations

• Affected offspring must have an affected parent unless they posses a new mutation

Rules of Inheritance

Autosomal Dominant

• When one parent is affected (het.) and the other parent is unaffected, approx. 1/2 of the offspring will be affected

• Unaffected parents do not transmit the trait

Rules of Inheritance

Rules of InheritanceAutosomal Recessive

•Appears in both sexes with equal frequency • Trait tend to skip generations

• Affected offspring are usually born to unaffected parents

• When both parents are heterozygous, approx. 1/4 of the progeny will be affected

• Appears more frequently among the children of consanguine (blood) marriages

X-Linked Dominant

• Both males and females are affected; often more females than males are affected • Does not skip generations.

• Affected sons must have an affected mother;

• affected daughters must have either an affected mother or an affected father

Rules of Inheritance

X-Linked Dominant

• Affected fathers will pass the trait on to all their daughters

• Affected mothers if heterozygous will pass the trait on to 1/2 of their sons and 1/2 of their daughters

Rules of Inheritance

X-Linked Recessive

• More males than females are affected

• Affected sons are usually born to unaffected mothers, thus the trait skips generations

• Approximately 1/2 of carrier mothers’ sons are affected

Rules of Inheritance

X-Linked Recessive

• It is never passed from father to son

• All daughters of affected fathers are carriers

Rules of Inheritance