Pedigree Analysis

Pedigree

Help us visualize modes of inheritance. In a pedigree, we consider complete penetrance and narrow expressivity.

Modes of inheritance

How traits are passed between generations. Help us predict the chances that an individual will have a disease or be a carrier of a particular allele.

Penetrance

How much or how little a certain trait manifests itself in individuals.

Complete penetrance vs incomplete

All see the disease vs some may see the disease while others won’t.

Variable expressivity

When all individuals may have the same genotype, but their phenotypes may vary

Autosomal recessive

Heterozygous have a recessive allele but they only exhibit their dominant and unaffected allele/phenotype. Another way to say this is that if a heterozygote does not show disease, then we have autosomal recessive.

• if both parents express the recessive, then 100% of the children will also express the recessive

• Relatively equal numbers of affected males and females

• ¼ probability of affected offspring

Autosomal dominant

Heterozygotes exhibit the recessive phenotype

• affected children have at least one affected parent

• Unaffected parents do not produce affected children

• ½ of children are affected if one parent is affected

• Equal numbers of affected males and females

X linked recessive

• In a heterozygous individual, the dominant allele leads to an unaffected individual

• All sons of affected females will be affected

• Affected daughters will have affected fathers

• More affected males than females

• Can skip generations

X linked dominant

In a heterozygous individual, the dominant allele leads to an affected individual

All daughters of affected males will be affected

Does not skip generations

Expect twice as many affected females as affected males

Y linked dominant

Traits on the Y chromosome are only found in males, never females

The fathers traits are passed to all sons

Dominance is irrelevant: there is only 1 copy of each Y linked gene (hemizygous)

Mitochondrial

Have their own DNA from matrilineal bloodline.

Chi square test is used for what?

To determine if the variation in your data is caused by error or a real statistical significance

How do we use chi square?

1. Set null hypothesis that your value match the expected value

2. Get chi value

3. Get df: phenotypes-1. Example phenotype: yellow vs green peas

4. Determine probability

5. Critical chi> chi fail to reject null. Other way around = reject null.