Pedigrees are Not Just a Brand of Dog Food
Bob Bobby and Jen Jenny are having a baby. Bob Bobby and Jen Jenny each have a sister with cystic fibrosis, a genetic disease that causes the lungs to fill with mucus. Cystic fibrosis is an autosomal recessive disorder; therefore, two people without the disease can be carriers for the trait. The couple makes an appointment to see a genetic counselor who will make a genetic pedigree to determine if they will have a child with cystic fibrosis.
Wait, did you say pedigree?
Yes, I did.
Isn’t that dog food?
Um, yes, but I am talking about a genetic pedigree.
Did you know that you can survive by eating just dog food? I did it when I was lost in a Macy’s for three months.
Wait. What?
It pares well with apple juice.
Anyway, a genetic pedigree is a diagram that traces the appearance of certain single-gene traits through generations. A few key elements of a pedigree need to be defined before solving it:
Autosomal Recessive Traits
A few rules about autosomal recessive disorders:
- Recessive traits can skip generations.
- If a child has the trait, but neither parent has it, then the parents are carriers of the trait (heterozygous).
The genetic counselor applies three generations of Bob Bobby’s `and Jen Jenny’s ancestry into a pedigree. The filled-in individuals have cystic fibrosis, which is an autosomal recessive disorder.
Albinism is an autosomal recessive disorder that prevents the production of the pigment called melanin. Without melanin, a person’s skin, hair, and iris’ will be colorless.
1. How many generations are seen in the pedigree above?
2. How many children did the first generation (I-1 and I-2) have?
3. Complete the genotypes for all the individuals in the pedigree.
4. How many are known carriers of albinism?
Autosomal Dominant Disorders
A few Rules about dominant traits
- Individuals with the genotypes AA or Aa will have the trait, while homozygous recessive (aa) individuals will not.
- There are no carriers of a dominant trait because heterozygous individuals (Aa) have the trait.
- Dominant traits CANNOT skip generations. If neither parent has the trait, then none of their offspring will get the trait
Huntington’s Disorder is an autosomal dominant disease where part of the brain starts to degrade. This is a dominant disorder only one allele inherited means the individual will have the disease.
Recessive Sex-Linked Traits
A few rules about recessive sex-linked traits:
- Males will have the trait more often than females
- Males inherit the trait from their mothers.
- Only females can be carriers of the recessive allele.
Red-Green colorblindness is a recessive sex-linked trait.
Summary
- Autosomal traits affect males and females equally.
- Autosomal recessive traits can skip generations, and both males and females can be carriers of the trait.
- Solve for the individuals without the trait (not shaded in)
- Autosomal dominant traits cannot skip generations, and there are no carriers of the trait.
- Solve for the individuals with the trait (shaded in)
- Recessive sex-liked traits primarily affect males because they only need one allele to get the trait.
- Can skip generations.
- Only females are carriers of the trait (heterozygous).
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