How to Analyze Autosomal Recessive Pedigree Trait?

In order to analyze this pedigree, we have to first check whether it is dominant or recessive in nature.

The criteria for dominant pedigree trait is:

a. There should not be skipping of generation
b. Either or both the parents should be affected to pass a disease to the next generation. 

Note: when two normal parents transfer a disease to the offspring, it is a recessive characteristic.

Now, when you see the above pedigree, you can see that in the second generation, two parents 8 and 9 are normal, but passing a disease to their daughter (17) in the third generation. So, you can say that the pedigree is recessive.

The second step is to check whether it is autosomal recessive or X-linked recessive. The best way is to first check for X-linked. If it is X-linked, it should follow two conditions:

a. More boys affected than girls
b. Affected mom passes the disease to all her sons

You can see that in the first generation, an affected mom does not pass disease to all her sons, so, it is not an X-linked trait.

So, we can conclude that the above pedigree shows autosomal recessive trait.

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PEDIGREE ANALYSIS

There are four different types of inheritance that are considered in pedigree analysis. These are mentioned below:

• Autosomal Dominant

(Examples: Polycystic kidney disease, Huntington's disease, hereditary spherocytosis)

• Autosomal Recessive

(Examples: Sickle cell anemia,cystic fibrosis, Tay-Sachs disease)

• X-linked Dominant

(Example: Rett syndrome, Vitamin-D resistant rickets, Alport syndrome)

• X-linked Recessive

(Example: hemophilia, color blindness)


Pedigree Symbols:

Tips to solving Pedigree Problems:

Autosomal dominant

- when father transfers disease to his son -- it is always an autosomal trait. 
- affected parents can have unaffected children.

- Pedigrees of autosomal dominant disorders show affected males and females in each generation and also show affected men and women transmitting the condition to equal proportions of their sons and daughters.

Autosomal recessive:

- there is skipping of generation 
- unaffected parents can gave affected children 

X-linked dominant:

- the disorder never tranfers from father to son
- it follows DDD rule (D= dominant, D= diseased dad, D= diseased daughters). It means, if father is affected, all daughters will be affected.

X-linked recessive: 

- males are more affected than females
- disease tends to tranfer from mother to son and father to daughter
- disease never tranfers from father to son


If both parents are shown healthy -- it is often a recessive trait 

When one or both the parents are shown as diseased -- it is often a dominant trait 

Now solve the following problem:

-- Since this is the first problem, we would check for all the 4 traits:

1. Autosomal dominant
2. Autosomal recessive
3. X-linked dominant
4. X-linked recessive

A. Let's check for the autosomal dominant trait. 

In the picture shown above, there are 4 generations. We are considering whether the mode is autosomal dominant or not. We choose 'A' as dominant allele and 'a' as recessive allele. We are only concerned with autosomes and not with sex chromosomes.

Generation I: Mom is diseases and dad is healthy. So the allele for mom could be 'Aa' or 'AA', but we consider 'Aa'. For dad, we take recessive alleles 'aa'.

Generation II: The offspring is a healthy daughter. So, her genotype would be 'aa' -- one 'a' comes from mom and one 'a' comes from dad. She marries a healthy man.

Generation III: there are two sons and one daughter. However, one son is diseased and rest others are healthy. Since one son is diseased, his genotype should be 'Aa', which is not possible because his parents possess only 'aa' genotype. That is why, it is not an autosomal dominant trait. 

B. Let's check for the autosomal recessive trait. 

In the above picture, we are checking whether its an autosomal recessive trait or not.

Generation I: Since mom is diseased, so her recessive genotype would be 'aa' and the genotype of dad would be 'Aa'. (For a recessive trait to appear, it has to be homozygous recessive).

Generation II: The offspring is a daughter and she is healthy and therefore, her genotype would be 'Aa' ('A' comes from father and 'a' comes from the mother). She marries a healthy man.

Generation III: there are two sons and one daughter. However, one son is diseased and rest others are healthy. The genotypes of all the offspring in the third generation can be satisfied from their mom and dad. For example, since mom and dad for generation III has a genotype of 'Aa' so, the genotypes of offspring could be 'Aa' and 'aa'.

Generation IV: All the genotypes are possible from their parental cross. And therefore, it is an autosomal recessive trait. 

C. Let's check for the X-linked dominant trait. 

D. Let's check for the X-linked recessive trait. 

Let's discuss another question on Pedigree. Look at the following pedigree:

In the above pedigree, we have to find any one out of the following four given situations:

a) Autosomal Dominant

b) Autosomal recessive

c) X-linked dominant

d) X-linked recessive

If you see the pedigree, there are three generations; however, there is no skipping of disease. It means it is a DOMINANT condition. So, we can discard the recessive conditions. 

Let's check first for X-linked dominant condition. The trick is DDD rule -- the first D represents dominant; the second D represents diseased dad and the third D represents disease daughther. In other words, if the situation is dominant, so a disease dad will transfer disease to all his daughters. This condition is not shown above, it means, it's an AUTOSOMAL DOMINANT trait.

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