GAMSAT Section 3: Studying Pedigree Charts

What is a Pedigree Chart?

A Pedigree chart is a diagrammatic presentation exhibiting the biological connections amongst the members of the family. It is a means of studying the inheritance pattern of traits from parents to the offspring. GAMSAT will test your ability to carefully observe and analyse a pedigree and also your prior knowledge of inheritance pattern of different genetic diseases to answer the questions.

What is the Significance of Pedigree Charts?

Pedigree charts are vital in tracking the probability of a disorder in an individual. Also, these charts can help study the nature of the disorder (dominant or recessive). If a disease is dominant, it implies that the mutation is carried in both the alleles of the gene and is clinical manifested and if the disease is recessive, the mutation is carried in only one allele of the gene and its phenotypic expression is dependent on the nature of the other allele. Neither of the parents has to have the disease.

In genetic counselling, pedigree chart mapping is very crucial. By studying the number of infected individuals in a family and their sex, the inheritance pattern of a particular disease in the following generations can be determined. In GAMSAT, a case study can be given as a vignette where you need to understand the pattern of trait inheritance across generations and determine the ratio of affected: unaffected offspring from different mating pairs.

Also, researchers in the field of genetics need to study the transmission trait of a particular disease in a family and then they can compare the affected and unaffected individuals to get an idea of the nature of diseased gene and the genetic factor is responsible for the disease. GAMSAT questions can also be framed to determine the trait of one of the parents by observing the nature of offspring and the disease that is being studied.

What Does a Pedigree Chart Look Like?

A pedigree chart is a collection of different symbols with each one having a significant contribution in analysing a pedigree chart.

Pedigree Chart

An example of a pedigree chart is given below:

Pedigree Chart in gamsat

In the above example, you need to observe the following features:

1. Two mating pairs (I and II) are shown here with a normal (unaffected) female married to an affected male in mating pair I and an affected male married to a normal (unaffected) female in couple II.
2. The three offspring of couple I (encircled by red dotted lines) include one unaffected son and two daughters (one affected and the other unaffected).
3. Mating pair II also has three offspring (encircled by blue dotted lines) of which one is an affected male and the other two are normal (unaffected) daughters.
4. The mating between the unaffected female and affected male (mating pair III) gives rise to an affected daughter.
5. The mating between this affected daughter with a normal (unaffected) male gives rise to two affected daughters.

How to Determine Inheritance Patterns from Pedigree Charts?

1. Autosomal dominant trait:

 

Here, you see that the male is diseased. So, this is an X-linked trait.Next, you observe that approx. half of the offspring are diseased which implies that the disease is autosomal (defect in the non-sex chromosome) dominant in nature.Both the affected males and females have equal probability of getting the disease and transmitting the trait to the next generation.Since the disease is dominant, it can be manifested in an individual even if only one defective allele is inherited.

Example: Huntington’s disease

For answering GAMSAT questions here, you need to know that autosomal diseases are not sex-linked diseases and that the defect or mutation is in the autosome and not the sex-chromosome.

2. Autosomal recessive trait:

 

Here, you see that the female is not diseased but is the carrier.The half black colour of the circle indicates that only one of the two alleles is defective. So, this is a recessive trait and no clinical manifestation is found.Next, you observe that both the males and females have equal chances of being the carrier of the disease. This implies that the trait is autosomal in nature.

Example: Phenyl ketonuria (PKU)

For answering GAMSAT questions here, you need to have a clear concept of recessive traits and its impact on males and females. Males will always have a higher probability of clinical manifestation of such diseases than females.

3. X-linked recessive trait:

 

Here, you see that the female is not diseased but is the carrier. X-linked traits are clinically manifested in males as they have only one X chromosome. Females are generally the carriers of the disease as only one of the allelic pairs is mutant. Manifestation is possible only if both the alleles are mutant/defective.

Example: Phenylketonuria (PKU)

GAMSAT questions here will test your prior knowledge of X linked disease inheritance pattern.

4. X-linked dominant trait:

 

Here, you see that the male is diseased. X-linked traits are clinically manifested in males as they have only one X chromosome. Females with one mutant allele are also diseased and not carriers as the trait is dominant and only one infected allele is required for the clinical manifestation of the disease.

Example: Fragile X-syndrome

Answering GAMSAT questions here will test your concept of dominant traits and how they differ from recessive ones.

Probable GAMSAT Style Questions that can be Asked:

1. Determine the trait of the offspring by studying the trait of the parents.
2. Identify the trait of the parent by observing the traits of the offspring.
3. What are the probable combinations of mating pairs that can give rise to an offspring with a particular trait?
4. Determine the ratio of diseased:normal individuals in different generations.
5. Determine the nature of the trait and the pattern of inheritance.

Prior Knowledge Required for Answering GAMSAT Questions?

1. Sex-genes (X and Y)-linked diseases and autosomal diseases
2. The differences between dominant and recessive traits and their effects on the offspring
3. Nature of genetic diseases and the ratio of males and females affected in them should be understood
4. Types of autosomal diseases and their inheritance pattern

video source: youtube.com

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