Experiment 12: Study of Prepared Pedigree Charts of Genetic Traits ( Rolling of tongue, Blood groups, Widow's peak, and Colour blindness.)

AIM

To study of prepared pedigree charts of genetic traits such as rolling of tongue, blood groups, widow's peak and colour blindness.

PRINCIPLE

Pedigree analysis is study of inheritance of traits through family history over generations. Standard symbols are used to represent male, female, affected individuals and mating patterns to trace mode of inheritance.

Traits may be autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant or Y-linked. Dominant traits appear in every generation and affect both sexes equally, while recessive traits may skip generations and appear in homozygous condition.

Analysis of pedigree charts of widow's peak, rolling of tongue, blood groups and colour blindness helps to determine whether gene is autosomal or sex-linked, dominant or recessive and predicts probability of transmission to offsprings.

REQUIREMENTS

• Questionnaire about a particular disorder
• A family with genetic disorder for more than one generation
• Paper, pencil, etc.
• Prepared pedigree charts of genetic traits

PROCEDURE

1. The careful examination of chart would suggest whether the gene for the character is autosome-linked dominant or recessive.
2. Examine whether the gene is X-chromosome linked dominant or recessive.
3. Examine whether the gene is Y-chromosome linked or not.

OBSERVATION

Standard Pedigree Symbols Observed:

• Square - Male
• Circle - Female
• Diamond - Sex unspecified
• Filled square, circle, diamond - Affected individuals
• Square connected to circle - Mating
• Double line between square and circle - Mating between relatives (consanguineous mating)
• Vertical line with horizontal sibship line - Parents above and children below in order of birth left to right
• Square with affected son below - Parents with male child affected with disease
• Diamond with number 5 - Five unaffected offspring

A. Autosome Linked Dominant Traits

• The encoding gene of this trait is present on any one of the autosomes.
• The mutant allele is dominant and the wild type allele is recessive for such traits.
• In this pedigree chart, the female being interviewed is exhibiting the trait and is indicated by an arrow mark in the chart.
• Transmission of trait occurs from any of the parent.
• Male and female are equally affected. e.g., Widow's Peak.

Widow's Peak

It is a 'V'-shaped hairline across the forehead. It is a dominant trait. Thus, both homozygous dominant (AA) and heterozygous (Aa) individuals have widow's peak, while homozygous recessive (aa) individuals have a straight hairline.

Pedigree of Taylor family shows Taylor (affected male) and Mary (affected female) transmitting trait to Susan (affected), Joseph (affected) and others, indicating autosomal dominant inheritance where affected individuals appear in every generation.

B. Autosome Recessive Traits

• In this, mutant allele is recessive to its wild type allele.
• These traits occur in equal proportions of male and female siblings, whose parents are carriers of the allele, i.e., they have only a single copy of the allele and therefore they are normal.
• The siblings are homozygous for the defective allele, but their parents, though some may appear normal are obviously heterozygous, i.e., are merely carriers of the trait.
• The marriage between man and woman genetically related with each other, occasionally results in the appearance of such traits, e.g., albinism and rolling of tongue.

The Rolling of Tongue

Some persons are able to roll their tongue in U-shape. The inability to roll the tongue is caused by an autosomal recessive allele 'a'. Thus, both homozygous dominant (AA) and heterozygous (Aa) individuals are able to roll the tongue, while homozygous recessive (aa) individuals are unable to roll their tongue in this way.

Pedigree shows normal parents Aa × Aa producing affected child aa, confirming recessive inheritance. Free ear lobe and affected ear lobe symbols differentiate normal and affected individuals.

X-Linked Recessive Traits

• These are the traits whose encoding gene is present on the X-chromosome and its mutant allele is recessive to its wild type allele.
• The females expresses the trait only when they are homozygous for the mutant allele whereas, males do so even when they are hemizygous for it.
• About half of the sons of the carrier (heterozygous for the trait) females are affected.
• In case of homozygous females showing the trait, 50% of her daughters and all of her sons are likely to be affected. Therefore, it can be concluded that males are most affected population in this category.
• Affected persons are related to one another through the maternal side of their family.
• Any evidence of male to male transmission of the trait rules out the X-linked inheritance.
• Examples are haemophilia and red-green colour blindness.

Colour Blindness

The colour blindness is a sex-linked recessive disorder of humans. In this, the affected individual is unable to differentiate between red and green colours. It results in the absence or malfunctioning of one or more of the three types of cone cells responsible for colour vision.

Pedigree analysis of colour blindness shows mother XX normal and father XY normal producing carrier daughters and colour blind sons. Affected males are connected through maternal side and male to male transmission is absent.

BLOOD GROUP INHERITANCE

• Blood group inheritance is independent of sex of organism.
• The ABO blood groups are controlled by gene I which have three alleles I^A, I^B, I^O.
• I^A and I^B are completely dominant over I^O. I^A and I^B are codominant to each other.

Pedigree shows I: AB × O parents producing II: A, B, A, A, A children and III: AB, B, A, O grandchildren, demonstrating autosomal inheritance with multiple alleles.

RESULT

The families studied showed the transmission of traits responsible for genetic diseases in the successive generations in each category. Pedigree charts confirmed autosomal dominant, autosomal recessive and X-linked recessive patterns.

CONCLUSION

Widow's peak is autosomal dominant, rolling of tongue inability and albinism are autosomal recessive, colour blindness and haemophilia are X-linked recessive and ABO blood group shows multiple allelism and codominance. Pedigree analysis helps to predict risk and mode of inheritance of genetic traits.

PRECAUTIONS

• Collect accurate family history for at least three generations using questionnaire.
• Confirm the trait physically and avoid assumption based on verbal information only.
• Use standard pedigree symbols and indicate proband with arrow.
• Draw pedigree with proper order of birth from left to right.