A Guide to the Use of Pedigree Analysis in the Study of Human Inheritance
One often-used technique to study patterns of human inheritance is to analyze pedigrees. In this context, a pedigree is defined as a pictorial representation of family history, which basically means that it is a family tree which shows the occurrence of (an) inherited trait(s). To investigate the appearance of a certain characteristic, a geneticist will study the family of the affected individual and draw a pedigree. The person where the pedigree initiates is called the proband.
Through the analysis of the inheritance patterns that come to the surface in a pedigree analysis, several possible modes of inheritance are able to be identified. These are:
- Autosomal dominant traits,
- Autosomal recessive traits,
- X-linked dominant traits,
- X-linked recessive traits, and
- Y-linked traits.
Autosomal Dominant Traits
Autosomal (not on the sex chromosomes) dominant traits appear in both sexes with equal frequency and can be passed on to the next generation by both males and females. Since everyone who possesses such a trait must have received the allele from at least one parent, this type of trait does not skip a generation. If an autosomal dominant trait is rare, most affected individuals will be heterozygous.
One example of an autosomal dominant trait is familial hypercholesterolemia, a condition where the blood cholesterol is greatly increased due to an effect in cholesterol transport.
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Autosomal Recessive Traits
Autosomal recessive traits also appear at equal frequency in both sexes. These traits, however, only appear when the affected individual has received one allele from each parent. If the trait is relatively uncommon, most people carrying it will be heterozygous and consequently unaffected. Due to this phenomenon, autosomal recessive traits sometimes appear to skip generations. It often happens that a recessive allele is passed for several generations without causing the trait.
A well-known example of a condition cause by autosomal recessive alleles is Tay-Sachs disease, a metabolic disorder resulting in the swelling of the brain and subsequently causing neurological disorders, leading to an early death.
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X-Linked Dominant Traits
X-linked dominant traits occur in both sexes, but affect more females than males. This is a result of the fact that women receive an X-chromosome from both their father and mother, whereas men only receive an X-chromosome from their mother. This type of trait does not skip a generation, as the mother potentially passes it on to all her children, and the father to all his daughters.
Hypophosphatemia, a bone-deforming condition resulting from defective transport of phosphate is an example of such an X-linked dominant trait.
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X-Linked Recessive Traits
X-linked recessive traits have a quite distinct pattern of inheritance. Firstly, they are more frequent in males, since they need only one allele to express the trait. Secondly, these traits are not passed on from father to son, as the son inherits the Y-chromosome from his father and not the X-chromosome. As the trait is passed on from unaffected mother to affected son to unaffected granddaughter, the trait tends to skip generations.
Hemophilia A, or classical hemophilia is an example of a condition that is inherited in this manner. This condition is characterized by the absence of a protein required to make blood clot.
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Y-Linked Traits
Y-linked traits show an easily recognizable pattern of inheritance. Only males are affected and the trait is passed on from father to son. These traits do not skip generations and when a man is affected, so are all his sons.
As few genes reside in the Y-chromosome, few human traits are Y-linked. A supposed example of an Y-linked traits are hairy ears.
Image Sources
All images: https://www.migeneticsconnection.org
References
- Maynard Smith, J. (1999) Evolutionary Genetics. Second Edition. Oxford University Press.
- Pierce, B.A. (2002) Genetics: A Conceptual Approach. First edition. W.H.Freeman Publishing.
