Analysing a Pedigree
By observing the above pedigree, we are able to determine which members of the family have been affected by albinism. The first observation that can be made is that the skin pigmentation characteristic is autosomal recessive. This is identifiable due to the fact that the allele for albinism has been masked by the allele for normal pigmentation and hence skipped the second generation of the family. As the parents in the second generation produced two children with albinism, a recessive trait, they are identified as being heterozygous.
The term heterozygous refers to the situation where a person posses different alleles for a particular characteristic (In this case, skin pigment). This observation therefore states that at least one of the parents (in generation 2) must have also been heterozygous due to the fact that the recessive trait for albinism was passed on to the third and fourth generations. Single gene inheritanceSingle gene inheritance is commonly referred to as Mendelian inheritance as it follows the patterns observed by Mendel during his research on pea plants. There are four types of Mendelian Inheritance patterns:
- Autosomal dominant - Autosomal recessive - X-linked dominant - X-linked recessive Definitions: - Autosomal: The gene responsible for the phenotype is located on one of the 22 pairs of autosome chromosomes (chromosomes that are not sex related) - X-linked: A gene located on the X chromosome. An X-linked disorder is associated with or caused by a gene on the X chromosome. |
What is a Pedigree?
A pedigree is a family tree that displays the characteristics of the members of a family. A pedigree is often constructed using symbols to represent different family members. For example, a square represent males, a circle represents females and coloured in symbols represent a male/female with the particular characteristic under study. Through the use of pedigrees, scientists are able to identify hundreds of human traits that are inherited by a single pair of genes. By constructing a pedigree, people are able to observe the pattern of inheritance within a family and how these traits are passed on through generations.
A table that displays the various symbols used in a pedigree are accessible below By Observing a pedigree, we are able to identify whether the trait(s) displayed are/is recessive or dominant. A recessive trait will not be seen in parents but will be seen in their children whilst a dominant trait will be seen in both parents but not in their children |
Autosomal Dominant
In an autosomal dominant trait, individuals posses just one copy of the mutant allele. Both males and females have an equal probability of passing the trait on to offspring. As affected individuals posses one mutated allele and one normal allele, their offspring have a 50% change of inheriting the condition. An autosomal dominant disorder is very likely to be passed on and most likely will not skip generations like a recessive disorder does.The symptoms of this disorder do not usually appear before about the age of 40. Symptoms of autosomal dominant disorder include dementia, abnormal movements of the hands and difficulty making voluntary movements of limbs. An example of an autosomal dominant disorder is Huntington's disease.
Autosomal Recessive
In an autosomal recessive trait, individuals posses two copies of the mutant allele, one copy from each parent. An affected individual usually has two unaffected parents. Someone who carries a recessive allele but not show the recessive phenotype is termed a carrier. Autosomal recessive disorders are not usually seen in every generation and therefore frequently skip generations. If both parents carry a recessive allele for the same characteristic, each of their offspring have a 25% chance of inheriting the trait but also have a 25% to not inherit it. In addition, the offspring also have a 50% change to inherit one of the abnormal alleles but still be unaffected (they become carriers)