In the pedigree analysis, the inheritance of a particular trait is represented in the family tree over generations. It is useful for the genetic counsellors to advise intending couples about the possibility of having children with genetic defects like haemophilia, colour blindness, phenylketonuria, thalassaemia, sickle cell anaemia (recessive traits), myotonic dystrophy and polydactyly (dominant traits).
STEPS FOR IDENTIFYING THE TYPE OF TRAITS:
a. In the case of a recessive trait, the characteristic condition can appear in the progeny of apparently unaffected parents. Moreover, two affected individuals cannot have an unaffected child, i.e., either parents or children do not show the characteristic/disorder.
b. If both parents and progeny are affected then the trait is of dominant type.
c. If equal number of males and females are affected in a generation then the trait is of autosomal type.
d. If either of the sex is only affected in a generation i.e., only male or female is affected in a generation, then the trait is X-linked.
e. If only males are affected in a family then the trait is Y-linked. It is also called holandric inheritance. The genes are called holandric genes.
In the above pedigree chart given, the offspring is affected (equal number of males and females) but the parents are not affected. From this, it can be interpreted that the trait traced in the above pedigree chart is autosomal recessive.
In the pedigree shown above, offspring is affected and parents are unaffected which means that either one or both parents are a carrier.
It also depicts recessive pattern of the trait. If the trait had been dominant it would have been expressed by individuals who are heterozygous in nature. But in heterozygous individuals, parents serve as carrier only. It cannot be X-linked and Y-linked as none of the parent is affected.
Autosomal recessive disorder is that disorder which is expressed in homozygous condition.
It occurs when a person has defect in both copies of an autosomal gene. This results in 'loss of function'.
If both the copies of the gene have the same deleterious mutation, the defect is termed homozygous.
For example: cystic fibrosis, sickle cell anaemia, etc.
