The record of occurrence of a trait in several generations of a human family is called pedigree analysis. The inheritance of a particular trait is represented in the family tree over generations.
Mendelian disorders are mainly determined by alteration or mutation in the single gene. Mendelian disorders can be traced in a family by the pedigree analysis.
Most common and prevalent Mendelian disorders are :
- Cystic fibrosis
- Sickle-cell anaemia
- Colour blindness
This sex linked recessive disease, which shows its transmission from unaffected carrier female to some of the male progeny has been widely studied. In this disease, a single protein that is a part of the cascade of proteins involved in the clotting of blood is affected. Due to this, in an affected individual a simple cut will result in non- stop bleeding.
The heterozygous female (carrier) for haemophilia may transmit the disease to sons. The possibility of a female becoming a haemophilic is extremely rare because mother of such a female has to be at last carrier and the father should be haemophilic.
This is an autosome linked recessive trait that can be transmitted from parents to the offspring when both the partners are carrier for the gene(or heterozygous). Heterozygous individuals appear apparently unaffected but they are carrier of the disease as there is 50 per cent probability of transmission of the mutant gene to the progeny, thus exhibiting sickle-cell trait.
The defect is caused by the substitution of Glutamic acid by Valine at the sixth position of the beta globin chain of the haemoglobin molecule.
This inborn error of metabolism is also inherited as the autosomal recessive trait.the affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives.
Accumulation of these in brain results in mental retardation. These are also excreted through urine because of its poor absorption of kidney.
The chromosomal disorders on the other hand are caused due to absence or excess or abnormal arrangement of one or more chromosomes. Failure of segregation of chromatids during cell division cycle results in the rain or loss of a chromosome, called Aneuploidy. Failure of cytokinesis after telophase stage of cell division results in an increase in a whole set of chromosomes in an organism and this phenomenon is known as Polyploidy.
The cause of this genetic disorders is the presence of an additional copy of the chromosome number 21.
The affected individual is short statured with small round head, furrowed tongue and partially open mouth. Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.
This genetic disorder is also caused due to the presence of an additional copy of X-chromosome resulting into a karyotype of 47, XXY. Such an individual has overall masculine development, however, the feminine development (development of breast) is also expressed. Such individuals are sterile.
Such a disorder is caused due to the absence of one of the X chromosomes, i.e., 45 with X0. Such females are sterile as ovaries are rudimentary besides other features including lack of other secondary sexual characters.