In XLR diseases, a modified inheritance pattern called 'pseudodominance' occurs when
Ans. b (One parent is unaffected heterozygote and another is affected homozygote). (Fig. Harrison 18th/61/l 1)In most instances of AR inheritance, an affected individual is the offspring of heterozygous parents. In this situation, there is a 25% chance that the offspring will have a normal genotype, a 50% probability of a heterozygous state, and a 25% risk of homozygosity for the recessive alleles, in the case of one unaffected heterozygous and one affected homozygous parent, the probability of disease increases to 50% for each child. In this instance, the pedigree analysis mimics an autosomal dominant mode of inheritance (pseudodominance). In contrast to autosomal dominant disorders, new mutations in recessive alleles are rarely manifest because they usually result in an asymptomatic carrier state.PSEUDODOMINANCE# The sudden appearance of a recessive phenotype in a pedigree, due to deletion of a masking dominant gene.# The phenomenon in which a recessive allele shows itself in the phenotype when only one copy of the allele is present, as in hemizygous alleles or in deletion heterozygotes.# An autosomal recessive condition present in individuals in two or more generations of a family, thereby appearing to follow a dominant inheritance patterns.Common explanations include:# A high carrier frequency.# Birth of an affected child to an affected individual and a genetically related (consanguinous) reproductive partner.# In populations with high frequencies of heterozygotes for autosomal recessive conditions, such as in geographically isolated communities, the mating of a homozygote with a heterozygote may result in an inheritance pattern mimicking dominant inheritance (pseudodominance).Examples of diseases with pseudodominance# Storgordt disease (caused by ABCR mutations)# Pseudoxanthoma elasticum (is caused by mutations in the ABCC6 gene, inherited in an autosomal recessive manner and presence of disease in two generations is due to pseudodominance.)# Glycogen storage disease type VII (Tarui's) - it is autosomal recessive, although a few cases with pseudodominance or sumptomatic heterozygous individuals have been found.
One parent is affected homozygote and another is unaffected homozygote
One parent is unaffected heterozygote and another is affected homozygote
One parent is normal and another is unaffected het- erozygote
Both parents are unaffected