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FREQUENTLY ASKED QUESTIONS
 
  1. What is autosomal disease?

    The offspring of consanguinous relationships are at greater risk of certain genetic disorders. Autosomal recessive disorders occur in individuals who are homozygous for a particular recessive gene mutation. This means that they carry two copies (alleles) of the same gene. Except in certain rare circumstances (new mutations or uniparental disomy) both parents of an individual with such a disorder will be carriers of the gene. Such carriers are not affected and will not display any signs that they are carriers, and so may be unaware that they carry the mutated gene. As relatives share a proportion of their genes, it is much more likely that related parents will be carriers of an autosomal recessive gene, and therefore their children are at a higher risk of an autosomal recessive disorder. The extent to which the risk increases depends on the degree of genetic relationship between the parents; so for incestuous relationships where the parents share 1/2 of their DNA the risk is great, but for relationships between second cousins where the parents only share 1/32 of their DNA the risk is less (although still greater than the general population).

  2. Recessive trait

    The term "recessive allele" refers to an allele that causes a phenotype (visible or detectable characteristic) that is only seen in homozygous genotypes (organisms that have two copies of the same allele) and never in heterozygous genotypes. Every diploid organism, including humans, has two copies of every gene on autosomal chromosomes, one from the mother and one from the father. The dominant allele of a gene will always be expressed while the recessive allele of a gene will be expressed only if the organism has two recessive forms. (Thus, if both parents are carriers of a recessive trait, there is a 25% chance with each child to show the recessive trait.)

    The term "recessive allele" is part of the laws of Mendelian inheritance formulated by Gregor Mendel. Examples of recessive traits in Mendel's famous pea plant experiments include the color and shape of seed pods and plant height.

    Autosomal recessive allele

                              

    Relationship between two carrier parents and probabilities of children being unaffected, carriers, or affected.

    Autosomal recessive is a mode of inheritance of genetic traits located on the autosomes (the pairs of non-sex determining chromosomes - in humans 22).

    In opposition to autosomal dominant trait, a recessive trait only becomes phenotypically apparent when two similar alleles of a gene are present. In other words, the subject is homozygous for the trait.

    The frequency of the carrier state can be calculated by the Hardy-Weinberg formula: p2 + 2pq + q2 = 1 (p is the frequency of one pair of alleles, and q = 1 - p is the frequency of the other pair of alleles.)

    Recessive genetic disorders occur when both parents are carriers and each contributes an allele to the embryo, meaning these are not dominant genes. As both parents are heterozygous for the disorder, the chance of two disease alleles landing in one of their offspring is 25% (in autosomal dominant traits this is higher). 50% of the children (or 2/3 of the remaining ones) are carriers. When one of the parents is homozygous the trait will only show in his/her offspring if the other parent is also a carrier. In that case, the chance of disease in the offspring is 50%.

  3. Nomenclature of recessiveness

    Technically, the term "recessive gene" is imprecise because it is not the gene that is recessive but the phenotype (or trait). It should also be noted that the concepts of recessiveness and dominance were developed before a molecular understanding of DNA and before molecular biology, thus mapping many newer concepts to "dominant" or "recessive" phenotypes is problematic. Many traits previously thought to be recessive have mild forms or biochemical abnormalities that arise from the presence of the one copy of the allele. This suggests that the dominant phenotype is dependent upon having two dominant alleles, and the presence of one dominant and one recessive allele creates some blending of both dominant and recessive traits.

  4. What to do?

    Genetic counseling. The job of a genetic counselor is to take a careful family history and counsel couples on their risk based on their history.



 
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