Cystic fibrosis is associated with recessive mutations in the CFTR gene, whereas sickle-cell anemia is associated with recessive mutations in the beta hemoglobin HBB gene. Interestingly, although individuals homozygous for the mutant HBB gene suffer from sickle-cell anemia, heterozygous carriers are resistant to malaria.
This fact explains the higher frequency of sickle-cell anemia in today's African Americans, who are descendants of a group that had an advantage against endemic malaria if they carried HBB mutations. Finally, oculocutaneous albinism is associated with autosomal recessive mutations in the OCA2 gene. This gene is involved in biosynthesis of the pigment melanin, which gives color to a person's hair, skin, and eyes. Autosomal dominant single-gene diseases occur in individuals who have a single mutant copy of the disease-associated gene.
In this case, the presence of a single nonmutant or "wild-type" copy of the gene is not enough to prevent the disease. Individuals can inherit the mutant copy of the disease-associated gene from either an affected mother or an affected father. Huntington's disease , a progressive neurodegenerative disorder, is a well-known example of an autosomal dominant single-gene disease; most individuals with a single copy of the mutant huntingtin gene HTT will have Huntington's disease later in life.
Typically, autosomal dominant diseases affect individuals in their early years and prevent them from living past infancy or childhood, which in turn precludes these individuals from reproducing and potentially passing on the mutation to their offspring.
In the case of Huntington's disease, however, the late onset of the disorder means that many affected individuals have already had children before they are even aware that they carry the mutation.
Disease-associated changes in the huntingtin gene consist of a special type of mutation called triplet repeats; these mutations are simply extra repetitions of the three-base DNA sequence CAG. The number of CAG repeats in a mutated huntingtin gene determines the age at which a person will develop Huntington's disease, as well as how severe the condition will be.
Genetic tests can be used to determine how many CAG repeats are in an individual's huntingtin gene, thereby providing a highly accurate assessment of the individual's disease risk. Genetic testing can either provide immediate relief in knowing that one is free from the disease, or the confirmation that one will certainly suffer from the condition at some point in the future.
Myotonic dystrophy, familial hypercholesterolemia, neurofibromatosis , and polycystic kidney disease serve as additional examples of autosomal dominant single-gene diseases. Myotonic dystrophy is associated with dominant mutations in the dystrophia myotonica protein kinase DMPK gene; familial hypercholesterolemia is associated with dominant mutations in both the low-density lipoprotein receptor LDLR gene and the apolipoprotein B APOB gene; and neurofibromatosis is associated with dominant mutations in the neurofibromin NF1 gene.
Autosomal dominant polycystic kidney disease can be caused by mutations in either the polycystic kidney disease 1 PKD1 gene or the polycystic kidney disease 2 PKD2 gene; the PKD1 gene is located on human chromosome 16, whereas the PKD2 gene is located on human chromosome 4.
Table 1. Single-gene diseases that involve genes found on the sex chromosomes have somewhat different inheritance patterns than those that involve genes found on a person's autosomes.
The reason for these differences lies in the genetic distinction between males and females. Recall that females have two copies of the X chromosome, and they receive one copy from each parent. Therefore, females with an X chromosome-linked recessive disease inherit one copy of the mutant gene from an affected father and the second copy of the mutant gene from their mother, who is most often a carrier heterozygous but who might be affected homozygous.
Males, on the other hand, have only one copy of the X chromosome, which they always receive from their mother. Therefore, males with an X chromosome-linked disease always receive the mutant copy of the gene from their mother. Moreover, because men don't have a second copy of the X chromosome to potentially "cancel out" the negative effects of X-linked mutations, they are far more likely than women to be affected by X chromosome-linked recessive diseases.
The blood-clotting disorder hemophilia A is one of several single-gene diseases that exhibit an X chromosome-linked recessive pattern of inheritance. In contrast, women are rarely affected by this disease, although they are most often carries of the mutated gene.
Duchenne muscular dystrophy is another example of a single-gene disease that exhibits an X chromosome-linked recessive inheritance pattern. This condition is associated with mutations in the dystrophin gene DMD. Few dominantly inherited forms of human disease are X chromosome linked.
Females with an X chromosome-linked dominant disease can inherit the mutant gene from either an affected mother or an affected father, whereas males always inherit such diseases from an affected mother. Examples of X chromosome-linked dominant diseases are rare, but several do exist. For instance, dominant mutations in the phosphate-regulating endopeptidase gene PHEX , which resides on the X chromosome, are associated with X-linked dominant hypophosphatemic rickets.
Similarly, Rett syndrome , a neurodevelopmental disease, is associated with dominant mutations in the methyl-CpG-binding protein 2 gene MECP2. Rett syndrome almost exclusively affects females, because male embryos with a dominant mutation in the MECP2 gene rarely survive. Like X-linked dominant diseases, Y chromosome-linked diseases are also extremely rare.
Because only males have a Y chromosome and they always receive their Y chromosome from their father, Y-linked single-gene diseases are always passed on from affected fathers to their sons.
It makes no difference whether the Y chromosome-linked mutation is dominant or recessive, because only one copy of the mutated gene is ever present; thus, the disease-associated phenotype always shows.
One example of a Y-linked disorder is nonobstructive spermatogenic failure, a condition that leads to infertility problems in males. With the complete sequence of the human genome in hand, scientists are now poised to match monogenic disease phenotypes to their corresponding genes. By analyzing complex pedigrees, geneticists can correlate changes in gene sequence with particular disease states. After all, once a disease-associated change in the DNA sequence of a gene is identified, it is much easier to determine how the structure of the corresponding gene product protein might be changed in a manner that alters its biological function.
The nature of disease-associated changes in protein structure and function can in turn enhance our ability to design drugs that effectively and specifically target mutant proteins. Dictionary Articles Tutorials Biology Forum. Table of Contents. Biology definition: Polygenic inheritance is a non-Mendelian pattern of inheritance in which a particular trait is produced by the interaction of genes at many loci i. Compare: monogenic inheritance.
Related term: polygene. Polygenic inheritance is also involved in quantitative traits , in which multiple gene loci each contribute in a similar way to the phenotype so that the total number of contributing alleles determines the phenotype. In humans, height, weight, and skin color are examples of quantitative traits. For instance, the height of an adult human is determined by not just a single gene but by more than genes apart from the other non-genetic factors such as environment and nutrition.
In quantitative traits, the Mendelian ratios are replaced by a normal distribution curve, with the two ends of the curve defined by the two extremes possible for the phenotype. The trait is a result of the cumulative effects of many genes Mendelian inheritance. Polygenic inheritance. Monogenic inheritance. Alleles that contribute to continuous variation Allelic pair. Contributing alleles. Non-contributing alleles. Genes producing a phenotype when expressed together Genome. Which of the following shows polygenic inheritance?
Free earlobes as a dominant trait vs. Pink flower color trait as a result of a cross between white-flowering and red-flowering plants. Varying skin colors ranging from very dark to very light. In humans, the height is determined by Send Your Results Optional. Your Name.
To Email. Time is Up! Genetics and Evolution Humans are diploid creatures. Genetic Information and Protein Synthesis Genes are expressed through the process of protein synthesis. Related Articles No related articles found See all Related Topics. Controlled by a number of genes. The expression of an individual gene is minor and is usually undetectable. Mammals are a diverse group of organisms, where most of them develop their offspring within the uterus of the mother.
Animals living in aquatic habitats have diversified and evolved through time. They eventually occupy ecological niches a.. The arthropods were assumed to be the first taxon of species to possess jointed limbs and exoskeleton, exhibit more adva..
Skip to content Main Navigation Search. Dictionary Articles Tutorials Biology Forum. Movement of Molecules Across Cell Membranes Molecules move within the cell or from one cell to another through different strategies. Multifactorial inheritance is based on the synergy of genes and environmental factors. Extra nuclear mitochondrial heredity can only be transmitted by the mother whose cells contain a number of mitochondria. Several factors can modify the expected individual phenotypes.
There will undoubtedly be important advances in our knowledge of the pattern of inheritance of characters and diseases given a better understanding of gene structure and role, interaction of genes between them and with the environment.
Eukaryotes have 2 copies of the hereditary message by contrast with prokaryotes and viruses , 1 paternal and 1 maternal: 2 alleles are 2 alternates of a gene at the same locus on both copies of the genome; any change of a hereditary character at the level of one of the two copies of the genome homologous chromosomes can: either modify the phenotype: this is expressed as a dominant pattern D or not modify the phenotype: recessive gene R If 2 alleles are expressed simultaneously, the genes are co-dominant ex: ABO blood group.
One individual who at the same locus has 2 identical alleles is known as homozygous HOZ for this allele. One individual who has 2 different alleles at the same locus is called heterozygous HEZ for this allele. This general picture refers to autosomal inheritance; but sex chromosomes are different in male and female: in the woman, XX, recessivity and dominance of X linked characters will be expressed as an autosomal pattern; the male, XY, is hemizygous for the X, the phenotype will be the expression of the X genotype.
The character is apparent in each generation does not skip a generation, except when the penetrance is reduced. There are as many daughters and sons affected. In a sibship one finds as many affected as normal individuals. Half of descents of an affected individual will be affected. All children of a normal individual will be normal.
Consanguinity is not elevated. The character can be expressed if there is a mutation and be transmitted or eliminated if the defect is severe.
Remarks : Most of the time one ignores what would be a HOZ individual for a dominant character. Some observations suggest that the individual would be affected earlier and more severely or that the disease would progress more rapidly.
Penetrance and expressivity play a role. If a disease is not compatible with reproduction, its frequency equals the mutation rate. Parental genotype: Aa x Aa Fig. In a sibship there are usually one affected and three normal individuals. An affected individual who marries a normal, non consanguineous person, usually has normal children. However the disease can affect only one individual who has mutant genes: due to the small number of sibs in families this does not mean that this situation is due to a de novo mutation.
The frequency of consanguineous families is elevated the risk of matings between 2 individuals, carriers of the same mutation, and with common ancestors is increased and more so if the disease is rare.
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