How does the inheritance pattern of mitochondrial dna differ from inheritance of single-gene traits?
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Mode of Inheritance is the manner in which a genetic trait or disorder is passed from one generation to the next. Autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, multifactorial, and mitochondrial inheritance are examples. Each mode of inheritance results in a characteristic pattern of affected and unaffected family members.3 Show
Autosomal DominantAutosomal dominant inheritance refers to genetic conditions that occur when a variant is present in one copy of a given gene (i.e., the person is heterozygous).3 Autosomal RecessiveAutosomal recessive inheritance refers to genetic conditions that occur only when variants are present in both copies of a given gene (i.e., the person is
homozygous for a variant, or carries two different variants of the same gene, a state referred to as compound heterozygosity).3 First Degree RelativeA first-degree relative is a family member who shares about 50 percent of their genes with a particular individual in a family. First degree relatives include parents, offspring, and siblings.4 MitochondrialMitochondria make most of the energy for the cell and have their own genetic material that is different from the genetic material found in the nucleus.
Mitochondrial DNA is the small circular chromosome found inside mitochondria. The mitochondria, and thus mitochondrial DNA, are passed from mother to offspring.2,4 Second Degree RelativeThe aunts, uncles, grandparents, grandchildren, nieces, nephews, or half-siblings of an individual. Also called SDR.3 Third Degree RelativeThird degree relatives in a genetic family history are identified as those individuals with two other family members between them, for example first cousins. These relatives share on average 12.5% of their DNA. X-Linked DominantX-linked dominant inheritance refers to genetic conditions associated with variants in genes on the X chromosome.3 X-Linked RecessiveX-linked recessive inheritance refers to genetic conditions associated with variants in genes on the X chromosome.3 Subcategory - Family HistoryConsanguinityGenetic relatedness between individuals who are descendants of at least one common ancestor.3 Founder EffectThe founder effect is the reduction in genetic variation that results when a small subset of a large population is used to establish a new colony. The new population may be very different from the original population, both in terms of its genotypes and phenotypes. In some cases, the founder effect plays a role in the emergence of new species.4 PedigreeA graphic illustration of family history.2 ProbandA proband is an individual being studied or reported on. A proband is usually the first affected individual in a family who brings a genetic disorder to the attention of the medical community.4 Quick links
DNA, genes and chromosomesDNA (deoxyribonucleic acid) is the hereditary material in humans and most other organisms. It is composed of four chemical bases/letters called adenine (A), guanine (G), cytosine (C) and thymine (T). The human genome is made of over 3 million letters of DNA. These letters are arranged in various sequences and length to make up genes. Genes are the basic physical and functional units of heredity. We have more than 20,000 genes, some of which provide instructions to cells in to make specific proteins that control the structure and function of our bodies. However, most genes do not code for any proteins and we are still trying to learn about their functions. Genes are located in small thread-like structures called chromosomes, which in turn are located inside the nucleus of our cells.
Most cells in our body contain 23 pairs of chromosomes. In each pair, we inherit one chromosome from each parent. We therefore have two copies of most of our genes. In contrast, our germ cells (egg and sperm cells) only have one set of chromosomes (and therefore one set of genes) due to a division process called meiosis. When the egg is fertilised by a sperm cell, the fertilised egg will contain two sets of chromosomes, one from each parent. 22 of these pairs are identical in males and females and these are called autosomes. The 23rd pair are the sex chromosomes and they determine gender. Females have two copies of X chromosome (one from each parent) while males have one X chromosome (from the mother) and one Y chromosome (from the father). Credit: US National Library of Medicine Jump to top How are genetic disorders inherited?A genetic disorder is usually due to changes (mutations) or “spelling mistakes” in a single gene. These changes can either be inherited from parent(s) or they can be “de novo” (i.e. due to a new change in that person). The faulty gene can be located in one of the 22 autosomes, in the X chromosome or even in a structure called the mitochondria. Mitochondria are small structures present in most cells in our body, providing energy for these cells to function normally (much like a battery). Genetic conditions can be inherited in different ways depending on where the faulty gene is located and how many copies of the faulty gene are required to cause disease. These include:
Jump to top Autosomal dominant inheritanceIn autosomal dominant conditions, the faulty gene is in one of the autosomes (chromosomes 1-22) and only one copy of the faulty gene is required to cause disease. This can be inherited from either parent. There is a 50% chance of an affected individual passing the faulty gene on to a child and a 50% chance of passing on their normal gene. The risk is the same whether the child is male or female. This is a random process that occurs for each pregnancy. Occasionally, conditions inherited in this manner have variable expressivity and incomplete penetrance. Variable expressivity means a range of severity from very mild to very severe while incomplete penetrance means that some people with the faulty gene have symptoms and other people with the same exact genetic change have no symptoms at all. Jump to top Autosomal recessive inheritanceIn autosomal recessive conditions, two copies of the faulty gene are required to cause disease, one from each parent. A parent who has one faulty gene copy and one normal gene copy is known as a heterozygote or a carrier and they usually do not have any symptoms. Carriers are usually not aware of their status until they have an affected child or have undergone genetic testing. For a child to be affected, both parents would need to be carriers of a mutation in the same gene. If both parents are carriers (most likely scenario):
 If one parent is affected and the other is not a carrier:
If one parent is affected and the other is a carrier (common with inter-marriages or in small isolated populations):
This is a random process and these risks apply to each new pregnancy. Jump to top X-linked recessive inheritanceThe faulty gene is located in the X chromosome in such instances. If a female has a faulty gene on one of her two X chromosomes, she is considered a heterozygote carrier and usually does not show any symptoms. This is because of a process called X-inactivation which ensures that females, like males, only have one functional copy of the X chromosome in each cell. X-inactivation is a random process and therefore, the X chromosome inherited from the mother may be active in some cells while in others the active X chromosome is inherited from the father. On the other hand, if a male has a faulty gene on his single X chromosome, he is termed “hemizygous” and will show symptoms as he does not have another X chromosome to compensate. Generally, X-linked recessive conditions only manifest in males but a minority of female carriers can be affected (usually to a milder extent) due to non-random X-inactivation. If the mother is a carrier and the father is healthy:
If the father is affected and the mother is healthy:
This is because an affected male cannot transmit the disorder to his male offspring who inherit his Y chromosome but all his daughters will receive his X chromosome. To summarise, in X-linked recessive inheritance:
Jump to top X-linked dominant inheritance
Jump to top Mitochondrial inheritanceMitochondria are energy producing structures inside our cells which have their own DNA with 37 genes. Mutations in mitochondrial genes are passed on to the next generation through the mother (maternal inheritance). This is because only the egg cells but not the sperm cells contribute mitochondria to the developing embryo. Both males and females can be affected with mitochondrial inheritance. Jump to top How does the inheritance of mitochondrial DNA differ from the inheritance of DNA found in the nucleus of cells?The mitochondrial mode of inheritance is strictly maternal, whereas nuclear genomes are inherited equally from both parents. Therefore, mitochondria-associated disease mutations are also always inherited maternally.
How would you distinguish mitochondrial inheritance from other modes of inheritance?Unlike nuclear genes, which are inherited from both parents, mitochondrial genes are inherited only from the mother. If there is a mutation in a mitochondrial gene, it is passed from a mother to all of her children; sons will not pass it on, but daughters will pass it on to all of their children, and so on.
How is mitochondrial DNA different from genomic DNA?Mitochondrial DNA is short compared to the nuclear DNA. The main difference between mitochondrial DNA and nuclear DNA is that mitochondrial DNA is encoded for the genetic information required by mitochondria whereas nuclear DNA is encoded for the genetic information required by the entire cell.
How is inheritance of mitochondrial and chloroplast genes different from inheritance of genes in the nucleus?The inheritance of mitochondrial and chloroplast genes differs from that of nuclear genes in showing vegetative segregation, uniparental inheritance, intracellular selection, and reduced recombination.
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