Contact a health care provider if you have questions about your health. What are the different ways a genetic condition can be inherited? From Genetics Home Reference. These conditions are usually inherited in one of several patterns, depending on the gene involved: Patterns of inheritance Inheritance pattern Description Examples Autosomal dominant One altered copy of the gene in each cell is sufficient for a person to be affected by an autosomal dominant disorder.
Huntington disease , Marfan syndrome Autosomal recessive In autosomal recessive inheritance , variants occur in both copies of the gene in each cell. Y chromosome infertility , some cases of Swyer syndrome Codominant In codominant inheritance , two different versions alleles of a gene are expressed, and each version makes a slightly different protein.
ABO blood group, alpha-1 antitrypsin deficiency Mitochondrial Mitochondrial inheritance , also known as maternal inheritance, applies to genes in mitochondrial DNA. Leber hereditary optic neuropathy LHON Many health conditions are caused by the combined effects of multiple genes described as polygenic or by interactions between genes and the environment.
Topics in the Inheriting Genetic Conditions chapter What does it mean if a disorder seems to run in my family? Why is it important to know my family health history? If a genetic disorder runs in my family, what are the chances that my children will have the condition?
What are reduced penetrance and variable expressivity? What do geneticists mean by anticipation? What are genomic imprinting and uniparental disomy?
Are chromosomal disorders inherited? Share This Video Whatsapp. Text Solution. Autosomal recessiveAutosomal dominantY-linkageSex-linked recessive Answer :. Very Important Questions. National Education Day: e-learning Transforming Educational Landscape National education day: e-learning transforming educational landscape. By using this site you agree to the use of cookies. Mitochondrial Inheritance. All of these inheritance patterns can be very confusing, and it isn't always easy to know how a specific condition is inherited.
Sometimes genetic conditions can be inherited in more than one way. If you have questions about a specific condition in your family and the possible type of inheritance pattern, please call the nearest Genetics Department for a confidential consultation. Genetics Northern California. Home Resources Inheritance Patterns.
Autosomal Dominant Autosomal Recessive X-linked inheritance "X-linked" refers to traits determined by genes located on the X chromosome only and not the other 22 pairs of chromosome known as autosomes. And he repeatedly came up with the same results—among the traits he studied, one was always dominant, and the other was always recessive.
Remember, however, that this dominant—recessive relationship between alleles is not always the case; some alleles are codominant, and sometimes dominance is incomplete. Using his understanding of dominant and recessive traits, Mendel tested whether a recessive trait could be lost altogether in a pea lineage or whether it would resurface in a later generation.
By crossing the second-generation offspring of purebred parents with each other, he showed that the latter was true: recessive traits reappeared in third-generation plants in a ratio of three offspring having the dominant trait and one having the recessive trait. If an individual receives two recessive alleles, then the recessive trait will be expressed in the phenotype. In this case, the dominant trait will be expressed, and the individual will be phenotypically identical to an individual who possesses two dominant alleles for the trait.
It is common practice in genetics to use capital and lowercase letters to represent dominant and recessive alleles. A dwarf pea plant must be homozygous because its dwarfism can only be expressed when two recessive alleles are present tt.
A heterozygous pea plant Tt would be tall and phenotypically indistinguishable from a tall homozygous pea plant because of the dominant tall allele. Mendel deduced that a ratio of dominant to recessive would be produced by the random segregation of heritable factors genes when crossing two heterozygous pea plants. In other words, for any given gene, parents are equally likely to pass down either one of their alleles to their offspring in a haploid gamete, and the result will be expressed in a dominant—recessive pattern if both parents are heterozygous for the trait.
Because of the random segregation of gametes, the laws of chance and probability come into play when predicting the likelihood of a given phenotype. Consider a cross between an individual with two dominant alleles for a trait AA and an individual with two recessive alleles for the same trait aa. All of the parental gametes from the dominant individual would be A , and all of the parental gametes from the recessive individual would be a.
All of the offspring of that second generation, inheriting one allele from each parent, would have the genotype Aa , and the probability of expressing the phenotype of the dominant allele would be 4 out of 4, or percent. This seems simple enough, but the inheritance pattern gets interesting when the second-generation Aa individuals are crossed. Because segregation and fertilization are random, each offspring has a 25 percent chance of receiving any of these combinations.
The genotypic ratio for this inheritance pattern is However, we have already established that AA and Aa and aA individuals all express the dominant trait i. Figure 2. In the formation of gametes, it is equally likely that either one of a pair alleles from one parent will be passed on to the offspring. This figure follows the possible combinations of alleles through two generations following a first-generation cross of homozygous dominant and homozygous recessive parents.
The recessive phenotype, which is masked in the second generation, has a 1 in 4, or 25 percent, chance of reappearing in the third generation. The law states that the members of one pair of genes alleles from a parent will sort independently from other pairs of genes during the formation of gametes. Applied to pea plants, that means that the alleles associated with the different traits of the plant, such as color, height, or seed type, will sort independently of one another.
This holds true except when two alleles happen to be located close to one other on the same chromosome. Independent assortment provides for a great degree of diversity in offspring. Although all diploid individuals have two alleles for every gene, allele pairs may interact to create several types of inheritance patterns, including incomplete dominance and codominance.
Secondly, Mendel performed his studies using thousands of pea plants. He was able to identify a phenotypic ratio in second-generation offspring because his large sample size overcame the influence of variability resulting from chance. In contrast, no human couple has ever had thousands of children. If we know that a man and woman are both heterozygous for a recessive genetic disorder, we would predict that one in every four of their children would be affected by the disease.
In real life, however, the influence of chance could change that ratio significantly. For example, if a man and a woman are both heterozygous for cystic fibrosis, a recessive genetic disorder that is expressed only when the individual has two defective alleles, we would expect one in four of their children to have cystic fibrosis. However, it is entirely possible for them to have seven children, none of whom is affected, or for them to have two children, both of whom are affected.
For each individual child, the presence or absence of a single gene disorder depends on which alleles that child inherits from his or her parents. In the case of cystic fibrosis, the disorder is recessive to the normal phenotype. However, a genetic abnormality may be dominant to the normal phenotype. When the dominant allele is located on one of the 22 pairs of autosomes non-sex chromosomes , we refer to its inheritance pattern as autosomal dominant.
An example of an autosomal dominant disorder is neurofibromatosis type I, a disease that induces tumor formation within the nervous system that leads to skin and skeletal deformities.
Consider a couple in which one parent is heterozygous for this disorder and who therefore has neurofibromatosis , Nn , and one parent is homozygous for the normal gene, nn. The heterozygous parent would have a 50 percent chance of passing the dominant allele for this disorder to his or her offspring, and the homozygous parent would always pass the normal allele.
Therefore, four possible offspring genotypes are equally likely to occur: Nn , Nn , nn , and nn. That is, every child of this couple would have a 50 percent chance of inheriting neurofibromatosis. This inheritance pattern is shown in the table below, in a form called a Punnett square , named after its creator, the British geneticist Reginald Punnett.
Figure 3. Inheritance pattern of an autosomal dominant disorder, such as neurofibromatosis, is shown in a Punnett square. Because autosomal dominant disorders are expressed by the presence of just one gene, an individual with the disorder will know that he or she has at least one faulty gene.
When a genetic disorder is inherited in an autosomal recessive pattern, the disorder corresponds to the recessive phenotype. Heterozygous individuals will not display symptoms of this disorder, because their unaffected gene will compensate. Such an individual is called a carrier.
Carriers for an autosomal recessive disorder may never know their genotype unless they have a child with the disorder.
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