- Slides: 41
An allele is one of the possible forms of a gene. Most genes have two alleles, a dominant allele and a recessive allele. If an organism is heterozygous for that trait, or possesses one of each allele, then the dominant trait is expressed. Alleles were first defined by Gregor Mendel in the law of segregation.
Codominance is a form of dominance wherein the alleles of a gene pair in a heterozygote are fully expressed. This results in offspring with a phenotype that is neither dominant nor recessive. A typical example showing codominance is the ABO blood group system. Pseudodominance is the situation in which the inheritance of a recessive trait mimics a dominant pattern. 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.
Dominant alleles A dominant allele masks or hides the expression of a recessive allele and it is represented by an uppercase letter. A recessive allele is an allele the effects of only in the homozygous state and in heterozygous condition its expression is masked by the effects of dominant allele. Wild type vs. mutant alleles wild type An individual having the normal phenotype; that is, the phenotype generally found in a natural population of organisms. mutant An individual having a phenotype that differs from the normal phenotype. Wild type is designated with a “+” for any allele.
Incomplete dominance In the case of incomplete dominance, heterozygotes exhibit both alleles simultaneously, blended together. This is unlike codominance, where the traits are independently expressed together. Therefore, heterozygotes express entirely new phenotypes(physical expressions) that are not like the parent organisms. Incomplete dominance, while not the most common form of expression, is seen in many organisms, including plants, animals, and humans.
Each gamete acquires one of the two alleles as chromosomes separate into different gametes during meiosis. Heterozygotes, which posess one dominant and one recessive allele, can receive each allele from either parent and will look identical to homozygous dominant individuals; the Law of Segregation supports Mendel’s observed 3: 1 phenotypic ratio. Mendel proposed the Law of Segregation after observing that pea plants with two different traits produced offspring that all expressed the dominant trait, but the following generation expressed the dominant and recessive traits in a 3: 1 ratio. law of segregation: a diploid individual possesses a pair of alleles for any particular trait and each parent passes one of these randomly to its offspring Mendel's law of independent assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene.
MENDELIAN SEGREGATION IN HUMAN FAMILIES In humans, the number of children produced by a couple is typically small. Today in the United States, the average is around two. In developing countries, it is six to seven. Such numbers provide nothing close to the statistical power that Mendel had in his experiments with peas. Consequently, phenotypic ratios in human families often deviate significantly from their Mendelian expectations. As an example, let’s consider a couple who are each heterozygous for a recessive allele that, in homozygous condition, causes cystic fibrosis, a serious disease in which breathing is impaired by an accumulation of mucus in the lungs and respiratory tract. If the couple were to have four children, would we expect exactly three to be unaffected and one to be affected by cystic fibrosis? The answer is no. Although this is a possible outcome, it is not the only one. There are, in fact, five distinct possibilities: 1. Four unaffected, none affected. 2. Three unaffected, one affected. 3. Two unaffected, two affected. 4. One unaffected, three affected. 5. None unaffected, four affected.
Intuitively, the second outcome seems to be the most likely, since it conforms to Mendel’s 3: 1 ratio. We can calculate the probability of this outcome, and of each of the others, by using Mendel’s principles and by treating each birth as an independent event For a particular birth, the chance that the child will be unaffected is 3/4. The probability that all four children will be unaffected is therefore (3/4) (3/4)4 81/256. Similarly, the chance that a particular child will be affected is 1/4; thus, the probability that all four will be affected is (1/4)4 1/256. To find the probabilities for the three other outcomes, we need to recognize that each actually represents a collection of distinct events.
Unaffected Affected Probability Cc X Cc 4 children How many unaffected? How many affected? Parents Number of children that are: Probability distribution: 43210 01234 1 x (3/4) = 81/256 1 x (1/4) = 1/256 4 x (3/4) x (1/4) = 108/256 6 x (3/4) x (1/4) = 54/256 4 x (3/4) x (1/4) = 12/256
Test Cross Definition The test cross is an experiment first employed by Gregor Mendel, in his studies of the genetics of traits in pea plants. Mendel’s theory, which holds true today, was that each organism carried two copies of each trait. One was dominant trait, while one could be considered recessive. The dominant trait, if present, would determine the outward appearance of the organism, or the phenotype. Thus, Mendel became interested in the question of determining which organisms with the dominant phenotype had two dominant alleles, and which have one dominant allele and one recessive allele. His answer came in the form of the test cross.
Huntington's disease is inherited in an autosomal dominant fashion. The probability of each offspring inheriting an affected gene is 50%. Inheritance is independent of gender, and the phenotype does not skip generations. Huntington’s disease is a single gene disorder? caused by a mutation? in the HD (also known as HTT) gene? on chromosome? 4. It is an autosomal dominant? disease. This means that a single defective gene copy will cause disease. Huntington’s disease is caused by a mutation in the HD gene in which the same three bases? (CAG) are repeated many more times than normal. This is known as a CAG trinucleotide repeat expansion. In people who don’t have Huntington’s disease this section of CAG repeats in the gene is usually only repeated 10 to 35 times. In people with Huntington’s disease, this section is repeated over 36 times and can be repeated more than 120 times.
Cystic fibrosis is caused by mutations in the gene that produces the cystic fibrosis transmembrane conductance regulator (CFTR) protein. . Cystic fibrosis is an example of a recessive disease. That means a person must have a mutation in both copies of the CFTR gene. What Is Cystic Fibrosis? Cystic fibrosis is a progressive, genetic disease that causes persistent lung infections and limits the ability to breathe over time. In people with CF, a defective gene causes a thick, sticky buildup of mucus in the lungs, pancreas, and other organs. In the lungs, the mucus clogs the airways and traps bacteria leading to infections, extensive lung damage, and eventually, respiratory failure. In the pancreas, the mucus prevents the release of digestive enzymes that allow the body to break down food and absorb vital nutrients.
The sickle cell anemia trait is found on a recessive allele of the hemoglobin gene. This means that you must have two copies of the recessive allele — one from your mother and one from your father — to have the condition. People who have onedominant and one recessive copy of the allele won't have sickle cell anemia. What is Sickle Cell Disease? SCD is a group of inherited red blood cell disorders. Healthy red blood cells are round, and they move through small blood vessels to carry oxygen to all parts of the body. In someone who has SCD, the red blood cells become hard and sticky and look like a C-shaped farm tool called a “sickle”. The sickle cells die early, which causes a constant shortage of red blood cells. Also, when they travel through small blood vessels, they get stuck and clog the blood flow. This can cause pain and other serious problems such infection, acute chest syndrome and stroke.
What is polygenic inheritance? Polygenic inheritance occurs when one characteristic is controlled by two or more genes. Often the genes are large in quantity but small in effect. Examples of human polygenic inheritance are height, skin color, eye color and weight. Polygenes exist in other organisms, as well.
Blood group genetics
Multiple Alleles are alternative forms of a gene, and they are responsible for differences in phenotypic expression of a given trait (e. g. , brown eyes versus green eyes). A gene for which at least two alleles exist is said to be polymorphic. Instances in which a particular gene may exist in three or more allelic forms are known as multiple allele conditions. It is important to note that while multiple alleles occur and are maintained within a population, any individual possesses only two such alleles (at equivalent loci on homologous chromosomes).
Characteristics of Multiple Alleles 1. The study of multiple alleles may be done in population. 2. Multiple alleles are situated on homologous chromosomes at the same locus. 3. There is no crossing over between the members of multiple alleles. Crossing over takes place between two different genes only (inter-generic recombination) and does not occur within a gene (intragenic recombination). 4. Multiple alleles influence one or the same character only.
5. Multiple alleles never show complementation with each other. By complementation test the allelic and non-allelic genes may be differentiated well. The production of wild type phenotype in a trans -heterozygote for 2 mutant alleles is known as complementation test. 6. The wild type (normal) allele is nearly always dominant while the other mutant alleles in the series may show dominance or there may be an intermediate phenotypic effect. 7. When any two of the multiple alleles are crossed, the phenotype is of a mutant type and not the wild type. 8. Further, F 2 generations from such crosses show typical monohybrid ratio for the concerned character.
Lethal alleles (also referred to as lethal genes or lethals) are alleles that cause the death of the organism that carries them. They are usually a result of mutations in genes that are essential for growth or development. pseudoalleles allele that is functionally but not structurally allelic, that is wild-type recombinants can be recovered by intragenic recombination from heterozygotes containing two different pseudoalleles.
Recombination causes allelic variation
In genetics, complementation occurs when two strains of an organism with different homozygous recessive mutations that produce the same mutant phenotype (for example, a change in wing structure in flies) produce offspring with the wild-type phenotype when mated or crossed. Complementation will occur only if the mutations are in different genes. In this case, each strain's genome supplies the wild-type allele to "complement" the mutated allele of the other strain's genome. Since the mutations are recessive, the offspring will display the wild-type phenotype. A complementation test (sometimes called a "cis-trans" test) can be used to test whether the mutations in two strains are in different genes. Complementation will not occur if the mutations are in the same gene. The convenience and essence of this test is that the mutations that produce a phenotype can be assigned to different genes without the exact knowledge of what the gene product is doing on a molecular level. The complementation test was developed by American geneticist Edward B. Lewis.
crossing over, process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids.