Non mendelian inheritance
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Non-Mendelian Inheritance: Incomplete Dominance, Co-dominance, Multiple Alleles, Pleiotropy, Lethality, Polygenic Traits, and Environmental Factors
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Non mendelian inheritance
- 1. N D E L I ANON MEN I N H E R I T AN C E
- 3. It is when a dominant allele, or form of a gene, does not completely mask the effects of a recessive allele, and the organism’s resulting physical appearance shows a blending of both alleles. SEMI DOMINANCE PARTIAL DOMINANCE
- 5. A. A pink flower produced from red and white flowers B. A flower that is both red and white produced from red and white flowers C. Curly-haired and straight-haired individuals producing wavy-haired offspring D. A highly spotted dog and a non-spotted dog producing puppies with a few spots Which is NOT an example of incomplete dominance? B. A flower that is both red and white produced from red and white flowers
- 6. It occurs when two versions, or “alleles,” of the same gene are present in a living thing, and both are
- 8. A. A child of parents with blood types A and B, who has AB blood type. B. A calf of a red cow and a white cow, who has a roan coat consisting of red and white hairs. C. A child of a parent with blue eyes and a parent with brown eyes, who has brown eyes. D. A flower offspring of red and white flowers, which has both red and white petals. Which of the following is NOT an example of codominance? C. A child of a parent with blue eyes and a parent with brown eyes, who has brown eyes.
- 9. They exist in a population when there are many variations of a gene present. In organisms with two copies of every gene, also known as diploid organisms, each organism has the ability to express two alleles at the
- 11. A. It is not important. B. Stable organisms ensure that the experiment can be repeated. C. More variety is good for research. Often, breeders of animals aim to breed “true” lines. This means that generation after generation, the animals will look almost exactly the same, and the number of different alleles in a population is reduced. Why would this be important for scientific research? B. Stable organisms ensure that the experiment can be repeated.
- 12. It refers to the expression of multiple traits by a single gene. These expressed traits may or may not be GENE PLEITROPY DEVELOPMENT AL PLEITROPY SELECTIONAL PLEITROPY ANTAGONISTIC PLEITROPY
- 14. Inheriting two copies of mutated genes that are nonfunctional can have lethal consequences.
- 15. These are traits that are controlled by multiple genes instead of just one. The genes that control them may be located near each other or even on separate chromosomes.
- 17. A. Height B. Skin color C. Eye color D. Widow’s peak Which of these is NOT a polygenic trait? D. Widow’s peak
- 18. U H A N K Y O T
Editor's Notes
- This is called Non-mendelian inheritance and it plays an important role in several disease processes. Non-mendelian inheritance: 1) Incomplete dominance 2) Co-dominance 3) Multiple alleles 4) Pleiotropy 5) Lethality 6) Polygenic traits 7) Environmental factors
- Incomplete dominance is an important concept in the study of genetics. It refers to a circumstance in which the two copies of a gene for a particular trait, or alleles, combine so that neither dominates the other. This creates a new phenotype or set of observable characteristics caused by the interaction of genetics and environment. In short, incomplete dominance is when neither gene is fully dominant, and the result is a brand new trait. Non-mendelian inheritance can manifest as incomplete dominance, where offspring do not display traits of either parent but rather, a mix of both. Two alleles produce an intermediate phenotype, rather than either one exerting a specific dominance. Incomplete dominance will give a 1:2:1 phenotype ratio with the homozygous genotypes each showing a different feature and the heterozygous showing one more distinct phenotype. It is also called semi-dominance or partial dominance. One example is shown in roses. The allele for red color is dominant over the allele for white color, but heterozygous roses, which have both alleles, are pink. Note that this is different from codominance, which is when both alleles are expressed at the same time.
- It's important to note that most observable traits in any living thing are caused by more than one gene. Incomplete dominance is specific to traits that occur on just one gene. However, there are many such traits, and incomplete dominance occurs in every sort of organism that has genes, including plants, animals and even human beings. Why does incomplete dominance occur? As we have seen, it does not always occur with flower color; roses (and tulips, carnations, and snapdragons, among others) show incomplete dominance, but Mendel’s pea plants showed complete dominance. Incomplete dominance can occur because neither of the two alleles is fully dominant over the other, or because the dominant allele does not fully dominate the recessive allele. This results in a phenotype that is different from both the dominant and recessive alleles, and appears to be a mixture of both. In Humans A child born to a parent with straight hair and a parent with curly hair will usually have wavy hair, or hair that is a little curled, due to the expression of both curly and straight alleles. Incomplete dominance can be seen in many other physical characteristics such as skin color, height, hand size, and vocal pitch. In Other Animals The Andalusian chicken, a type of chicken native to the Andalusia region of Spain, shows incomplete dominance in its feather color. A white male and a black female will often produce offspring that have blue-tinged feathers. This is caused by a dilution gene that partially dilutes the pigment melanin and makes the feathers lighter.
- B is correct. This is an example of codominance, not incomplete dominance, because both phenotypes are shown instead of one intermediate phenotype.
- Non-Mendelian Inheritance is applicable in co-dominance where two alleles may be expressed simultaneously i.e. heterozygous. There is no mixing or blending involved. The human blood type AB, where types A and B are both codominant, is an example of this. A cross between AA and BB will produce AB offspring, with both alleles being expressed equally. Often, co-dominance is linked with a characteristic that has multiple alleles of a given gene. In many cases one of those alleles will be recessive and two others will be dominant. This gives the trait the ability to follow the Mendelian Laws of heredity with simple or complete dominance or, alternatively, to have a situation where co-dominance occurs. For example, coat colour of rabbits can appear as four common phenotypes: black CC, chinchilla CchCch, himalyan ChCh and albino cc. In this case, the black C allele is completely dominant to all the others. The chinchilla cch is incompletely dominant to the Himalayan ch and albino c alleles. The Himalayan ch, allele is completely dominant to the albino c allele Instead of one trait being dominant over the other, both traits appear. Codominance is easy to spot in plants and animals that have more than one pigment color. Spotted cows and flowers with petals of two different colors are examples of codominance, for example. Codominance also occurs in some less visible traits, such as blood type. The A and B alleles for blood type can both be expressed at the same time, resulting in type AB blood. In genetics, “dominant” genes are those that are always expressed if they are found in an organism. Dominant genes may be expressed as co-dominant – where two different traits are both expressed alongside each other – or as dominant/recessive, where the presence of a dominant gene completely masks the presence of a recessive gene.
- Livestock When a chicken with white feathers breeds with a chicken with black feathers, the result is an offspring chicken that grows up to have both black and white feathers. Likewise, when a red cattle breeds with a red cattle, the resulting offspring may show both red and white hairs, resulting in a mixed coat pattern called “roan.” Rhododendron Rhododendrons and other flowers may also exhibit codominance. In the case of rhododendrons, the crossing of a red and white flower may yield a flower that has both red and white patches. Many flowers show similar patterns of codominance, where both of the parental flower colors show up in different parts of the plant. Blood Type An example of codominance that occurs in humans is that of blood type. There are three different versions of the gene for proteins that appear on the outside of our blood cells and help our body to identify the cells as their own. These alleles are A, B, and O. The “O” allele actually does not code for any protein at all, so people with the “O” trait lack both A and B proteins. The A and B proteins, on the other hand, code for two different proteins. These proteins, like different colors in a flower, can appear together. Someone who inherits an A allele from one parent and a B allele from the other will express both proteins in a codominant fashion, resulting in an AB blood type. The “O” trait, on the other hand, is a good example of a dominant/recessive relationship: if either A or B is expressed, the “O” trait is not expressed. This chart below illustrates how codominance can occur between A and B traits, while a dominant/recessive relationship exists between those traits and the O trait:
- C is correct. This is not an example of codominance, because the child does not express both parents’ traits. She only expresses the brown-eyed gene inherited from one parent.
- Multiple alleles exist in a population when there are many variations of a gene present. In organisms with two copies of every gene, also known as diploid organisms, each organism has the ability to express two alleles at the same time. They can be the same allele, which is called a homozygous genotype. Alternatively, the genotype can consist of alleles of different types, known as a heterozygous genotype. Haploid organisms and cells only have one copy of a gene, but the population can still have many alleles. Multiple alleles combine in different ways in a population, and produce different phenotypes. These phenotypes are caused by the proteins encoded for by the various alleles. Although each gene encodes for the same type of protein, the different alleles can cause high variability in the functioning of these proteins. Just because a protein functions at a higher or lower rate does not make it good or bad. This is determined by the sum of the interactions of all the proteins produced in an organism and the effects of the environment on those proteins. Some organism, driven by multiple alleles in a variety of genes, do better than others and can reproduce more. This is the basis of natural selection, and as new mutations arise and new lines of genetics are born the origin of species takes place.
- Coat Color in Cats In domestic cats, breeding has taken place for thousands of years selecting for different and varied coat colors. Cats can be seen with long hair, short hair, and no hair. There are genes that code for whether or not a cat will have hair. There are multiple alleles for this gene, some that produce hairless cats, and some that produce cats with hair. Another gene regulates the length of the hair. Long haired cats have two recessive alleles, while a dominate allele will produce short hair. Other genes control the color of coat. There is a gene for several colors of pigment: red, black and brown. Each gene has multiple alleles in the population, which express the protein responsible for making the pigment. Each allele changes the way the protein works, and therefore the expression of the pigment in the cat. Other genes, in similar ways, control traits for curliness, shading, patterns, and even texture. The amount of combinations and expressions of different genotypes together creates an almost infinite variety of cates. For this reason, cat breeders have been successfully attempting for thousands of years to create new and strange varieties of cats, and dogs for that matter. Even with only 4 alleles between two parents at each gene, the variety can be incredible. Just look at the kittens in the photo above. All these kittens came from the same parents. Fruit Flies In the year 2000, scientist finally succeeded in mapping the complex genome of the common fruit fly, Drosophilia melanogaster. The fruit fly had been, and continues to be, a valuable laboratory animal because of its high reproduction rate and the simplicity of keeping and analyzing large quantities of flies. At about 165 million base pairs, the DNA of a fruit fly is much smaller than that of a human. While a human has 23 chromosomes, a fruit fly only has 4. Still, in only 4 chromosomes, there exists around 17,000 genes. Each gene controls a different aspect of the fly, and is subject to mutation and new alleles arising. In the picture above, all the flies are the same species Drosophilia melanogaster. The variation seen between the flies is caused by multiple alleles, in different genes. For instance, the gene for eye color determines if the fly will have an orange/brown eye, a red eye, or a white eye. Both the white and orange alleles are recessive to the wild type red eye allele. The two flies at the top have wild type bodies, a tan with dark stripes. In the gene that controls body color, two other alleles are present. The fly on the far right is showing a homozygous recessive genotype that causes a dark body. The three flies on the bottom show another homozygous recessive genotype, the yellow body mutation.
- B is correct. In a research environment, you want as little variation as possible. This makes your results more meaningful. If lines are bred “true”, then organisms can be bred for generations and produce the same results as when the experiments were started. Without this decrease in variation through artificial selection, many experiments would not be reproducible. Being able to reproduce an experiment is the basis of all good science.
- Pleiotropy refers to the expression of multiple traits by a single gene. These expressed traits may or may not be related. Pleitropy was first noticed by geneticist Gregor Mendel, who is known for his famous studies with pea plants. Mendel noticed that plant flower color (white or purple) was always related to the color of the leaf axil (area on a plant stem consisting of the angle between the leaf and upper part of the stem) and seed coat. The study of pleitropic genes is important to genetics as it helps us to understand how certain traits are linked in genetic diseases. Pleitropy can be spoken of in various forms: gene pleiotropy, developmental pleiotropy, selectional pleiotropy, and antagonistic pleiotropy. Pleiotropy is the expression of multiple traits by a single gene. Gene pleiotropy is focused on the number of traits and biochemical factors impacted by a gene. Developmental pleiotropy is focused on mutations and their influence on multiple traits. Selectional pleiotropy is focused on the number of separate fitness components affected by a gene mutation. Antagonistic pleiotropy is focused on the prevalence of gene mutations that have advantages early in life and disadvantages later in life.
- An example of pleiotropy that occurs in humans is sickle cell disease. Sickle cell disorder results from the development of abnormally shaped red blood cells. Normal red blood cells have a biconcave, disc-like shape and contain enormous amounts of a protein called hemoglobin. Hemoglobin helps red blood cells bind to and transport oxygen to cells and tissues of the body. Sickle cell is a result of a mutation in the beta-globin gene. This mutation results in red blood cells that are sickle-shaped, which causes them to clump together and become stuck in blood vessels, blocking normal blood flow. The single mutation of the beta-globin gene results in various health complications and causes damage to multiple organs including the heart, brain, and lungs. Phenylketonuria, or PKU, is another disease resulting from pleiotropy. PKU is caused by a mutation of the gene responsible for the production of an enzyme called phenylalanine hydroxylase. This enzyme breaks down the amino acid phenylalanine that we get from protein digestion. Without this enzyme, levels of the amino acid phenylalanine increase in the blood and damage the nervous system in infants. PKU disorder may result in several conditions in infants including intellectual disabilities, seizures, heart problems, and developmental delays. The frizzled feather trait is an example of pleiotropy seen in chickens. Chickens with this particular mutated feather gene display feathers that curl outward as opposed to lying flat. In addition to curled feathers, other pleiotropic effects include a faster metabolism and enlarged organs. The curling of the feathers leads to a loss of body heat requiring a faster basal metabolism to maintain homeostasis. Other biological changes include higher food consumption, infertility, and sexual maturation delays.
- Combinations of genes are often needed to promote the survival of organisms. If an allele that contributes to a gene is not expressed it can lead to harmful activity. This is known as lethality. A classic example of an allele that affects survival is the lethal yellow allele, a spontaneous mutation in mice that makes their coats yellow. Mice that are homozygous die early in development. Although this particular allele is dominant, lethal alleles can be dominant or recessive, and can be expressed in homozygous or heterozygous conditions.
- Some characteristics are polygenic, meaning that they’re controlled by a number of different genes. In polygenic inheritance, traits often form a phenotypic spectrum rather than falling into clear-cut categories. are traits that are controlled by multiple genes instead of just one. The genes that control them may be located near each other or even on separate chromosomes. Because multiple genes are involved, polygenic traits do not follow Mendel’s pattern of inheritance. Instead of being measured discretely, they are often represented as a range of continuous variation. Some examples of polygenic traits are height, skin color, eye color, and hair color.
- Height Human height is controlled by many genes; in fact, there are over 400 genes related to height, and all of these genes interact to make up a person’s phenotype. This is a very large number, but it makes sense because height is a compilation of the lengths of many different body parts, such as leg bones, the torso, and even the neck. Polygenic traits can also be influenced by an organism’s environment. If a person gets inadequate nutrition during childhood, they can have stunted growth and end up smaller and shorter than they would otherwise. It is estimated that 90% of a person’s adult height is controlled by genetics, and 10% is affected by the environment. Skin Color In humans, skin color is influenced by many things, but the pigment melanin influences most of a person’s phenotype. In general, the more melanin a person has, the darker their skin is. Albino people produce no melanin at all. The body creates more melanin to protect against the sun’s UV rays, which is why skin darkens after prolonged sun exposure. The amount and type of melanin that a person produces, such as eumelanin, pheomelanin, and neuromelanin, is controlled by multiple genes, and the different types of melanin interact to form the final phenotype. For example, people with red hair have more pheomelanin and often have a pinkish skin tone. Eye Color There are 2 major human eye color genes, OCA2 and HERC2, but at least 13 other genes also play a role. The colored part of a person’s eye is the iris. It is a muscle that changes the size of the pupil in order to change the amount of light that is absorbed by the retina. A person’s eye color is determined by the pigmentation of their irises, but also by the way the cells in their irises scatter light. As with skin color, eye color is affected by the presence of melanin. People with brown eyes have a lot of melanin, while people with blue eyes have low melanin in the front part of the iris that is visible. Green eyes are caused by multiple factors; they are the result of a light brown iris combined with a blue tone given by light scattering.
- D is correct. Having a widow’s peak, which is a V shaped front of the hairline, is not a polygenic trait. Individuals that have at least one dominant allele have a widow’s peak, while individuals with two recessive alleles have a hairline that is straight across the forehead.