Non-Mendelian Inheritance PDF

Gregor Mendel knew how to keep things simple. In Mendel's work on pea plants, each gene came in just two different versions, or alleles, and these alleles had a nice, clear- cut dominance relationship (with the dominant allele fully overriding the recessive allele to determine the plan...

Gregor Mendel knew how to keep things simple. In Mendel's work on pea plants, each gene came in just two different versions, or alleles, and these alleles had a nice, clear- cut dominance relationship (with the dominant allele fully overriding the recessive allele to determine the plant's appearance). Today, we know that not all alleles behave quite as straightforwardly as in Mendel’s experiments. For example, in real life: Allele pairs may have a variety of dominance relationships (that is, one allele of the pair may not completely “hide” the other in the heterozygote). There are often many different alleles of a gene in a population. In these cases, an organism's genotype, or set of alleles, still determines its phenotype, or observable features. However, a variety of alleles may interact with one another in different ways to specify phenotype. NON-MENDELIAN INHERITANCE CODOMINANCE  When the alleles for a particular trait are codominant, they are both expressed equally rather than dominant allele taking complete control over recessive allele. This means that when an organism has two different alleles, both will be expressed at the same time.  For example: If a person is blood group A, it means the RBC surface consists of antigen-A.But this is decided by the gene I. The gene I have three types of alleles namely, IA, IB and i. The alleles IA and IB produce two different antigens while the allele-i do not produce any antigen. Hence, alleles IA and IB are dominant over the allele i. As we know, each diploid organism bears two pairs of alleles. Hence, in humans, there are two types of alleles of any combination. Depending on the combination and dominance of allele blood type of an individual could be determined. The different combination of alleles and their type of blood groups are given. In the example, A person with blood group A indicates that he has an IA and i pair of alleles. This is because the allele i is recessive in character and no antigen is produced. However, a person who possess both the alleles IA and IB, they have blood group AB. This is because of alleles IA and IB­are codominant. Both the gene will produce their type of antigen. Source: https://byjus.com/biology/co-dominance-and-multiple-alleles/  Codominance means that neither allele can mask the expression of the other allele. An example in humans would be the ABO blood group, where alleles A and alleles B are both expressed. So if an individual inherits allele A from their mother and allele B from their father, they have blood type AB. BLACK AND WHITE COAT In other animals, codominance is exemplified by a mix of coat colors in a progeny of parents with different coat colors. For example, a cross between a black-furred male dog and a white-furred female dog could produce an offspring with a black-and-white coat. This means that the color coat traits of the parents are codominant as both were expressed together in the progeny.  White-spotted red flower Codominance is exemplified by a plant that bears flowers with two distinct color phenotypes. For instance, a white-spotted red flower could be caused by a cross between a red flower and a white flower. The alleles for the red and white color INCOMPLETE DOMINANCE  Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike complete dominance inheritance, one allele does not dominate or mask the other.  With incomplete dominance, the traits are blended together, rather than occurring in discrete patches like speckled flowers. Example: if a flower shows incomplete dominance then two different flowers crossed together will produce a hybrid that’s between both od the parents. (white flower crossed with a red flower will produce a pink flower) Example: Snapdragons As an example, incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. In this monohybrid cross, the allele that produces the red color (R) is not completely expressed over the allele that produces the white color (r). The resulting offspring are all pink. The genotypes are: Red (CRCR) X White (CWCW) = Pink (Rr). When the first filial (F1) generation consisting of all pink plants is allowed to cross-pollinate, the resulting plants (F2 generation) consist of all three phenotypes [1/4 Red (CRCR): 1/2 Pink (CRCW): 1/4 White (CWCW)]. The phenotypic ratio is 1:2:1. When the F1 generation is allowed to cross-pollinate with true breeding red plants, the resulting F2 plants consist of red and pink phenotypes [1/2 Red (CRCR): 1/2 Pink (CRCW)]. The phenotypic ratio is 1:1. When the F1 generation is allowed to cross-pollinate with true breeding white plants, the resulting F2 plants consist of white and pink phenotypes [1/2 White (CWCW): 1/2 Pink (CRCW)]. The phenotypic ratio is 1:1. Incomplete Dominance in Animals Incomplete dominance in animals is most widely studied in domestic animals since it's important for their health, appearance, and value. Here are several examples of the effects of incomplete dominance in animals: Chickens with blue feathers are an example of incomplete dominance. When a black and a white chicken reproduce and neither allele is completely dominant, the result is a blue- feathered bird. When a long-furred Angora rabbit and a short-furred Rex rabbit reproduce, the result can be a rabbit with fur longer than a Rex, but shorter than an Angora. That's a classic example of incomplete dominance producing a trait different from either of the parents.  Tail length in dogs is often determined by incomplete dominance. Pups of long-tailed and short-tailed parents often split the difference and have medium-length tails. On the subject of dogs, lots of labradoodles have wavy hair. Just like humans, that comes from having straight- haired and curly-haired parents. The result is an intermediate inheritance: the wavy-haired labradoodle. The cream gene in horses is a classic incomplete dominant. When paired with a red allele, the cream allele produces horses with golden coats such as palominos and buckskins. Incomplete Dominance in Plants The science of genetics began with plants. People have been interbreeding plants for particular traits since we first started farming more than 11,000 years ago. Gregor Mendel, one of the founders of genetic science, began his studies by recording the ways he planted his garden. Whether for food, other uses, or simple beauty, humans have employed genetic selection of plants, including incomplete dominance, throughout our history. Incomplete dominance was first recorded in plants. The German scientist Josef Kolreuter bred red and white carnations, expecting to get offspring with the dominant red coloration. Instead, many came up pink! Kolreuter found that neither allele was fully dominant in his flowers and identified the concept of incomplete dominance.  Four-o-clocks are flowering plants that get their funny name from their inclination to bloom in the late afternoon. Wild four-o-clocks tend to have red flowers, while "pure" four-o-clocks with no coloration genes are white. Mixing the two results in pink flowers, just like Dr. Kolreuter's carnations. Those pink flowers are a result of incomplete dominance. However, mixing the pink flowers results in ¼ red, ¼ white and ½ pink. That 1:2:1 ratio - a quarter like one parent, a quarter like the other, and the remaining half different from either - is common in cases of incomplete dominance.  Pink snapdragons are a result of incomplete dominance. Cross- pollination between red snapdragons and white snapdragons result in pink when neither the white or the red alleles are dominant. The fruit color of eggplants is another example of incomplete dominance. Combining deep purple eggplants with white eggplants results in eggplants of a light violet color. Incomplete dominance is a key element of improving crops such as corn. Corn with multiple incompletely dominant traits is generally healthier and provides greater yields than "purer" strains with fewer such traits. Just compare the original plant, teosinte, with a modern ear of corn to see the genetic difference! Incomplete Dominance in Humans Incomplete dominance is rare in humans; we're genetically complex and most of our traits come from multiple genes. However, there are a few examples. Incomplete dominance is just part of what makes our species so complicated and interesting. The disease familial hypercholesterolemia (FH) is an example of incomplete dominance. One allele causes liver cells to be generated without cholesterol receptors, while another causes them to be generated normally. The incomplete dominance causes the generation of cells that do not have enough receptors to remove all dangerous cholesterol from the bloodstream. Incomplete Dominance in Humans Tay-Sachs Disease is an example of incomplete dominance in humans. This neurological disease is caused by an enzyme imbalance and is autosomal recessive; that is, people who actually suffer from the disease have two recessive genes that cause it. However, one or both of their parents may have been carriers who had incompletely dominant genes, causing them to produce one half of the necessary enzyme, which is enough for a normal life. When one parent with straight hair and one with curly hair have a child with wavy hair, that's an example of incomplete dominance. MULTIPLE ALLELES  A gene with more than two alleles are said to have multiple alleles 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). Examples of Multiple Alleles Two human examples of multiple-allele genes are the gene of the ABO blood group system, and the human- leukocyte-associated antigen (HLA) genes. The ABO system in humans is controlled by three alleles, usually referred to as IA, IB, and i (the "I" stands for isohaemagglutinin). IA and IB are codominant and produce type A and type B antigens, respectively, which migrate to the surface of red blood cells, while IO is the recessive allele and produces no antigen.  POLYGENIC INHERITANCE Many traits and phenotypic characters present in plants and animals such as height, skin pigmentation, hair and eye colour, milk and egg production are inherited through many alleles present in different loci. This is known as polygenic inheritance. Polygenic inheritance is defined as quantitative inheritance, where multiple independent genes have an additive or similar effect on a single quantitative trait.” Characteristics Polygene refers to a gene that exerts a slight effect on a phenotype along with other genes Effect of a single gene is too small, so it is difficult to detect Multiple genes produce an equal effect Each allele has a cumulative or additive effect Polygenic inheritance differs from multiple alleles, as in multiple alleles, three or more alleles are present in the same locus of which any two alleles are present in an organism, e.g. ABO blood group system, which is controlled by three alleles There is no epistasis involved, i.e. masking of the expression of an allele of the different locus There is no linkage or dominance, rather there exist contributing and non-contributing alleles, which are known as active or null alleles respectively Polygenic inheritance is characteriZed by the continuous variation of the phenotype of a trait The polygenic inheritance pattern is complex. It is difficult to predict phenotype The statistical analysis can give the estimate of population parameters Polygenic Inheritance in Humans There are many traits in humans, which show polygenic inheritance, e.g. skin and hair color, height, eye color, the risk for diseases and resistance, intelligence, blood pressure, bipolar disorder, autism, longevity, etc. Skin pigmentation: inheritance of skin pigmentation is polygenic inheritance. Around 60 loci contribute to the inheritance of a single trait. If we take an example of a pair of alleles of three different and unlinked loci as A and a, B and b, C and c. The capital letters represent the incompletely dominant allele for dark skin color. The more capital letters show skin color towards the darker range and small letters towards the lighter color of the skin. Parents having genotype AABBCC and aabbcc will produce offspring of intermediate colour in the F1 generation, i.e. AaBbCc genotype. In the F2 generation of two triple heterozygotes (AaBbCc x AaBbCc) mate, they will give rise to varying phenotypes ranging from very dark to very light in the ratio 1:6:15:20:15:6:1. 1.Height: There are around 400 genes responsible for the phenotype and environment greatly influences the expression of genes. 2.Eye color: The color of the eye is determined by polygenes. At least 9 colors of eye color are recognized in humans. There are two major eye color genes and 14 more genes that determine the expression of the phenotype. A different number of alleles contribute to each color. These are found to be X-linked.  In this example, there are three genes that make reddish pigment in wheat kernels, which we’ll call A, B, and C. Each comes in two alleles, one of which makes pigment (the capital-letter allele) and one of which does not (the lowercase allele). These alleles have additive effects: the aa genotype would contribute no pigment, the Aa genotype would contribute some amount of pigment, and the AA genotype would contribute more pigment (twice as much as Aa). The same would hold true for the B and C genes\ [^{1,4}\]. Polygenic Inheritance in Plants Polygenic inheritance in plants includes the colour and shape of the stem, pollen, flower, yield, oil content, size of a seed, time to mature or flower, etc. Brief description of some of the traits: 1.Kernel colour of the wheat: The three independent pairs of alleles are involved in the expression of kernel colour of wheat. They show independent assortment. When dark red wheat kernel (AABBCC) is crossed with the white wheat kernel (aabbcc) the F1 generation has an intermediate red colour kernel (AaBbCc). When F1 generation is crossbred, F2 generation has 63 red kernel plants having different shades of red and 1 white kernel Effect of environment on Polygenic Inheritance The expression of polygenes is greatly influenced by environmental conditions. The genotype sets the range for a quantitative trait, but the environmental conditions decide the phenotype within its genetic limits. Genes function differently in different environmental conditions. Environment regulates the activity of certain genes and sets them on or off. The range of phenotype possible under the different environmental conditions from the same genotype is termed as ‘norm of reaction’. The norm of the reaction is narrow for certain genotypes and broad for some genotypes, e.g. genotypes involved in human height have a very broad norm of reaction. Identical twins raised in two different environments show that individuals may have genetic potential or vulnerability, but environmental conditions influence the expression of genotype. Human characters such as intelligence, depression, height, skin colour, schizophrenia show the effect of the environment on gene expression. Phenotypic expression is dependent on both nature and nurture.  SEX LINKAGE Sex linkage applies to genes that are located on the sex chromosomes. These genes are considered sex-linked because their expression and inheritance patterns differ between males and females. While sex linkage is not the same as genetic linkage, sex-linked genes can be genetically linked. Source: learn.genetics.utah.edu SEX CHROMOSOMES Sex chromosomes determine whether an individual is male or female. In humans and other mammals, the sex chromosomes are X and Y. Females have two X chromosomes, and males have an X and a Y. Non-sex chromosomes are also called autosomes. Autosomes come in pairs of homologous chromosomes. Homologous chromosomes have the same genes arranged in the same order. So for all of the genes on the autosomes, both males and females have two copies. SEX CHROMOSOMES A female’s two X chromosomes also have the same genes arranged in the same order. The X and Y chromosomes, however, have different genes. So, for the genes on the sex chromosomes, males have just one copy. The Y chromosome has few genes, but the X chromosome has more than 1,000. Well-known examples in people include genes that control color blindness and male pattern baldness. These are sex- linked traits.  Inheritance of Sex Chromosomes in Mammals Meiosis is the process of making gametes, also known as eggs and sperm in most animals. During meiosis, the number of chromosomes is reduced by half, so that each gamete gets just one of each autosome and one sex chromosome. Female mammals make eggs, which always have an X chromosome. And males make sperm, which can have an X or a Y. Inheritance of Sex Chromosomes in Mammals Egg and sperm join to make a zygote, which develops into a new offspring. An egg plus an X-containing sperm will make a female offspring, and an egg plus a Y-containing sperm will make a male offspring. Female offspring get an X chromosome from each parent Males get an X from their mother and a Y from their father X chromosomes never pass from father to son Y chromosomes always pass from father to son SEX-LINKED TRAITS Are genetic characteristics determined by genes located on sex chromosomes. Like traits that originate from genes on autosomes, sex-linked traits are passed on from parents to offspring through sexual reproduction. Genes that are found on sex chromosomes are called sex-linked genes. These genes can be either on either the X chromosome or Y chromosome. Y-linked gene (inherited only by males) X-linked genes (can be inherited by both male and female) X-LINKED RECESSIVE TRAITS The phenotype is expressed in males because they only have one X chromosome. The phenotype may be masked in females if the second X chromosome contains a normal gene for the same trait. Example is HEMOPHILIA.  CRITERIA for an X-LINKED RECESSIVE TRAIT Always expressed in the male. Expressed in a female homozygote and very rarely in a heterozygote. Affected male inherits trait from heterozygote or homozygote mother. Affected female inherits trait from affected father and affected or heterozygote mother. ICHTHYOSIS X-LINKED DOMINANT TRAITS Dominant X-linked conditions and traits are rare. The phenotype is expressed in both males and females who have an X chromosome that contains the abnormal gene. A female who inherits a dominant X-linked allele or in whom the mutation originates has the associated trait or illness, but a male who inherits the allele is usually more  CRITERIA for an X-linked dominant trait Expressed in females in one copy. Much more severe effects in males. High rates of miscarriage due to early lethality in males. Passed from male to all daughters but not to sons.  EXAMPLES OF SEX-LINKED DISORDERS MALE INFERTILITY (Y-linked) Duchenne muscular dystrophy, Color blindedness, and fragile-X syndrome (X-linked recessive) COLOR BLINDEDNESS Has difficulty seeing color differences. Red-green color blindness is the most common form and is characterized by the inability to distinguish shades of red and green. DUCHENNE MUSCULAR DYSTROPHY Is a condition that causes muscle degeneration. It is the most common and severe form of muscular dystrophy that quickly worsens and is fatal FRAGILE X SYNDROME Is a condition that results in learning, behavioral, and intellectual disabilities. It affects about 1 in 4, 000 males and 1 in 8, 000 females. SEX-INFLUENCED TRAITS  An allele is dominant in one sex but recessive in the other. Such gene may be X-linked or autosomal. The difference in expression can be caused by hormonal differences between sexes.  EXAMPLE An autosomal gene for hair growth pattern has two alleles, one that produces hair all over the head and another that causes baldness. The baldness allele is dominant in males but recessive in females, which is why more males are bald than females. A heterozygous male is bald but a heterozygous female is not. SEX-LIMITED TRAITS  Affects a structure or function of the body that is present in only males or females. The gene for such trait mat be X-linked or autosomal. EXAMPLE: In humans, beard growth is sex-limited. A woman does not grow a beard because she does not manufacture the hormones required for facial hair growth. She can however, pass to her sons the genes specifying heavy beard growth. References https://www.khanacademy.org/science/ap-biology/ heredity/environmental-effects-on-phenotype/a/ polygenic-inheritance-and-environmental-effects https://byjus.com/neet/polygenic-inheritance/ https://learn.genetics.utah.edu/content/pigeons/ sexlinkage/