MENDELIAN AND NON MENDELIAN INHERITANCE DIFFERENCE.

 

                                                      

MENDELIAN AND NON MENDELIAN INHERITANCE DIFFERENCE.

Mendlians inheritance is type of biological inheritance that follows the laws originally proposed by Gregor Mendel in 1865 and 1866 and rediscovered in 1900. These laws were controversial. Mendel discovered that, when he crossed pubered white flower with purple flower pea plants (the parental or P generation), the result was not a blend. Rather than being a mix of two, the offspring (known as F₁ generation) was purple flowered. When Mendel self-fertilized the F₁ generation pea plants he obtained a purple flower to white flower ratio in F₂ generation of 3 to 1. The result of this cross are tabulated in the Punnett square to the right. He then conceived the idea of heredity units which he called “factors”.  Mendel found that there are alternative forms of factors – now called genes that account for variations in inherited characteristics. For example the gene flower colour in pea plants exists in two forms, one for purple and the other for white. The alternative forms are now called alleles. For each biological trait, an organism inherits two alleles one from each parent. These alleles may be same or different.  An organism that has two identical alleles for a gene is said to be homozygous for that gene (and is called a homozygote). An organism that has two different alleles for a gene is said to be heterozygous for that gene (and is called a heterozygote).

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Mendel hypothesized that allele pairs separate randomly or separate randomly or segregate from each other during the production of gametes egg and sperm. Because allele pairs separate during gamete production, a sperm and egg carries only one allele for each inherited trait. When sperm and egg unite at fertilization, each contributes its allele restoring the paired condition in the offspring. This is called the Law of segregation. In it during gamete formation, the alleles for each other so that each gamete carries only one allele for each gene. Mendel also found that each pair of alleles segregates independently of the other pairs of alleles during gamete formation. This is known as the Law of independent Assortment.  In this law genes of different traits can segregate independently during the formation of gametes. The genotype of an individual^, s physical appearance, or phenotype, is determined by its alleles as well as by its environment. The presence of an allele does not mean that the trait will be expressed in the individual that possesses it.

If the two alleles of an inherited pair differ then one determines the organism appearance and I called the recessive allele. Thus in the example above the dominant purple flower allele hide the phenotypic effect of the recessive white flower allele. This is known as the Law of Dominance but it is not transmission law it concerns the expression of the genotype. The upper letter are used to represent the dominant allele and the lower case letter to represent recessive alleles.

NON MENDELIAN INHERITANCE

Mendel explained inheritance in terms of discrete factors-genes- that are passed along from one generation to generation according to the rules of probability. Mendel laws are valid for all sexually reproducing organisms including garden peas and human beings. However Mendel laws stop short of explaining some patterns of genetic inheritance. For most sexually reproducing organisms cases where Mendel laws can strictly account or the patterns of inheritance are relatively rare often inheritance patterns are more complex. The F₁ offspring of Mendel peas crosses always look like one of the two parental varieties. In this situation of “complete dominance “the dominant allele had the same phenotypic effect weather present in one or two copies. But for some characteristics, the F₁ hybrids have an appearance in between the phenotypes of the two parental varieties. A cross between two four clock plants shows this common exception to Mendel principles. Some alleles are neither dominant nor recessive the F₁ generation produced by a cross between red flowered and white flowered Mirabilis jalapa plants consist of pink coloured flower. Which allele is dominant in this case nether one? The inheritance of characteristics are not as simple as it is for the characteristics that Mendel studied in pea plants. Each characteristic that Mendel investigated was controlled by one gene that had two possible alleles, one of which was completely dominant to the other. This resulted in just two possible phenotypes for each characteristic. Each characteristic Mendel studied was also controlled by a gene on a different chromosome .a characteristic may be controlled by one gene with two alleles, but the two alleles may have a different relationship than the simple dominant recessive relationship

CODOMINANCE

Codominance occurs when both alleles are expressed equally in the phenotype of the heterozygote. The red and white petals has codominant alleles. The flower has red and white petals because of codominance of red and white petal alleles

INCOMPLETE DOMINANCE

Incomplete dominance occurs when the phenotype of the offspring is somewhere in between the phenotypes of both parents; a completely dominant allele does not occur. For example when red snapdragons C^RC^R are crossed with white the F₁hybrids are all pink heterozygotes for flower colour C^RC^W.The pin colour is an intermediate between the two parent colours when the two F₁ hybrids are crossed they will produce red pink and white flowers. The genotype of an organism with incomplete dominance can be determined from its phenotype

MULTIPLE ALLELES

Many genes have multiple alleles. In humans ABO blood type is an example. Three common alleles for the gene that controls this characteristic. The alleles I^AI^B are dominant over i. a person who is homozygous recessive ii has a type o blood. Type A and type B parents can have a type AB child. Type A and type B can also have a child with type O blood. If they are both heterozygous.

POLYGENIC CHARACTERISTICS

Polygenic characteristics are controlled by more than one gene and each gene may have two are more alleles. The genes may be on the same chromosome or on non-homozygous chromosomes.

If the genes are located close together on the same chromosome they are likely to be inherited together. However it is possible that they will be separated by crossing over during meiosis, in which case they may be inherited independently of one another.

If the genes are on non-homologous chromosomes, they may be recombine in various ways because of independent assortment. For these reasons the inheritance of polygenic characteristics is very complicated. Skin colour and adult height are examples of polygenic characteristics in humans.

Environmental factors such as sunlight and food availability can affect how genes are expressed in the phenotype of individual. Genes play important part in determining our adult height. However factors such as poor nutrition can prevent us from achieving our full genetic potential.

(Q. 2) DEFINE THE FOLLOWING TERMS IN YOUR OWN WORDS.

i) HIGH ORDER CHROMATIN FOLDING

Higher order chromatin folding is defined any assemblage of nucleosomes that assumes a reproducible conformation in 3D space. The most obvious chromatin higher order structure is the mitotic meiotic chromosome in which the DNA is compacted some 10,000 to 20,000 fold. Metaphase chromosomes have characteristic shapes banding patterns, and location of specific genes. The concept of primary secondary tertiary and quaternary structures used for proteins can also be usefully applied to chromatin  folding with the beads  on a sting organization of nucleosomes and their linker DNA constituting the primary structure and resulting from interactions between nucleosomes giving rise to secondary structure. Chromatin consist of repeating chains of more or less identical nucleosomes and form highly order secondary structures. Every 200 nucleotides the DNA duplex is coiled around a core of eight histone proteins forming a complex known as nucleosomes. Unlike proteins which have overall negative charge histones are positively charged due to an abundance of the basic amino acids arginine and lysine . Thus they are are strongly attracted to the negatively charged phosphate groups of the DNA. The histone core thus acts as magnetic forms that promote and guide the coiling of the DNA. Further coiling occurs when the sting of nucleosomes wraps up into higher order coils called supercoils.DNA wraps around histone proteins forming nucleosomes and the so called “beads on a sting” structure. Multiple histones wrap into a 30- nanometre fibre consist of nucleosomes arrays in their most compact form. Higher level DNA supercoiling of the 30 nm fibres produces the metaphase chromosome.

Ii) INTERPHASE

Interphase is the phase of the cell cycle in which cell spends most of its life. During this phase cell copies DNA in preparation for mitosis. Interphase is the daily living or metabolic phase of the cell in which the cell obtains nutrients and metabolizes them grows reads its DNA and conducts “normal” cell function. Eukaryotic cells spend most of their time in interphase. This phase was formerly called the resting phase. In interphase the cell gets itself ready for mitosis or meiosis. Somatic cells or normal diploid cells of the body go through mitosis in order to reproduce themselves through cell division, whereas diploid germ cells primary spermatocytes and primary oocytes go through meiosis in order to create haploid gametes for the purpose of sexual reproduction.

iii) LAMPRUSH CHROMOSOME

Lamprush chromosomes are special form of chromosomes found in the growing oocytes of most animals, except mammals. Walther Fleming described them in 1882. Lamprush chromosomes of amphibians, birds and insects are described best of all. Chromosomes transform into the lamp rush form during the diplotene stage of meiotic prophase I due to an active transcription of many genes. Lamprush chromosomes are visible in light microscope, where they are seen to be organized into a series of chromosomes with large chromatin loops extended laterally. Giant chromosomes in the lamp rush form are useful model for studying chromosomes organization, genome function and gene expression during meiotic prophase since they allow the individual transcription units to be visualized. Lamprush chromosomes are widely used for high resolution mapping of DNA sequence and construction of detail cytological maps of individual chromosomes.

Iv) MENDEL’ S LAW OF DOMINANCE

Mendel law of dominance states that recessive alleles will always be masked by dominant alleles. Therefore a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype. the law of dominance can be explained with the help of mono hybrid cross experiment in a cross between two organisms pure for any pair of contrasting traits  the character that appears in the F₁ generations called dominant and the one which is not expressed is called “recessive”  . Each character is controlled by a pair of dissimilar factors. Only one of the characters expresses. The one which expresses in the F1 generation is called Dominant. However law of dominance is not universally applicable.