Genetics and Mendelian Inheritance

      GENETICS:

Genetics is the study of inheritance, the transmission of traits from parent to offspring and the expression of these traits.  The hereditary material, DNA (deoxyribonucleic acid), found in chromosomes is organized into units called genes. Gene is a segment of the DNA molecule, and its location on specific chromosome is called locus. For any genetic character, the offspring will have two genes, one from one parent and another from another parent for that gene character on the homologous chromosome. Genes often exist in at least two alternate forms known as alleles.

The modern genetics started from Gregor Mendel (1822-1884). Mendel selected strains of garden pea with seven clearly contrasting pairs of traits. He studied only one or two of these pairs of traits at a time. Mendel carried out a series of monohybrid crosses, mating individuals that differed in only in one trait. When crossing dominant (Yellow) with recessive (green), F1 generation or first filial generation gave all dominant seed crops and F2 generation gave 3:1 or (Yellow: Green=3:1). Mendel also used test cross to support his hypothesis. A testcross involves mating an individual with an unknown genotype to a homozygous recessive individual.

Mendel also analyzed a series of dihybrid crosses, mating that involved parents that differed in two independent traits. Phenotypically offspring in F1 was with dominant character but genotypically it was heterozygous for both characters. At F2 generation phonotypical ratio was 9:3:3:1 for four possible combinations of traits. This gave to a principle called principle of independent assortment, which states that members of one gene pair segregate independently from other gene pairs during gamete formation.

There are lot of development and findings after Mendel. Whenever the heterozygous phenotype is intermediate, the genes are said to show incomplete dominance. Sometimes when neither allele is dominant, both alleles are expressed independently in the heterozygote, this condition is known as co-dominance. In Incomplete dominance, the heterozygote shows an intermediate phenotype, but in co-dominance both phenotypes are expressed.

Sometimes more than two alleles exist for a given character. For example in fruit fly, a large number of alleles affect the eye color by determining the amount of pigment produced , this is due to multiple alleles. Often a character is controlled by more than one pair of genes, and each allele has an additive effect on the same character, this is called polygenic inheritance. Chromosome are inherited as units, so genes that occur on one chromosome tend to be inherited together, a condition known as linkage. Because the genes are on the same chromosome. They move together through meiosis and fertilization.

Molecular genetics is the greatest achievement in biology.  It has identified the DNA- the genetic material. Hershey and Chase provided information about gene and Watson and Crick described the structure of DNA in 1953. They suggest that nitrogen bases always pair up in a specific pattern, with one purine base( adenine or guanine) hydrogen bonding to one pyrimidine base(thymine or cytosine).

Genes control for the proteins. The Beadle and Tatum found that, for each individual gene identified, only one enzyme was affected. Their hypothesis was later modified to state that each gene codes for one polypeptide chain.

The sequence of bases in the DNA molecule determines the sequence of amino acids in proteins, but the information in the DNA is not used directly. A molecule of messenger RNA (mRNA) is made as a complimentary copy of a gene, a proportion of one strand of the double helix. The process of RNA synthesis from DNA is called transcription. The enzyme RNA polymerase is responsible for attaching nucleotide together in the sequence of specified by DNA. The molecule of mRNA represents a gene, and each gene in an organism is represented by a different mRNA molecule. Each mRNA contains in its sequence of bases information that will be translated into sequence of amino acids that constitute a specific protein. mRNA has introns (removing segments) and Extron (expressed segments). Each of three consecutive bases of mRNA molecule constitutes a code word, or codon, that specifies a particular amino acid. This genetic code contains a total of 64 codons, with 61 of them coding for amino acids. Three codons do not code for amino acids but act to signal termination (UAA, UAG, and UGA). One codon (AUG) codes for amino acid methionine, acts as a signal to start translation.

The translation of the mRNA codons into an amino acid sequence occurs on ribosomes in the cytoplasm of the cell. In addition to mRNA, two other RNA, rRNA and tRNA function in translation. Ribosomal RNA (rRNA) joins with number of proteins to form ribosomes, the site of protein synthesis. Transfer RNA (tRNA) molecules are transport molecules that carry specific amino acids to a ribosome and align the amino acids to form a polypeptide chain.  In addition to three types of RNA involved in protein synthesis, two additional classes of RNA molecule are also involved i.e. microRNA (miRNA) and small interfering RNA (siRNA).

Mutations are the changes in DNA. Once a DNA sequence has changed DNA replication copies the altered sequence and passes it along to future generations of that cell line. The smallest mutation are called point mutation. A mutation in which a small segment of the DNA is list is known as a deletion and a mutation in which a segment is added is called an insertion. The insertion modify the mRNA reading frame, known as frame shift mutation. Mutations can occur in any cell. If they occur in cells that do not lead to gametes, they are called somatic mutations.

Recombinant DNA entails the introduction of genes from one organism into the DNA of a second organism. The formation of recombinant DNA makes use of proteins called restriction enzymes to cut a gene from its normal location. Transferring the isolated gene to another species requires the use of a vector, usually a plasmid, which is small, circular strand of DNA that also occurs in bacterial cells. The ends of the plasmid join to the ends of the gene, with the result being a recombinant DNA molecule that is transferred to a cell in another organism.

     MENDELIAN INHERITANCE

The concept of heredity started from pangenesis to blending hypothesis of inheritance, according to which, the factors that dictate hereditary traits can blend together from generation to generation. However, the pioneer work of Gregor Mendel would prove instrumental in refuting this viewpoint.

Mendel work started with hybridization concept. When two distinct individuals with different characteristics are bred, or crossed, to each other- a process called a hybridization experiment-their offspring are referred to as hybrids. Mendel chose pea plant as his experimental organism. There were couple of reason for choosing pea. It was easy for Mendel to carry out self-fertilization or cross-fertilization experiments and they were available in several varieties in which a character existed in two distinct variants. There are many laws derived from Mendel.

Law of segregation

Along with qualitative experimentation, Mendel also conducted empirical study (quantitative study). Mendel conducted single-factor crosses in which he followed the variants for single character. The results of his single factor crosses showed that the dominant trait was always observed in the F1 generation and displayed a 3:1 ratio in the F2 generation. Based on the results of his single-factor crosses, Mendel proposed three key ideas regarding inheritance.

i.                    Traits may be dominant or recessive.

ii.                  Genes are passed unaltered from generation to generation.

iii.                Two copies of a given gene segregate (or separate) from each other during transmission from parent to offspring. This third idea is known as law of segregation.

A Punnett square can be used to deduce the outcome of crosses. Mendel’s 3:1 phenotypic ratio is consistent with the law of segregation. Each of the seven character that Mendel studied is influenced by different genetic materials, known as gene.

Law of Independent Assortment:

Mendel investigated the pattern of inheritance by conducting two-factor crosses and proposed the law of independent assortment, which states that two different genes randomly assort their alleles during the formation of haploid cells. A Punnett square can be used to predict the outcome of two factor crosses.  The multiplication method and forked-line method can be used to predict the outcome of crosses involving three or more genes.

Chromosome Theory of Inheritance:

The chromosome Theory of inheritance describes how the transmission of chromosome can explain Mendel’s law. Mendel’s law of segregation is explained by the separation of homologs during meiosis. Mendel’s law of independent assortment is explained by the random alignment of different chromosomes during metaphase of Meiosis. The chromosome theory of inheritance is based on a few fundamental principles.

i.                    Chromosome contains the genetic material that transmitted from parent to offspring and from cell to cell.

ii.                  Chromosome are replicated and passed along, generation after generation, from parent to offspring. They are also passed from cell to cell during development of a multicellular organism. Each type of chromosome retains its individuality during cell division and gamete formation.

iii.                The nuclei of most eukaryotic cells contain chromosomes that are found in homologous pairs-they are diploid. One member of each pair is inherited from the mother, the other from the father. At Meiosis, one of the two members of each pair segregates into the other daughter nucleus. Gametes contain one set of chromosome-they are haploid.

iv.                 During the formation of haploid cells, different types of (nonhomologous) chromosomes segregate independently of each other.

v.                   Each parent contributes one set of chromosomes to its offspring. The material and parental sets of homologous chromosomes are functionally equivalent: each set carries a full complement of genes.

Studying Inheritance Pattern in Humans

Human inheritance pattern are determined by analyzing family trees known as pedigrees analysis. This is commonly used to determine the inheritance pattern of human genetic disease.

Conclusion: Thus heredity and Inheritance has been defined by Mendel and passed through many generation and have many advancement like recombinant DNA techniques, nowadays.

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