Tuesday 22 September 2009

Heredity

An important result of the reproductive cycle is production of similar featured individuals for generations. The transmission of characteristics through successive generations refers to heredity. These characteristics include all physical, physiological and psychological characteristics in organisms. We refer to them as traits. The study of the mechanism of transmission of characteristics from parents to the next generations is called genetics.In case of height, the tall plants of P generation are represented as TT and the short plants as tt. Each parent produces only one type of gamete. Tall plants produce gametes having allele 'T' and short plants produce allele having t gametes. When these two types of gametes fuse, the resultant plants are heterozygous (Tt). However, only tallness is expressed, as all plants of this generation are tall. Thus, tallness is the dominant allele and shortness is the recessive allele. Thus the recessive allele remains hidden in the first generation and becomes expressed on self-pollination in the next generation. The F1 generation plants are then self-pollinated. This results in tall and short plants in the ratio 3:1. This is called the phenotypic ratio. Of the tall plants, 1/3rd of the plants are homozygous for tallness and the remaining 2/3rd are heterozygous. The heterozygous plants on self-fertilization again result in offspring that show 3:1 ratio for tallness to dwarf ness. Thus the genotypic ratio in the F2 generation is 1:2:1. This means that 1/3rd are tall and homozygous, 2/3rd are tall and heterozygous and 1/3rd are dwarf and homozygous. Note that recessive traits are phenotypically expressed only in the homozygous genotype. Similar crossing experiments were also carried out with more than one trait. However, it is not possible to follow all the traits at the same time. The other traits studied by Mendel in garden pea were colour of the seed, the shape of the seed, colour of the seed coat, the flower colour, flower position, pod colour and pod shape. The crosses that study two traits together are called the dihybrid crosses. In the dihybrid crosses, it was found that the two traits were inherited independent of each other. The dominant alleles for each of the two traits asserted their dominance independent of the other. Based on his experiments Mendel explained the mechanism of inheritance with the help of laws he formulated. These laws summarizing his observations are: Law of Unit Characters Law of Dominance Law of Segregation Law of Independent Assortment Law of Unit Characters According to this law, all the traits are separate entities or units by themselves. Their inheritance is controlled by 'factors', now known as genes. Law of Dominance Each gene has more than one form of expression. These forms of expressions are called alleles or allelomorphs. Each pair of alleles will have a dominant allele and a recessive allele. In the presence of the dominant gene, the recessive gene will never express itself. Only if both the alleles are recessive, the recessive trait will be exhibited. Law of Segregation This is also called the law of purity of gametes. According to this law, the gametes are pure for a particular trait. This is because the alleles of a pair separate during gamete formation and again come together after fertilization. Therefore, in each gamete only either of the two alleles is present and it is pure for that trait. Law of Independent Assortment This law explains how more than one trait is inherited. According to this law, when there are two pairs of alleles, all four alleles assert themselves independently and are inherited independently. How do these Traits get Expressed? The genetic material that controls a characteristics or a trait is a nucleoprotein called chromatin. The chromatin material just before cell division forms into chromosomes. Each chromosome is made up of two longitudinal strands called the chromatids. Inherited Traits A man's characteristics, such as height, colour of the eyes, intelligence, etc. are all inheritable. Often we see that the children of parents with blue eyes would get blue eyes; baldness passes on in the family from one generation to another; even disorders like diabetes and heart diseases are often inherited. One should know how these traits pass on and the rules that govern the inheritance of traits. Rules for the Inheritance of Traits - Mendel's Contribution Gregor Mendel, an Austrian monk during 1850s was the first to carry out scientific studies on transmission of characteristics from the parent to the offspring. He selected garden peas as they showed many contrasting traits that were easy to track through the generations. Moreover, these plants are naturally self-pollinated. This made it easy for getting pure lines. Pure line plants will produce only plants of their own type. That is, if a plant bears red flowers, the new plants from its seeds also will bear only red flowers. Thus, these plants are pure for the red flower colour. For cross-pollination, Mendel opened the bud and removed the stamens. This is called emasculation. The emasculated flower was covered with plastic bag. When the flower opened, the stigma was dusted with the pollen of another flower. This produced crossbreeds. Monohybrid Cross In the first experiment, Mendel considered the phenotype - an externally exhibited trait only one at a time. Some of the traits he considered were the height, flower colour, shape of the seed, etc. He first ensured that he had pure-bred tall and pure-bred short plants by selecting seeds from plants that had been self-pollinating for many generations. (Self-pollination is ensured by keeping the flower covered with plastic till seed formation). He selected one tall and one short plant from the pure-bred. He called this the P or parental generation. He cross-pollinated the tall and short plants. In the first generation, he got all tall plants. This generation was called the first filial or F1 generation. Then the plants from F1 generation were self-pollinated and the next generation was called the second filial or the F2 generation. In this generation, the ratio of tall to short plants was found to be 3:1. Of the tall plants, one-third was found to breed true and the other two-thirds on self pollinating again produced plants in 3:1 ratio of tall and short plants. The plants that are produced in the F1 generation are called hybrids as they have a mixture of traits of both the parents. Since, in this case only one trait, i.e., height was considered; this cross is called the monohybrid cross. Physical Basis of Heredity According to Mendel, the genotype i.e., the combination of genes or the genetic constitution is made of certain structures called factors. The factors controlled the inheritance of all traits. These factors are present in pairs. The factors are now called genes. Thus the physical basis of heredity is the genes or factors. The different expressions of the same genes are called alleles. Each trait may be represented with an alphabet. If the alphabet T represents height, T represents tallness and t represents shortness. If the letter R represents the colour of the flower, R represents red and r represents white. General convention is to represent the dominant character or the character expressed in F, generation by capital letter. Further, each individual for sexual reproduction produces gametes. Gametes are haploid. Each gamete will have only one allele for each trait. Thus the gametes will be of two kinds - having allele T or t. Since each individual is formed by the fusion of gametes, they are represented by writing two letters - TT, Tt or tt depending on their origin. If the plants are pure breeds producing only one type of offspring, they are called homozygous (TT or tt). If the plants produce both tall and short plants among the offspring, then they are heterozygous (Tt).The chromosomes are present in pairs. The pairs are called the homologous pairs. A species will always have the same number of chromosomes. This is called the chromosome number and it will always be an even number. This number is called the diploid number. During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the haploid number. Thus the somatic or the vegetative cells of all organisms are diploid and the gamet Heredity An important result of the reproductive cycle is production of similar featured individuals for generations. The transmission of characteristics through successive generations refers to heredity. These characteristics include all physical, physiological and psychological characteristics in organisms. We refer to them as traits. The study of the mechanism of transmission of characteristics from parents to the next generations is called genetics. Inherited Traits A man's characteristics, such as height, colour of the eyes, intelligence, etc. are all inheritable. Often we see that the children of parents with blue eyes would get blue eyes; baldness passes on in the family from one generation to another; even disorders like diabetes and heart diseases are often inherited. One should know how these traits pass on and the rules that govern the inheritance of traits. Rules for the Inheritance of Traits - Mendel's Contribution Gregor Mendel, an Austrian monk during 1850s was the first to carry out scientific studies on transmission of characteristics from the parent to the offspring. He selected garden peas as they showed many contrasting traits that were easy to track through the generations. Moreover, these plants are naturally self-pollinated. This made it easy for getting pure lines. Pure line plants will produce only plants of their own type. That is, if a plant bears red flowers, the new plants from its seeds also will bear only red flowers. Thus, these plants are pure for the red flower colour. For cross-pollination, Mendel opened the bud and removed the stamens. This is called emasculation. The emasculated flower was covered with plastic bag. When the flower opened, the stigma was dusted with the pollen of another flower. This produced crossbreeds. Monohybrid Cross In the first experiment, Mendel considered the phenotype - an externally exhibited trait only one at a time. Some of the traits he considered were the height, flower colour, shape of the seed, etc. He first ensured that he had pure-bred tall and pure-bred short plants by selecting seeds from plants that had been self-pollinating for many generations. (Self-pollination is ensured by keeping the flower covered with plastic till seed formation). He selected one tall and one short plant from the pure-bred. He called this the P or parental generation. He cross-pollinated the tall and short plants. In the first generation, he got all tall plants. This generation was called the first filial or F1 generation. Then the plants from F1 generation were self-pollinated and the next generation was called the second filial or the F2 generation. In this generation, the ratio of tall to short plants was found to be 3:1. Of the tall plants, one-third was found to breed true and the other two-thirds on self pollinating again produced plants in 3:1 ratio of tall and short plants. The plants that are produced in the F1 generation are called hybrids as they have a mixture of traits of both the parents. Since, in this case only one trait, i.e., height was considered; this cross is called the monohybrid cross. Physical Basis of Heredity According to Mendel, the genotype i.e., the combination of genes or the genetic constitution is made of certain structures called factors. The factors controlled the inheritance of all traits. These factors are present in pairs. The factors are now called genes. Thus the physical basis of heredity is the genes or factors. The different expressions of the same genes are called alleles. Each trait may be represented with an alphabet. If the alphabet T represents height, T represents tallness and t represents shortness. If the letter R represents the colour of the flower, R represents red and r represents white. General convention is to represent the dominant character or the character expressed in F, generation by capital letter. Further, each individual for sexual reproduction produces gametes. Gametes are haploid. Each gamete will have only one allele for each trait. Thus the gametes will be of two kinds - having allele T or t. Since each individual is formed by the fusion of gametes, they are represented by writing two letters - TT, Tt or tt depending on their origin. If the plants are pure breeds producing only one type of offspring, they are called homozygous (TT or tt). If the plants produce both tall and short plants among the offspring, then they are heterozygous (Tt). In case of height, the tall plants of P generation are represented as TT and the short plants as tt. Each parent produces only one type of gamete. Tall plants produce gametes having allele 'T' and short plants produce allele having t gametes. When these two types of gametes fuse, the resultant plants are heterozygous (Tt). However, only tallness is expressed, as all plants of this generation are tall. Thus, tallness is the dominant allele and shortness is the recessive allele. Thus the recessive allele remains hidden in the first generation and becomes expressed on self-pollination in the next generation. The F1 generation plants are then self-pollinated. This results in tall and short plants in the ratio 3:1. This is called the phenotypic ratio. Of the tall plants, 1/3rd of the plants are homozygous for tallness and the remaining 2/3rd are heterozygous. The heterozygous plants on self-fertilization again result in offspring that show 3:1 ratio for tallness to dwarf ness. Thus the genotypic ratio in the F2 generation is 1:2:1. This means that 1/3rd are tall and homozygous, 2/3rd are tall and heterozygous and 1/3rd are dwarf and homozygous. Note that recessive traits are phenotypically expressed only in the homozygous genotype. Similar crossing experiments were also carried out with more than one trait. However, it is not possible to follow all the traits at the same time. The other traits studied by Mendel in garden pea were colour of the seed, the shape of the seed, colour of the seed coat, the flower colour, flower position, pod colour and pod shape. The crosses that study two traits together are called the dihybrid crosses. In the dihybrid crosses, it was found that the two traits were inherited independent of each other. The dominant alleles for each of the two traits asserted their dominance independent of the other. Based on his experiments Mendel explained the mechanism of inheritance with the help of laws he formulated. These laws summarizing his observations are: Law of Unit Characters Law of Dominance Law of Segregation Law of Independent Assortment Law of Unit Characters According to this law, all the traits are separate entities or units by themselves. Their inheritance is controlled by 'factors', now known as genes. Law of Dominance Each gene has more than one form of expression. These forms of expressions are called alleles or allelomorphs. Each pair of alleles will have a dominant allele and a recessive allele. In the presence of the dominant gene, the recessive gene will never express itself. Only if both the alleles are recessive, the recessive trait will be exhibited. Law of Segregation This is also called the law of purity of gametes. According to this law, the gametes are pure for a particular trait. This is because the alleles of a pair separate during gamete formation and again come together after fertilization. Therefore, in each gamete only either of the two alleles is present and it is pure for that trait. Law of Independent Assortment This law explains how more than one trait is inherited. According to this law, when there are two pairs of alleles, all four alleles assert themselves independently and are inherited independently. How do these Traits get Expressed? The genetic material that controls a characteristics or a trait is a nucleoprotein called chromatin. The chromatin material just before cell division forms into chromosomes. Each chromosome is made up of two longitudinal strands called the chromatids. The chromosomes are present in pairs. The pairs are called the homologous pairs. A species will always have the same number of chromosomes. This is called the chromosome number and it will always be an even number. This number is called the diploid number. During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the haploid number. Thus the somatic or the vegetative cells of all organisms are diploid and the gametes are haploid. The chromosome numbers of some of the plants and animals are given below: Plant Somatic cell(2n) Chromosome No. Animal Somatic (2n) Chromosome No. Field bean 12 Ascaris 2 Garden pea 14 Mosquito (Culex) 6 Onion 16 Fruit fly 8 cabbage 18 housefly 12 Maize 20 frog 24 Rice 24 House bee 32 Wheat 42 cat 38 Potato 48 mouse 40 Cotton 52 man 46 sugarcane 80 horse 64 Whatever may be the number of chromosomes, it is possible to keep track of the behaviour of these chromosomes as they are all different in some re The pictorial representation of the entire set of chromosomes is called the karyogram. Autosomes and Sex Chromosomes As can be seen from the human karyogram, the chromosomes are classified into two types, autosomes and sex chromosomes. The autosomes have homologous chromosomes as pairs whereas the sex chromosomes are of two different types - X and Y. A female has two X-chromosomes and a male has an X and a Y chromosome. Each chromosome has a double helical DNA molecule. DNA or Deoxyribonucleic Acid DNA is a double stranded molecule (double helix) with each strand being made up of many nucleotide units. Each nucleotide is made up of a nitrogenous base, a sugar molecule and a phosphoric acid molecule. Each DNA molecule has two such polynucleotide chains joined to each other by hydrogen bonds. The hydrogen bonds are between the nitrogenous bases of the opposite nucleotides. If the polynucleotide chains are the sides of the ladder, the bonds can be compared to the rungs of the ladder. The whole molecule can be likened to a rope ladder twisted around to from a helical structure. The DNA molecule is twisted around a core of proteins. Short chains of the DNA form the genes. Each gene is, therefore, a series of nucleotides in a particular sequence. The genes are arranged in a linear manner on the chromosomes. The position of a gene on the chromosome is called the locus. It always remains the same for a gene. The alleles of a gene are present on the homologous chromosomes at the same loci. Each gene codes for a particular trait. The proteins part of the DNA control the characteristics of a trait. Just when Mendel was making his observations it was the American scientist William Sutton who noticed the similarities between the behaviour of the Mendelian factors or genes and that of the chromosomes during meiosis. He noticed the following similarities: Both occurred in pairs - genes as alleles and chromosomes as homologous chromosomes. Both separated from the pairs and entered the gametes independently - each gamete received only one of the alleles of a pair and also received only one of the homologous chromosomes during meiosis. The pairs of both are restored during fertilization - the zygotes receive one allele each from the two parents through the gametes and the zygotes also receive the two homologous chromosomes, one from each parent, during fertilization. The above observations led Sutton and Boveri to formulate the Chromosomal theory of inheritance, independently in 1902. According to this theory, Each adult organism grows from a zygote. The zygote is a diploid cell having two sets of chromosomes - maternal and paternal. The zygote by mitotic divisions results in the formation of an adult and thus, all the vegetative or somatic cells of an organism are diploid with two sets of chromosomes. The chromosomes maintain their individuality through out the life cycle of the organism. The chromosomes contain the Mendelian factors or genes, which determine the various characteristics of an individual. In asexual reproduction similar rules of inheritance are followed. Here the DNA material is distributed equally to their progeny. Here, the paternal and maternal difference is not present as the chromosomes involve a single parent. Sex Determination From the above observations of production of different traits, it is clear that in sexual reproduction, the genetic materials from the two sexes participating are different from each other. All human chromosomes are not paired similarly, the sex chromosomes in men is odd in not always being a perfect pair. It is a mismatched pair in which one is a normal-sized X while the other is a short one called Y. Women have a perfect pair of sex chromosomes, both called X. So women are XX, while men are XY. The sex of organisms is dependent on different factors of environment, temperature or choice. However, in human beings, the sex of the individual is largely genetically determined. Parental genes decide whether the offspring will be boy or girl. How is the sex of a newborn human determined by genetic inheritance? During the formation of gametes, the females will have only one type of gametes, all with one X chromosome. However, the males will produce two types of gametes or sperms, half with X chromosome and half with Y chromosome. Thus the sex of a zygote is determined by which male gamete fuses with the female gamete. If the X gamete fuses with the female gamete (also X) the zygote will be a female. If the Y gamete fused with the female gamete, the zygote will be a male. This type of sex determination is called XX-XY type. The sex of organisms is dependent on different factors of environment, temperature or choice. However, in human beings, the sex of the individual is largely genetically determined. Parental genes decide whether the offspring will be boy or girl. How is the sex of a newborn human determined by genetic inheritance? During the formation of gametes, the females will have only one type of gametes, all with one X chromosome. However, the males will produce two types of gametes or sperms, half with X chromosome and half with Y chromosome. Thus the sex of a zygote is determined by which male gamete fuses with the female gamete. If the X gamete fuses with the female gamete (also X) the zygote will be a female. If the Y gamete fused with the female gamete, the zygote will be a male. This type of sex determination is called XX-XY type.

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