Heredity and Evolution (PDF)

CHAPTER – 9 HEREDITY AND EVOLUTION All the living organisms produce offspring which resemble their parents, but are not carbon copy of them. These variations may appear due to changes in DNA or due to sexual reproduction. During reproduction, the variations are produced due to the follo...

CHAPTER – 9 HEREDITY AND EVOLUTION All the living organisms produce offspring which resemble their parents, but are not carbon copy of them. These variations may appear due to changes in DNA or due to sexual reproduction. During reproduction, the variations are produced due to the following reasons. *Errors in DNA copying. *Random fertilization. *Crossing over during meiosis. Variations produced during sexual reproduction are much as compared to variations produced during asexual reproduction. The variations are less among the individual plants of sugarcane field because these plants are propagated from the same stock by asexual reproduction. But quite distinct variations are visible among different human beings which are reproducing sexually. In this chapter, we shall be studying the mechanism by which variations are created and inherited. Inheritance can be defined as the process by which characters pass from one generation to the next. The inheritance of characters from parents to offspring is called heredity. Both heredity and variations go side by side. The branch of biology that deals with the study of heredity and variations is called genetics. ACCUMULATION OF VARIATIONS DURING REPRODUCTION. Inheritance form parents to offspring provides two main features: A common basic body design Slight changes(variations) – When this new generation reproduce it will pass variations of first generation and new variations along with common basic design to the next generation. Although the offspring inherit the characters of the parents and resemble them very closely but the resemblance is not complete in all respects. The offspring are never a true copy of the parents. In fact, no two individuals are exactly alike and the members of any one species in some characters or the other. These differences are known as Variation. The differences in the characters among the individuals of the species is called Variation. Eg: Variation in human beings which involves our ears.  The lowest part of our ear is called earlobe. In most of the people, the earlobe is “hanging” and it is called Free earlobe. In some people the earlobe is closely attached to the side of the head and it is called “Attached earlobe”. So the free earlobes and the attached earlobe are the 2 variations found in human population. All the variations produced do not have equal chances of survival. Some variations are useful while others are not. Nature selects those variants (individuals with variations) which are advantageous and others are eliminated from the population. HEREDITY Gregor Mendel (Father of genetics) was the first scientist to make a systematic study of pattern of inheritance which involved the transfer of characters from parents to progeny. The branch of biology that  deals with the study of heredity and variation is called Genetics. Mendel chose pea plants (Pisum sativum) for studying inheritance. There are so many reasons for selecting the garden pea as the experimental material. Easy to grow Short lifespan Easily distinguishable characters Large size of flowers. Self-pollination. Mendel selected 34 varieties of garden pea for his experiments. The garden pea contains a number of contrasting trades. He selected and studied only 7 traits. The 7 traits are the following:  More about Genetical Terms ▪ Trait: The specific characteristic of an organism. Eg: Tall, dwarf etc… ▪ Phenotype: External appearance of an individual. ▪ Genotype: Genetic makeup of an individual. ▪ F1: First generation. ▪ F2: Second generation. ▪ Selfing: Crossing of two organisms of same kind. ▪ Homozygous: Individual with two similar factors for a trait. ▪ Heterozygous: Individual with 2 dissimilar factors of a trait. Mendel assumed from his experiments that each organism is formed of a number of traits. Each trait is determined by a physical body called Factor. These factors are now called Genes. Genes are the hereditary units of organisms. RULES FOR THE INHERITANCE OF TRAITS – MENDEL’S CONTRIBUTION 1. Principle or rule of paired factor: Each character is controlled by a pair of factors. The factors may be similar or dissimilar. CHARACTER SIMILAR DISSIMILAR FACTOR FACTOR TALL TT Tt DWARF tt Tt Paired condition of factors is present in the parents like TT, tt or Tt. Paired condition is broken at the time of gamete formation. Each gamete gets only factor of that character, either T or t. PARENT: Tt x Tt GAMETE: T t T t Paired condition is restored after fertilization. 2. Principle of Dominance: When 2 dissimilar factors of a character are present in an organism, only one expresses itself and is called Dominant factor, while other remains unexpressed and is called Recessive Factors. For eg: If one plant has 2 dissimilar factors for height, ie, Tt, then out of these 2 factors T expresses itself and is called Dominant Factor, while t remains unexpressed and is called Recessive Factor. The dominant gene is represented by a capital letter and the corresponding recessive gene is represented by corresponding small letter. 3. Principle of Segregation (Law of purity of gametes): Two factors of a character are separated at the time of gamete formation and each gamete gets only one factor for that character. It means gametes are always pure for their character. These principles can be explained by Monohybrid cross. MONOHYBRID CROSS  A cross involving one character (with two factors). Here Mendel cross pollinated a pure- breeding tall plant with a pure breeding dwarf plant. The progeny obtained is called the First filial generation (F1). All the F1 plants were tall. In the next stage Mendel allowed F1 hybrid plants to self-pollinate and raised the second generation (F2 generation). Mendel counted the number of offspring in each generation. He observed that 75% were tall plants and 25% were dwarf plants which occurred in 3:1 ratio. 4. Principle of Independent assortment: When more than one pair of characters are involved in a cross- factor pairs assort independent of each other. This can be explained by dihybrid cross. DIHYBRID CROSS A cross involving 2 traits is called Dihybrid cross. Mendel selected the traits such as seed shape and seed color. Mendel cross- pollinated a pure breeding round and yellow - seeded pea plant with a pure breeding wrinkled and green - seeded pea plant. All the F1 offspring were round and yellow – seeded. In the next stage the F1 offspring were self- pollinated and F2 generation was raised. In the F2 generation 4 types of plants were produced  How do these traits get expressed? Mendel proposed that inheritance of traits from parents to offspring occurs by hereditary units called Factors. Mendel suggested that every character is controlled by a pair of factors. Genes work in pairs. There is a pair of genes for each characteristic of an organism. Each parent possesses a pair of genes for each  characteristic on pair of chromosomes. However, each parent passes only one of the two genes of the pair for each characteristic to its progeny through gametes. Thus, the male gamete and female gamete carry one gene for each characteristic from the gene pairs of parents. But when a male gamete fuses with a female gamete during fertilization, they make a new cell called Zygote with a full set of genes. This zygote grows and develops to form a new organism having characteristics from both the parents which it has inherited through genes. (please note that though the progeny inherits 2 genes for each trait from its parents but the trait shown by the progeny depends on which inherited gene is dominant of the two).  How does the mechanism of heredity work? ▪ Genes are the segments of DNA on a chromosome occupying specific position. ▪ There are about 30000 – 40000 genes present in human beings. ▪ DNA is the information source in the cell for making proteins. ▪ A dominant factor controls the synthesis of a fully functional structural protein or enzyme. ▪ A recessive factor is able to contribute partially and thus produces defective, incomplete and less efficient structural protein or enzyme. EG: ▪ The plant height depends on the amount of a particular plant hormone which further depends on the efficiency of a particular enzyme synthesized by the gene present on any chromosome. If this enzyme works efficiently a lot of hormone will be made and the plant will be tall. If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less and the plant will be short. Thus, genes control the traits. Sex determination In human beings, the sex is determined genetically by chromosomal methods. Each human cell contains 46 chromosomes or 23 pairs. Out of 23 pairs, 22 pairs are common in males and females. These are called Autosomes. They are not concerned with the sex determination. The chromosomes which are concerned with sex determination are called Allosomes/Sex chromosome. It is the 23rd pair in human beings. In females, 23rd pair consist of 2 similar chromosomes designated as XX. In males, 23rd pair consist of 2 dissimilar chromosomes designated as XY. The males can produce 2 types of gametes/sperms, either X type / Y type.  (Organisms which produce two types of gametes are called Heterogametic) The females produce only one type of gametes or Ova, ie, X type. (Organism which produce only one type of gametes are called Homogametic) If X type sperm fuses with the ovum, then the sex of the baby will be female. If Y type sperm fuses with the ovum, then the sex of the baby will be male. So the male parent(father) is responsible for the sex of the baby which is decided at the time of fertilization. Sex of some animals is determined by various factors like environment. In some reptiles, the temperature at which the fertilized egg is incubated before hatchings plays a role in determining the sex of the offspring. 1. Eg: In turtle, high incubation temperature leads to the development of female offspring 2. Eg: In the case of lizard high incubation temperature results in male offspring In some animals such as snails individuals can change sex indicating that sex is not determined genetically. -----------------------------------------------