Bio 101 Lesson 2 (Dr. Maria) - Evolution and Heredity PDF
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Evangel University Akaeze
Dr. Maria
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This document is a lesson on evolution and heredity, covering fundamental biological concepts, including Mendelian genetics and the importance of these topics in modern biology. It explores the principles of heredity through examples such as Gregor Mendel's experiments.
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TOPIC: CONCEPT OF EVOLUTION AND HEREDITY ======================================== OBJECTIVES: At the end of the class, the student is expected to =============================================================== - Understand the concept of Heredity as well as concept of Evolution =============...
TOPIC: CONCEPT OF EVOLUTION AND HEREDITY ======================================== OBJECTIVES: At the end of the class, the student is expected to =============================================================== - Understand the concept of Heredity as well as concept of Evolution ================================================================== - Understand Mendelian genetics and its importance in modern day biology ====================================================================== - Be able to distinguish the types of Evolution and its importance to life. ========================================================================= INTRODUCTION ============ Heredity and Evolution are the most essential and fundamental branches of biology and are linked to each other through inheritable traits. These two terminologies help us to learn and understand about the continuity of the life cycle on the planet Earth. Both terms are correlated to each other. Without heredity, there can be no evolution. Genetics is that branch of biology that deals with the term's heredity, genes, variation, and evolution. THE CONCEPT OF HEREDITY ----------------------- The natural phenomena of transferring traits or characteristics through genes from both parents to their offspring are defined as Heredity. It is a tool for continuity of features from one generation to the next generations which are observed in [sexual reproduction](https://byjus.com/biology/sexual-reproduction-an-overview/) and occurs during meiosis- cell division and fertilization process. The study of heredity is referred to as "genetics". Genetics is the branch of biology that deals with the study of genes, [heredity](https://byjus.com/biology/heredity/) and variations. Gregor Johann Mendel (1822 & 1884), a monk and botanist, who is known as the Father of Genetics conducted many research and several experiments on plant breeding and hybridization using pea plants (*Pisum sativum*) to prove the inheritance of traits. Traits are genetically determined characteristics which are transferred from both parents to their offspring through the genes during the sexual mode of reproduction. DNA is the carrier of this genetic information and is found in the chromosomes of our cells. Mendel used a number of contrasting characters to study the garden pea. 1. 2. 3. 4. - Law of Dominance - Law of Segregation - A principle of Independent Assortment - Law of Dominance states that a gene has two contrasting alleles and one always expresses itself in the organism. It is called the dominant gene and is expressed in any possible combination. - Law of Segregation states that traits get segregated completely during the formation of gametes without any mixing of alleles. - Law of Independent Assortment states that the traits can segregate independently of different characters during gamete formation. - Genes: Gene is the functional unit of heredity. Every gene control one or several particular characteristic features in living organisms. - Traits are characteristic features of an organism, manifested in a physical form that is visible or in a physiological aspect of the organism. - Variation is the measure of the difference between individuals of the same species. Offspring is not identical to parents, there exist some variations. Each individual in a population differs from the others. Recombination and mutation are the main causes of variations. - Acquired characters: The traits that are acquired by an organism over the period of its lifetime. These characters that are not passed on to the DNA of germ cells do not get transferred to the next generation. E.g., loss of muscles and less weight due to starvation, loss of limb or tails due to injury, etc. - Inherited characters: The traits that are inherited from the parents. These traits always get transferred to the next generation but depending on the dominance or recessiveness, they may or may not be expressed. Examples are height, skin colour and eye colour. - Dominant traits: The traits that express themselves in an organism in every possible combination and can be seen are called Dominant traits. - Recessive traits: A trait which is not expressed in the presence of a dominant allele is known as recessive. They are usually not seen. - ![](media/image2.png)Monohybrid cross: When only one character is considered while crossing two organisms, then such a cross is known as a monohybrid cross. E.g., if a tall plant (TT) is crossed with a dwarf plant (tt), we get 3 tall: 1 short plant at the end of the F2 generation. So, 3:1 is a monohybrid ratio. - Dihybrid cross: When two characters are considered while crossing two organisms, then such a cross is known as a dihybrid cross. E.g., If a plant with round and green pea is crossed with a plant with wrinkled and yellow pea. The first generation plants would all have round and green peas. On crossing the same for an F2 generation, we would observe four combinations of characters in the ratio of 9:3:3:1. Thus, 9:3:3:1 is the dihybrid ratio. During reproduction, the genes of biological parents combine to form a new unique individual. This shuffling of genes is the reason all of us are different. THE BASIS OF GENETICS ===================== There are cases when Mendel's second law does not appear to be true i.e. the genes fail to segregate and assort independently. These situations tell us much more about the relationship between chromosomes and genes. They can be described as follows: 1. 2. **Sex chromosomes**: In human beings, there are 23 pairs of chromosomes. Out of these, 22 chromosomes pairs are autosomes (body chromosomes) and the last pair of chromosomes that help in deciding gender is called sex chromosomes (allosomes). When a cross between the two sexes occurs, half of the gender will be boys and half will be girls. However, the sex is always determined by the 'y' gene offsprings receive from their father. They will always receive the 'x' gene from their mothers. Gametes: X X X Y ---------------- XX XX Girl Girl Boy Boy Chromosome Abnormalities: ------------------------- The phenomenon where, on some occasions the paired chromosomes fail to separate properly to poles during anaphase 1 of meiosis is called non-disjunction. Non-disjunction creates chromosomal aberrations as well as variations in chromosome numbers. For instance, It produces one cell with an extra chromosome on the 21^st^ chromosome or a triploid condition known as Down syndrome. A condition known as Turners syndrome can also occur where the X chromosome is absent in an egg i.e. the offspring has a genetic constitution of XO in which case the females are said to be sterile since they do not mature sexually. Other defects include the Klinefelter syndrome (XXY), Phenylketonuria (PKU), Sickle cell Anaemia. Multiple Alleles: ----------------- This is the existence of alleles (Three or more) for one gene. The ABO blood group system in man is controlled by the multiple allele A, B, O. The A gene produces type A antigen in a person's red blood cells. Similarly, the B gene produces the type B antigen. The third gene, O, produces neither antigen A or B. Only two alleles can be present in any individual and the following combinations are possible. AB " AB ======= A B A O **SEX LINKAGE:** Characteristics whose genes are carried on the sex chromosomes (allosomes) are said to be sex linked. These genes are considered sex linked because their expression and inheritance patterns differ between males and females. Examples include the inheritance of red-green colour blindness, hemophilia and premature balding. They are more prevalent in males than females because it is usually an abnormality on X chromosome and so for a female to have it, both X chromosomes must be affected. For example, hemophilia is an x- linked condition; It is known as X- linked dominant if the father and mother is affected, hence affecting their daughters since they each inherit one x from each of their parents. If only one X from one parent is affected, then it is x- linked recessive meaning that the daughters are carriers. If the x affected comes from the mother is present in the son, the son will have hemophilia. If a daughter receives her mother's affected X chromosome, she will be a carrier. IMPORTANCE OF HEREDITY ====================== THEORY OF EVOLUTION ------------------- LAMARCK THEORY ============== In 1809, Jean-Baptiste Lamarck influenced evolutionary thought stating that physical changes in organisms during their lifetime such as greater development of an organ or a part through increased use could be transmitted to their offspring. For example, the giraffe stretches its neck in an attempt to reach the tree to seek for leaves to feed on and as a result of these continued habit , the giraffes front limbs and neck has grown longer. Birds also that need to rest on water to find food spread out their feet when they wish to swim, the skin becomes accustomed to being stretched and forms webs between the toes. The horns of ruminants have formed from them butting their heads together during combats. These examples, though naïve constitute some of evidence on which Lamarcks theories were based on. **Lamarck theory of Use and Disuse**: Lamarck postulated that changes in environmental conditions create new needs and new changes in the species, these changes cause organisms to develop new structures to function effectively. Structures that are subjected to constant use become well developed whereas those structures not used tend to degenerate. **Lamarck theory of acquired character**: Lamarck also went further to suggest that new characteristics or traits acquired during an organism's lifetime through the use of these structures could be inherited by subsequent generation. For example, an ancestral giraffe had short neck but according to Lamarck, it developed long neck through many generations due to its constant stretching of its neck in order to feed on leaves on high branches of trees. Also, birds have come to start resting on water to find water and food and so has developed webbed feet through the constant spreading of toes and stretching of skin. However, Lamarck theory was not popular because there was no decisive evidence to proof that offspring inherit acquired characters of their parents. DARWIN'S THEORY =============== Darwin's theory of natural selection (1859) in his book the origin of species was proposed by Charles Darwin and his contemporary, Alfred Russell Wallace in the nineteenth century after their visit to the Gelapagos islands in 1835 where he found several spp. of finches adapted to different environmental niches, these finches also differed in beak shape, food source and how food was taken in. Darwinism is a theory of biological evolution stating that all species of organism arise and develop through the natural selection of small, inherited variations that increase the individual's ability to compete, survive and reproduce (theory of origin and perpetuation of new species). Darwin put forward some conditions needed for natural selection to occur; reproduction, heredity, variation in fitness or organisms, variation in individual characters among members of the population. Consider the giraffe, a darwinian theory of evolution posits that it was through random variation that some giraffes had longer necks than others and due to the long necks were able to reach tall trees in their environment. Darwin put forward the following ideas: 1. Organisms reproduce enormous number of offspring and increase their number geometrically yet the number of specie tends to remain relatively constant over a long period of time (multiplication of species). 2. There is continuous competition between individuals for food, water, oxygen, space hence struggle for survival. 3. In this struggle, only those better suited for life in their environment will survive (Survival of the fittest), the less successful ones will die out or become extinct (natural selection). 4. All species exhibit both structural and functional variation through many generations i.e they vary in terms of their behavior and physiology and these often affect their chances of survival (common descent) 5. Advantageous variations are inherited by subsequent generations who keep improving them while the less successful variants are selected out for losing the battle of existence (gradualism and natural selection) 1. Fossil records- Fossils are remains, casts or impressions of plants and animals preserved in rocks when they were formed. Life can be traced far back using these records to find out the sequence of occurrence (Geneology and Paleontology). 2. Embryology: The study of embryos of vertebrates shows a distinct degree of similarity in the structures formed and steps followed. It is said that before birth all vertebrates pass through a stage of development in which they have visceral arches, gill clefts and gill pouches meaning that these organisms are descendants of organisms that live in water. 3. Comparative anatomy, morphology and vestigial structures: Similarity in the general plan of vertebrate skeleton, muscular system, digestive and nervous system seems to indicate common ancestry especially when vestigial organs like the appendix, coccyx, ear muscle in man persist even when the individual has no use for them. Some anatomical features of evolutionary significance include the wings- modified for flying; forelimb in whale- modified for steering and balancing when swimming, arms of human- modified for grasping and holding etc. 4. Geographical distribution of plant and animal- Long periods of geographical isolation of related groups by some barriers like mountain, ocean, desert etc often brings about great differences between the related groups under different conditions in different environment. 5. Mutation- Changes in gene can lead to spontaneous appearance of distinct characters in large population of organisms. Mutation can be passed onto future generations when it occurs in gene or gametes leading to production of new species. 6. Cross breeding of domesticated animals and plants: New varieties can be raised by cross breeding. Animals and plants can be selected based on milk production, meat production, color of fruits, plant height etc. and mated with each other to produce offspring's of greater characteristics. Some offsprings of subsequent generations tend to change environment probably unsuitable for their breed and their former characteristics tend to disappear from the population. 7. Serological tests: In finding possible evolutionary affinities, the blood proteins of different animals can be compared. The closer two species are physiologically, the more alike their blood proteins are liable to be. CONVERGENT AND DIVERGENT EVOLUTION ---------------------------------- **Convergence:** This is a phenomenon of developing structures in organisms that are not closely related as a result of living in similar ecological conditions or niche. It is termed similarity by analogy- the structures serve same purpose but have different evolutionary origins and thus a different embryological origin and structure. Ex the wings of birds and butterfly have independently adapted to life in the air, they both use wings for flight but have been constructed quite differently and operate on different principles. **Divergence:** This is a phenomenon of developing structures that though in different species are believed to have the same origin in a common ancestor. It is termed similarity by Homology- Different lines of evolution led to modifications of basic pattern to serve different functions thereby enabling descendants to fill a wide variety of ecological niches. Ex. The forelimbs and hind limbs of all land vertebrates are said to be homologous i.e. being constructed on the same five- digit (pentadactyl limb) pattern but they no longer serve same purpose. **Evolution by Artificial Selection:** Humans have the power to modify wild species to suit their own requirement by selecting some desirable qualities to appear in the next generation. Any organism that its DNA has been altered is known as genetic modified organism. Importance of Evolution ----------------------- Evolution helps us to better understand how new species come into existence, how it becomes extinct, and various other aspects such as their habitats and behavior. It can also be observed that there is an inbuilt tendency for variation during the reproduction process and these are mainly caused by the combining of genetic material from two parents and errors in DNA copying during sexual reproduction.