Chapter-4, Biology- Genetics.pptx biology

Biology

1. Swami Keshvanand Institute of Technology, Management & Gramothan , Jaipur B.Tech . IV Semester Biology 4EE2-01 Credit: 2 Max. Marks: 100 (IA: 20 , ETE: 80) Chapter-4 Genetics Presented by Prof. Archana Saxena Department of Chemistry

2. RAJASTHAN TECHNICAL UNIVERSITY, KOTA I V Semester Electrical Engineering 4EE2-01: Biology Credit: 2 2 L Max. Marks: 1 00 ( IA: 2 , ETE: 8 ) End Term Exam: 3 Hours Content Hrs. Genetics: Purpose: To convey that “Genetics is to biology what Newton’s laws are to Physical Sciences”. Mendel’s laws, Concept of segregation and independent assortment. Concept of allele. Gene mapping, Gene interaction, Epistasis. Meiosis and Mitosis be taught as a part of genetics . Emphasis to be give not to the mechanics of cell division nor the phases but how genetic material passes from parent to offspring. Concepts of recessiveness and dominance. Concept of mapping of phenotype to genes. Discuss about the single gene disorders in humans. Discuss the concept of complementation using human genetics.       3

3. Genetics

4. Content Mendel’s laws Concept of segregation and independent assortment Concept of allele Gene mapping Gene interaction Epistasis Meiosis and Mitosis Concepts of recessiveness and dominance . Concept of mapping of phenotype to genes . Single gene disorders in humans Concept of complementation using human genetics .

5. Genetics Genetics is the study of inheritance and variation. Heredity: The transfer of genetically controlled characters from one generation to next generation. Inheritance: The process by which the characters pass from one generation to the next is called inheritance. Variation: Offspring differ from their parents and siblings

6. Some important terms Chromosomes: These are thread like structures in the cell nuclei of all living organisms. They carry the genes which control the development and maintenance of the organism. Each type of plant or animal has a fixed no. of chromosomes in its cells. Genes: Genes are discrete structures in the chromosome. The fundamental physical and functional unit of heredity, which carries information from one generation to the next. Each gene controls one or more characteristics of the organism and as a result determine what kind of organism is produced. When cell divides by mitosis, the chromosomes and genes are copied exactly and each new cell gets a full set. At meiosis, only one chromosome of each pair goes into the gamete. Alleles : These are alternative forms of the same gene, which affect a particular characteristics of the organism but in different ways. Alleles are genes controlling the same characteristic e.g. (hair colour) but producing different effects (e.g. black or red) and occupying corresponding position on homologous chromosomes .

7. Mendel’s Law Before Mendel (Theory of blending inheritance): It was believed that characters get mixed during their transmission to the offspring from parents Gregor Johann Mendel explained his principles of heredity and variation through his experiments on pea plant, Pisum sativum . He is called the father of genetics .

8. Reason for using Pea plant for study It is easy to cultivate pea plant and it completes its life cycle in one season only. It has sharply distinct heritable differences seen morphologically. Variety of characteristics in various plants. Petals of the flower close down tightly, self fertilization is promoted.

9. Mendel confined his attention to the single character at a time. Characters are the visible features that are represented by two alternate and clearly identifiable traits called contrasting traits ( allelomorphic traits) An allele is the two contrasting traits or alternate forms of a gene which appears on the same location. For example, Character-length of stem, tall and dwarf, two contrasting traits.

10. Mendel chose 14 pea plants to focus on seven clearly definable characters, each of which occurred in two allelomorphic traits.

11. Mendel’s law of inheritance Mendel proposed the mechanism involved in heredity by the transmission of units in the reproductive cells. The genotype of an organism is made up of genes. Mendel postulated- law of dominance, law of independent assortment, law of segregation.

12. Law of Dominance When two contrasting traits for a character come together in an organism only the trait which is dominant is expressed. Tallness- Dominant character Dwarfness - Recessive character

13. Law of segregation (purity of gametes) When a pair of contrasting genes or factors or allelomorphs are brought together in a heterozygote the two members of the allelic pair remain together without being contaminated and when gametes are formed from hybrid, two allele separate from one another so each gamete has one allele.

14. Law of independent assortment If the inheritance of more than one pair of characters is studied simultaneously, the factors or genes for each pair of characters sort out independently of the other pairs.

15. Gene Interaction In some type of inheritance, Mendelian ratio is not observed (3:1 for monohybrid and 9:3:3:1 for dihybrid in F2), because sometimes a particular allele can be partially or equally dominant on the other or due to the existence of more than two alleles or due to lethal alleles.

16. Interallelic or Intragenic interaction Incomplete dominance (1:2:1): F1 hybrids are not related to either of the parents, but exhibit a blending of characters of two parents. For example a cross between red and white flowers of Four O’ clock plant Mirabilis jalapa , produced pink flowers in F1 generation.

17. Interallelic or Intragenic interaction Co-dominance (1:2:1): Both the genes of an allelomorhic pair express themselves equally in F1 hybrids. 1:2:1 ratio, both genotypically and phenotypically in F2 generation. For example, MN blood type in man. The person with MN genotype produce both antigen M and N and not some intermediate product indicating that both the genes are functional at the same time.

18. Complementary genes (9:7 ratio) The complementary genes are two pairs of non-allelic dominant genes (present on separate gene loci), which interact to produce only one phenotypic trait, but neither of them if present alone produces the phenotypic trait in the absence of the other. For example gene for flower colour in sweet pea. The flower of this plant are either purple (if they contain anthocyanin pigment) or white. Two independently assorting genes (C and P) are involved in anthocyanin synthesis and each gene has a recessive allele to inhibit pigment production. When two genes are present together, purple flowered plants were obtained. The presence of one alone is not sufficient to produce coloured variety. On self-crossing the hybrid, they produce a ratio of 9(purple): 7 (white) in F2 generation.

19. Supplementary genes (9:3:4 ratio) Here only one factor is sufficient to produce a phenotypic expression but addition of another factor causes the change in expression. Supplementary genes are two independent dominant genes interacting to produce a phenotypic expression different from that produced by either gene alone. In supplementary gene action, the dominant allele of one gene is essential for the development of the concerned phenotype, while the other gene modifies the expression of the first gene.

20. For example, The development of grain colour in maize is governed by 2 dominant genes ‘R’ and ‘P’. The dominant allele ‘R’ is essential for red colour production; homozygous state of the recessive allele ‘r’ ( rr ) checks the production of red colour . The gene ‘P’ is unable to produce any colour on its own but it modifies the colour produced by the gene ‘R’ from red to purple. The recessive allele ‘p’ has no effect on grain colour .

21. Epistasis (inhibiting genes) Epistasis is the interaction between non-allelic genes (present on separate loci) in which one gene inhibits the expression of other gene. The gene that suppresses the other gene is known as inhibiting or epistatic factor and the other which is prevented from exhibiting itself, is known as hypostatic. Although it is similar to dominance and recessiveness , but the two factors occupy two different loci.

22. Two types of epistasis Dominant epistasis (12:3:1 or 13:3): Out of two pairs of genes, the dominant allele, (i.e. gene A) of one gene masks the activity of other allelic pair (Bb), for example dominant epistasis in dogs.

23. Recessive epistasis (9:3:4 ratio) Epistasis due to recessive gene is known as recessive epistasis, that is out of the two pair of genes, the recessive epistatic gene masks the activity of the dominant gene of the other gene locus, for example in mice agouti colour.

24. Duplicate genes (15:1 ratio): Two pairs of genes located on different chromosomes determine the same phenotype are called as duplicate genes. For example in shepherd’s purse ( Bursa pastoris ), the seed capsules have two shapes-triangular and oval. If the genotype is aabb , that is only recessive alleles in both genes are present, then oval seed capsules were formed However, if the dominant allele of either gene is present, triangular seed capsules are formed.

25. Cell division: Mitosis In sexual reproduction, a new organism starts life as a single celled called zygote. All the tissues and organs are produced by cell division from the one cell. Zygote divides and produces an organism consisting of thousands of cells and this type of cell division is called mitosis. It does not take lace in a zygote but occurs in all growing tissues. Mitosis takes place in any part of plant or animal which is producing new cell for growth or replacement, e.g. bone marrow produces new blood cells by mitosis, growth of muscles or bones in animals and root, leaf, stem or fruit in plants results from mitotic cell division. Mitosis takes lace only in somatic cells.

26. Mitosis:

27. Meiosis: There is a fixed number of chromosomes in each species. Each human body cell contains 46 chromosomes. The number of chromosomes in a species is the same in all of its body cells. The chromosomes have different shapes and sizes. The chromosomes are always in pairs. The 46 chromosomes of human body consists of 23 from mother and 23 from father. The number of chromosomes in each body cell of a plant or animal is called the diploid number, because the chromosomes are in pairs, it is always in even number. The chromosomes of each air are called homologues chromosomes.

28. The zygote is formed at fertilization when a male gamete fuses with female gamete. Each gamete brings a set of chromosomes to the zygote. The gamete, therefore must contain only half the diploid number of chromosomes. The process of cell division which gives rise to gametes is different from mitosis because it results in the cells containing only half the diploid number of chromosomes. This number is called haloid number and the process of cell division which gives rise to gametes is called meiosis. Meiosis takes place in reproductive cells only.

29. Meiosis

30. Gene Mapping A genome is the total genetic content of any cell in an organism. It consists of all the genes on all the chromosomes. It is thought that the human genome consists of up to 40,000 genes distributed between 46 chromosomes. The human genome project aims to locate the position of all the genes on each chromosome, this involves mapping. Work out the entire sequence of bases for the whole genome, this is called sequencing.

31. It has long been possible to recognise in chromosomes, regions which have a specific function, e.g. it has been possible to observe that if certain bands are missing in the chromosomes from salivary gland of a fruit fly, there is mutation in the fruit fly and corresponding changes are seen in the phenotype. This shows that the bands in the chromosome include, at least, the gene or genes controlling the specific feature. Cross breading experiments also enable geneticists to identify positions of genes on chromosomes. Modern biochemical techniques allow many more regions of chromosomes to be identified. Also the recognizable (mapped) regions are not necessarily genes because only about 3 per cent of the human genome consists of genes. The mapped regions may represent genes, stretches of DNA or even lengths of DNA which have no known function.

32. Single gene disorder in human beings Single gene disorders are caused by DNA changes in one particular gene. Over 10,000 human disorders are caused by a change known as mutation in a single gene. Individually single gene disorders are very rare but as a whole they affect about one per cent of the population. Single gene disorders can be divided into: dominant, recessive and X-linked.

33. Dominant disease Single gene disorder that occur in heterozygous state, when an individual has one mutant copy of the relevant gene and one healthy copy. The effect of the mutant version of the allele overrides the effect of the healthy version of the gene. So, the mutant allele causes disease symptoms even though a healthy allele is present. Dominant diseases tend to crop up in every generation of an affected family because everyone carrying a dominant mutant allele shows the symptoms of the disease. ( in rare cases when an individual has two copies of the mutant gene being homozygous case, the disorder symptoms are generally more severe. E.g. Huntington’s disease , It is characterized by progressive deterioration of mental and physical health. However the gene is not expressed until the carrier reaches middle age. Disease is rare, affecting 1 person in 10,000.

34. Recessive disease Single gene disorders that occur in the homozygous state only, i.e when an individual receives both recessive alleles, one from each of their parents. Thus, in turn both parents must be heterozygous for the gene. These are described as carriers, because they do not exhibit characteristics of the disease although they carry the recessive gene. E.g. Sickle cell anaemia, haemophilia, cystic fibrosis, colour blindness.

35. Sickle cell anaemia results from a defective haemoglobin molecule which causes the red blood cells to distort when subjected to a low concentration of oxygen.

36. Haemophilia is a genetic disease in which blood does not clot easily. The blood of people with the disease lacks one of the plasma proteins, called factor VIII, which plays a part in clotting.

37. Cystic fibrosis is the commonest inherited disease in white people- affecting about one in every 2500 children. The disease occurs when a person inherits two copies of a recessive allele which controls the production of a protein in cell membranes. (Damages lungs and digestive system

38. X-linked disorders Single gene disorders that result from the presence of a mutated gene on the X-chromosome. These can also be either dominant or recessive E.g. of X-linked recessive disorders are haemophilia, muscular dystrophy etc. X-linked dominant disorders are rare, e.g. Rett syndrome that affect brain development.

39. Concept of complementation using human genetics In complementary gene action, some genes act together to produce an effect that neither gene can produce separately. Complementation will occur only if the mutation are in different genes. It will not occur if the mutations are in same genes. If the combination of two genes containing different recessive mutations yield a mutant phenotype, the there are three possibilities: Mutations occur in the same gene One mutation affects the expression of the other. One mutation may result in an inhibitory product.

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