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VARIATION, HEREDITY AND GENETICS

VARIATION, HEREDITY AND GENETICS. What is variation?. Variation is the characteristics that differ between organisms of the same species. e.g. height , weight, shape of face, nose, ears, skills etc It is a result of both genetic and environmental factors influencing the organism.

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VARIATION, HEREDITY AND GENETICS

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  1. VARIATION, HEREDITY AND GENETICS

  2. What is variation? • Variation is the characteristics that differ between organisms of the same species. • e.g. height , weight, shape of face, nose, ears, skills etc • It is a result of both genetic and environmental factors influencing the organism

  3. Genetic variation • The complete genetic code of an organism is all the instructions required for the functioning of organism. Referred to as the Genotype. • Human genome consists of 20,000 identified genes. • The genotype is inherited from parents when fusion of the haploid gametes occur during meiosis • The new individual develops and grows into adulthood with a set of genetic material from each parent

  4. Environmental variation • These are characteristics gained during the organisms lifetime. • They include knowledge, skills, scars, habits, morals • Are also referred to as acquired characteristics • The phenotype is the observable or physical expression of the genes. They result from both genetic and environmental factors

  5. Types of variation • There are two types: • Continuous variation • Discontinuous variation

  6. Continuous variation • This type of variation is seen where the differences in the characteristic are slight or can be graded from one extreme to the next • E.g. of characteristics which show continuous variation include height , skin colour, leaf size, pod size in legumes.

  7. Continuous variation in shape, color and size of peppers.

  8. Discontinuous variation • The organism either possess the characteristic or not. • The difference between individuals are clear-cut. There is no merging or halfway stage in the characteristic. • E.g. tongue-rolling ability, cattle with horns, fingerprint: arch, loop, whirl, double whorl

  9. THE STRUCTURE OF CHROMOSOME: GENES & DNA • The somatic or body cells contains 23 pairs of chromosomes. • Each pair are called homologous pairs, one derived from the mother and the other from the father. They are the same size and shape • Therefore each body cell has two copies of the gene

  10. DNA: Di-oxyribonucleic acid Chromosomes are long fragments made up of molecules of DNA. It is shaped in the form of a double helix. DNA is composed of a sugar, phosphate and base Imagine a twisting ladder The sugar and phosphate form the sides and the base forms the rung on the ladder.

  11. DNA: Di-oxyribonucleic acid There are four types of bases: Adenine, Guanine, Thymine, Cytosine A-T, G-C combination of bases The bases, arranged in different patterns of three, will code for the formation of different amino acids which eventually make proteins: the building blocks of the organism Enzymes which are needed to catalyse all meyabolic reactions are made up of proteins, hence the importance of DNA.

  12. DDNA STRUCUTRE

  13. What are genes? Genes are the genetic information or segments of DNA on the chromosome that codes for specfic characteristics . It is the unit of inheritance.e.g. genes for eye colour, hair colour, length of arm etc Sometimes a single pair of genes may give instructions to a characteristic or several genes combined will control the characteristic. The position of the gene on the chromosome is its locus

  14. Alleles • Each gene can have one or more alternate forms or alleles. Also known as an allelomorphic pair • E.g. Gene for eye colour: alleles may be brown, black, green, blue eye color • Hence a cell can have two alleles that are the same ,i.e., the organism is Homozygous for the characteristic • Or if the two alleles on the chromosome pair are different than the organism is Heterozygous for the characteristic.

  15. Homozygous • E.g. On one chromosome of the homologous pair, the allele may have the code for black hair and on the allele will also be black hair

  16. Heterozygous • E.g. On one chromosome of the homologous pair, the allele may have the code for black hair and on the allele will also be red hair

  17. Dominance • If the alleles are different, often one will have a stronger influence and will mask the expression of the other. • The allele which is expressed is referred to as the dominant allele and the other is referred to as the recessive allele

  18. When representing the alleles, capital letters is used for the dominant allele, common for the recessive. E.g. the gene for hair colour: B Where B= black and b=red hair Which is dominant ? If a person is homozygous for black hair then the alleles represented are: BB If is heterozygous then rep’d as: Bb

  19. Genetic cross using Punett table

  20. Incomplete dominance • Incomplete dominance is where for a heterozygous characteristic, the phenotype is a blending of both alleles. • Hence neither allele is completely dominant over the other. • This occurs in flower colour of some plants, Impatiens

  21. E.g. if two parent flowers: red with genotype RR and white genotype(rr) were mated, the offspring could be red (RR), white (rr) or pink (Rr).

  22. Co- Dominance • Co- dominance is where for a heterozygous characteristic, the phenotype is an expression of both alleles • E.g. impatiens, ABO blood group found in Man. Blood has three alleles: IA, IO, IB where IA and IB are equally or co-dominant and IO is recessive • Hence man has four possible blood combinations: A, B, AB, O

  23. Genetic diagrams • A Genetic diagram shows the process/cross when two parents are mated, their genotype and phenotype, resulting gametes and also the possible combination of gametes. Finally the resulting genotype and phenotype of the offspring. • It basically shows the inheritance of a single pair of characteristics • The drosophila fruit fly is used often to show inheritance because of the numerous offspring hatched-more accurate and also the small no. of chromosomes make it easier to isolate and work with the characteristics

  24. e.g. Using Drosophila fly wing length • Cross between homozygous dominant longwinged- ( LL) and homozygous recessive short winged ( ll): • Phenotype LW * SW • Genotype LL * ll • Gametes L l • Offspring/F1 genotype Ll • Offspring/ F1 phenotype LW • 100% probability of LW

  25. Genetic cross using Punett table

  26. If both parents are heterozygous • Phenotype LW * L W • Genotype L l * L l • Gametes L, l L, l • Offspring/F1 genotype LL, Ll, Ll, ll • Homozygous, heterozygous, heterozygous, homozygous • Offspring/ F1 phenotype 3 LW, 1 SW • 25% probability of SW

  27. Genetic cross using table

  28. Test cross/ Back cross • Homozygous dominant and heterozygous individuals both show same phenotype. • Using a back cross with a HOMOZYGOUS RECESSIVE individual determine the genotype of individuals with the same phenotype by examining the offspring’s phenotype

  29. e.g. Individual with black hair can be BB or Bb • Cross with homozygous recessive : bb • Cross can either be with BB • Where all offspring comes out with black hair indicating that parent was homozygous dominant • Or with Bb • offspring is 50%black and 50% red hair indicating heterozygous dominant

  30. e.g. of co-dominant cross using blood groups • What are possible blood groups if parents are blood group: Homozygous A and heterozygous A? • Homozygous A: IAA • Heterozygous A: IAO • Parent genotype IAA * IAO • Gametes A,A A,O • Offspring genotype IAA,IAA IAO,IAO • Cross parents with Heterozygous A and homozygous B

  31. Sex-linked characteristics • Of the 23 prs of chromosomes, one pair is ref’d as the sex-chromosomes. • There are two types- X and Y • Females have XX and males have XY • The Y chromosome is shorter than the X on males hence the X may not have a corresponding allele on the Y chromosome.

  32. A boy child will receive his X chromosome from his mother and his Y from his father, therefore he cannot receive his sex-linked characteristics from his father. • A girl child XX can however receive her sex-linked characteristic from her father.

  33. Sex-linked diseases • Haemophilia is one example of sex-linked characteristics that is passed on. • The blood doesn’t have the ability to clot • The disease is carried on the X chromosome and is a recessive gene. • This means that it can be passed down to boy child but if it isn’t expressed then he is referred to as a Carrier.

  34. Only if the child has double recessive genes then it is expressed as the disease. Female haemophiliacs are rare as both parents must be carriers for this to occur.

  35. The genotype and phenotype of a haemophiliac, H-normal clot , h-haemophilia

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