Genetics - Form 4 Biology Notes

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Introduction

  • Genetics is the study of inheritance.
  • The fact that the offspring of any species resemble the parents indicates that the characters in the parents are passed on to the offspring.
  • Factors that determine characters (genes) are passed on from parent to offspring through gametes or sex cells.
  • In fertilisation the nucleus of the male gamete fuses with the nucleus of the female gamete.
  • The offspring show the characteristics of both the male and the female.
  • Genetics is the study of how this heritable material operates in individuals and their offspring.




Variations within Plant and Animal Species

Variation

  • The term variation means to differ from a standard.
  • Genetics alsodeals with the study of differences between organisms belonging to one species.
  • Organisms belonging to higher taxonomic groups e.g. phyla or classes are clearly different.
  • Although organisms belonging to the same species are similar, they show a number of differences or variations such that no two organisms are exactly the same in every respect.
  • Even identical twins, though similar in many aspects, are seen to differ if they grow in different environments.
  • Their differences are as a result of the environment which modifies the expression of their genetic make-up or genotype.
  • The two causes of variations are the genes and the environment.
  • Genes determine the character while the environment modifies the expression of that character.

Continuous and Discontinuous Variation

Continuous Variations

  • The differences between the individual are not clear-cut.
  • There are intermediates or gradations between any two extremes.
  • Continuous variations are due to action of many genes e.g. skin complexion in humans.
  • In continuous variation, the environment has a modifying effect in that it may enhance or suppress the expressions of the genes.
  • Continuous variation can be represented in form of a histogram.
  • Example of continuous variation in humans is weight, height and skin complexion.
  • Linear measurements:
  • In humans, height shows gradation from tall, to tallest.
  • So does the length of mature leaves of a plant.
  • In most cases, continuous variation is as a result of the environment.

Discontinuous Variations

  • These are distinct and clear cut differences within a species.
  • Examples include:
    • Ability to roll the tongue.
    • An individual can either roll the tongue or not.
    • Ability to taste phenylthiourea (PTC); some individuals can taste this chemical others cannot.
    • Blood groups - and individual has one of the four blood groups A, B AB or O. There are no intermediates.
    • Albinism - one is either an albino or not.
  • Discontinuous variations is determined by the action of a single gene present in an individual.


Structure and Properties of Chromosomes

  • These are threadlike structures found in the nucleus.
  • They are normally very thin and coiled and are not easily visible unless the cell is dividing.
  • When a cell is about to divide, the chromosomes uncoil and thicken.
  • Their structure, number and behaviour is clearly observed during the process of cell division.
  • The number of chromosomes is the same in all the body cells of an organism.
  • In the body cells, the chromosomes are found in pairs.
  • Each pair is made up of two identical chromosomes that make up a homologous pair.
  • However sex chromosomes in human male are an exception in that the Y- chromosome is smaller.

Number of Chromosomes

Diploid Number (2n)

  • This is the number of chromosomes found in somatic cells.
  • For example, in human 2n = 46 or 22 pairs (44 chromosomes) are known as autosomes (body chromosomes") while 1 pair is known as the sex chromosomes.
  • In Drosophila melanogaster, 2n = 8.

Chromosome Structure

  • All chromosomes are not of the same size or shape.
  • In human beings; each of the twenty three pairs have unique size and structure .
  • On this basis they have been numbered 1 to 23.
  • The sex chromosomes formthe 23rd pair.

Properties of Chromosomes

  • Chromosomes are very long and thin.
  • They are greatly and loosely coiled and fit within the nucleus.
  • During cell division they shorten, become thicker and are easily observable.
  • Each consists of two chromatids.
  • The two chromatids are held at same position along the length, at the centromere.
  • Chromatids separate during cell division in mitosis and in the second stage of meiosis.
  • Chromosomes take most dyes and stain darker than any other part of the cell.
  • This property has earned them the name "chromatin material"
  • Each chromosome is made up of the following components:
    • Deoxyribonucleic acid (DNA) - this carries the genes. It is the major component of the genetic material.
    • Protein e.g. histones.
    • Ribonucleic acid (RNA) is present in very small amounts.
    • Enzymes concerned with DNA and RNA replication - these are DNA and RNA polymerases and ligases.

Structure of DNA

  • The structure of DNA was first explained in 1953 by Watson and Crick.
  • DNA was shown to be a double helix that coils around itself.
  • The two strands are parallel and the distance between the two is constant.

Components of DNA

  • DNA is made up of repeating units called nucleotides.
  • Each nucleotide is composed of:
    • A five-carbon sugar (deoxyribose).
    • Phosphate molecule.
    • Nitrogenous base, four types are available i.e,
      • Adenine - (A)
      • Guanine - (G)
      • Cytosine - (C)
      • Thymine - (T)
  • The bases are represented by their initials as A, G, C and T respectively.
  • The sugar alternates with the phosphate, and the two form the backbone of the strands.
  • The bases combine in a specific manner, such that Adenine pairs with Thymine and Guanine pairs with Cytosine.
  • The bases are held together by hydrogen bonds. A gene is the basic unit of inheritance consisting of a number of bases in linear sequence on the DNA.
  • Genes exert their effect through protein synthesis.
  • The sequence of bases that make up a gene determine the arrangement of amino acids to make a particular protein.
  • The proteins manufactured are used to make cellular structures as well as hormones and enzymes.
  • The types of proteins an organism manufactures determines its characteristics.
  • For example, albinism is due to failure of the cells of an organism to synthesise the enzyme tyrosine required for the formation of the pigment melanin.
  • The diagram below shows a coiled double helix strand structure of the DNA and the structure of the RNA.
    dna vs rna


First Law of Heredity

  • It is also known as Law of Segregation (Mendel's First Law).
  • The characters of an organism are controlled by genes occurring in pairs known as Alleles.
  • By definition, an allele is an alternative form of a gene controlling a particular characteristic. Of a pair of such alleles, only one is carried in each gamete.
  • This is explained by first meiotic anaphase stage, when the homologous chromosomes are separated so that each carries one of the allelic genes.

Monohybrid Inheritance

  • This is the study of the inheritance of one character trait that is represented by a pair of genes on homologous chromosomes.
  • Gregor Mendel (an Austrian monk) was the first person to show the nature of inheritance.
  • He did this through a series of experiments using the garden pea, Pisum sativum.
  • As opposed to others before him, the success in his work lay in the fact that:
  • He chose to study first a single character at a time (monohybrid inheritance).
  • He then proceeded to study two characters at time (dihybrid inheritance) .
  • He quantified his results by counting the number of offspring bearing each trait.
  • Each character he chose was expressed in two clearly contrasting forms.

Examples

  • Stem length: some plants were tall while others were short.
  • Colour of unripe pods: some were green, others yellow. There were no intermediates.

Mendel's Procedure

  • For each character, Mendel chose a plant that bred true.
  • A true or pure breed continues to show a particular trait in all the offspring in several successive generations of self-fertilisation.
  • He made one plant to act as the female by removing the stamens before the ovary was mature and protecting (e.g. by wrapping with paper).
  • The female plant from contact with any stray pollen.
  • When the ovary was mature, he carefully dusted pollen from the anthers of the selected male plant and transferred it to the stigma of the female plant.
  • Observations were then made on the resulting seeds or on the plants obtained when those seeds were planted.

Results

  • For each pair of contrasting characters he studied, Mendel obtained the same results.
  • For example, when he crossed pure breeding tall plants with pure breeding short plants, the first offspring, known as the first filial generation (F1) were all tall.
  • When these were selfed i.e. self-fertilisation allowed to take place, the second generation offspring also know as the second filial generation or F2 occurred in the ratio of 3 tall:1 short.
  • The same ratio was obtained for each of the other characters studied.
  • From this it is clear that one character i.e. tall is dominant over the short character.
  • A dominant character is that which is expressed alone in the offspring even when the opposite character is represented in the genotype.
  • The unexpressed character is said to be recessive.
  • From these results and others obtained when he studied two characters at the same time, Mendel concluded that gametes carry factors that are expressed in the offspring.
  • These factors are what we know today as genes.
  • Mendel put forward the following laws of inheritance:
    • Of a pair of contrasting characters, only one can be represented in a gamete.
    • For two or more pairs of such contrasting characters, each factor (gene) in the gamete acts independently of the others and may combine randomly with either of the factors of another pair during fertilisation.
  • Genetic experiments carried out to date confirm Mendel's Laws of inheritance e.g. T.H. Morgan's work on inheritance in the fruit fly Drosophila melanogaster.

Terms Used in Genetics

Genotype

  • The genes present in an individual. The genetic constitution of an individual. It is expressed in alphabetical notation.e.g TT,Tt 

Phenotype

  • The observed character or appearance i.e. the expression of the genes in the structure and physiology of the organism.
  •  In some cases the phenotype is the product of the genotype and the environment. Phenotype is expressed in words.eg TALL,SHORT,RED WHITE .etc.

Alleles

  • These are alternative forms of the same gene that control a pair of contrasting characters e.g. tall and short.
  • They are found at the same position or gene - locus on each chromosome in a homologous pair.

Homozygous

  • This is a state where the alleles in an individual are similar e.g. TT (for tall)

Heterozygous

  • This is a state where the alleles are dissimilar i.e. each of the two genes responsible for a pair of contrasting characters are present e.g. Tt. (T for tall; t for short)

Hybrid

  • This is the offspring resulting from crossing of two individuals withcontrasting characters.

Hybrid vigour or Heterosis

  • The hybrid develops the best characteristics from both parents i.e. it is stronger or healthier, or yields more than either parent.

Use of Symbols

  • To represent genes in the chromosomes, letters are used.
  • It is customary to use a capital letter for the dominant characteristic and small letter for the recessive one.
  • The gametes are encircled.
  • For example,a cross between a tall and a short pea plant is illustrated as follows;
    • Let – T- represent gene for tallness.
    • Let - t- represent gene for shortness.
      cross between a short and tall pea
      Fertilization - using checker board or Punnet square
      F1 genotype Tt
      F1 Phenotypic ratio = All tall.
      2
      F2 Genotype TT, 2Tt, tt
      F2 Phenotypic ratio; 3 Tall : 1 short

Test Cross or Back Cross

  • This is a eras made between the F1 bearing the dominant trait with the homozygous recessive parent.
  • It is called a back cross because of using the first parent.
  • It is also a test cross because it tests the genotype of the individual.
    test cross

Complete Dominance

  • Mendel happened to choose characters that showed complete dominance, i.e. the dominant trait completely masked the recessive one in the F1 generation.
  • In man, certain characters are inherited in the same way e.g. colour of the skin; normal colour is dominant to albinism (lack of skin pigment).
    complete dominance
  • The children are all normal but have the gene for albinism.
  • Such individuals are referred to as carriers.
  • Other characters that show complete dominance in humans are:
    • Ability to roll the tongue.
    • Polydactyly (having more than 5 digits in one limb).
    • Brachydactyly - having short fingers.
    • Achondroplasia - dwarf with bow legs.

Incomplete Dominance

  • In this kind of inheritance there is no dominant or recessive gene but the two are expressed equally in the offspring,
  • Resulting in blending of the characters.
  • The gene for red colour (R) in cattle and the gene for white colour(W) show incomplete dominance or co-dominance.
    incomplete dominance
  • The offspring are neither red nor write but are intermediate between the two. They are said to be roan.
  • In humans, the sickle cell gene and the normal gene are co-dominant.

Inheritance of ABO Blood Groups in Humans

  • Blood groups in human are determined by three alleles, A, B, and O.
  • An individual can have only two of these genes.
  • Genes A and B are codominant, while gene 0 is recessive to A and B.
  • These are referred to as multiple alleles.
     Blood group  Gossible genotype  Type of Antigen on red blood cells  Type of antibody in plasma
     A  AA; AO  A  b
     A  BB; BO  B  a
     AB  AB  AB  none
     O  OO  None  a and b

The ABO Blood Group System

Rhesus Factor

  • The Rhesus factor is responsible for the presence of a protein (Antigen D) in the red blood cells.
  • If blood from a Rhesus positive (Rh+) person is transferred into a person without the Rhesus factor (Rh-);the recipients' body produces antibodies against the Rhesus factor.
  • This causes agglutination of red blood cells which can be fatal if subsequent transfusion with Rh+ blood is done.

Sex Determination in Humans

  • XY type e.g. human male
  • In males, two types of sperms are produced.
  • Half of then containing X chromosomes and half Y chromosomes.
  • During fertilisation only one sperm fuses with the egg.
  • If it is an X-carrying sperm then a female zygote is formed;
  • If it is a Y-carrying sperm then a male zygote is formed.
  • It follows then that the chances of getting a boy or girl are half or fifty-fifty.
  • Note also that it is essentially the type of sperm that fertilises the egg that determines the sex.
    sex determination in humans

Linkage

  • The term linkage describe the situation where genes or certain characters are located on the same chromosome.
  • Offspring produced by sexual reproduction show only the parental characteristics and only sometimes few new recombinants. i.e. offspring with combinations of characteristics not found in either of the parents due to crossing over in first prophase of meiosis.
  • Genes are said to be linked when they are located close together on the same chromosome such that they are always inherited together.

Sex linked genes

  • These are genes that are located on the sex chromosomes.
  • Sex-linkage - refers to carrying of the genes on the sex-chromosome.
  • Gene for a trait may be present, yet offspring does not show the trait.
  • This happens in human females (XX) where a gene for the trait is recessive.
  • The female acts as a carrier.
  • In human, sex linked characters found on the X chromosome include:
    Haemophilia
  • This is a disease that affects the rate of clotting of blood, leading to excessive bleeding even from a minor cut.
  • Haemophilia is more common in males than in females.
  • A female my have the gene for haemophilia and not show the trait because the normal gene is dominant over the gene for haemophilia.
  • Such females are referred to as carriers.
  • If the carrier female offspring will be carriers while the other half will be normal.
  • Half the males will be normal and the other heamophilic.
    Red-green colour-blindness
  • Red-green colour-blindness is caused by a recessive gene found on the X chromosome.
  • It is inherited in the same way as haemophilia.
  • More males 1:10,000, less female 1: 100 million afflicted.
  • It is the inability to distinguish between red and green colours in humans.
    Genes found on y-chromosome include:
  • Hairy pinna and hairy nose are carried on the Y - chromosome.
  • Premature balding.



Mutations

  • Mutations are sudden changes in the genotype that are inherited.
  • Mutations are rare in nature and mutated genes are usually recessive to the normal (wild type) genes.
  • Most mutations are generally harmful and some are lethal.
  • A somatic mutation is a genetic change in somatic cells.
  • Somatic mutations are only inherited if asexual reproduction takes place e.g. as in plants and unicellular animals.
  • A gene mutation is a change in genes of reproductive cells and is always inherited.
  • The resultant individual is called a mutant.
  • The mutant has different characteristics from the rest of the population.

Types of Mutations

  • Chromosomal mutations - are changes in number or structure of chromosomes.
  • Gene mutations - also called point mutations - are changes in the chemical nature of the gene.

Mutagens

  • These are agents that cause mutations.
  • They include ultra-violet light, Gamma rays., x-rays and cosmic rays.
  • Certain chemicals e.g. mustard gas and colchicines also induce mutations.

Causes and Consequences of Chromosomal Mutations

  • There are three main types of chromosomal mutations.
    • Changes in the diploid number of chromosomes (allopolyploidy).
      • The diploid number changes to 3n (triploid) or 4n (tetraploid) and so on.
      • This results from the doubling of the chromosome number in the gamete (2n).
      • This is due to failure of the chromosome sets to separate during meiosis.
      • The phenomenon is known as polyploidy.
      • It is common in plant's and has been employed artificially to produce varieties of crops with hybrid vigour e.g. bread wheat is hexaploid (6n). (This is allopolyploidy).
    • Change in the total number of chromosomes involving the addition or loss of individual chromosomes (autopolyploidy).
      • This is due to failure of individual chromosomes to separate during meiosis.
      • One gamete gains an extra chromosome while the other loses a chromosome.
      • The term non-disjunction is used to describe the failure of chromosomes to separate.
      • Non-disjunction results in several disorders in humans:
        Down's syndrome
      • The individual has 47 chromosomes due to non-disjunction of chromosome 21.
      • It is also known as trisomy 21.
      • The individual has slanted eyes with flat and rounded face, mental retardation and large tongue and weak muscles.
        Turner's Syndrome
      • This brings about to a sterile and abnormally short female.
      • It is due to loss of one of the sex chromosomes i.e. the individual has one X chromosome (44 + X) instead of two (44 + XX).
        Klinefelter's Syndrome
      • This results in a sterile male who may be mentally retarded.
      • It is due to an additional X chromosome i.e. the individual i.e. 47 chromosomes (44 + XXY) instead of 46 (44 + XY).
    • Changes in the structure of a chromosome during meiosis.
      • A portion of a chromosome may break off and fail to unite again or it may be joined in the wrong way or to the wrong chromosome.
      • These mutations are described as follows:
        Deletion
      • This is the loss of a portion of a chromosome,
      • Deletion results in individuals born with missing body parts. e.g. limbs in the extreme of cases.
        Inversion
      • A portion may break from a chromosome and then rejoin to it after turning though an angle-of 180o.
        Translocation
      • This is when aportion is joined to a non-homologous chromosome.
        Duplication
      • A certain section of an intact chromosome replicates such that the genes are repeated.

Gene Mutations

  • A gene mutation is a change in the structure of a gene.
  • It may involve only a change in one base, e.g. adenine in place of thyamine yet the effect on the individual is profound e.g. sickle cell anemia .
  • There are two main type of gene mutations:
    • Due to insertion or deletion of one or more (base) pairs.
    • Substitution of base pairs e.g. purine for pyrimidine.

Genetically Inherited Disorders in Humans

  • Albinism is a mutation that alters the gene responsible for synthesis of skin pigment (melanin).
  • The gene for albinism is recessive.
  • Sickle cell anemia is a common condition in Kenya. Individuals with the sickle-cell gene produce abnormal haemoglobin.
  • It is due to gene mutation caused by substitution of the base adenine for thymine.
  • The result is the inclusion of the amino acid valine (in place of glutamic acid) in the haemoglobin synthesised.
  • As a result the red blood cells become sickle shaped when oxygen concentration becomes low i.e. inside tissues.
  • This leads to blockage of capillaries.
  • Tissues do not get sufficient oxygen.
  • Homozygous individuals are seriously anaemic and die in early childhood.
  • Heterozygous individuals have a mixed population of normal and sickled red blood cells.
  • They are not seriously anaemic and can lead fairly normal lives.
  • Haemophila (bleeder's diseases) is due to lack of gene for production of proteins responsible for blood clotting.


Practical Applications of Genetics

  • Study of genetics has been put into a wide variety of uses en-compasing plants and animals and in particular humans.

Blood transfusion

  • Blood groups are genetically determined.
  • As discussed earlier a person of blood group A can only get blood from another one of A or O.
  • In case of emergencies and unavailability of blood, a patient may be given blood group A + when he/she is A-.
  • First transfusion is fine since, by the time enough antibodies are produced most of the red blood cells of donor have completed their lifespan but a subsequent transfusion of A+ blood is fatal.

Plant and Animal breeding

  • Genetics is applied mostly in plant and animal breeding in order to produce varieties that are most suitable to man's needs.
  • This is done through artificial selection.
  • Varieties are developed that are resistant to pests, diseases or harsh climatic conditions.

Genetic Counselling

  • Genetic counselling involves advising about hereditary diseases and disorders so that they can make informed decisions.
  • This is done through:
    • Taking family history.
    • Screening for genotypes e.g. through amniocentesis.
    • In amniocentesis, cells are obtained from amniotic fluid during pregnancy.
    • Conditions such as Down's syndrome can be detected using microscopy.

Genetic Engineering

  • This is a technology that involves the manipulation of the genotype of an organism to get the desired trait.
  • It also involves the transfer of gene coding for the desired trait from one organism to another.
    Application of Genetic Engineering
    1. Pharmaceutical industries
      • Making of hormones e.g. Human insulin and human growth hormone.
      • Enzymes e.g. Alph-Anti-Trypsin (AAT) used to treat emphysema. (c) Proteins.
      • Drugs and vaccines.
    2. Agricultural industries
      • Transgenic animals and plants are produced which are also called Genetically Modified Organisms (GMO's).
      • A variety of tomato with improved paste and a longer shell life.
      • Sheep for producing desired proteins in milk.
      • Plants resistant to pests and diseases.
    3. Cloning
      • This is the making of identical copies of genes, DNA and whole organisms.
      • Cloning is used in plants - that is tissue culture e.g. in development of various varieties of bananas and Eucalyptus trees.
      • The first mammal to be cloned successfully was Dolly - the sheep.
      • A nucleus from the cell obtained from the udder of the sheep was inserted in an unfertilised egg without a nucleus.
      • This zygote was introduced into the uterus of a sheep and developed to full term.
    4. Gene therapy
      • Involves injecting genes into patients of certain diseases e.g. Parkinson's diseases.
      • The injected gene alters metabolism to bring about the cure of the disease.


Practical Activities

To Demonstrate Continuous Variations - Height of students

  • Students should work in pairs, use chalk and metre rule to mark level of top of head onto the wall or door as one student stands straight without shoes, next to the wall or door.
  • The height for each student is recorded on chalk board.
  • The frequency distribution of height is recording as the height is grouped into various classes.
  • A histogram to represent frequency against height is drawn.
  • The normal bell shaped curve is observed.

Discontinuous Variations - Ability to Roll Tongue

  • The number of students who can roll their tongue is recorded as well as the number of non-tongue rollers.
  • The ratio of tongue-rollers to non tonguerollers is worked out.
  • Gene for the ability to roll the tongue is dominant, therefore is expected more tongue rollers.

Demonstration of Mitosis and Meisosis

Mitosis

  • Plasticene is used to represent number and shapes of various chromosomes e.g. 8 in Drosophila melanogaster
  • Each stage of mitosis illustrated e.g. interphase,
  • Each is rolled to appear long is and coiled, prophase is each made into a ball and then shaped to the appropriate length; and split into two to represent chromatids.
  • Centromeres for different chromosomes can be illustrated in different positions.
  • Each stage of mitosis is illustrated and telophase can be illustrated by surrounding the "chromosomes" with a long many drawn plasticene to represent cell membrane.
  • It is manipulated to show how telophase takes place.

Meiosis

  • The same procedure is followed.
  • Plasticine with contrasting colours is used to show clearly gene mixing in crossing over.
  • Each pair of homologous chromosomes is represented by plasticene with two different colours e.g. red (paternal) blue for maternal chromosome.
  • All the steps in the two stages of meiosis are illustrated up to the production of four haploid gametes.

Human Finger Prints

  • The finger prints for each student's thumb, forefinger and middle fingers of the left hand is imprinted on a white paper.
  • A rubber stamp with ink is used to and each finger -tip phalange is rolled onto the inkpad.
  • For best results students work in pairs.
  • Observations are made at all forefingers, thumb prints and differences noted.
  • The main patterns are noted. It is also noted that no two, fingerprints are exactly similar.

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