- Variations within Plant and Animal Species
- Structure and Properties of Chromosomes
- First Law of Heredity
- Practical Applications of Genetics
- Practical Activities
- 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.
- 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.
- 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.
- 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.
- 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.
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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Stem length: some plants were tall while others were short.
- Colour of unripe pods: some were green, others yellow. There were no intermediates.
- 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.
- 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.
- The genes present in an individual. The genetic constitution of an individual. It is expressed in alphabetical notation.e.g TT,Tt
- 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.
- 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.
- This is a state where the alleles in an individual are similar e.g. TT (for tall)
- 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)
- 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.
- 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.
Fertilization - using checker board or Punnet square
F1 genotype Tt
F1 Phenotypic ratio = All tall.
F2 Genotype TT, 2Tt, tt
F2 Phenotypic ratio; 3 Tall : 1 short
- 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.
- 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).
- 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.
- 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.
- 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.
- 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 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.
- 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.
- 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:
- 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 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 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.
- 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.
- 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.
- 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:
- 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.
- 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).
- 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:
- 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.
- A portion may break from a chromosome and then rejoin to it after turning though an angle-of 180o.
- This is when aportion is joined to a non-homologous chromosome.
- A certain section of an intact chromosome replicates such that the genes are repeated.
- Changes in the diploid number of chromosomes (allopolyploidy).