Genetics

Crime cases can be solved by collecting DNA evidence: forensic scientists compare the DNA in fragments of skin, hair or droplets of blood left at the crime scene with the suspect’s DNA p. 75.

5.1 Cell Division

The nucleus contains chromosomes p. 76. Chromosomes are fine thread-like structures which consist of nucleic acids and protein, located in the nucleus, and each chromosome contains a long DNA molecule p. 76. Deoxyribonucleic acid or DNA is made up of basic units known as nucleotides, and each nucleotide consists of deoxyribose sugar, a nitrogenous base and a phosphate group p. 77. DNA is made of two twisted antiparallel polynucleotide chains forming a double helix p. 77.

A gene is a basic hereditary unit that determines an individual’s characteristics, such as tongue-rolling ability in humans, the shape of the green pea and the colour of the cat’s fur p. 77. Genes are arranged as segments in the DNA along the chromosomes, and there are thousands of genes in a DNA molecule p. 77. Genetics is a study of genes, heredity and variations in organisms p. 77.

The human body cell contains 46 chromosomes, and each chromosome exists in pairs of similar shapes and sizes called homologous chromosomes p. 78. The arrangement of human homologous chromosomes according to their shapes and sizes is called karyotype p. 78. Autosomes carry genes that control characteristics such as the colour of the eyes, the ability to roll the tongue and the type of hair, while sex chromosomes carry genes that determine gender p. 78.

Mitosis is a cell division process that forms two identical daughter cells, each having the same number of chromosomes and genetic content as the parent cell p. 79. Mitosis takes place in the somatic cells of the human and animal bodies, and in meristematic tissue such as the tip of the roots and the shoot of plants p. 79. Meiosis is a process where the cell divides into four distinct cells, each containing half the number of chromosomes in the parent cell, to produce gametes for sexual reproduction p. 79. Meiosis in humans takes place in the testes and the ovary, and in plants in the anther and the ovary p. 79. The human somatic cell has 46 chromosomes but the gametes, sperm and ovum, have only 23 chromosomes p. 79. Interphase is the stage before mitosis and meiosis, when DNA replication occurs p. 79. Cancer is caused by uncontrolled and abnormal cell division p. 79.

In prophase of mitosis, chromosomes shorten and thicken, each made of two chromatids joined at the centromere, and the nuclear membrane and nucleolus disintegrate p. 80. In metaphase, chromosomes are arranged in an equatorial plane with spindle fibres attached to the centromere, and in anaphase each sister chromatid separates and moves towards the opposite poles of the cell p. 80. In telophase, the chromatids reach the opposite poles and the nuclear membrane and nucleolus are formed again; then the cytoplasm divides into two daughter cells with the same chromosome number and genetic information as the parent cell p. 81.

In prophase I of meiosis, homologous chromosomes pair together and crossing over occurs, that is the exchange of genetic information between the homologous chromosomes p. 80. In anaphase I the homologous chromosomes separate and move towards the opposite poles, and in telophase I the cytoplasm divides p. 80. Meiosis II ends in telophase II with four daughter cells that are not identical (gametes), each with half the number of chromosomes of the parent cell p. 81.

Mitosis and meiosis both involve DNA replication and cell division p. 82. Mitosis produces two genetically identical daughter cells with no crossing over and no variation, while meiosis produces four daughter cells that are genetically not identical and has variations p. 82. Mitosis is important for the growth of organisms, to replace damaged or dead cells when we are injured, and for asexual reproduction in organisms such as Amoeba sp., while meiosis is important for the production of gametes p. 82.

5.2 Inheritance

The transmission of characteristics from the parents to the children is known as inheritance p. 84. Genes are the basic units of inheritance found in the chromosomes p. 84. The pair of genes at the same locus or position in the homologous chromosome is known as the allele, and alleles can be found in dominant or recessive forms p. 84.

The characteristics controlled by the dominant allele will be shown, whereas the recessive allele will be covered, so recessive characteristics are only shown when both alleles are recessive p. 85. The dominant allele is represented by capital letters and the recessive allele by small letters, for example ‘T’ for tall and ‘t’ for short p. 85. For every trait, we inherit two copies of the genes, one from our mother and the other from our father p. 85. Dominant traits in humans include the ability to roll the tongue, black hair and free earlobes p. 85.

Gregor Mendel (1822-1884), the “Father of Genetics”, was the first person to discover the inheritance mechanism in a systematic and detailed manner p. 86. Mendel used pure-breeding pea plants in his experiment on monohybrid cross, which involves research on one characteristic at a time, such as height p. 86. He crossed a pure-breeding tall pea plant (TT) with a pure-breeding dwarf pea plant (tt), and all the first filial generation (F1) were tall (Tt) p. 86. When individuals from F1 are crossed, the second filial generation (F2) is formed p. 86. Crossing Tt with Tt gives the F2 genotypes TT, Tt, Tt and tt, so three are tall and one is dwarf p. 87. Genotype is the genetic information in an organism, and phenotype is the physical characteristics shown in an organism p. 86.

The characteristics inherited by a person depend on the genes inherited from his parents p. 87. A father who can roll his tongue (BB) and a mother who cannot (bb) have F1 children who are all Bb and able to roll the tongue p. 87.

A male and a female have 46 chromosomes each, made up of 44 autosomes and two sex chromosomes; the male has X and Y, while the female has two X chromosomes p. 88. After meiosis, the sperm has 23 chromosomes (either 22 + X or 22 + Y) and the ovum has 23 chromosomes (22 + X) p. 88. The gender of a child depends on the type of sperm that fertilises the ovum: 44 + XX is a female and 44 + XY is a male p. 88. The chances of getting a male or a female offspring at each pregnancy is equal, which is 50% p. 89.

5.3 Mutation

Mutation is a spontaneous and random change that takes place in the genes and chromosomes that can cause changes of characteristics to the offspring who inherits the modified genes p. 90. There are two types of mutations, chromosome mutation and gene mutation p. 90. The birth rate of Down syndrome babies is 1 to 800, and the risk is higher if the mother is more than 35 years old p. 90.

Chromosome mutation occurs when there are changes in the number or structure of the chromosome, as a result of a defect during the cell division process p. 91. A person with Down syndrome has an extra chromosome number 21, giving 47 chromosomes, with physical and mental retardation, a short neck and slanted eyes p. 91. A person with Turner syndrome is a female with a missing X chromosome, 45 chromosomes (44 + XO) only p. 91. A person with Klinefelter syndrome is a male with an extra chromosome X, 47 chromosomes (44 + XXY), and is sterile p. 91.

Gene mutation is caused by chemical changes that occur to a gene p. 92. A colour blind person cannot differentiate between red and green, a disease controlled by a recessive gene on the X chromosome that usually occurs in males p. 92. Sickle cell anaemia gives sickle-shaped red blood cells, affecting the transport of oxygen, and is caused by a recessive gene on the autosome p. 92. A person with thalassemia has small red blood cells with a shorter lifespan, which can cause severe blood deficiency p. 92. A person with haemophilia has difficulty in the clotting of his blood and will lose blood continuously if wounded p. 92. In November 2016, the Ministry of Health Malaysia launched a thalassemia check for Form 4 students p. 92.

Mutation may occur spontaneously (naturally) during cell division, and can also occur due to external factors known as mutagens p. 93. Factors that cause mutation include radioactive rays, X-rays, ultraviolet rays, carcinogens and pregnancy at a late age p. 93.

Traits whose genes are located on the sex chromosomes are known as sex-linked traits, for example haemophilia p. 94. Haemophilia is only inherited when there is a recessive allele that causes the disease on the X chromosome p. 94. With X^H as the dominant (normal) allele and X^h as the recessive (haemophilia) allele, a normal father and a carrier mother can have a son with haemophilia p. 95. If the mother is a haemophiliac, every son has haemophilia and every daughter is a carrier p. 95.

Amniocentesis is used to identify foetal cell abnormalities from the 15th to the 20th week of pregnancy, after the position of the foetus is detected using the ultrasound technique p. 96. Genetic research that can increase the quality of human life includes forensic science, gene therapy and genetic genealogy p. 97. Forensic science studies crime investigation by confirming the chronology of an incident based on scientific evidence p. 97. Gene therapy, still at an experimental stage, aims to repair mutated genes that cause diseases such as cystic fibrosis, haemophilia and sickle cell anaemia by introducing a normal gene into the patient’s cells p. 98. Genetic genealogy uses DNA tests to determine the family pedigree and its history p. 99.

5.4 Genetic Engineering Technology

Genetic engineering is the term used for genetic modification of an organism, first introduced around 1971-1973 p. 100. It is usually related to recombinant DNA, genetically modified organisms (GMO) and gene therapy p. 100. Recombinant DNA technology combines two different species to produce a new genetic characteristic, such as the production of human insulin using bacteria for people with diabetes mellitus p. 100. The DNA and the plasmid DNA are cut with restriction enzyme, the recombinant DNA is introduced into a bacterial cell, and the recombinant bacteria multiply in a fermentation tank to produce human insulin p. 100.

GMO is a Genetically Modified Organism, that is an organism (plant, animal, bacteria or virus) which has been genetically modified for a particular purpose p. 101. Plants that resist pests or diseases are grown in paddy, maize and palm oil cultivation p. 101. One benefit of genetically modified crops is a reduction in the use of pesticides, which can reduce environmental pollution p. 101.

Advantages of genetic engineering technology include producing quality plants and livestock and producing insulin and enzymes from bacteria p. 102. Disadvantages include allergy and side effects to consumers, new species that cause the original species to become extinct, and unethical use to produce biological weapons p. 102. Genetic research needs to be carried out with high integrity, and ethics and values must be practised so that it does not touch on religious and moral sensitivities p. 103.

5.5 Variation

Variation is the differences in characteristics among individuals from the same species p. 103. Continuous variation shows differences that are not distinct between individuals in the same population, such as height and body weight p. 104. Continuous variation is quantitative and is represented by a normal distribution curve p. 104. It is determined by genes but can also be influenced by environmental factors, as with skin colour p. 104. Discontinuous variation shows differences that are distinct between individuals, such as the ability to roll the tongue, types of fingerprints, earlobes and blood groups p. 104. Discontinuous variation is qualitative, is represented by a bar chart, and is determined by genetic factor only p. 105.

Genetic factors that cause variation are crossing over during meiosis, independent assortment of chromosomes, random fertilisation and chromosome and gene mutations p. 107. Environmental factors such as pH, sunlight, temperature, water, climate and nutrition can also change the characteristics of organisms; for example, the Hydrangea sp. flower is blue in acidic soil and pink in alkaline or neutral soil p. 107.

Variation enables organisms to adapt themselves to their environment, and if all individuals are the same, they will become extinct when a drastic change happens p. 108. Variation lets us recognise each individual in the same species, allows natural selection and evolution, and helps organisms to camouflage from predators p. 108. Through genetic engineering and selective breeding, animals and plants with new genetic characteristics can be produced, increasing the quality and quantity of the product p. 108.