Contraceptive methods are used to prevent unwanted pregnancy. The main types are: (1) natural methods, such as the rhythm method (periodic abstinence) and withdrawal; (2) barrier methods, such as condoms and diaphragms, which prevent the sperm and ovum from meeting; (3) intrauterine devices (IUDs), such as the copper-T; (4) hormonal methods, such as oral contraceptive pills; and (5) surgical methods (sterilisation), such as vasectomy in males and tubectomy in females. An ideal contraceptive should be safe, effective and reversible.

Sexually transmitted diseases (STDs), also called venereal diseases, are infections that are transmitted mainly through sexual contact. Examples include gonorrhoea, syphilis, genital herpes, hepatitis B and AIDS (caused by HIV). They can be prevented by: avoiding sexual contact with unknown or multiple partners, using condoms, avoiding sharing needles, and getting early diagnosis and complete treatment. Awareness and safe practices are the best prevention.

Assisted reproductive technologies (ART) are special medical techniques used to help infertile couples have a child. In IVF (in vitro fertilisation), the ovum and sperms are collected and fertilisation is carried out outside the body in the laboratory (in a culture dish); the resulting early embryo is then transferred into the uterus (embryo transfer). A baby born by such a technique is commonly (though misleadingly) called a test-tube baby. Other ART methods include GIFT, ZIFT and ICSI.

Law of dominance: characters are controlled by discrete units called factors (genes) which occur in pairs; in a dissimilar pair (heterozygous) of factors, one factor is dominant and expresses itself while the other is recessive and remains masked. Law of segregation: during the formation of gametes, the two factors (alleles) of a pair separate (segregate) so that each gamete receives only one factor of the pair; the factors do not blend and are passed on unchanged.

Mendel's law of independent assortment states that when two pairs of contrasting characters (traits) are considered together, the alleles of one pair of characters segregate (assort) independently of the alleles of the other pair during the formation of gametes. In other words, the inheritance of one character is not affected by the inheritance of another. This law is based on the results of a dihybrid cross, which gives a phenotypic ratio of 9 : 3 : 3 : 1 in the F2 generation.

A test cross is a cross in which an individual showing a dominant phenotype (whose genotype is unknown) is crossed with a homozygous recessive individual. It is used to determine whether the individual with the dominant phenotype is homozygous or heterozygous. If all the offspring show the dominant character, the individual is homozygous; if the offspring show a 1 : 1 ratio of dominant to recessive phenotypes, the individual is heterozygous.

In incomplete dominance, neither allele is completely dominant, so the heterozygote shows an intermediate (blended) phenotype; for example, in the four o'clock plant (Mirabilis jalapa) a cross between red and white flowered plants gives pink flowers in the F1. In codominance, both alleles express themselves fully and independently in the heterozygote (there is no blending); for example, in the human ABO blood group, the AB blood group shows both the A and the B antigens.

Linkage is the tendency of two or more genes located on the same chromosome to be inherited together (as a group) because they do not assort independently; such genes are said to be linked. Recombination is the formation of new combinations of genes (different from the parental combinations) in the offspring, mainly as a result of crossing over between homologous chromosomes during meiosis. The frequency of recombination between two genes is used to estimate the distance between them on a chromosome.

In humans, sex is determined by the sex chromosomes. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY); this is the XX-XY type of sex determination. All the eggs produced by the female carry one X chromosome, while the sperms are of two types: half carry an X chromosome and half carry a Y chromosome. If an X-bearing sperm fertilises the egg, the child is a female (XX); if a Y-bearing sperm fertilises the egg, the child is a male (XY). Thus the father (male) determines the sex of the child.

Down syndrome is caused by the presence of an additional copy of chromosome number 21 (trisomy 21, i.e. 47 chromosomes); affected persons show mental retardation and characteristic facial features. Turner syndrome is caused by the absence of one X chromosome in females (45, X0), giving sterile females with underdeveloped features. Klinefelter syndrome is caused by an additional X chromosome in males (47, XXY), giving males with some feminine features and usually sterile.

Pleiotropy is the phenomenon in which a single gene affects (controls) more than one character or trait; for example, the gene for sickle-cell anaemia (or the gene causing phenylketonuria) affects several traits at once. Polygenic inheritance is the phenomenon in which a single character or trait is controlled by two or more genes, each adding to the effect; for example, human skin colour and human height are controlled by many genes and show a range of variation.

According to the Watson and Crick model (1953), DNA is a double helix made of two polynucleotide chains coiled around a common axis. The two strands are antiparallel (run in opposite directions) and have a sugar-phosphate backbone on the outside, with the nitrogenous bases pointing inward. The two strands are held together by hydrogen bonds between complementary bases (A pairs with T by two hydrogen bonds, and G pairs with C by three hydrogen bonds). The helix has about 10 base pairs per turn, with a pitch of about 3.4 nm.

In DNA, the base pairing rule (complementary base pairing) states that adenine (A) always pairs with thymine (T) through two hydrogen bonds, and guanine (G) always pairs with cytosine (C) through three hydrogen bonds. As a consequence, Chargaff's rule states that in a DNA molecule the amount of adenine equals the amount of thymine (A = T) and the amount of guanine equals the amount of cytosine (G = C); therefore the total purines equal the total pyrimidines.

A = T; G = C

The central dogma of molecular biology, proposed by Francis Crick, states that the genetic information generally flows in one direction: from DNA to RNA to protein. DNA is first copied into a messenger RNA (mRNA) by the process of transcription, and the information in the mRNA is then used to synthesise a protein by the process of translation. (In some viruses called retroviruses, the flow can be reversed from RNA to DNA by reverse transcription.)

DNA -> RNA -> protein

The genetic code is the set of rules by which the sequence of bases in mRNA is translated into the sequence of amino acids in a protein. Its features are: (1) it is a triplet code - each codon of three bases codes for one amino acid (there are 64 codons); (2) it is degenerate - one amino acid may be coded by more than one codon; (3) it is universal - the same codons code for the same amino acids in almost all organisms; (4) it is non-overlapping and read in a continuous manner; and (5) AUG is the start codon, and there are three stop (termination) codons.

The three main types of RNA are: (1) messenger RNA (mRNA), which carries the genetic message (codons) from the DNA in the nucleus to the ribosomes for protein synthesis; (2) transfer RNA (tRNA), also called the adaptor molecule, which brings specific amino acids to the ribosome during translation according to the codons; and (3) ribosomal RNA (rRNA), which is a structural and functional (catalytic) component of the ribosomes where proteins are synthesised.

The lac operon is a segment of DNA in the bacterium E. coli that controls the metabolism of lactose; it consists of a promoter, an operator and three structural genes (z, y and a). It is an inducible operon. In the absence of lactose, a repressor protein binds to the operator and blocks transcription, so the genes are switched off. When lactose is present, it acts as an inducer: it binds to the repressor and inactivates it, so the repressor leaves the operator and transcription of the genes takes place, allowing the bacterium to use lactose.

DNA fingerprinting is a technique used to identify an individual on the basis of the unique patterns in certain highly variable regions of their DNA (called repetitive DNA or VNTRs), which differ from person to person (except in identical twins). Applications: (1) in forensic science, to identify criminals or victims from biological samples such as blood, hair or semen; and (2) in settling disputes of parentage (paternity or maternity testing). It is also used to study genetic diversity and evolution.

Darwin's theory of natural selection (survival of the fittest) states that: (1) organisms produce far more offspring than can survive, so there is a struggle for existence for limited resources; (2) individuals within a species show variations, some of which are useful (favourable); (3) individuals with favourable variations are better adapted, so they survive, reproduce more and pass on these variations to their offspring (natural selection); and (4) over many generations these favourable variations accumulate, leading to the origin of new species.

(1) Fossil evidence (palaeontological): fossils are the preserved remains of organisms of the past found in rocks; the study of fossils in different layers shows a gradual change in organisms over time and provides direct evidence of evolution. (2) Morphological and anatomical evidence: the presence of homologous organs (similar in structure and origin but different in function, such as the forelimbs of a whale, bat and human) shows common ancestry (divergent evolution), while vestigial organs (reduced, functionless organs like the appendix in humans) also indicate evolution.