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.

In a monohybrid cross, a pure tall plant (TT) is crossed with a pure dwarf plant (tt). All the F1 plants are Tt and are tall, because T (tall) is dominant over t (dwarf). When the F1 plants (Tt) are self-pollinated, the gametes T and t combine in all possible ways to give the F2 generation: TT, Tt, Tt and tt. This gives a phenotypic ratio of 3 tall : 1 dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt.

In a dihybrid cross, two pairs of contrasting characters are studied together (for example, seed shape and seed colour in peas: round yellow RRYY crossed with wrinkled green rryy). All F1 plants are round and yellow (RrYy). When the F1 are self-pollinated, the F2 generation shows four phenotypes in the ratio 9 (round yellow) : 3 (round green) : 3 (wrinkled yellow) : 1 (wrinkled green). This 9 : 3 : 3 : 1 ratio illustrates Mendel's law of independent assortment.

F2 dihybrid ratio = 9 : 3 : 3 : 1

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.

Sex-linked disorders are genetic disorders caused by genes located on the sex chromosomes (usually the X chromosome), so their inheritance is linked to the sex of the individual. Haemophilia is an X-linked recessive disorder in which the blood does not clot properly (a clotting factor is missing), so there is prolonged bleeding even from minor injuries. Colour blindness (red-green) is also an X-linked recessive disorder in which a person cannot distinguish between red and green colours. Because these genes are on the X chromosome, such disorders are more common in males (who have only one X chromosome).

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

Semiconservative replication is the method of DNA replication in which each of the two strands of the parent DNA acts as a template, and the newly formed DNA molecule contains one old (parental) strand and one newly synthesised strand. During replication, the enzyme helicase unwinds and separates the two strands; then the enzyme DNA polymerase adds new nucleotides to each template strand following complementary base pairing, forming two identical daughter DNA molecules. This mode was experimentally proved by Meselson and Stahl.

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

Transcription is the process of copying the genetic information from one strand of DNA into a molecule of messenger RNA (mRNA). Only one strand of the DNA (the template strand) is copied. The enzyme RNA polymerase binds to the promoter region and moves along the template, adding ribonucleotides according to complementary base pairing (A of DNA pairs with U of RNA, and so on), forming the RNA. The three steps are initiation, elongation and termination. In eukaryotes the RNA is then processed (splicing) before it leaves the nucleus.

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.

Translation is the process of synthesis of a protein (polypeptide) from the information present in the mRNA. It takes place on the ribosomes. The mRNA attaches to a ribosome, and transfer RNA (tRNA) molecules bring specific amino acids according to the codons on the mRNA (each tRNA has an anticodon complementary to a codon). The amino acids are joined one by one by peptide bonds as the ribosome moves along the mRNA (initiation, elongation and termination), forming a polypeptide chain. Translation stops when a stop codon is reached.

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.