Genetics is the study of heredity, which is how traits are passed from parents to children. Gregor Mendel is known as the father of genetics. He carried out his classic experiments on the garden pea plant (Pisum sativum). A gene is a section of DNA that controls a trait, and the different forms of a gene are called alleles. The genetic makeup of a person is called the genotype, and the physical expression of a trait is called the phenotype. A person with genotype Aa, which has two different alleles, is called heterozygous (a hybrid). A person with two identical alleles, such as AA or aa, is called homozygous.
Mendel’s laws explain how traits are inherited. The law of segregation states that the two alleles of a gene separate from each other during gamete formation, so each gamete carries only one allele. The law of independent assortment states that genes for different traits are inherited independently. In a typical dihybrid cross, the phenotypic ratio in the F2 generation is 9:3:3:1.
DNA stands for deoxyribonucleic acid. Its double helix structure was discovered by James Watson and Francis Crick in 1953. DNA is made of nucleotides, which contain a sugar, a phosphate, and a nitrogenous base. The bases pair in a fixed way: adenine (A) always pairs with thymine (T), and guanine (G) pairs with cytosine (C). A and T are joined by two hydrogen bonds, while G and C are joined by three hydrogen bonds. Chargaff’s rules state that in DNA the amount of adenine equals thymine and the amount of guanine equals cytosine.
Genes are used to make proteins. The start codon is AUG, which also codes for the amino acid methionine. The stop codons, which end translation, are UAA, UAG, and UGA. A mutation is a change in the DNA sequence. Mutations can be harmless, harmful, or sometimes useful.
A normal human somatic (body) cell has 46 chromosomes, which is 23 pairs. Females have the sex chromosomes XX, and males have XY. Some conditions are caused by a change in chromosome number. Down syndrome is caused by an extra copy of chromosome 21 (trisomy 21). A person with the sex chromosome pattern XXY is affected by Klinefelter syndrome.
Colour blindness and haemophilia are examples of X-linked (sex-linked) recessive inheritance. The genes are carried on the X chromosome, so these conditions are more common in males, who have only one X chromosome.
The ABO blood group system is controlled by three different alleles: IA, IB, and i. The alleles IA and IB show codominance, which means both are fully expressed together in a person with genotype IAIB (blood group AB). Blood group AB is considered the universal recipient, because it has both A and B antigens and no anti-A or anti-B antibodies in the plasma.
Gene linkage means that genes located close together on the same chromosome tend to be inherited together. This was first demonstrated by Thomas Hunt Morgan using the fruit fly (Drosophila melanogaster).