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Genetics and Evolution

Genetics (The Science of Heredity)

Genes, Chromosomes, and the Genome

Genetics is the science of heredity; it deals with the structure and function of the genes. The cells of all living things contain a program that guides their functioning. This program is genetically determined, i.e., it is transmitted to both newly formed cells during every cell division. The transmission must be precise, since otherwise it leads to disturbances in function (mutations, see below). The genetic program consists of individual information units, the genes (= hereditary characters), with each gene determining a specific function. The sum total of all genes is the genome (the human inheritance includes within a single set of chromosomes approximately 30000–40000 genes; see Chapter 1: The Cell Nucleus), which is contained in the sum of chromosomes in each cell nucleus. Genes are arranged along the chromosomes in a linear fashion and have a definite location and structure. They represent the smallest functional genetic unit; each comprises on an average 1000–10000 base pairs (300–3000 base triplets), a comparatively short chromosome segment (a single set of chromosomes, that is, 23 chromosomes, contains a double strand of DNA with a total length of around 3 billion base pairs). A single gene might, for instance, contain the information for one protein (i.e., how many amino acids it contains and how they are arranged). A single character, on the other hand, may be determined by several genes.

The Allele

With the exception of the sex cells, human cells contain 46 chromosomes: 23 maternal and 23 paternal. In this way, every gene is present on the corresponding paired homologous chromosome in identical or slightly modified form. The genes that are localized at the same site on both the maternal and paternal chromosomes are called alleles. If both alleles are completely identical in their genetic information, the carrier of such a character is called homozygous; if they differ, the carrier for that character is heterozygoous.

Dominance, Recessiveness, and Codominance

When an allele in a heterozygote always prevails over the other allele, so that it is solely responsible for the expression of a character, it is called dominant. The allele that is not expressed in the phenotype (see below), that is, is not in evidence, is called recessive (suppressed). When in a heterozygote both alleles are expressed in the phenotype, the alleles are called codominant.

Phenotype and Genotype

The two concepts genotype and phenotype refer to the genetic information of a character at the site of each gene (gene locus). The observed character, the appearance, is called the phenotype; this might be a hair color, a certain blood group, or the color of the flower of a plant. The genotype is the genetic information on which the phenotype is based.

The Mendelian Rules

If the transmission of individual hereditary factors (genes) is followed from generation to generation, the distribution of chromosomes during maturation division (meiosis; see Chapter 1) will be seen to follow certain laws. These pertain to the random distribution of the homologous chromosomes during meiosis and the combinatory possibilities when a sperm cell meets an ovum. The Augustinian monk Gregor Johann Mendel (1822–1884) recognized these laws in 1866 during cross-breeding studies with garden peas, even though he did not know about the processes that occur during meiotic maturation division.

To discover the rules for the distribution of hereditary characters, certain conditions must be met: the crossbreeding experiments must be performed with purebred (homozygous) organisms, so that all germ cells receive the same hereditary characters; the hereditary characters studied must be visible externally (genes were unknown at the time); and the factors or genes that determine these characters must be located on different chromosomes. For crossbreeding studies, the first generation is known as the parental generation (P generation), the first offspring as the first filial generation (F1 generation), and the next offspring as the second filial generation (F2 generation).

Mendel’s first law: the law of uniformity (dominance) (phenotypic uniformity of the F1 generation)

Mendel’s second law: the law of segregation (phenotypic segregation in the F2 generation after dominant–recessive or intermediate hereditary transmission

Mendel’s third law: the law of independence (independent transmission of nonlinked genes)

The Rule of Uniformity (Dominance)

When two different homozygous lines that differ in one or more alleles are crossbred, the result is a heterozygous F1 generation with a uniform (i.e., dominant) phenotype. If, for instance, a red-flowered homozygous pea plant (RR1) is crossed with a white-flowered (rr) homozygous pea plant in generation P, the F1 heterozygote will be uniformly red (Rr) like the red parent (Fig. 2.1). The character of the white parent is suppressed and so cannot become expressed. Hence the white flower character is suppressed in the F1 generation and so cannot be expressed. Hence the phenotypic character appearing in the F1 generation (red in this case) is called dominant, while the other is the recessive (suppressed) character.

Dominant–recessive hereditary transmission is by far the commonest form of heredity. Thus, in Mendel’s studies of peas not only did red flowers predominate over white, but yellow seed color predominated over green, smooth seeds predominated over wrinkled seeds, and tall plants predominate over short ones.

However, when a purebred red four o’clock plant is crossed with a purebred white, the heterozygous generation’s flowers are uniformly pink (Fig. 2.2). Such a case, in which the heterozygous F1 generation differs in phenotype from the two homozygous parents is called intermediate inheritance. In the F1 generation the pink coloration of the flower is produced by a blending of the two genes inherited from the P generation (white and red coloration of the flower).

Fig. 2.1 Dominant–recessive hereditary transmission. A homozygous red-flowered (RR) garden pea is crossed with a homozygous white (rr) one. The heterozygous F1 generation is uniformly red, since the flower color red is dominant over white. The F2 generation splits in the ratio 3 : 1, that is 3 offspring are red-flowered (RR, Rr, and rR) while one offspring is white-flowered (rr)

On the other hand, when both alleles are of equal weight and both characters appear in the heterozygote side by side, the condition is called codominance. An example is given by the A and B blood groups. If a child receives the blood group A allele from the father and the blood group B allele from the mother, the child will have the blood group AB.

Fig 2.2 Intermediate hereditary transmission. Crossing of a homozygous red-flowered (rr) four o’clock flower with a homozygous white-flowered one (ww). The F1 generation is uniformly pink-flowered (rw) as both flower colors are expressed in the phenotype. The F2 generation splits in a ratio of 1:2:1, i.e., one plant is red-flowered (rr), one white-flowered (ww) and two more are pink-flowered (rw, wr)

Rule of Segregation

When the peas plants of the F1 generation are crossed with each other (Rr × Rr), the next generation (F2 generation) will show three-quarters red-flowered and one-quarter white-flowered plants (Fig. 2.1). The 3 : 1 numerical ratio becomes more exact the more offspring are examined. The phenotypic segregation ratio depends on whether a gene (allele) is dominant or recessive. The dominance of the red-flowered gene of the pea over the white-flowered gene results in a red (R) to white (r) ratio close to 3 : 1 because the combinations RR and Rr both result in the phenotype R.

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