Genetics VIII The rules of inheritance
Blending hypothesis of inheritance (the hit of 1800s): parental characters are mixed similar to mixing of paints by an artist A logical inference from the blending hypothesis is that over time all individuals in a population will be uniform. As we know, it does not happen. Everyone is unique!!!
Gregor Mendel and the gene idea Augustinian St. Thomas's Abbey monks Johann (Gregor) Mendel 1822-1884 Gregor Mendel was a German-speaking teacher (Austrian Empire) with the deep knowledge in physics, astronomy, meteorology, and breeding.
Mendel s initial plan was to study genetic phenomena using mice, because he had experience with animal breeding. However, monastery s authorities declared experiments with mice as «inappropriate» for the monastery (watching how mice make love). For that reason, Mendel had to use plants to study genetical principles.
Major features of Mendel s experiments 1. simple cross-pollination technique + 2. use of true-breeding (pure) lines + 3. large number of plants tested + 4. precise quantification of crossing results discovery of the fundamental rules of inheritance
Mendel counted occurance of parental traits in the first (F 1 ) and second (F 2 ) generations after a cross (P)
The results of Mendel s F 2 crosses for 7 characters in pea plants All recessive traits were absent in the first generation (F 1 ), but were reappearing in the second generation (F 2 ) in the proportion of 1 to 3 relative to the dominant traits.
Useful genetic vocabulary genotype = a set of genes responsible for a trait (character) phenotype = a set of phenes (traits) = appearance conditioned by the genotype alleles = copies of the same gene on homologous chromosomes loci (single: locus) = locations/positions of genes homozygous = one, whose zygote was made of equal ( homo ) gametes heterozygous = one, whose zygote was made of different ( hetero ) gametes
A box, called a Punnett square, is very useful for calculation of all possible combinations of alleles after a cross. PP Pp pp Reginald Crundall Punnett 1865-1967
The test cross A technique used to discriminate between homozygous dominant (AA) and hetrozygous (Aa) gentotypes. It uses a known recessive pure line (aa) as a crossing partner.
Mendel s conclusions were made without knowledge about chromosomes, but were generally correct and now are known as Mendel s gene model: 1. Alternative versions of genes (A and a, alleles) account for variations in inherited characters. 2. For each character, an organism inherits two copies of a gene, one from each parent. 3. If the two alleles at a given locus (position) are different, then one, called the dominant allele, determines the organism s appearance; the other, called the recessive allele, has no noticable effect on the organism s appearance. 4. The law of segregation: the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. These conclusions were derived from monohybrid crosses (experiments in which only one character has been followed).
Mendelian inheritance is governed by laws of probability Pp = 1 x 1 = 1 Pp = 1/2 x 1 = 1/2 pp = 1/2 x 1 = 1/2 Probabilities of simultaneous events are multipied (and = x). Example: probability of the first allele coming from the egg and the second one coming from the sperm. Probabilities of alternative events are added (or = +). Example: probability of the first allele coming from the egg and the second one coming from the sperm or the other way round. PP = 1/2 x 1/2 = 1/4 Pp = 1/2 x 1/2 + 1/2 x 1/2 = 2/4 = 1/2 pp = 1/2 x 1/2 = 1/4 PP = 1 x 1/2 = 1/2 Pp = 1 x 1/2 = 1/2
Mendel s experiments with dihybrids The law of independent assortment: Each pair of alleles segregates independently of each other pair of alleles during gamete formation.
x Can we apply probability rules for calculation of genotype frequencies in dihybrid crosses? Yes! YYRR = [1/2 x 1/2] (probability of YY) x [1/2 x 1/2] (probability of RR) = 1/16 YyRR = [1/2 x 1/2] + [1/2 x 1/2] (probability of Yy) x [1/2 x 1/2] (probability of RR) = 2/16 = 1/8
In practice, calculation of genotype frequencies based on probability rules is necessary for solving complex genetic problems Let us consider offspring frequencies from a trihybrid cross: PpYyRr x Ppyyrr. What fraction of the progeny is expected to show recessive phenotypes for at least two characters? Punnett squares for monohybrid crosses will help us:
Thomas Hunt Morgan 1866-1945 How does linkage between two genes affect inheritance of characters? Predicted ratios: If genes are located on different chromosomes If genes are located on the same chromosome Experimentally observed by T. Morgan
Chromosomal basis for the results observed by T. Morgan
Recombination frequencies can be used to map relative positiones of genes on a chromosome. A unit for genetic distance between two genes is called cm (1 centimorgan = 1% recombination). Genetic recombination between two genes is detected only if there are an odd number (1, 3, 5 ) of chromosomal crossovers between the two genes. The distance in centimorgans does not correspond exactly to the probability of genetic recombination. Organism 1cM = kb (thousands bp) Human 769 Tomato 510 Corn 2140
Possible gametes no crossingover AB, ab one crossingover Ab, ab two crossingovers AB, ab three crossingovers Ab, ab Genetic recombination between two genes is detected only if there are an odd number (1, 3, 5 ) of chromosomal crossovers between the two genes. The distance in centimorgans does not correspond exactly to the probability of genetic recombination, because some of the recombination events remain unnoticed (do not change gamete types).
Both frequency of crossing over and gene density are non-uniform along different regions of a chromosome. Centromeric regions are the least active in recobmination and have the lowest density of genes.
Thank you for your attention!