Билет 1

1. Основные законы генетики. Генетическая связь. Изменчивость генотипов, мутации и рекомбинация. Закон Менделя.

2. Гено-экологические взаимодействия. Наследование многофакторных признаков и заболеваний у человека

The following points highlight the three fundamental laws of genetics proposed by Mendel. The laws are: 1. Law of Segregation 2. Law of Dominance 3. Law of Independent Assortment and Di-Hybrid Cross.

Mendel's First Law - the law of segregation; during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete

 Law of Segregation

Instead of crossing homzygous pea plants the 2nd law refers to the crossing of the heterozygous F1 Generation. When crossing 2 heterozygous pea plants we can expect the following:

T= the colour yellow t=colour green Parents: Tt x Tt Offspring (genotype): TT;Tt;Tt;tt (ratio 1:2:1) Offspring (phenotype): three yellow one green (ratio 3:1)

Law of Dominance . While crossing/reproducing pea plants he discovered something interesting. When he crossed yellow peas with green peas the offspring (F1) would only consist of yellow peas. The same results occured when he crossed round one with wrinkeld ones. He refered to the properties as factors and stated that some factors are dominant over others. Nowadays we call those "factors" alleles. The dominant allele is expressed by a capital letter and the rezessive allele by a small letter.

Example of crossing: (dominant allele)T= the colour yellow (recessive allele)t=colour green Considering purely(homozygous) yellow and purely(homozygous) green the genotype of the parental generation would be TT and tt. When crossed the genotype of the whole offspring would be Tt. Since T is dominant over t they would all be yellow. So their phenotype would also be the same.

Law of Independent Assortment

So far we've been dealing with one trait at a time. For example seed shape (round or wrinkled)or color (green or yellow). Mendel noticed during all that the shape of the plant and the color had no impact on each other. The different traits seem to be inherited INDEPENDENTLY. This Law describes the crossing between 2 pea plants who are heterozygous for 2 properties, in this case shape and colour.

AaBb x AaBb

A = dominant allele for yellow a = recessive allele for green B = dominant allele for round seeds b = recessive allele for wrinkled seeds

The Offspring Results in 16 different Genotypes. 9/16 show dominant phenotype for both traits (round & green), 3/16 show dominant phenotype for first trait & recessive for second (round & yellow), 3/16 show recessive phenotype for first trait & dominant form for second (wrinkled & green) 1/16 show recessive form of both traits (wrinkled & yellow).

Ratio 9:3:3:1

2. Genetic linkage is the tendency of DNA sequences that are close together on a chromosome to be inherited together during the meiosis phase of sexual reproduction. Two genetic markers that are physically near to each other are unlikely to be separated onto different chromatids during chromosomal crossover, and are therefore said to be more linked than markers that are far apart. In other words, the nearer two genes are on a chromosome, the lower the chance of recombination between them, and the more likely they are to be inherited together. Markers on different chromosomes are perfectly unlinked.

Genetic linkage is the most prominent exception to Gregor Mendel's Law of Independent Assortment. The first experiment to demonstrate linkage was carried out in 1905. At the time, the reason why certain traits tend to be inherited together was unknown. Later work revealed that genes are physical structures related by physical distance.

Genotype variation, mutations and recombination.