Билет 1
Основные законы генетики. Генетическая связь. Изменчивость генотипов, мутации и рекомбинация. Закон Менделя.
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 ofSegregation2. Law of Dominance3. Law of Independent Assortment and Di-HybridCross.
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.
Genetic Variability
Genetic variability is a measure of the tendency of individual genotypes in a population to vary from one another. The variability of a trait describes how much that trait tends to vary in response to environmental and genetic influences. It is important to note that the sequence of nuclear DNA between any two humans is nearly 99.9% identical, and yet it is that 0.01% of DNA sequence differences that cause genetically determined variability among humans. On the other hand some DNA sequence differences have little or no effect on phenotype whereas others are directly responsible for causing disease. Between these two extremes the difference in DNA sequence is responsible for variation in phenotype, character, talents, susceptibility to specific diseases etc.
Basic forms of variation
1. Continuous variation: This is the case where the individuals in a population show a graduation from one extreme to another. For example, height of individuals in the human population follows a normal distribution curve (bell-shaped curve). Characteristics which show continuous variation are controlled not by one but by the combined effect of a number of genes and is called a polygene. Thus any characteristic which results from the interaction of many genes is called polygenic inheritance. The variable assortment of the genes during prophase 1 of meiosis ensures that individuals posses a range of genes from any polygenic complex.
2.Discontinuous variation: This is the case where there is a limited number of distinct forms within the population in other words there are no intermediate phenotypes. For example humans may be separated into groups according to their blood type i.e. 4 groups.
Recombination
Genetic recombination is the process by which the combinations of alleles observed at different loci in two parental individuals become shuffled in offspring individuals. Such shuffling can be the result of recombination via intra-chromosomal recombination (crossing over) and via inter-chromosomal recombination (also called independent assortment). In other words, it is a process by which a breaking of a strand occurs and then rejoined to a different DNA molecule therefore the offspring now having a different combination of alleles from their parents.
The crucial events of meiosis are those which are responsible for recombination, which means that the combinations of alleles passed by individuals to their offspring differ from those that were passed to the individuals by their parents. This helps to a level of genetic variation.
Independent assortment
Each pair of homologous chromosomes consists of one chromosome inherited from the father and one inherited from the mother. When a pair of homologous chromosomes separate/segregate at anaphase I, one member of each pair moves to opposite poles of the cell. It is important to note that the process is not selective to which chromosome of the homologous pair, paternal or maternal, is going to move to a specific pole of the cell. Therefore the two daughter cells contain new combinations of maternally and paternally inherited chromosomes. Hence we say that we have recombination due to independent assortment (on the equatorial plate) in metaphase I
Mutation
It is defined as a change in the DNA sequence of a cell's genome. Mutations can be divided into 3 classes or categories:
§ Genome mutations: Mutations that affect the number of chromosomes.
§ Chromosome mutations: Mutations that alter the structure of individual chromosomes. Also known as Gross mutations.
§ Gene mutations: Mutations that alter individual genes.
All 3 types of mutations occur quite often in many different cells. However, if a mutation occurs in a germline cell, it may be passed on to future generations. On the other hand somatic mutations occur by chance in a subset of cells in certain tissues and result in somatic mosaicism that cannot be transmitted.
It is important to note the fact that many types of mutations are represented among the millions of DNA variants found throughout the genome in the normal population as well as among the vast numbers of alleles at individual loci in thousands of genetic disorders. Also another important pointer is that, mutations are the drive force of evolution but they can also be pathogenic.
Билет 2сурак
1. Gene–environment interaction (or genotype–environment interaction or G×E) is when two different genotypes respond to environmental variation in different ways.
There are two different conceptions of gene–environment interaction. Tabery has labeled them biometric and developmental interaction, while Sesardic uses the terms statistical and commonsense interaction.
The biometric (or statistical) conception has its origins in research programs that seek to measure the relative proportions of genetic and environmental contributions to phenotypic variation within populations. Biometric gene–environment interaction has particular currency in population genetics and behavioral genetics. Any interaction results in the breakdown of the additivity of the main effects of heredity and environment, but whether such interaction is present in particular settings is an empirical question. Biometric interaction is relevant in the context of research on individual differences rather than in the context of the development of a particular organism.[8]
Developmental gene–environment interaction is a concept more commonly used by developmental geneticists and developmental psychobiologists. Developmental interaction is not seen merely as a statistical phenomenon. Whether statistical interaction is present or not, developmental interaction is in any case manifested in the causal interaction of genes and environments in producing an individual's phenotype.