Methodology for solving problems of the exam in biology

The section “Genetics” of the school biology course is one of the most difficult for students to understand. Facilitating the assimilation of this section can be facilitated by knowledge of the terminology of modern genetics, as well as solving problems of different levels of complexity.

At the moment, most of the textbooks used to study the sections of genetics in the senior classes of general education schools contain few training tasks in genetics. As a rule, they are not enough for successful development of skills in solving genetic problems for monohybrid, dihybrid and sex-linked inheritance of traits.

The solution of genetic problems develops logical thinking in schoolchildren and allows them to better understand the educational material, enables teachers to effectively monitor the level of students’ achievements.

The manual provides the basic terminology necessary for understanding and successfully solving genetic problems, generally accepted conventions, as well as exemplary algorithms for solving problems for different types of inheritance.

For each task, an approximate number of points is given that a student can earn if the task is successfully completed. Also, sizing will help to differentiate the knowledge of students.

This teaching aid has been compiled to help biology teachers, high school students and applicants.


Alternative signs are mutually exclusive, contrasting.

Analyzing cross – crossing an individual of an indeterminate genotype with an individual homozygous for recessive alleles.

Autosome – any paired chromosome that is not related to the sex chromosomes in diploid cells. In humans, the diploid chromosome set (karyotype) is represented by 22 pairs of chromosomes (autosomes) and one pair of sex chromosomes (gonosomes).

Mendel’s second law (the law of splitting) – when two hybrids of the first generation are crossed among themselves among their descendants – hybrids of the second generation – splitting is observed: the number of individuals with a dominant trait is related to the number of individuals with a recessive trait as 3: 1 (splitting by genotype 1: 2 :1, phenotype 3:1).

Gamete – a germ cell of a plant or animal organism that carries one gene from an allelic pair.

A gene is a section of a DNA molecule (in some cases, RNA) that encodes information about the biosynthesis of one polypeptide chain with a specific amino acid sequence.

Genome – a set of genes contained in the haploid set of chromosomes of organisms of the same biological species.

Genotype – a set of geneslocated in the haploid set of chromosomes of a given organism. Unlike the concepts of genome and gene pool, it characterizes an individual, not a species.

Heterozygous organisms are organisms that contain different allelic genes.

Homozygous organisms are organisms that contain two identical allelic genes.

Homologous chromosomes – paired chromosomes that are identical in shape, size and set of genes.

Dihybrid crossing – crossing of organisms that differ in two ways.

Morgan’s law (linkage law) – linked genes located on the same chromosome are inherited together (linked).

The law of purity of gametes – during the formation of gametes, only one of the two allelic genes enters each of them.

Karyotype – a set of features (number, size, shape, etc.) of a complete set of chromosomes, inherent in the cells of a given biological species (species karyotype), a given organism (individual karyotype) or a line (clone) of cells. A karyotype is sometimes also called a visual representation of the complete chromosome set (karyograms).

Codominance is a type of interaction of allelic genes, in which signs of the genes of both parents appear in the offspring.

Complementary (additional) interaction of genes – such an interaction of genes, as a result of which new signs appear.

Locus is the region of the chromosome where the gene is located.

Monohybrid crossing – crossing of organisms that differ in one trait (only one trait is taken into account);

Incomplete dominance – incomplete suppression by the dominant gene of the recessive of the allelic pair. In this case, intermediate signs arise, and the sign in homozygous individuals will not be the same as in heterozygous ones.

Mendel’s first law (the law of uniformity of hybrids of the first generation) – when crossing parents of pure lines that differ in one contrasting trait, all hybrids of the first generation will be uniform and they will show the trait of only one of the parents.

Pleiotropy (multiple gene action) is an interaction of genes in which one gene affects several traits at once.

Polymeria – duplicative actions of non-allelic genes in the manifestation of this trait.

Polyhybrid crossing – crossing organisms that differ in several ways.

Sex-linked inheritance is the inheritance of a gene located on the sex chromosome.

Mendel’s third law (law of independent inheritance, combination of traits) – each pair of contrasting (alternative) traits is inherited independently of each other in a number of generations; as a result, among the hybrids of the second generation, descendants appear with new combinations of traits in the ratio 9 : Z : Z : 1.

Phenotype – the totality of all external and internal features of an organism.

Pure lines – organisms that do not cross with other varieties, homozygous organisms.

Epistasis is such an interaction of genes when one of them suppresses the manifestations of another, non-allelic to it.


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