16 topics with concise breakdowns: definitions, key facts, and class questions.
This section is dedicated to classical Mendelian genetics—the mechanisms of transmission of traits encoded in nuclear DNA. Here, we analyze the core principles that form the modern understanding of hereditary pathologies and human phenotypic variability.
The material covers everything from fundamental genetic concepts, Mendel's laws, and the purity of gametes hypothesis to complex patterns of interaction between allelic and non-allelic genes. Special emphasis is placed on topics critically important for future physicians: inheritance of blood groups and the Rh factor, multiple allelism, pleiotropy, and the chromosomal theory of heredity, including sex genetics and sex-linked inheritance.
In practical classes and exams, instructors rarely ask you to simply "recount the theory." You will be required to proficiently solve genetic problems. Be sure to understand how a test cross works, why segregation ratios change in incomplete dominance, and how to properly construct a pedigree for an X-linked trait.
What is the difference between homozygous and heterozygous individuals?
Homozygous organisms have two identical alleles of a gene (e.g., AA or aa), whereas heterozygous organisms carry different alleles—one dominant and one recessive (Aa).
Does Mendel's First Law hold true in incomplete dominance?
Yes, the law is fully preserved. Despite the appearance of a new intermediate trait, all offspring of the first generation ($F_1$) remain completely uniform both in genotype (all $Aa$) and in phenotype.
What does the term "pure" gamete mean?
This means that the sex cell does not contain the second component of the gene pair. Alleles in a heterozygous state do not mix and enter the gamete one by one.
How to externally distinguish a heterozygote from a dominant homozygote?
Under complete dominance—not at all. The heterozygote (Aa) will phenotypically look identical to the dominant homozygote (AA), as the recessive trait is completely masked.
What is the formulation of Mendel's Third Law?
It states that hereditary traits are transmitted to successive generations independently of each other, combining in all possible combinations.
How many genotypes are possible in the Rh system?
In the Rh factor system, due to the combination of three pairs of closely linked non-allelic genes (Cc, Dd, Ee), 27 different genotypes can be formed.
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