MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I.Fedonyuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Basics of Human Genetics

Linked Inheritance of Genes

Genes located on the same chromosome are referred to as linked genes and are predominantly inherited together (linked inheritance). All genes on a single chromosome form a single linkage group. Traits whose genes belong to the same linkage group do not follow THE PRINCIPLE OF independent assortment in dihybrid crosses (AaBb x AaBb) in a 9:3:3:1 ratio and in dihybrid test crosses (AaBb x aabb) in a 1:1:1:1 ratio. Linked genes are not always transmitted together.

The phenomenon of trait linkage was discovered in 1906 by W. Bateson and R. Punnett in experiments with sweet peas by crossing two of its strains that differed in two pairs of traits: pollen shape and flower color. Instead of the expected segregation In the second generation F₂ (9:3:3:1), segregation was observed in a ratio close to 3:1. Thus, the traits did not exhibit independent inheritance.

T. Morgan analyzed the phenomenon of Gene linkage and gave it this name. In Drosophila, the gray body color allele (B) dominates over the black body color allele (b), and the normal wing length allele (V) dominates over the vestigial wing allele (v). Crossing a homozygous gray fly with normal wings (BBVV) and a black fly with vestigial wings (bbvv) yielded in the first generation F₁ hybrids with gray bodies and normal wings (BbVv), which confirmed Mendel's law of uniformity of first-generation hybrids. The experimental results did not depend on the sex of the recessive homozygote. Next, two dihybrid test crosses were performed. In the first one, the male was an F₁ dihybrid (gray body and normal wings), and the female was homozygous for the recessive alleles (black body and vestigial wings). Two phenotypic classes, analogous to the initial parental forms, were obtained from this cross in an equal ratio: 50% gray with normal wings (BbVv) and 50% black with vestigial wings (bbvv). In the second dihybrid test cross, the female was an F₁ dihybrid (gray with normal wings), and the male was a recessive homozygote (black with vestigial wings). Four phenotypic classes were obtained from this cross in the following ratio:

1) gray with long wings (BbVv) 41.5%;

2) gray with vestigial wings (Bbvv) 8.5%;

3) black with long wings (bbVv) 8.5%;

4) black with vestigial wings (bbvv) 41.5%

The results of both dihybrid test crosses did not match the expected phenotype ratios—25% gray with normal wings, 25% gray with vestigial wings, 25% black with normal wings, and 25% black with vestigial wings, as occurs in independent assortment (G. Mendel). Morgan explained the deviation from the expected segregation (1:1:1:1) by the fact that the GENES OF THE studied traits (B and V) are located on the same chromosome and are inherited together (linked). The strength of linkage between genes is inversely proportional to the distance between them on the chromosome (Morgan's rule or law).

Gene linkage can be complete or incomplete. In the first test cross (♀ bbvv x ♂ BbVv), complete gene linkage occurred. In the second test cross (♀ BbVv x ♂ bbvv), the number of individuals with phenotypes repeating the parental phenotypes predominated (83%), also indicating a tight linkage of the B and V alleles. T. Morgan, drawing upon F. Janssens' discovery of chiasmata in Meiosis (1909), explained the appearance in smaller numbers (17%) of offspring with phenotypes combining the traits of both parents as a disruption of linkage resulting from chromosome Crossing-over at a point between the B and V genes. Morgan called the crossing-over process crossing-over. The differences in the results of the 1st test cross, where the male was the dihybrid, and the 2nd test cross, where the female was the dihybrid, are explained by a specific feature of Drosophila biology, which is the absence of crossing-over in male gametogenesis. Therefore, the dihybrid male (BbVv) produces only non-crossover Gametes (BV and bv) of two types at 50% for each type, while the dihybrid female (BbVv) produces 83% non-crossover gametes of two types (41.5% BV and 41.5% bv) and 17% crossover gametes of two types (8.5% Bv and 8.5% bV). Gametes formed As a result of crossing-over are called crossover gametes, whereas non-crossover gametes are formed without crossing-over. Accordingly, individuals arising with the participation of crossover gametes are called crossovers, and those formed without them are non-crossovers. Thus, in T. Morgan's experiment, there were 17% crossovers and 83% non-crossovers in the offspring.