Parental strains and experiment start
G. Mendel studied how two traits of pea plants are simultaneously transmitted to offspring. He examined seed color (yellow or green) and seed shape (smooth or wrinkled). Homozygous plants were taken as the parental generation, designated by the Latin letter P.
One of them possessed exclusively dominant characteristics: yellow smooth peas with the genotype RRYY. The second plant carried only recessive traits: green wrinkled peas with the genotype rryy. During meiosis, homozygous organisms are capable of producing only one type of gametes. The first plant forms RY type gametes, while the second forms ry type gametes.
Formation of first-generation hybrids
When fertilization occurs, gametes fuse. This results in the first generation of offspring, $F_1$. Since each parental individual contributed strictly one allele variant, the genotype of all first-generation hybrids is identical: RrYy.
Such organisms are called diheterozygotes. Phenotypically, all the resulting plants look identical: they produce exclusively yellow smooth seeds. This clearly demonstrates the law of uniformity.
Gamete formation in diheterozygotes
The key stage explaining the essence of independent assortment occurs during gamete formation in $F_1$ hybrids. During meiosis, chromosomes assort completely independently.
Due to this, the diheterozygous organism produces exactly four types of gametes in equal proportions:
- (RY)
- (Ry)
- (rY)
- (ry)
Second generation and phenotypic segregation
When crossing first-generation hybrids with each other ($RrYy \times RrYy$ fertilization), the second generation, $F_2$, is obtained. Externally, a clear phenotypic segregation of offspring into four groups is observed in a 9:3:3:1 ratio:
- 9/16 — Yellow smooth seeds (genotype $R-Y-$).
- 3/16 — Green smooth seeds ($rrY-$).
- 3/16 — Yellow wrinkled seeds ($R-yy$).
- 1/16 — Green wrinkled seeds ($rryy$).
Mathematical foundation of the law
Data analysis shows that a dihybrid cross is essentially a combination of two monohybrid crosses. The 9:3:3:1 ratio has a strict mathematical foundation. It represents the product of probabilities of two independent events — squaring the classic monohybrid segregation: $(3:1)^2$. This confirms that traits are inherited independently.
Briefly: answers to topical questions
What specific genotypes correspond to plants with yellow wrinkled seeds in the F2 generation?
Plants with yellow wrinkled seeds in the F2 generation correspond to the generalized genotype (phenotypic radical) $R-yy$.
They are formed as a result of a dihybrid cross of first-generation hybrids (diheterozygotes $RrYy \times RrYy$). In this generation, phenotypic segregation occurs, and the proportion of offspring with yellow wrinkled seeds is 3/16 of the total number (with an overall phenotype ratio of 9:3:3:1). Specific complete genotypes for this phenotype are not detailed in the study materials.
What cytological foundations ensure the independent assortment of traits?
The independent assortment of traits is ensured by the independent distribution of chromosomes in meiosis and the behavior of bivalents. Key cytological mechanisms include:
- Random arrangement of bivalents — occurs at the equatorial plane of the mitotic/meiotic spindle.
- Disjunction of homologous chromosomes — takes place in anaphase I of meiosis, which, in the case of heterozygosity, leads to the formation of gametes differing in alleles.
- Recombination — independent segregation ensures the recombination of parental linkage groups in the haploid set of the forming gametes.
In what cases is the law of independent assortment of traits violated?
The law of independent assortment of traits does not hold when inheriting genes localized in the same chromosome. In such cases, linked inheritance is observed.
According to T. Morgan's law, genes located on the same chromosome are inherited together (linked). The strength of this linkage is inversely proportional to the genetic distance between them: the farther apart the genes are located on the chromosome map, the weaker the linkage and the higher the probability of crossing over.
Class and exam questions
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.
What phenotypic segregation is characteristic of a dihybrid cross in the second generation?
When crossing diheterozygotes, phenotypic segregation occurs in a 9:3:3:1 ratio.
How many types of gametes does an organism with the RrYy genotype form?
As a result of meiosis, a diheterozygote forms 4 types of gametes in equal proportions: RY, Ry, rY, and ry.
What else to study on this topic
- Law of uniformity (manifestation in the first generation)
- Monohybrid cross
- Polyhybrid and trihybrid cross
- Test cross
- Independent segregation of chromosomes in meiosis
- Crossing over
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