INHERITANCE OF TWO GENES
The inheritance of two genes, also known as dihybrid inheritance, involves the simultaneous consideration of the inheritance of two different traits controlled by two different genes, each with two or more alleles. Dihybrid crosses can be more complex than monohybrid crosses, but they still follow Mendel’s laws of inheritance. Here are the key points to understand about the inheritance of two genes:
Independent Assortment: Mendel’s Law of Independent Assortment states that genes located on different chromosomes segregate independently during gamete formation. This means that the inheritance of one gene does not affect the inheritance of the other gene.
Dihybrid Cross: A dihybrid cross involves the mating of individuals that are heterozygous for both of the genes under consideration. For example, if you are studying the inheritance of seed color and seed shape in pea plants, you would perform a dihybrid cross by crossing individuals heterozygous for both traits (e.g., AaBb x AaBb).
Phenotypic Ratios: The phenotypic ratios in dihybrid crosses typically follow a 9:3:3:1 ratio. This means that you can expect to see different combinations of the two traits in the offspring. The 9:3:3:1 ratio represents:
- 9 offspring with both dominant traits (A_B_)
- 3 offspring with the first dominant trait and the second recessive trait (A_bb)
- 3 offspring with the first recessive trait and the second dominant trait (aaB_)
- 1 offspring with both recessive traits (aabb)
Test Cross: Dihybrid crosses can be used to determine the genotype of an individual with an unknown genotype for two traits. By crossing the individual in question with a known homozygous recessive individual for both traits (e.g., aabb), you can analyze the phenotypes of the offspring to deduce the genotype of the unknown individual.
Linked Genes: In some cases, genes located on the same chromosome may not segregate independently, which can lead to non-Mendelian ratios. These genes are said to be linked, and they tend to be inherited together more often than expected under independent assortment.
Scenario: Let’s consider a dihybrid cross involving pea plants with two different traits: seed color and seed shape. The seed color trait is controlled by one gene with alleles “Y” (yellow) and “y” (green), and the seed shape trait is controlled by another gene with alleles “R” (round) and “r” (wrinkled). We’ll perform a dihybrid cross between individuals heterozygous for both traits (YyRr x YyRr).
| Parental Generation (P) | Genotype | Phenotype |
|---|---|---|
| Male (Father) | YyRr | Yellow, Round Seeds |
| Female (Mother) | YyRr | Yellow, Round Seeds |
| F1 Generation (First Filial Generation) | Genotype | Phenotype |
|---|---|---|
| Offspring | YyRr | Yellow, Round Seeds |
| F2 Generation (Second Filial Generation) | Genotype | Phenotype |
|---|---|---|
| Offspring | YYRR, YyRR, YyRr, Yyrr, YyRr, yyRR, yyRr, yyrr | Different combinations of seed color and seed shape |
In this dihybrid cross, the F1 generation consists of offspring with the genotype YyRr, and they display the dominant phenotypes for both traits: yellow seeds (Y_) and round seeds (R_).
The F2 generation results from crossing the F1 generation individuals among themselves. This generation exhibits a 9:3:3:1 phenotypic ratio. The different combinations of alleles in the F2 generation lead to various phenotypes for seed color and seed shape, following the independent assortment of the two genes.
- 9 offspring have both dominant traits (yellow and round): YYRR, YyRR, YyRr
- 3 offspring have the first dominant trait (yellow) and the second recessive trait (wrinkled): Yyrr
- 3 offspring have the first recessive trait (green) and the second dominant trait (round): yyRR, yyRr
- 1 offspring has both recessive traits (green and wrinkled): yyrr
Chromosomal Theory of Inheritance
Challenges Faced by Mendel:
- Communication was limited during Mendel’s time, making it difficult to widely publicize his work.
- Mendel’s concept of genes (which he referred to as “factors”) as stable and discrete units controlling traits was not initially accepted.
- His use of mathematics to explain biological phenomena was considered unconventional.
- Mendel could not provide physical proof for the existence of factors (genes) or describe their composition.
Rediscovery of Mendel’s Work:
- In 1900, three scientists, de Vries, Correns, and von Tschermak, independently rediscovered Mendel’s results on the inheritance of characters.
- This rediscovery brought Mendel’s work to the forefront of scientific attention.
Advancements in Microscopy:
- Advancements in microscopy during this period allowed scientists to observe cell division more closely.
- These observations led to the discovery of structures within the nucleus called chromosomes, which appeared to double and divide before each cell division.
Chromosomes and Genes:
- Walter Sutton and Theodore Boveri noted that the behavior of chromosomes during cell division was parallel to the behavior of genes during inheritance.
- Chromosome movement during meiosis helped explain Mendel’s laws and the behavior of genes.
Chromosomes and Genes Occur in Pairs:
- The passage emphasizes that both chromosomes and genes occur in pairs.
- Alleles of a gene pair are located on homologous sites on homologous chromosomes, and the principles of segregation and independent assortment operate based on this arrangement.
In summary, Mendel’s groundbreaking work on inheritance was initially overlooked due to communication limitations, a lack of understanding about the nature of genes, and Mendel’s mathematical approach. However, advancements in microscopy and the independent rediscovery of his work by other scientists eventually led to the acceptance of Mendel’s principles and the connection between genes and chromosomes, paving the way for modern genetics.
Historical development of the chromosomal theory of inheritance and the contributions of various scientists
Sutton and Boveri’s Ideas: Sutton and Boveri proposed that the pairing and separation of a pair of chromosomes during cell division would lead to the segregation of a pair of factors (genes) they carried. This idea combined the knowledge of chromosomal behavior with Mendelian principles, leading to the development of the chromosomal theory of inheritance.
Experimental Verification: Thomas Hunt Morgan and his colleagues conducted experimental research to verify the chromosomal theory of inheritance. They used Drosophila melanogaster, commonly known as fruit flies, for their studies.
Advantages of Using Fruit Flies:
- Fruit flies were well-suited for genetic studies due to several factors:
- They could be easily grown in a laboratory setting on a simple synthetic medium.
- Their short life cycle (about two weeks) allowed for the rapid generation of offspring.
- Fruit flies exhibited clear sexual dimorphism, making it easy to distinguish between males and females.
- Various types of hereditary variations were observable in fruit flies, even with low-power microscopes.
Morgan’s research with Drosophila played a crucial role in establishing the connection between genes and chromosomes, providing strong experimental evidence for the chromosomal theory of inheritance. The study of fruit flies helped elucidate the genetic basis for the variations observed in sexual reproduction and contributed significantly to the field of genetics.
Linkage and Recombination
Linkage and recombination are concepts in genetics that describe how genes on the same chromosome can be inherited together or undergo independent assortment during meiosis. These processes are crucial in understanding the inheritance of multiple genes and the formation of genetic diversity. Here’s an explanation of linkage and recombination:
Linkage:
- Definition: Linkage refers to the tendency of genes located on the same chromosome to be inherited together as a unit, deviating from the principle of independent assortment.
- Genes on the Same Chromosome: When two or more genes are physically close to each other on the same chromosome, they are said to be linked. These genes are often inherited as a group, meaning that the combination of alleles for these genes stays relatively constant from generation to generation.
- Genetic Mapping: Linkage is fundamental to genetic mapping. By analyzing how often linked genes are inherited together versus independently, geneticists can determine the relative positions of genes on a chromosome.
Recombination:
- Definition: Recombination, or genetic recombination, refers to the process by which alleles at different loci (gene locations) on the same chromosome can be exchanged during meiosis, leading to the generation of new combinations of alleles.
- Crossing Over: Recombination occurs through a process called crossing over. During meiosis, homologous chromosomes exchange segments of genetic material. This exchange leads to the creation of chromosomes with a mix of alleles from both parents, promoting genetic diversity.
- Independent Assortment: Recombination is the mechanism by which the principle of independent assortment is maintained, allowing alleles at different loci to be assorted independently during gamete formation.
- Map Units: Geneticists use the concept of map units or centimorgans to quantify the degree of recombination between linked genes. One map unit corresponds to a 1% chance of recombination occurring between two genes.
