Inheritance of Two Gene
Inheritance of Two Gene
Mendel's Dihybrid Cross:
- Mendel also conducted experiments with pea plants that exhibited differences in two characters, such as seed colour and seed shape.
- One parent plant had yellow-coloured and round-shaped seeds, while the other had green-coloured and wrinkled-shaped seeds.
- Mendel observed that the resulting seeds from the cross had a yellow colour and round shape, suggesting the dominance of these traits.
Dominance in Dihybrid Cross:
- In this dihybrid cross, Mendel determined that yellow colour was dominant over green, and round shape was dominant over wrinkled.
- The results mirrored those obtained in separate monohybrid crosses involving single traits (yellow/green seed colour and round/wrinkled seed shape).
- Mendel used genotypic symbols to represent the alleles for these traits: Y for dominant yellow seed colour, y for recessive green seed colour, R for round seed shape, and r for wrinkled seed shape.
- The parental genotypes were RRYY (round and yellow) and rryy (wrinkled and green).
- When Mendel crossed these parental plants (RRYY x rryy), the gametes RY and ry combined to produce the F1 hybrid genotype RrYy.
- Upon self-pollination of the F1 plants, Mendel found that 3/4th of the F2 plants had yellow seeds, while 1/4th had green seeds.
- The yellow and green seed colors are segregated in a 3:1 ratio, just like in a monohybrid cross.
- Similarly, round and wrinkled seed shapes are also segregated in a 3:1 ratio in the F2 generation.
- Mendel's dihybrid cross experiments reinforced the principles of dominance and segregation for two different traits simultaneously.
- The results supported the idea that traits are controlled by discrete units (genes) and that these genes segregate independently during gamete formation.
- Mendel's Law of Independent Assortment:
- In dihybrid crosses, Mendel observed that the phenotypes of offspring appeared in a specific ratio, such as 9:3:3:1 (in the case of round, yellow; wrinkled, yellow; round, green; and wrinkled, green seeds).
- This ratio can be derived as a combination series of two independent segregations: 3 yellow: 1 green (for seed colour) and 3 round: 1 wrinkled (for seed shape).
- To understand this derivation, consider two pairs of traits: seed shape (R for round and r for wrinkled) and seed colour (Y for yellow and y for green).
- The first pair segregates independently as 3 Round: 1 Wrinkled.
- The second pair also segregates independently as 3 Yellow: 1 Green.
- Combine these segregations: (3 Round: 1 Wrinkled) (3 Yellow: 1 Green) = 9 Round, Yellow : 3 Wrinkled, Yellow : 3 Round, Green: 1 Wrinkled, Green.
- Based on observations from dihybrid crosses, Mendel proposed the Law of Independent Assortment, stating that "when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters."
- This means that the inheritance of one trait (e.g., seed colour) is not dependent on the inheritance of another trait (e.g., seed shape).
- The Punnett square is a helpful tool to comprehend the independent segregation of two pairs of genes during meiosis and the formation of eggs and pollen in the F1 hybrid (e.g., RrYy).
- For example, when considering the segregation of seed shape (R/r) and seed colour (Y/y), each pair of alleles segregates independently.
- Gametes can have combinations like RY, Ry, rY, and ry, each with a frequency of 25% or 1/4th of the total gametes produced.
- These four types of gametes give rise to various combinations in the F2 generation.
- The Punnett square is a useful tool to visualise and predict the outcomes of such genetic crosses.
- Chromosomal Theory of Inheritance
Mendel's Work and Initial Challenges:
- Gregor Mendel published his groundbreaking work on the inheritance of traits in 1865.
- Mendel's work remained largely unrecognised for several reasons:
1. Limited means of communication in those times made it difficult to widely publicise his findings.
2. Mendel's concept of genes as stable and discrete units controlling traits, with non-blending alleles, was not accepted by contemporaries.
3. His use of mathematics to explain biological phenomena was novel and not well-received.
4. Mendel couldn't provide physical proof for the existence of factors (genes) or describe their composition.
- In 1900, three scientists independently rediscovered Mendel's results on inheritance (de Vries, Correns, von Tschermak).
- Advancements in microscopy allowed scientists to observe cell division, leading to the discovery of chromosomes.
- Chromosomes were visualised during cell division and seen to double and divide, similar to Mendel's genes.
- Walter Sutton and Theodore Boveri noted the parallel behaviour of chromosomes and genes, using chromosome movement to explain Mendel's laws.
- Chromosomes, like genes, occur in pairs, with two alleles located on homologous chromosomes.
- Sutton and Boveri proposed the chromosomal theory of inheritance, uniting chromosomal segregation with Mendelian principles.
- They argued that the separation of a pair of chromosomes during meiosis leads to the segregation of a pair of factors (genes) they carry.
- This theory provided a framework to explain the hereditary variation produced by sexual reproduction.
Experimental Verification by Thomas Hunt Morgan:
- Thomas Hunt Morgan and his colleagues experimentally verified the chromosomal theory of inheritance.
- They used Drosophila melanogaster (fruit flies) for their studies due to their suitability:
1. Rapid life cycle (about two weeks).
2. High reproduction rate.
3. Clear differentiation between male and female flies.
4. Various hereditary variations are observable under a microscope.
- Linkage and recombination
- Morgan's Dihybrid Crosses:
In his groundbreaking research, Thomas Morgan conducted dihybrid crosses in Drosophila (fruit flies) to explore genes that were sex-linked, similar to Mendel's dihybrid crosses in peas. He used crosses involving traits like body colour and eye colour.
Morgan noticed that when two genes were located on the same chromosome (specifically the X chromosome), their inheritance did not follow the expected Mendelian pattern. Unlike Mendel's 9:3:3:1 ratio for independent genes, Morgan observed deviations in the ratios of phenotypes in the F2 generation.
Morgan recognized that the deviation from the expected ratios occurred because these genes were physically associated or "linked" on the same chromosome. This led to the development of the term "linkage" to describe the physical proximity of genes on a chromosome.
In addition to linkage, Morgan identified the occurrence of "recombination," which represented the generation of non-parental gene combinations. During meiosis, crossing over between homologous chromosomes resulted in the exchange of genetic material, creating new combinations of alleles.
Morgan found that in linked genes, the proportion of parental gene combinations (those present in the parental generation) was much higher than the non-parental gene combinations (those generated through recombination).
Morgan's research also revealed that not all linked genes exhibited the same degree of linkage. Some genes were tightly linked, meaning they showed very low recombination rates, while others were loosely linked, showing higher recombination rates. This variation in linkage strength provided insights into the organisation of genes on chromosomes.
Alfred Sturtevant, a student of Morgan, used the frequency of recombination between gene pairs as a measure of the physical distance between genes on the chromosome. This allowed for the creation of genetic maps, which depicted the relative positions of genes on a chromosome.
- Linkage and Recombination in Drosophila: Results of Two Dihybrid Crosses
Crossing between Gene y and w (y+ w+ / y w)
- In Cross A, Thomas Morgan conducted a dihybrid cross between two genes, y (yellow body) and w (white eyes), on the X chromosome of Drosophila.
- The wild-type alleles for these genes are represented as y+ and w+.
- The "+" sign in superscript indicates the presence of the dominant wild-type allele.
Observations for Cross A:
- Morgan found that the genes y and w were very tightly linked, meaning they exhibited a very low rate of recombination.
- The result of this tight linkage was that most offspring inherited the same combinations of alleles as their parents. In this case, the dominant alleles (y+ and w+) were inherited together, resulting in yellow-bodied, white-eyed flies.
- The proportion of parental gene combinations (y+ w+ and y w) was much higher than the proportion of recombinant gene combinations (y+ w and y w+).
Crossing between Gene w and m (w+ m / w m+):
- In Cross B, Morgan conducted a dihybrid cross between genes w (white eyes) and m (miniature wing) on the same X chromosome.
- Again, the wild-type alleles for these genes are represented as w+ and m+.
Observations for Cross B:
- Morgan found that the genes w and m showed linkage but were less tightly linked than y and w in Cross A. This meant that they exhibited a higher rate of recombination compared to y and w.
- Consequently, in Cross B, a significant proportion of offspring exhibited recombinant gene combinations, such as white-eyed, miniature-winged flies (w m and w+ m+).
- The proportion of parental gene combinations (w+ m+ and w m+) was lower than the proportion of recombinant gene combinations (w m and w+ m+).
- Cross A demonstrated a very tight linkage between genes y and w, resulting in a high proportion of parental combinations and a low proportion of recombinants.
- Cross B showed a linkage between genes w and m but with a higher rate of recombination compared to Cross A. This led to a more balanced distribution of parental and recombinant gene combinations.
- The varying degrees of linkage and recombination observed in these crosses contributed to Morgan's understanding of genetic linkage and recombination, providing insights into the organisation of genes on the X chromosome of Drosophila.
- Mutation is a biological phenomenon resulting in alterations to DNA sequences, leading to changes in an organism's genotype and, consequently, its phenotype. It serves as a significant source of genetic variation.