bzyct-137-solved-assignment-2024-ss-020cab3d-1c01-486f-9bdf-7506d86b97ee
- a) Explain dihybrid cross and Mendel’s Law of independent assortment.
Answer:
A dihybrid cross is a genetic cross that examines the inheritance of two different traits simultaneously. Gregor Mendel, an Austrian monk and the founder of modern genetics, conducted dihybrid crosses with pea plants to study the inheritance of two traits, such as seed color and seed shape.
In a dihybrid cross, the parent generation (P) consists of individuals that are homozygous for two traits. For example, one parent might have yellow, round seeds (YYRR), and the other parent might have green, wrinkled seeds (yyrr). When these parents are crossed, the first filial generation (F1) will all be heterozygous for both traits (YyRr), displaying the dominant phenotypes (yellow and round seeds).
When the F1 generation is self-crossed or intercrossed, the resulting second filial generation (F2) will exhibit a phenotypic ratio of 9:3:3:1 for the four possible combinations of traits:
- 9/16 will be yellow and round (YYRR, YYRr, YyRR, YyRr)
- 3/16 will be yellow and wrinkled (YYrr, Yyrr)
- 3/16 will be green and round (yyRR, yyRr)
- 1/16 will be green and wrinkled (yyrr)
Mendel’s Law of Independent Assortment states that the alleles of two (or more) different genes get sorted into gametes independently of one another. In other words, the allele a gamete receives for one gene does not influence the allele received for another gene. This law explains the 9:3:3:1 phenotypic ratio observed in the F2 generation of a dihybrid cross. It is important to note that this law applies only to genes that are located on different chromosomes or are far apart on the same chromosome.
Mendel’s Law of Independent Assortment and his Law of Segregation (which describes the separation of alleles during gamete formation) form the foundation of classical genetics and explain how traits are inherited from one generation to the next.
b) Explain the types of epistasis.
Answer:
Epistasis is a form of genetic interaction where the expression of one gene is influenced by one or more other genes. It can modify the typical Mendelian ratios of phenotypes in the offspring. There are several types of epistasis, each with its own characteristic effects on phenotypic ratios:
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Recessive Epistasis:
- In recessive epistasis, the presence of two recessive alleles at one locus masks the expression of alleles at a second locus. A common example is the 9:3:4 phenotypic ratio seen in Labrador retriever coat color, where the presence of two recessive alleles for pigment deposition (ee) masks the expression of the B locus that determines black or brown coat color.
- Example Ratio: 9:3:4
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Dominant Epistasis:
- In dominant epistasis, a single copy of an allele at one locus masks the expression of alleles at a second locus. An example is the 12:3:1 ratio seen in fruit color in certain squash varieties, where a dominant allele for white color (W) masks the expression of color alleles at another locus.
- Example Ratio: 12:3:1
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Duplicate Recessive Epistasis:
- In this type of epistasis, two genes are required for the expression of a phenotype, and the presence of recessive alleles at either locus masks the expression of the dominant phenotype. An example is the 9:7 ratio seen in the flower color of certain plants, where both genes are required for pigment production.
- Example Ratio: 9:7
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Duplicate Dominant Epistasis:
- In duplicate dominant epistasis, the dominant allele of either of two genes is sufficient to produce the same phenotype, resulting in a 15:1 phenotypic ratio.
- Example Ratio: 15:1
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Dominant-Recessive Epistasis:
- In this type of epistasis, the dominant allele at one locus masks the expression of alleles at a second locus, but only if the second locus is homozygous recessive. An example is the 13:3 ratio seen in certain traits.
- Example Ratio: 13:3
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Polymeric (Cumulative) Epistasis:
- This occurs when two or more genes contribute additively to a single phenotype, often resulting in a continuous range of phenotypes rather than discrete classes.
The specific type of epistasis and the resulting phenotypic ratios depend on the genetic pathways involved and how the products of different genes interact. Understanding epistasis is crucial for unraveling the complexities of genetic regulation and trait inheritance.