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Mendel's 2nd Principle

a. Give one reason why genes may not follow Mendel's 2nd Principle. Explain clearly.
b. What is meant by the term double crossover as it relates to recombinant progeny. Explain your answer by giving the genotype of parental chromosomes and those chromosomes showing a double crossover event.
c. If the RF (recombinant frequency) between 2 genes is 0, what does this mean and explain your answer.
d. If the RF of two genes is 0.5, what are the two possibilities for the two genes? Explain clearly.

a. Give one reason why genes may not follow Mendel's 2nd Principle. Explain clearly.
Mendel's 2nd law principle is the law of independent assortment states that, "During the formation of gametes, two traits will segregate independently from each other if the segregation of one trait having no effect on the other" (1). This segregation happen only if the allele pairs is occur on different chromosomes. In other words, the law of independent assortment states that, "Separate genes for separate traits are passed independently of one another from parents to offsprings. That is the, "Selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait" (1).

For example, if you have two characteristic of pea plants.
Seed color - Yellow denoted Y; Green denoted y
Seed shape - Round denoted R ; wrinkled denote r

If you crossed a true breeding plant with round yellow seeds (YYRR) and true breeding plant with wrinked green seeds (yyrr), which is called a dihybrid cross. The F1 generation heterozygous for two alleles (Yy Rr). The dihybrid cross is a cross between two parents that differ by two pairs of alleles ( AABB x aabb)

In independent assortment, the F1 generation would have any combination of Y/y and R/r alleles in the crosses. The progeny would have YR, Yr, yR or yr. Both parents would produce four types of gametes, YR, Yr, yR, and yr. The phenotypic ratio for this cross is 9 yellow round to 3 green round; 3 yellow wrinked to 1 green wrinkled. The ratio is 9:3:3:1 supporting the independent assortment hypothesis (1).

Conclusion: The law of independent assortment state that, "During gamete formation, the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair" (2). Basically in order for the law of independent assortment to work, the alleles or genes must not be linked on the same chromosomes. Linked genes do not obey the law of independent assortment because they are linked to the same chromosomes and cannot be segregated independently (2). For example if the two genes YR are linked to the same chromosomes, when crossing over or meiosis occur, the two genes YR will not separate, but stay linked together and passed on to the offspring. You will have the YR genotype and yellow round pea phenotypes.

b. What is meant by the term double crossover as it relates to recombinant progeny. Explain your answer by giving the genotype of parental chromosomes and those chromosomes showing a double crossover event.

Gametes that are generated by a "trihybrid heterozygous individual. a+b+c+/ a b c
Meiotic tetrad Gametes Combinations

Referenc: http://www.mun.ca/biology/dinnes/B2250/exercise7.pdf
If the genotype of parental chromosome is a+ b+ c+ that mate with genotype a b c , then the double crossover offsprings contain the

a+ b c+
a b+ c
a b c
a+ b+ c+

c. If the RF (recombinant frequency) between 2 genes is 0, what does this mean and explain your answer.

If the RF or recombinant frequency between two genes is 0, this mean that the two genes cannot cross over or segregate from one chromosome to another. The two genes must be too close or adjacent to each other on the same chromosome in that they cannot recombine or crossover. They must also be linked to each other on the same chromosomes so they cannot recombine (2). In contrast, gene that are far apart on a chromosome have a recombination frequency of 50% (2). The "Relative distance between the linked genes will influences the amount of recombination observed (2). The recombination frequency is an estimate of the distance between two genes. So if the recombination frequency is 0, the genes are adjacent to each other.

d. If the RF of two genes is 0.5, what are the two possibilities for the two genes? Explain clearly.
In contrast, if the recombination frequency of the two genes is 0.5 or 50%, this means that the distance between the two genes are far apart. The two genes are not linked and can recombine or cross over efficiently. The two genes are far apart so they have a chance of recombination or crossing over when cell division occur.

Figure 1 is an example
Gene A and B are close to each other, so the recombination frequency is 0.2 or 2 % of recombination.
Gene A and C are far apart from each other, so the recombination frequency is 0.4 or 4% of recombination.

References:
1. https://www.boundless.com/physiology/reproduction-chromosomes-and-meiosis/sources-genetic-variation-1/law-independent-assortment
2. http://www.mun.ca/biology/dinnes/B2250/exercise7.pdf

Solution Preview

a. Give one reason why genes may not follow Mendel's 2nd Principle. Explain clearly.
Mendel's 2nd law principle is the law of independent assortment states that, "During the formation of gametes, two traits will segregate independently from each other if the segregation of one trait having no effect on the other" (1). This segregation happen only if the allele pairs is occur on different chromosomes. In other words, the law of independent assortment states that, "Separate genes for separate traits are passed independently of one another from parents to offsprings. That is the, "Selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait" (1).

For example, if you have two characteristic of pea plants.
Seed color - Yellow denoted Y; Green denoted y
Seed shape - Round denoted R ; wrinkled denote r

If you crossed a true breeding plant with round yellow seeds (YYRR) and true breeding plant with wrinked green seeds (yyrr), which is called a dihybrid cross. The F1 generation heterozygous for two alleles (Yy Rr). The dihybrid cross is a cross between two parents that ...

Solution Summary

Mendel's 2nd law principle is the law of independent assortment states that, "During the formation of gametes, two traits will segregate independently from each other if the segregation of one trait having no effect on the other" (1). This segregation happen only if the allele pairs is occur on different chromosomes. In other words, the law of independent assortment states that, "Separate genes for separate traits are passed independently of one another from parents to offsprings. That is the, "Selection of a particular gene in the gene pair for one trait to be passed to the offspring has nothing to do with the selection of the gene for any other trait" (1).

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