10.2 Dihybrid Cross
20/02/2013 § Leave a comment
To transition into the next unit (Genetics), this blog will cover shortly the concept of independent assortment. We know from the previous unit that Mendel discovered the law of segregation by crossing monohybrid pea plants together. He also discovered the law of inheritance by doing dihybrid crosses, which involves parents with two different characteristics, for example pea plants that are yellow or green and smooth or wrinkled. The ratios of his results were 9:3:3:1. In the case of the pea plants, that would be 9 yellow round peas : 3 green round peas : 3 yellow wrinkled peas : 1 green wrinkled. This ratio has been proven with both predicted and actual results, which turned out to be very close. This law supports that genes are inherited independently of each other and allows for new combinations of characters to form freely. Such a characteristic is necessary for variation.
We sort of already know this, but during meiosis, in metaphase 1 when the homologous chromosome pairs line up at the equator of the cell, the position each chromosome takes is completely random, as is the direction the pair faces. The entire positioning of where the chromosomes end up is completely random and don’t affect one another. This is called independent orientation. This is why, for two alleles, there are four possible daughter cells (therefore a 25% chance each can come to existence) that can contribute to the gamete cell.
These concepts are law. But there are always exceptions, which we’ll be learning about tomorrow, specifically gene linkage.
- Calculate and predict; genotypic and phenotypic ratios of offspring of dihybrid crosses involving unlinked autosomal genes.
- Identify which of the offspring in dihybrid crosses are recombinants.
- Describe the methods and aims of DNA profiling.
- Outline a technique for transferring genes between species.
- Describe the technique for the transfer of the insulin gene using E. coli.
- Discuss the potential benefits and possible harmful effects of genetic modification.
- Discuss the ethical arguments for and against the cloning of humans.
- Outline the ethical issues of cloning humans.
I have yet to know any of these essay questions. Crap.
DATA BASED QUESTIONS
Page 159, gene linkage in Zea mays
Corncobs are often used for showing inheritance patterns. All the grains on a cob have the same female parent, and with careful pollination they can also have the same male parent. A variety with coloured and starchy grains was crossed with a variety with white and waxy grains. The F1 grains were all coloured and starchy. The F1 plants were grown from these grains were crossed (F1 x F1).
1. Calculate the expected ratio of F2 plants, assuming that the genes for colours/white and starchy/waxy grains are unlinked. Use a genetic diagram to show how you reached your answer. —> I did this on my quiz and I got it right, I don’t need to show y’all, heheheh.
2. The actual frequencies were… coloured starchy: 1774, coloured waxy: 263, white starchy: 279, white waxy: 420. Using this data, deduce whether the genes for coloured/white and starchy/waxy are linked. —> The genes are linked because the colored waxy and white starchy recombinants imply that crossing over was done among linked genes.
A variety with coloured and shrunken grains was crossed with a variety with white and non-shrunken grains. The F1 grains were all coloured and non-shrunken. The F1 plants grown from these grains were test crossed using pollen from a homozygous recessive variety with white shrunken grains.
3. Calculate the expected ratio of F2 plants, assuming that the genes are unlinked, using a diagram to show how you reached your answer. —> I have the answers on my quiz, alL IS WELL, LADIES AND GENTLEMEN.
4. The actual frequencies were… coloured non-shrunken: 638, coloured shrunken: 21,379, white non-shrunken: 21,096, white shrunken: 672. Using this data, deduce whether the genes for coloured/white and non-shrunken/shrunken are linked. —> They are linked because the 1:1:1:1 ratio is not satisfied and there are more parental genes than recombinants.
5. Deduce whether the genes for starchy/waxy and non-shrunken/shrunken are linked. —> They are linked, eheheheheh.