Meiosis and Mendel

Question 1

What is Mendel’s Law of Independent assortment?

Answer 1

“During formation of gametes, the alleles of one gene segregate from each other independently of those for another gene”

• This means that independent assortment is a direct consequence of the random orientation of homologous pairs on the spindle at Metaphase 1 of meiosis.

Question 2

What evidence was put forward to suggest that Mendel may have been wrong?

Answer 2

• Through observation of two different flowers, one purple with long pollen and the other red with round pollen. It was shown that segregation occurred normally, however the segregation of flower colour was not independent of pollen shape- two different genes.
• This was made apparent through the observation of more parental phenotypes than recombinant genotypes in the F2 progeny.

Question 3

So why should there be more parental than recombinant phenotypes in the F2 Progeny?

Answer 3

• In 1911 it was shown by Thomas Hunt Morgan that this phenomenon only occurs then the two loci are on the same chromosome.
• The genes are said to be linked and tend to remain associated during meiosis.

Question 4

If both genes are on the same locus then how can recombinant gametes be produced?

Answer 4

Through crossing over of alleles between genes at prophase 1. This breaks the linkage.

Question 5

What are the effects of crossing over?

Answer 5

• Without crossing over, all gametes would carry the original parental arrangements of alleles of linked genes.
• This means that the combination of alleles on a single chromosome would remained fixed throughout the generations.
• This would greatly limit the genetic variability created by sexual reproduction

Question 6

What factor is the recombinant frequency dependant on?

Answer 6

How far apart on the chromosome the recombinant genes are.

• Closely linked genes rarely recombine
• Distantly linked genes show independent assortment

Question 7

How would you calculate the Recombinant Frequency (RF) from a dihybrid cross?

Answer 7

You would add the two total values of recombinant phenotypes together that are observed. Then you would divide this number by the total number of progeny and multiple by 100 to get a percentage.

Question 8

How would you calculate RF from a 3 locus test cross

Answer 8

To work out the recombinant frequency for A and C, you would add the total number of A recombinations with C ones, divide the sum by the total number of progeny and multiply by 100. This RF value can be used directly as a map unit called a Centrimorgan (cM). This would be repeated for A and B & B and C. From these values, a gene map can be produced.