20.4 Phenotypic Ratios

Question 1

Definition of linkage

Answer 1

Two different genes located on the same chromosome

Question 2

What is autosomal linkage?

Answer 2

The genes that are linked are found on one of the other pairs of chromosomes

Question 3

Explain linked genes

Answer 3

Inherited as one unit, there is no independent assortment, they are linked and they stay together
The parental combination of alleles is likely to be found in gametes, parents genotypes are seen in many more of the offspring then you would expect, small amount of recombinant offspring occur due to crossing over but the closer the genes are on the chromosome the less likely they are to be separated during crossing over and the fewer recombinant offspring that occur

Question 4

How does linkage affect phenotypic ratio?

Answer 4

Will produce an unexpected ratio as there are only two outcomes, so more gamete’s will contain the linked genes (parental) and fewer where they are separated (recombinant). Recombinant genes will only be formed through crossing over in meiosis I.

Question 5

What is the link between where genes are on a chromosome and linkage?

Answer 5

When genes are far apart, crossing over happens often but two genes that are close together are not very common, due to a very short distance between genes the ‘target; for crossover is small, few such events will take place. The closer together the loci are on the chromosome, the lower the change of choasma forming between them

Question 6

What do recombinant alleles provide evidence for?

Answer 6

Crossing over

Question 7

What is recombinant frequency?

Answer 7

A measure of the amount of crossing over that has occurred in meiosis

Question 8

How do you calculate recombinant frequency?

Answer 8

number of recombinant offspring / total number of offspring

Question 9

What does a recombinant frequency of 50% indicate?

Answer 9

There is no linkage and the genes are on separate chromosomes

Question 10

What does a recombinant frequency of below 50% indicate?

Answer 10

That there is gene linkage, two different genes are located on the same chromosome and independent assortment cannot occur

Question 11

Suggest reasons why the observed phenotypic ratios are not the same as the expected ratios?

Answer 11

Radom fertilisation, linkage, small sample size