Phenotypic ratio

Question 1

Why does the ratio observed in many dihybrid crosses differ significantly from those expected?

Answer 1

• due to linkage meaning the genes are located on the same chromosme
• inherited as a single unit

Question 2

What is an autosomal linkage?

Answer 2

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

Question 3

How are linked genes inherited?

Answer 3

• as one unit
• there is no independant assortment during meiosis unless the alleles are separated by chiasmata
• they tend to be inherited together

Question 4

How does this effect the gametes produced?

Answer 4

• homozygous parent
  
  • only produce one homozygous gamete
• hetrozygous parent
  
  • produced 2 gametes homozygous gametes

Question 5

Why will the hetrozygous parents produce a few hetrozygous gametes?

Answer 5

• due to crossing over
• results in the separation of some of the linked genes

Question 6

What are recombinant offspring?

Answer 6

• different combination of alleles than either parent

Question 7

What determines the number of recombinant chromosomes?

Answer 7

• the closer the genes are on a chromosome the less likely they are to be separated during crossing over
• therefore fewer recombinant offspring are produced

Question 8

What is recombinant frequency?

Answer 8

• a measure of the amount of crossing over that has happened in meiosis
• recombinant frequency = number of recombinant offspring / total number of offspring

Question 9

What do the different values of recombinant frequency reveal?

Answer 9

• recombinant frequency of 50% indicates that there is no linkage
  
  • genes are on separate chromosomes
• less than 50% indicates that there is a gene linkage
  
  • the random process of independant assortment has been hindered
• as the degree of crossing over reduced
  
  • the recombination frequency gets smaller
    *

Question 10

How can the recombination frequency be used to map the chromosome?

Answer 10

• recombinant frequency determines how close the genes are on a chromosome
• the recombination frequencies for a number of characteristics coded for by genes on the same chromosome can be used to map the genes on the chromosome
• a recombination frequency of 1% relates to a distance of one map unit on a chromosome