Recombination And Mendel’s Second Law

Question 1

Mendel’s dihybrid cross

Answer 1

• Crossed peas that differed in two attributes; colour and shape (yellow already found to be dominant to green; smooth to wrinkled)

           Phenotypes        Genotypes         Gametes P1     Smooth yellow x   SSYY x WWGG  SY and WG
       wrinkled green 

F1 All smooth yellow SWYG SY, SG, WY
and WG

Question 2

Mendels findings of dihybrid cross

Answer 2

• When Mendel self feritilised the F1 plants he found an unexpected phenotypic ratio in the F2 generation:
• 9 smooth yellow: 3 smooth green: 3 wrinkled yellow: 1 wrinkled green
• the new combinations of phenotypes not found in the parental generations are recombinant.

Question 3

Recombinants

Answer 3

A new combination of genotyopes not found in the previous generations

Question 4

Law of Independent assortment (Mendels second law)

Answer 4

During the formation of gametes, the segregation of alleles of alleles at one locus is independent of that of the segregation of alleles at any other

• Mendel then took all his seven characters in pairs (sometimes in threes) and repeated the experiment.
• for every cross there was independent assortment
• we can use chi squared test to statisctically test whether two alleles segregate independently

Question 5

Applications of Mendels second law

Answer 5

Independent assortment can generate huge numbers of different genotypes by recombination

Question 6

Recombination table

Answer 6

Look at notes

Question 7

What does the enourmous diversity mean?

Answer 7

No two individuals share the same set of alleles at freely recombining loci:

The famous DNA fingerprint turns on existance of many highy variable and freely recombining loci, which generate unique combinations

Question 8

Give examples of the usefulness of Mendels 2nd Law (Tomatoes)

Answer 8

• Agriculture depends on putting together advantageous characteristics from different lines selecting recombinants that have both

Examples:
- original tomato was small and green. Crossing different lines together (based on size, smell, colour, taste) has generated a remarkable amount of phenotypic relationship

Question 9

Give examples of the usefulness of Mendels 2nd Law (Maze in the US)

Answer 9

Before the 1940s farms had their own inbred strains, which were unproductive. Different inbred lines were crossed together and the size of the productivity of the corn greatly increased. This is due to outbreeding. The original lines were homozygous for rare deleterious alleles. Crossing lines produced heterozygosity at these alleles and fitness increased.

Question 10

Give examples of the usefulness of Mendels 2nd Law (India)

Answer 10

• once famine was common in India. Tall but productive pure lines of rice were available. However, rice is grown in flooded fields and strong winds blew over the rice and it would rot. Norman Borlaug crossed this tall productive strain with a wild strain of rice which was small but less productive variant. The small and productive recombinants were used to generate
  new lines.

Question 11

Give examples of the usefulness of Mendels 2nd Law (Bananas)

Answer 11

Similarly, bananas are also clones; Cavendish banana and are being killed off by a fungus. Recently a
group has inserted a resistance gene into the banana genome which gives hope for the future.

Question 13

Give examples of the usefulness of Mendels 2nd Law (Potatoes in Ireland)

Answer 13

Throughout the 1800s the potato was a staple of the Irish diet. Potatoes were infected clones and
lacked genetic diversity. This made them all were susceptible to the fungus Phytophthora infestans.
Today we can use recombination to cross in resistance genes found in wild populations of potatoes.

Question 14

Implications for humans (Flu)

Answer 14

• Flu is caused by the influenza virus and produces a wide variety of symptoms
  including death.
• Over decade ago there was the H5N1 swine flu outbreak.
• 1918 flu epidemic which killed 1 in 30 individuals in the world
• Influenza is an RNA virus about 13.5k bp long.

The 1918 flu epidemic differed in one base from the H5N1 strain. Bird and human influenza can simultaneously infect pig, where they can potentially recombine and form a new virulent version of the virus. In the same way, different
strains of HIV have recombined to become resistant to many antiviral drugs.

• Humans are all recombinants. Historically humans from different groups have mated. In Spain has shown a mixture of Jewish, African and Iberian genes.