Higher Biology Linkage Recombination Linkage Recombination By the

Higher Biology Linkage & Recombination

Linkage & Recombination By the end of this lesson you should be able to: ØKnow what is meant by linked genes. ØKnow the consequences of genes being linked. ØUnderstand the effect crossing over has on linked genes. ØKnow the effect that crossing over has on distantly located genes. ØKnow what cross-over values (COV) are. 2

Introduction ¡ ¡ ¡ In the early 1900 s it was noticed that the 9: 3: 3: 1 ratio from a dihybrid cross did not always occur. In some F 2 generations the parental phenotypes appeared more frequently than expected. Sometimes recombinant phenotypes did not appear at all. 3

Linked Genes ¡ Two or more genes which are carried on the same chromosome are said to be linked. simple gene map in peas 4

Linked Genes ¡ ¡ If there is gene linkage there is a lower probability that the alleles will be separated during meiosis. This means that more of the parental genotypes appear in the F 2 generation. Different linked genes are inherited as a group rather than as separate units. The closer 2 genes are on a chromosome, the more likely it is that they will be inherited together. 5

Linked Genes ¡ ¡ ¡ The opposite is also true: The further apart 2 genes are on a chromosome, the less likely it is that they will be inherited together as crossing over is more likely to separate them. REMEMBER THAT: When the homologous chromosomes pair up during the first meiotic division, non-sister chromatids are able to exchange pieces of DNA. 6

Linked Genes crossing over to separate linked genes 7

Linked Genes ¡ ¡ Consider two genes A and B that are linked on the same chromosome. A cross is carried out AABB ¡ ¡ X aabb The F 1 would all be Aa. Bb At this stage there is no indication that genes A and B are linked (on the same chromosome) 8

Linked Genes ¡ If two F 1 individuals are then self-crossed: Aa. Bb ¡ ¡ X Aa. Bb If A and B are very tightly linked, and there has been no crossing over, then the gametes will be: AB ab AB Draw a Punnett Square: ab 9

Linked Genes Male gametes (AB) Female gametes (ab) (AB)(AB)(ab)(ab) The F 2 will be: AABB Ratio 1 Aa. Bb : 2 aabb : 1 We sometimes use brackets around the gametes to show they are linked together. If the alleles A & B were not linked the ratio would be 9: 3: 3: 1 10

Linked Genes 1. Complete the “Dihybrid Cross- Linked Genes” sheet from your diagram pack. 11

Linked Genes- separating them 12

Recombination Try to answer the following questions: 1. What process separates linked genes? 2. How many different types of gametes would be formed from a heterozygous individual with linked genes? 3. How many different types of gametes would be formed from a heterozygous individual when the linked genes are separated? 4. What do we call the additional 2 types of gametes? 13

Recombination Recombinant gametes will change the expected phenotypic ratio. ¡ ¡ A backcross will have different results depending on whether the heterozygous individual has linked or unlinked genes Insert and complete the “Unlinked Backcross and Linked Backcross” sheet from your diagram pack. You can fill in the middle boxes now! 14

Summary table Unlinked genes (phenotypic ratio) 2 true breeding individuals crossed to F 2 Heterozygous in a back cross to F 1 Linked genes (phenotypic ratio) 9: 3: 3: 1 1: 1: 1: 1 15

Chromosome Mapping ¡ ¡ ¡ A genetic map is a diagram that shows the positions of genes on the chromosomes of an organism. How do we get the information to do this? Remember- the frequency of crossing over between two linked genes will increase if the genes are further apart. 16

Chromosome Mapping ¡ Broadly speaking: The frequency of crossing over between two genes is proportional to the distance between them. ¡ The recombination frequency for two genes (%) is a measure of crossing over between them. Number of F 2 recombinants Total number of F 2 X 100 17

Chromosome Mapping ¡ ¡ ¡ A recombinant frequency can also be called a crossover value (COV) A recombinant frequency (COV) of 1% represents one unit of measure on the genetic map. If genes A/a and B/b show a recombination frequency of 9%, then they will be: 9 units apart on the genetic map. 18

Constructing a Gene Map 1. 2. To construct a genetic map for a particular chromosome, a line is drawn and one gene is then placed on the map. (this is the reference point) The other genes are then placed on the map by positioning them at the correct number of units from the initial gene & from each other. 19

Constructing a Gene Map Example Genes Recombinant Frequency (%) A/a x B/b 5 A/a x C/c 13 A/a x D/d 3 B/b x C/c 18 B/b x D/d 8 C/c x D/d 10 20

Constructing a Gene Map 1. ¡ Identify the two most distantly linked genes. B/b & C/c- 18 units apart C/c B/b 18 units 21

Constructing a Gene Map 2. ¡ Identify the gene that is closest to the on the left (B/b) B/b & A/a- 5 units apart B/b C/c A/a 5 units 18 units 22

Constructing a Gene Map 3. ¡ ¡ Now place the remaining gene (D/d) on the map. B/b & D/d- 8 units apart D/d & C/c -10 units apart B/b A/a 5 units ¡ C/c D/d 3 units 18 units It is equally correct to draw the genes in the reverse order. 23

Constructing a Gene Map Now try this yourself. Genes COV (%) T/t x L/l 12 T/t x B/b 4 T/t x S/s 21 L/l x B/b 16 L/l x S/s 9 B/b x S/s 25 24

Constructing a Gene Map B/b L/l T/t 4 units 12 units S/s 25 units 25

Practice Questions ¡ 1. 2. 3. 4. Torrance TYK page 93 Q 3 & 4 TYK page 96 Q 2 a, b, c TYK page 101 Q 1 AYK pages 102 -3 Q 6 & 7 26

Linkage & Recombination Can you do it? ØKnow what is meant by linked genes. ØKnow the consequences of genes being linked. ØUnderstand the effect crossing over has on linked genes. ØKnow the effect that crossing over has on distantly located genes. ØKnow what cross-over values (COV) are. 27