Linkage and Recombination PDF

# Linkage and Recombination ## Gene Linkage - Genes may or may not be linked to one another. - Most of the progenies resemble the parentals. - Strong tendency for the parental combinations to be inherited. - Gene linkage causes low recombination rate. ## Gene Linkage - As a consequence, only few...

# Linkage and Recombination ## Gene Linkage - Genes may or may not be linked to one another. - Most of the progenies resemble the parentals. - Strong tendency for the parental combinations to be inherited. - Gene linkage causes low recombination rate. ## Gene Linkage - As a consequence, only few recombinants will be produced. - Some genes are "partially" linked (Saunders and Punnett, 1906). - Linkage may be complete or incomplete. ## Complete Linkage - Genes are so closely associated that they are always inherited together. | Parent 1 | Parent 2 | Offspring | |---|---|---| | prvg (purple eyes, vestigial wing) | prvg (normal) | prvg (normal) | | prvg | prvg | prvg | Testcross/BC₁: prvg (normal) x prvg (purple eyes, vestigial wing) | Offspring | |---|---| | prvg (normal) | 1 | | prvg (purple eyes, vestigial wing) | 1 | ## Incomplete Linkage - Due to the presence of recombinants caused by "crossing-over". - Crossing-over is a process wherein homologous chromosomes exchange parts. - If crossing-over involves sister chromatids, recombinant types are not produced. ## Incomplete Linkage - Therefore, recombinants are the result of non-sister chromatid crossing-over. ## Incomplete Linkage **Figure 4.1.** Crossovers of non-sister chromatids and the resulting gametic genotypes. | Crossover Type | Chromosomes | Genotypes | |---|---|---| | Single crossover (A & B) | A <br> B <br> C <br> a <br> b <br> C | A B C <br> A b C <br> a B C <br> a b C | | Two-strand double crossover (1 & B) | A <br> B <br> C <br> a <br> b <br> C | A b C <br> a B C <br> a b c <br> a b C | | Three-strand double crossover (2 & 3, 1 & 3) | A <br> B <br> C <br> a <br> b <br> C | A B C <br> A B c <br> a B c <br> a b C | | Four-strand double crossover (2 & 3, 1 & 4) | A <br> B <br> C <br> a <br> b <br> C | A b c <br> A B C <br> a b c <br> a B C | ## Determination of Linkage - Linkage of the segregating genes of a dihybrid leads to deviations from the 9:3:3:1 F2 phenotypic ratio. - Such deviation depends upon the frequency with which recombination occurs. ## Determination of Linkage - The strength of linkage is expressed by linkage value. Linkage value = Total no. of crossover types x 100 / Total no. of population ## Linkage values involving three genes - Valuable information on linkage relations can be obtained when the cross involves 3 genes in the same chromosome. For example, in corn, the following 3-point testcross was made: V G1 Va<br> v g1 va x V g1 va<br> v gi va | Testcross progeny | Phenotype | Crossover Type | No. | |---|---|---|---| | 1 | Normal | Parental | 235 | | 2 | Virescent, glossy, variable sterile | Parental | 270 | | 3 | Glossy, variable sterike | Single Crossover Region I | 60 | | 4 | Virescent | Single Crossover Region I | 62| | 5 | Variable sterile | Single Crossover Region II | 40 | | 6 | Virescent, glossy | Single Crossover Region II | 48 | | 7 | Glossy | Double Crossover | 7 | | 8 | Rough | Double Crossover | 4 | The computations of linkage values are as follows: Region I: (60+62+ 7+4) / 726 x 100 = 18.3% Region II: (40+48+ 7+4) / 726 x 100 = 13.6% d.c.o: (7+4) / 726 x 100 = 1.52% ## Factors Affecting Recombination Frequencies ### Sex - The heterogametic sex of a species has lower crossover frequencies ### Maternal Age - As maternal age increases, crossing-over tends to decrease. ### Temperature - In Drosophila, it was observed that temperatures, cooler or warmer than 22°C, tended to increase the rate of crossing over. ### Cytoplasmic Effect - In Drosophila, females selected for reduced recombination frequencies to pass this trait through their daughters, indicating that the crossover factors are carried in the cytoplasm. ### Nutrition - Young Drosophila females on high Ca showed decreased crossing over. - Larval starvation at certain ages generally increased crossing over. ### Genotype - Certain genes affected the preconditions for exchange (i.e., chromosome pairing) while others act after pairing has been completed. ### Centromere - Genes located close to the centromere show reduced crossing-over. ### Chemical Effects - Crossing-over increased upon injection of antibiotics (e.g. mitomycin C and antinomycin D). ### Radiation - Increases in crossing-over were observed when Drosophila were exposed to X-ray irradiation. ## Sex Determination ### Sexuality Theory (Hartan, 1931) - Every cell of sexually reproducing organisms was bisexual. - Had the potential for the male and the female sex, becoming male or female by the development of one with the other potential. ## Sex Determination - This theory helped explain bisexuality, hermaphroditism, and sex reversal in plants and animals. ## Genetic Sex Determination - Genetically regulated process. - Direct consequences of the genotypes while others are influenced by other factors in addition to the genotype. **Different types of sex determination:** - XX/XO - XX/XY - ZW/ ZZ ## Genetic Sex Determination ### Multiple genes - In molluscs (a) Crepidula & (b) Patella, sexuality showed continuous variation in the population, where hermaphrodites were most frequent. ## Genetic Sex Determination ### Haplo-Diploidy - Among bees & ants, the unfertilized eggs develop into males or drones (haploid). - These drones have no father, only godfathers from the maternal side. - No meiotic reduction of chromosome number. ## Environmental Sex Determination - In some animals, sex depends on environmental circumstances. ## Environmental Sex Determination ### Bonellia viridis-marine worm - Females - individuals that remain free swimming throughout their entire larval stage. - The larvae that attach themselves to mature females become males. - Maleness is due to masculinizing hormons absorbed from the from the females. ## Environmental Sex Determination ### Labroides dimidiatus-coral reef fish - When a male dies, any dominant female repels any new males that will approach the group. - If she succeeds, she will court the female and behave like a male. - In about 2 weeks, there will be a sex reversal. ## Chromosomal Sex Determination - McClung (1900) showed the association of sex characteristics & the presence or absence of a particular chromosome. - He proposed that sex factor is located in 1 chromosome. - That the number of spermatozoa containing this chromosome should be equal to those that do not contain it. ## Chromosomal Sex Determination ### XX-XO Mechanism - In bugs. Male has XO (X & Zero) chromosome- heterogametic sex. - Female has XX chromosome-homogametic since all eggs contain an X chromosome. ## Chromosomal Sex Determination ### XX-XY Mechanism - Found in most mammals. - Female has 2 X chromosomes. - Male has the XY chromosome composition. ## Sex Linkage - Sex chromosomes carry genes for characters other than sex determination. - Expected that the inheritance of certain characteristics shall be sex-linked or associated with that of sex. ## Sex Linkage 2 types of sex-linked inheritance may be observed as: - X-linked inheritance for genes located on the X. - Y-linked inheritance for those on the Y chromosome. ## Sex Linkage ### X-linked Inheritance - Because females have 2 X chromosomes, she may either be homozygous or heterozygous for an X-linked gene. - She becomes a carrier of the recessive allele when she is heterozygous. ## Sex Linkage ### X-linked inheritance (ex. Hemophilia in Humans) - Gene for hemophilia is located in X chromosome (xh). - It is recessive (h) and its expression is completely masked by the dominant allele. ## Sex Linkage ### X-linked inheritance (ex. Hemophilia in humans-carrier father) | Parent | Genotype | |---|---| | Father | XY | | Mother | XX | | Child | Genotype | |---|---| | Daughter | XX | | Son | XY | ## Sex Linkage ### Y-linked inheritance - Webbed toes: Characterized by a web-like connection between the 2nd and 3rd toes. - All sons have webbed toes. - All daughters are unaffected. ## Sex Linkage ### Y-linked inheritance - Hypertrichosis of the ears: A conspicuous growth of hair on the outer rim of the ear is transmitted only from father to son