Genetics Module: Dihybridism & Linked Genes 2024-2025 PDF

People's Democratic Republic of Algeria Ministry of Higher Education and Research Course n° 2 Dihybridisme Course n°3 linked genes Module: Genetics Second Level LMD Academic year 2024-2025 GENETIC L...

People's Democratic Republic of Algeria Ministry of Higher Education and Research Course n° 2 Dihybridisme Course n°3 linked genes Module: Genetics Second Level LMD Academic year 2024-2025 GENETIC L2 12/9/2024 1  Dihybrid cross (study of two traits) Case of unrelated genes GENETIC L2 12/9/2024 2 ❖ Mendel derives the law of segregation (the first law) by following a single trait (a single character): 2 alleles of a single gene at a segregated locus when gametes are formed The F1 progeny produced in this monohybrid cross are heterozygous for this trait ❖ Mendel identifies his second law of inheritance by following two characters at the same time (Dihybridism): Mendel was interested in determining whether the 2 alleles of two different genes segregate in a dependent or independent way By crossing two pure parents with two different traits, hybrids are produced in the F1 generation, which is heterozygous for both traits GENETIC L2 12/9/2024 3  Mendel's experiments - Dihybrid cross Dihybridism and interchromosomal mixing Dihybridism is the study of the transmission of 2 pairs of alleles (two characters). Interchromosomal mixing (recombination) corresponds to the independent disjunction of pairs of alleles carried by different homologous chromosomes (in the case of independent genes). Interchromosomal mixing takes place during metaphase 1 / anaphase 1 of meiosis and leads to the appearance of recombined phenotypes in quantities equal to parental phenotypes. GENETIC L2 12/9/2024 4 Dihybrid cross Mendel & Meiosis: Interchromosomal Mixing (Recombination) The pairs of chromosomes would be oriented in different ways in metaphase 1 and Anaphase 1 The Law of Character Matching It appears that the transmission of seed shape has no influence on the transmission of seed color The two characters are transmitted in an INDEPENDENT way The pairs of alleles that control these two traits occur independently GENETIC L2 12/9/2024 5  Using a dihybrid cross, Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles separates independently from the other pair of alleles during gamete formation Strictly speaking, this law applies only to genes on different non-homologous chromosomes Genes located in close proximity to each other on the same chromosome tend to be passed on together GENETIC L2 12/9/2024 6 GENETIC L2 12/9/2024 7  Test-cross Dihybride In monohybrid crosses, to find out whether a dominant trait is homozygous or heterozygous, it is necessary to perform a test cross This is done with a recessive homozygous The same is true for a dihybrid cross where the test-cross is done with an individual who is homozygous recessive for both traits (double homozygous recessive) GENETIC L2 12/9/2024 8 GENETIC L2 12/9/2024 9 Crossover interpretation There are 4 types of offspring in equiprobable proportions with as many parental phenotypes as recombined phenotypes. The double recessive male produces only one type of gamete. We deduce that the female F1 produces 4 types of gametes so that each trait is encoded by a different gene (so 2 genes here) The equiprobability of the parental and recombinant phenotypes shows that the female F1 produces as many parental gametes as recombinant gametes and therefore that the genes in question behave independently during meiosis (there are therefore no privileged allelic associations). The genes in question are therefore independent, i.e. carried by different pairs of chromosomes (it is the interchromosomal mixing that is at the origin of the different phenotypes observed in the progeny of the test-cross). GENETIC L2 12/9/2024 10 Case of linked genes:Recombination of linked genes(Intra-chromosomal recombination) GENETIC L2 12/9/2024 11 Discovery of gene linkage Morgan's Experience: The early 1900s The first to associate a specific gene with a specific chromosome Experiments conducted on Drosophila GENETIC L2 12/9/2024 12 Morgan's material was not varied Morgan had only a variety of Drosophila with red eyes, grey bodies and normal wings In nature, almost all fruit flies have these characteristics. So-called "wild" character or wild phenotype. The most prevalent phenotype in a population. Due to the abundance of "wild" alleles. What did Morgan do to get other varieties of fruit flies? He bred flies for a year while subjecting them to X-rays to cause mutations. The first mutant he found was a male fly with white eyes. In the wild, some fruit flies are mutants. A so-called "mutant" trait or mutant phenotype. Phenotype not very common in a population. GENETIC L2 12/9/2024 13  Morgan's Dihybrid Cross Crossover for two characters Gray (b+, dominant) or black (b, recessive) body character Character wings normal (vg+, dominant) or vestigial (vg, recessive) wings The presence of such mixing can be inferred of crossbreeding results Cross A: Drosophila [normal wings; grey body] ( vg+b+//vg+ b+ ) [ vestigial wings; blackbody ( vg b//vg b ) → 100% F1 [normal wings; grey body] → ( vg+b+//vg b ) Cross B: GENETIC L2 double recessive X F1 ( test cross ) 12/9/2024 14 ( vg+b+//vg b ) ( vg b//vg b ) GENETIC L2 12/9/2024 15 Interpretation of crossing A: The parents are pure strains so the F1 generation is heterozygous for the genes considered. However, only the alleles coding for the long wing and grey-body phenotype are expressed It is deduced that the allele coding for long wings is dominant over the allele coding for vestigial wings and that the allele coding for the gray body is dominant over the allele coding for the black body. Interpretation of the test-cross (B cross): The 2 traits studied in the Morgan dihybrid cross did not undergo the independent assortment of chromosomes since the 4 phenotypes are not in equal proportion, There are 4 types of offspring in non-equiprobable proportions with many more parental phenotypes than recombined phenotypes. However, the double recessive male produces only one type of gamete. We deduce that the female F1 produces 4 types of gametes so that each trait is encoded by a different gene (so 2 genes here) The non-equiprobability of the parental and recombinant phenotypes shows that the female F1 produces more parental gametes than recombined gametes and therefore that the genes in question behave in a related way during meiosis (there are therefore privileged allelic associations). GENETIC L2 12/9/2024 16 The genes in question are therefore linked, i.e. carried by the same pair of chromosomes. Remarks: In a given organism containing a large number of linked and independent genes, the effect of intrachromosomal mixing is added to the effect of interchromosomal mixing →Leads to a much higher number of allele combinations in the gametes formed GENETIC MAPPING There are two main types of cards: Genetic maps : distances are expressed in centimorgan (cM) Physical cards :Distances are expressed in base pairs (Pb) kilobases (kb) = 1000 bp Megabases (Mb) = 1000 kb GENETIC L2 12/9/2024 17 GENETIC L2 12/9/2024 18 GENETIC L2 12/9/2024 19