University:
Cameron University
Course:
1004 | General Biology
Academic year:

2023
Views:

245

Pages:

1
Author:

Clint W.

InHERITAnCE Unlinked Genes Linked Genes The inheritance of two different genes / traits will occur independently provided the genes are on separate chromosomes • This is because unlinked genes segregate independently during meiosis (random assortment of homologous pairs) A linkage group describes a group of genes whose loci are on the same chromosome (i.e. linked genes) • Linked genes will function as a single inheritable unit and will not follow the law of independent assortment Mendel’s Law of Independent Assortment • Separation of alleles for one gene occurs independently of the separation of alleles for another gene Linked genes are represented as pairs: AB one chromosome Dihybrid Crosses Dihybrid crosses determine allele combinations of offspring for two genes that are unlinked (on different chromosomes) • 2 genes × 2 alleles = 4 potential gamete combinations Gamete combinations are calculated via the FOIL method: • First (AaBb = AB) • Outside (AaBb = Ab) • Inside (AaBb = aB) A a B b • Last (AaBb = ab) Once gamete combinations are identified, a punnett square is then used to calculate genotype and phenotype frequencies Example: Pea seed cross (heterozygous) YR YyRr × YyRr Yr Y = yellow y = green R = round r = wrinkled yR yr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr one chromosome ab Linked genes can be separated (unlinked) by crossing over • The novel allele combinations are called recombinants Recombinant phenotypes will only be evident if crossing over has occurred and thus occur at lower frequencies • Frequency increases the further apart the two genes are Linkage Studies Thomas Morgan determined the concept of gene linkage via breeding experiments involving fruit flies (Drosophila) Morgan identified a number of different traits that did not conform to Mendelian ratios and surmised the following: • These traits represented linked genes (on same chromosome) • Linked genes can be uncoupled via recombination, but such events are uncommon (hence occur at low frequencies) Grey/normal wing (B.VG) × Black/vestigial wing (b.vg) Grey normal Black vestigial Normal offspring Grey vestigial Black normal Recombinants Polygenic Inheritance Variation can be discrete (finite patterns) or continuous (normal distribution) Monogenic traits are characteristics that are controlled by a single gene locus • They therefore have a finite pattern of expression (discrete variation) Polygenic traits are characteristics controlled by more than two gene loci • They exhibit a bell-shaped distribution pattern (continuous variation) Polygenic traits (e.g. human height) may also be influenced by environment 1 gene (monogenic) 4 genes (polygenic)