Lec 5

• Mendel's Law of Segregation

  Genes occur in pairs, each gamete receives only one allele of each pair (the alleles segregate), refers to a single gene locus
• Mendel's Law of Independent Assortment


  Loci that are far apart on the same chromosome or on different chromosomes segregate independently
• If locus A and locus B are unlinked, there is a 1:1:1:1 ratio of the types of gametes A1B1, A2B2, A1B2, and A2B1
• Linkage equilibrium


  Independent assortment of alleles at a locus irrespective of allele in the second locus
• Linkage and Recombination

  An exception or deviation from the 1:1:1:1 ratio of gametes suggests the two loci are linked, exchange of DNA occurs between two of the four chromatids during meiosis, a parental haplotype will become a new combination of the original maternal and paternal haplotypes, alleles at linked genes stay together during meiosis more often than not
• Linkage

  Genes on loci that are close enough together on a single autosomal chromosome will not exhibit independent assortment, chromatids on homologous pairs of autosomal chromosomes will likely undergo recombination/crossing-over during meiosis
• Gene Action - Linkage

  Physical connection that exists between genes whose loci are on the same chromosome, the closer together the genes are, the lower the probability that they will be separated during DNA repair or replication processes (meiosis), and hence the greater the probability that they will be inherited together
• Linkage disequilibrium

  When genotypes at the two loci are not independent of another, non-random association of alleles at adjacent loci, occurring more often than expected if the loci were segregating independently, measured by D (difference between observed frequency of a 2-locus genotype and expected frequency if segregating independently)
• Non-independent Assortment

  If loci are close together on the same chromosome there is an excess of non-recombinant gametes (parental) > 50% parental gametes, if located far apart there is a higher probability of crossing over/recombination resulting in more recombinant gametes (non-parental) < 50% parental gametes
• Recombination


  Producing non-parental types (recombinants)
• Sex chromosomes in mammals

  Females have two X chromosomes (homogametic), males have one X and one Y chromosome (heterogametic)
• Sex chromosomes in birds

  Females are ZW (heterogametic), males are ZZ
• 1. linked or sex linked inheritance

  Genes carried on the X-chromosome, since males only have one X chromosome, any recessive gene on the X-chromosome will be expressed, in females a recessive gene may be suppressed by a dominant gene on the other X-chromosome
• Sex Y-chromosome

  XY chromosomes separate during gamete formation, Y chromosomes have < 80 genes and are genetically barren, presence of sex determining region (SRY) gene converts the embryo into a male
• 1. linked or sex-linked inheritance


  • Hemophilia A, Duchenne Muscular Dystrophy, color blindness in humans
• Sex-linked recessive gene

  Presence of horns in sheep, alleles P (polled), p (horned), p' (horned in both sexes)
• Sex-linked locus

  • Hair colour in rabbits, T = black, t = orange
• Recessive Sex Linked

  Early embryonic lethals - causing death to embryos of affected individuals
• Sex-linked Inheritance

  Fathers pass x-linked alleles to only and all of their daughters, mothers pass x-linked alleles to both sons and daughters, if a sex-linked trait is due to a recessive allele, a female will express the trait if she is homozygous, because males have only one x-linked locus, any male receiving a mutant allele from his mother will express the trait, more males than females having sex-linked disorders
• Sex Limited Trait

  Phenotype is only expressed in one sex, sex limited genes are not expressed (not penetrant) in one of the two sexes, includes all sex specific secondary sexual characteristics and several production traits
• Sex influenced traits

  Whose expression of the phenotype depends on the sex of the individuals, not x-linked, not sex limited, found in both sexes but expressed differently or with a different inheritance pattern
• Pleiotropy

  An allele having an influence on more than 1 trait or characteristic, high genetic correlation between 2 traits could possibly be due to similar genes affecting the expression of the two traits
• Pleiotropy


  • Frizzle gene in fowl, body weight at weaning and post weaning gain in cattle, days to reach 100 kg and feed conversion in pigs, milk yield and % protein in milk in dairy cattle, gene for double muscling in cattle, Merle coat colour gene and eye defect in dogs, Overo coat colour gene and lethal intestinal obstruction in horses