Ch 5: Linkage and Genetic Mapping (INCOMPLETE)

5.1: Linkage and Crossing Over

  Linkage can have 2 related meanings (100-101): The phenomenon that 2 or more genes may be located on the same chromosome. The genes are physically linked to each other. Genes that are close together on the same chromosome tend to be transmitted as a unit. This indicates that linkage has an influence on inheritance patterns. Chromosomes are sometimes called linkage groups b/c chromosome contains a group of genes that are physically linked together (101). When geneticists follow the variants of 2 different traits in a cross, this is a dihybrid cross. When 3 traits are followed, it's a trihybrid cross. Outcome of cross depends on whether or not the genes are linked along the same chromosome (101).  Crossing over and Recombinants Homologous chromosomes can exchange pieces with each other (this is crossing over, which you know about) (101). Occurs in prophase of meiosis I. Sister chromatids associate with homologous sister chromatids to form a bivalent. In the cells in which the grouping of linked alleles has been changed (Ab or aB), a new combination of alleles is made. This is genetic recombination (101). Haploid cells carrying the new combination of alleles are called nonparental/recombinant cells (101). Offspring that have inherited the same combination of alleles found in the chromosome of their parents are called parental/nonrecombinant offspring Chi Square Telling Difference Between Linkage and Independent Assortment Without expected numbers of recombinantand parental offspring, we cannot conduct a chi square test. This can occur when 2 linked genes that have not been previously mapped (103).  A null hypothesis is when there is no real difference between real and expected value (103).  If the chi square value is low and we cannot reject the null hypothesis, we infer that the genes assort independently. If the chi square value is so high that our hypothesis is rejected, we will accept the alternative that the genes are linked (103).  It's not this black and white though. This is based on the assumption that only 2 outcomes as possible (105).  Crossing Over During Mitosis Mitosis  normally does not involve the homologous pairing of chromosomes to form a bivalent. But it can happen. Mitotic recombination is when mitotic crossing over occurs and a new pair of recombinant chromosomes that have a new combination of alleles are made (109). If this happens early on in embryonic development, the daughter cells continue to divide and a create patch of tissue different from the rest of the organism, 

Genetic Mapping in Plants and Animals The purpose of genetic mapping is to determine the linear order and distance of separation among genes that are linked to each other along the same chromosome (109). Each gene has its own unique location on the chromosome (locus) (109). The liner arrangement of genes is known as a genetic linkage map (111)