Chapter 10 BSC 1005 Mr Tidwell The particular combination of the two alleles carried by an individual is called the genotype genetic make up The physical expression of the genotype is the phenotype physical characteristic Monohybrid crosses genotypic ratio and a 3 1 Dominant recessive phenotypic ratio always have a 1 2 1 Dominant Heterozygous recessive A Punnett Square represents all possible combinations of gametes Dihybrid crosses phenotypic ratio always have a 9 3 3 1 D H H r genotypic ratio and a 3 1 D r A test cross is used to deduce whether an organism with a dominant phenotype is homozygous for the dominant allele or heterozygous The independent inheritance of two or more traits is called the Law of Independent Assortment o Multiple traits are inherited independently because the alleles of one gene are distributed to gametes independently of the alleles for other genes o Independent assortment will occur when the traits being studied are controlled by genes on different pairs of homologous chromosomes o Mendel s law of independent assortment works only for genes whose loci are on different pairs of homologous chromosomes o Alleles that are on the same chromosome do not line up independently of one another on the metaphase plate and are not separated at anaphase 1 o Different loci located on the same chromosome tend to be inherited together o Characteristics whose genes tend to assort together are said to be linked Mendel s work was published in 1865 but went unnoticed o Three biologists Carl Correns Hugo de Vries and Erich Tschermak independently of Mendel and each other rediscovered Mendel s principles of inheritance in 1900 o Mendel was credited in new papers as laying the groundwork of genetics 30 years earlier Crossing over creates new combinations of linked alleles o Genes on the same chromosome do not always sort together o Mendel was not only careful and clever but also lucky that the 7 traits he studied were located on 4 different chromosomes He observed independently assorted because those genes on the same chromosome were far apart In humans the X chromosome is much larger than the Y and carries over 1 000 genes Page 1 of 2 Chapter 10 BSC 1005 Mr Tidwell o In contrast the human Y chromosome is smaller and carries only 78 genes During embryonic life the action of the Y linked gene SRY Sex determining region of the Y sets in motion the entire male developmental pathway o Under normal conditions SRY causes the male gender to be linked 100 to the Y o Few of the genes on the X chromosome have a specific role in female o Most of the genes on the X chromosome have no counterpart on the Y chromosome reproduction chromosome Some of the genes found only on the X chromosome are important to both sexes such as genes for color vision blood clotting and certain structural proteins in muscles Page 2 of 2