General Genetics Chapter 14 Mendel and the Gene Idea

Objectives Understand Mendel's three Laws governing genetics Understand the meaning of the relevant vocabulary discussed in class Be able to predict the results of a mono & dihybrid cross using a Punnett square Understand how recombination of genes affect genetic variability Be familiar with special situations regarding genotype/phenotype predictions

Terms from Chapter 13 Diploid: having 2 of each chromosome type Haploid: having a single representative of each chromosome type Homologous chromosomes (homologs): chromosomes from different parents that are of the same type (contain similar information) Sister Chromatids: 2 “identical” strands of DNA connected by a centromere that contains a kinetochore. Makes up each member of a homologous pair

Descriptors The physical appearance of an organism reflects its genetic makeup Each gene codes for a different polypeptide Polypeptide combinations may alter the appearance of an organism Gene (Character): is a feature that is heritable Locus: specific area on chromosome where the gene is found Allele (Trait): is a variation of a character Genotype: the genetic makeup of an organism (combination of genes in its nucleus) Phenotype: the physical appearance of an organism

Mendelian Genetics Gregor Mendel described three Laws of Genetics Law of Segregation: each parent has two copies of a gene but only one is passed to the offspring via the gametes (separation of homologous pairs) Three possible genotypes for each gene in the diploid cell Homozygous dominant: both alleles of a gene are of the “Dominant” variety Homozygous recessive: both alleles of a gene are of the “Recessive” variety Heterozygous: the diploid cell has one dominant and one recessive allele for each gene

Law of Independent Assortment: genes residing on different chromosomes separate without regard for one another –describes the broad range of variation seen in organisms Law of Dominance: some alleles for a gene are fully expressed if present (dominant) in the phenotype while others may have their effect masked (recessive) –hierarchy of alleles

Genetics is Probability In a diploid organism (2n), each allele has a 50:50 chance of being found in a particular gamete (1/2). To calculate the likelihood of two alleles recombining during fertilization, we must multiply our probability for each allele together (1/2 x 1/2 = 1/4)

Punnett Square Device used to predict potential genotypes of offspring Along each axis are placed the gamete possibilities for each parent Internal boxes represent union of genotypes for offspring produced by the union of the corresponding axial gametes Phenotypes can be determined for each potential zygote

Special Situations Incomplete dominance: the phenotype of a heterozygous genotype is intermediate in appearance Codominance: each allele in the genotype for a particular gene will be expressed in the phenotype Pleiotropy: the ability of a gene to affect an organism in many ways Epistasis: gene at one locus influences the expression of a gene at another locus (different gene) Polygenic Inheritance: additive effect of 2 or more genes on a phenotypic character

Incomplete Dominance (heterozygous genotype is intermediate in appearance)

Codominance (each genotypic allele will be expressed)

Pleiotropy (single gene affects multiple phenotypes) Sickle Cell Disease –Production of abnormal hemoglobin Prevalent amongst African Americans, but rare in people of other races

Epistasis (one gene influences the expression of another gene)

Polygenic Inheritance (additive effect of 2 or more genes on a phenotypic character)