Single Gene Inheritance How Traits are Passed On 2 Genetics Units Heredity/Classical Genetics Inheritance patterns Based on the work of Gregor Mendel in the mid 1800s Molecular Genetics DNA/Chromosomal explanations for inheritance patterns Defines our modern understandings of genetics *Note We are covering the genetics of sexually reproducing eukaryotes like ourselves Bacteria/Archaea and asexual eukaryotes are a little different Chromosomes Human DNA is split into 23 pairs of chromosomes (46 total) Each codes for hundreds to thousands of genes We have 2 versions of each chromosome, called homologous chromosomes Human Chromosomes Gene A gene is a segment of DNA that codes for one protein That protein can determine what trait we express E.g. a protein produces brown pigments in eyes Inherited Traits Different versions of the gene are called alleles E.g. the brown allele is the DNA that codes for brown eyes We inherit one chromosome from each parent and thus 1 allele from each parent for every gene Homologous Vocabulary Homozygous to have 2 identical alleles for a particular gene Heterozygous to have 2 different alleles HomozygousHeterozygous So you have 2 alleles, which determines what you look like?? It depends. This is where genetics starts to get complicated Sometimes only 1 of the 2 alleles gets expressed Sometimes they both express themselves Case 1 Complete Dominance Only 1 trait is expressed The dominant trait OR the recessive trait is expressed Human Examples You have dimples or you dont You have freckles or you dont You are a dwarf or you arent You have attached earlobes or you dont You have cystic fibrosis or you dont Complete Dominance Dominant Allele Is expressed if it is present You can have 1 or 2 dominant alleles Represented by capital letters Recessive Allele Is expressed only if there is no dominant allele You must have 2 recessive alleles (usually) Represented by lower case letters The Dominant Phenotype Phenotype = appearance Is shown by individuals who have 1 or 2 dominant alleles Called homozygous dominant (AA) or heterozygous (Aa) genotypes The Recessive Phenotype Is only shown if the individual is homozygous recessive (aa) Genotypes Combinations of Alleles = homozygous dominant (shows dominant trait) =heterozygous (shows dominant trait) = homozygous recessive (shows recessive trait) Punnett Square Show the possible alleles each parent could give. This example shows an Aa (heterozygous) person mating with an aa (homozygous recessive individual) Parent 1 Parent 2 Dad Mom Potential sperm Potential eggs Potential offspring Say A codes for green eyes a codes for black eyes Draw a Punnett Square for Two Heterozygotes Mating Case 2 If 2 different alleles are both present, their affects are combined i.e. red allele + white allele pink flowers Straight + curly hair Wavy hair Genotypes Incomplete Dominance = AA (red flowers, or straight hair) =Aa (pink flowers, or wavy hair) = aa (white flowers, or curly hair) Incomplete Dominance R= red allele, r= white allele RR = red flowers, Rr=pink flowers, rr= white flowers If red flowers are mated with white flowers, what will the offspring be? RR r Rr r Draw a Punnett Square for 2 pink flowers being crossed Case 3 Both alleles are expressed at the same and do not mix/form intermediate Codominance I.e. blood types I A and I B alleles are both dominant There is a 3 rd, recessive allele called i (codes for blood type O) Blood Types 4 possible types A (I A I A or I A i individuals) B (I B I B or I B i) individuals AB (I A I B individuals) O (ii individuals) What parents could have children with type A or type B blood only Summary You have 2 copies of every gene, one from your mom, one from your dad Which version you express depends on the gene, sometimes only 1 (complete dominance), sometimes a mixture (incomplete dominance) and sometimes both (co-dominant) The traits are due to which protein is produced