Human Genetics • Phenotype: observed physical and functional traits • Genotype: complete set of genes and alleles • Alleles: Different versions of homologous genes ex. B and b Human genetics • How are gametes made? • How does chromosome behavior affect inheritance of traits? • Somatic cells are diploid. • Gametes are haploid, with only one set of chromosomes egg polar body spermatogonium primary spermatocyte secondary spermatocyte oogonium primary oocyte secondary oocyte polar bodies (will be degraded) spermatids meiosis ll meiosis l SPERMATOGENESIS OOGENESIS a b 1st law - segregation of alleles • Cells contain 2 copies (alleles) of each gene • Alleles separate during gamete formation (meiosis) • gametes carry only one copy of each gene Figure 19.2 Possible genotypes and their probabilities Punnett squares show parental gametes and the genotypes of next generation • Homozygous: BB and bb • Heterozygous: Bb Law of Independent Assortment • During gamete formation, genes for different traits separate independently into gametes • Why? random alignment of homologues at Meiosis I Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 9.17 Chromosome behavior accounts for Mendel’s principles Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings A B a b Tetrad Crossing over A B a ba B A b Gametes • Genes on the same chromosome tend to be inherited together = linked genes • Crossing over produces gametes with recombinant chromosomes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings • an offspring’s phenotype is intermediate between the phenotypes of its parents Incomplete dominance P GENERATION F 1 GENERATION F 2 GENERATION Red RR Gametes R r White rr Pink Rr R r R R r r 1 / 2 1 / 2 1 / 2 1 / 2 1 / 2 1 / 2 SpermEggs Pink Rr Pink rR White rr Red RR Figure 9.12A VARIATIONS ON MENDEL’S PRINCIPLES [...]... three alleles for ABO blood type in humans IA, IB, i Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Polygenic traits - A single trait may be influenced by many genes • Quantitative traits Fraction of population skin color, height, eye color Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Skin pigmentation Genetic traits in humans can be tracked through family...• Incomplete dominance in human hypercholesterolemia GENOTYPES: HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors PHENOTYPES: LDL LDL receptor Cell Normal Mild disease... color, height, eye color Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Skin pigmentation Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability Figure 9.8A Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings • Family pedigrees are used to determine patterns... alleles – Examples: achondroplasia, Huntington’s disease Figure 9.9B Copyright © 2003 Pearson Education, Inc publishing as Benjamin Cummings Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles – Ex: hemophilia, red-green color blindness These traits appear mostly in males Why? Figure 9.23A – If a male receives a single X-linked recessive allele from