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Inheritance Chapter 29 Gregor Mendel 1822 - 1884 “Father of Genetics” What Mendel did He bred peas in the monastery garden at Brno, Czech Republic (then part of the AustroHungarian Empire). Observed occasional variations in the appearance of these plants. Selectively bred plants to consistently produce “characteristics” that were unusual. Saw a pattern in the way that the unusual characteristics showed up. Was the first to propose that these characteristics were passed from one generation to another by the gametes. The Abby where Mendel worked What Mendel did not do He didn’t use the word “gene” to refer to subject of his work. He didn’t see chromosomes. He never used a Punnett square. He never achieved fame in his lifetime for his work. Charles Darwin 1809 - 1882 •Proposed the “Theory of Evolution”. • Actually, talked about “descent with modification from a common ancestor”. He didn’t use the word “evolution” very often. • Voyage of the Beagle 1831 – 1836. • Presented paper with Alfred Russell Wallace in 1858. • Published first edition of “Origin of Species” in 1859. Some Vocabulary – study of inheritance. Autosomes – the 22 pairs of chromosomes that do not determine genetic sex. Sex chromosomes – the 23rd pair, the X and the Y. Karyotype – the diploid chromosomes displayed in their condensed form and paired as homologs Genetics A typical karyotype More Vocabulary Alleles - a matched pair of two genes, coding for the same or alternative forms of a particular trait. Found at the same location (locus) on homologous chromosomes. Homozygous – having the same alleles for a trait Heterozygous – having different alleles for the same trait. More words – an allele that expresses itself and masks its partner. Example: brown hair is dominant over blond. Recessive – the reverse of the above. The allele that is masked Allele pairs are expressed as a pair of letters representing the trait. Example: Mendal’s peas came in tall and short. Tall is the dominant allele for height in peas. Therefore it is written as a capital “T”. A heterozyote for height would be Tt, with the lowercase t representing the recessive. Dominant Genotype vs. Phenotype – the actual alleles an organism has is it’s genotype. In our heterozygote pea plant that would be Tt. Phenotype – that which is expressed. Our pea plant maybe genotypically heterozygotic but phenotypically it is tall. Genotype Homozygote dominant = TT phenotype = tall Homozygote recessive = tt phenotype = short Heterozygote = Tt phenotype = tall Mendel’s Laws Mendal discovered that if you bred plants that had two alleles for each trait that you would get the same ratios of phenotypes & genotypes whenever you crossed heterozygotes. It was like clockwork! This was because of independent assortment and segregation, which became known as “Mendal’s Laws” It works like this… Phenotypic ratio = 3:1 or 3 tall : 1 short Genotypic ratio = 1:2:1 or 1 homozygote dominant 2 heterozygotes 1 homozygote recessive Example: PKU Violation of Mendel’s Laws Mendal’s laws only hold if: there is random fertilization the alleles are located on separate chromosomes the alleles have a simple dominant/recessive relationship there are only two alleles for that trait they are not lethal to the zygote Penetrance Percentage of individuals with particular genotype that shows “expected” phenotype Expressivity Extent to which particular allele is expressed Teratogens Factors that result in abnormal development Sources of variation: segregation & independent assortment Assortment leads to many possibilities as far as gamete formation goes. For any genome it can be calculated as 2n, where n=the number of chromosome pairs. So for a human with 23 chromosome pairs, the possible 23 combinations of gametes = 2 or 8,388,608! (and that’s with out recombination) Suppression 1 gene suppresses other Second gene has no effect on phenotype Complementary Gene Action Dominant alleles on 2 genes interact to produce phenotype different from when 1 gene contains recessive alleles Sources of Individual Variation During meiosis, maternal and paternal chromosomes are randomly distributed Each gamete has unique combination of maternal and paternal chromosomes Crossing Over and Translocation Figure 29–17 Genetic Recombination During meiosis, various changes can occur in chromosome structure, producing gametes with chromosomes that differ from those of each parent Greatly increases range of possible variation among gametes Can complicate tracing of inheritance of genetic disorders Crossing Over Parts of chromosomes become rearranged during synapsis When tetrads form, adjacent chromatids may overlap Translocation Reshuffling process Chromatids may break, overlapping segments trade places Genomic Imprinting During recombination, portions of chromosomes may break away and be deleted Effects depend on whether abnormal gamete is produced through oogenesis or spermatogenesis Chromosomal Abnormalities Damaged, broken, missing, or extra copies of chromosomes Few survive to full term Produce variety of serious clinical conditions Mutation Changes in nucleotide sequence of allele Spontaneous Mutations Result of random errors in DNA replication Errors relatively common, but in most cases error is detected and repaired by enzymes in nucleus Errors that go undetected and unrepaired have potential to change phenotype Can produce gametes that contain abnormal alleles A Map of Human Chromosomes Human Genome Project Goal is to transcribe entire human genome Has mapped more than 38,000 human genes Karyotyping Determination of individual’s complete chromosomal complement Types of inheritance Aside from simple dominant/recessive dominance – a dominant allele does not completely mask the recessive (red flower + white flower = pink flower). Codominance – both traits are expressed together (red flower + white flower = stripes). Multiple alleles – More than one allele for a trait. ABO blood group is an example. Polygene – several alleles interact to produce a trait. Results are a continuous or quantitative phenotype, as in skin color. Incomplete Carriers Individuals who are heterozygous for abnormal allele but do not show effects of mutation Incomplete dominance: Sickle Cell Codominance of multiple alleles Polygenic inheritance Sex-linked inheritance Males only have one X chromosome. Therefore, if a trait is found only on the X it will be expressed in a male regardless of whether it is dominant or recessive. X – inactivation occurs in females. Every normal woman has two Xs but they only need one. Therefore, one X chromosome turns off, forming a Barr body. Because X – inactivation is random in most cases, it leads to a fine mosaic of cells in females. Sex determination in humans Colorblindness: a sex-linked trait Environmental influences – Developmental influences impact genetic expression in ways that appear to be genetic but are not inheritable. Temperature, nutrition, non-genetic pathologies can have impacts that are expressed in ways that appear genetic. Phenocopy Genetic defects – a defective set of genes. Triploidy – an extra set of chromosomes Trisomy – an extra single chromosome Monosomy – a missing homolog Aneuploidy of the 23rd chromosome – XXX = “super female” XXY = Klinefelter’s syndrome Trisomy of the 21st chromosome leads to Down’s Syndrome. Trisomy Down syndrome Klinefelter’s a type trisomy affecting the sex chromosomes Turner Syndrome: monosomy of the 23rd chromosome, X_ Monosomy of the 23rd chromosome Name that condition! A Peek into the Future: Screening for genetic disorders Thanks for a great term!