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Mendelian Genetics.docx

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University of Waterloo
BIOL 239
Heidi Engelhardt

Mendelian Genetics (pg. 40-48) Genetic traits are passed on from one generation to the next  Artificial selection – purposeful matings o domesticated plants and animals o Dogs - domesticated wolves  Most compatible wolves were bred  selected for 1st non-wolf canine skull found dated at ~15,0000 years old in arctic  Next…..pigs, goats, sheep, cattle ~ 10,0000 years ago o 1950s Belyaev  Scientific American  Man's new best friend? A forgotten Russian experiment in fox domestication o Crops ~ 13,000 years ago (wheats)  Middle east - Iraq, Iran, Syria  Gregor Mendel (1822-1884) o Augustinian monk and expert plant breeder (Austria 1800s) o Experiments with garden peas o Advantages to Mendel’s approach  Garden peas  easy to cross-fertilize  large numbers of offspring  short growing season  Clear-cut alternative forms of particular traits :  purple vs. white flowers, yellow vs. green peas,  round vs. wrinkled peas = phenotype  Establishment of pure-breeding lines  Offspring carry parental traits that remain constant from generation to generation  Carefully controlled breeding  Use of reciprocal crosses and self-fertilization  Phenotype o observable characteristic  largely determined by genotype  commonly referred to as a trait  Genotype o genetic make-up o description of the genetic information carried by an individual  Mendel’s Experiments o Cross-fertilization o Mendel used selective breeding to produce true-breeding lines of peas  offspring always showed same phenotype as the parent plants o Found a consistent pattern of inheritance, from which he based his theories  Prior to Mendel: o theories that one parent contributes most to an offspring’s inherited features  disproved through reciprocal crosses o parental traits become mixed and forever changed in the offspring  disproved through reappearance of recessive traits Monohybrid Crosses  Matings between individuals that differ in only one trait o Parental (P) pure breeding (tall x dwarf) o All first filial (F1) resemble one of the parental strains (tall)- monohybrids o Hybrids self-fertilize (tall x tall) o In second filial (F2), lost trait appears (3 tall: 1 dwarf)  Alleles o Discrete units of inheritance are alleles of genes. o Alleles are alternative forms of a single gene.  Gene: eye pigment Allele: brown or blue  However, most human traits (including the above) are determined by multiple genes with multiple alleles. o Homologous chromosomes carry same gene with different alleles (different DNA sequence) o Polymorphic  A gene may have several alleles that normally occur in a population, but a maximum of two for one gene can exist in a diploid individual (humans) o Monomorphic  Some genes have only one allele that is normally present in a population.  Mendel’s law of segregation o The two alleles for each trait separate (segregate) during gamete formation (gene separates), then unite at random, one from each parent, at fertilization.  describes how alleles of one gene behave o Each parent carries 2 copies of each gene (genes are in pairs)  specific allele exists for each gene  each individual receives 1 from each parent  Punnett Squares  Phenotypic effect o Homozygous Dominant  GG (green) o Homozygous Recessive  gg (yellow) o Heterozygous  Gg (G is dominate allele and g is recessive allele) (green) o 3:1 phenotypic ratio will always occur for simple dominant and recessive traits that result from a cross between monohybrids (one gene)  DNA o At the DNA level, different alleles differ in nucleotide sequence. o This results in changes in the amino acid sequence or the amount of protein. o Example:  trait: texture; round (smooth) or wrinkled  alleles: round (W) or wrinkled (w)  gene: codes for the enzyme starch-branching enzyme 1 (SBE1)  Mutation causes amino acid change resulting in inactive SBE1  WW= active enzyme which changes soluble starch into insoluble starch= round pea  ww=inactive enzyme which doesn’t change soluble starch= wrinkled pea  In ww, soluble starch builds up. This causes excess
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