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GENETICS The Science of Heredity TRAIT A characteristic that an organism can pass on to its offspring through its genes. GENE A segment of DNA on a chromosome that codes for a specific trait. DOMINANT ALLELE An allele whose trait always shows up in the organism when the allele is present. PROBABILITY The likelihood that a particular event will happen. PUNNETT SQUARE A chart that shows all the possible combinations of alleles that can result from a genetic cross. GENOTYPE An organism’s genetic makeup or allele combinations. PHENOTYPE An organism’s physical appearance, or visible traits. MUTATION A change in a gene or chromosome. FATHER OF GENETICS Gregor Mendel’s work was the foundation of genetics Understand why offspring have traits similar to those of their parents. ST 1 Mendel cross-pollinated purebred tall pea plants with purebred short pea plants. Traits are controlled by alleles of genes. Organisms inherit one allele from each parent. Some alleles are dominant capital letter. Some alleles are recessive lowercase letter. The principles of probability can be used to predict the results of genetic crosses The likelihood that a particular event will happen. Geneticists use Punnett squares to show all the possible outcomes of a genetic cross and to determine the probability of a particular outcome. PHENOTYPE PHYSICAL APPEARANCE VISIBLE TRAIT GENOTYPE GENETIC MAKE-UP ALLELE COMBINATIONS HOMOZYGOUS HETEROZYGOUS CODOMINANCE The alleles are neither dominant nor recessive. HOW DO GENES EXPRESS THEMSELVES ? An organism has two alleles for each trait . . . They may be dominant or recessive. ONLY THE DOMINANT ALLELE IS EXPRESSED ! The genotype of a black rabbit is either two dominant alleles (BB) or one dominant allele & one recessive allele (Bb); the genotype of a brown rabbit consists of two recessive alleles (bb). USING PUNNETT SQUARES Used to identify possible combinations of dominant and recessive alleles in offspring. Used to calculate the probability that an offspring will inherit a certain trait. USING PUNNETT SQUARES 1) Divide a square into four sections. 2) Write the letters that represent the alleles of the parents across the top & down the side of the square. USING PUNNETT SQUARES 3) Combine the alleles from one parent with those from the other in every combination. 4) The letters in the Punnett square show the genotypes of the offspring. LAW OF DOMINANCE The trait that appears is the dominant trait and the hidden trait is the recessive trait. IN RABBITS, THE ALLELES FOR BLACK COAT COLOR (B) IS DOMINANT OVER THE ALLELE FOR BROWN COAT COLOR (B). RABBITS (BB) ARE MATED. PREDICT ABOUT THE TWO WHAT OFFSPRING? BLACK CAN YOU _____________________________________ A) Most of the offspring will be brown. B) Most of the offspring will be black. C) All of the offspring will be black. D) All of the offspring will be brown. WHAT IS A PUREBRED PLANT? _______________________________________ WHY WAS IT IMPORTANT FOR MENDEL TO PREVENT HIS PEA PLANTS FROM SELF POLLINATING? _______________________________________ IS YELLOW SEED COLOR CONTROLLED BY A DOMINANT ALLELE OR A RECESSIVE ALLELE? WHAT TYPE OF ALLELE CONTROLS PINCHED POD SHAPE? ______________________________________ WHAT COMBINATION OF ALLELES MUST THE WHITE RABBIT HAVE? _______________________________________ IF A PEA PLANT HAS A TALL STEM, WHAT POSSIBLE COMBINATIONS OF ALLELES COULD IT HAVE? _______________________________________ WHAT IS A DOMINANT ALLELE? WHAT IS A RECESSIVE ALLELE? GIVE AN EXAMPLE OF EACH. _______________________________________ THE ALLELE FOR ROUND SEED IS REPRESENTED BY SUPPOSE R. THAT A PEA PLANT INHERITED TWO RECESSIVE ALLELES FOR WRINKLED SEEDS. HOW WOULD YOU WRITE THE SYMBOLS FOR ITS ALLELES? _______________________________________