- Slides: 12
Genetics � Genetics is the study of how traits are passed from parents to offspring. � Heredity is the actual passing of traits from parents to offspring.
Gregor Mendel � Gregor Mendel is known as the father of modern genetics. � Experimented with pea plants
� Because of Mendel’s experiments we now know that inherited traits are controlled by different versions of genes known as alleles. � Alleles can be dominant or recessive. ◦ Dominant alleles can hide alleles for other traits. Use capital letters to represent them. ◦ Recessive alleles can be hidden by alleles for other traits. Use lower case letters to represent them.
� The results of most of Mendel’s experiments looked the same. � The F 1 generation showed all of the dominant traits, while the F 2 generation showed the dominant and recessive traits in a ratio of 3: 1.
Phenotype and Genotype � Phenotype—an organism’s appearance (word description) � Genotype—an organism’s genetic make-up (letter code) ◦ Genotype for purple flowers: PP or Pp ◦ Genotype for white flower: pp
Homozygous and Heterozygous � Homozygous—both alleles are the same (pure) PP or pp � Heterozygous—two alleles are different (hybrid) Pp
Monohybrid Crosses � Monohybrid Crosses —only one characteristic is tracked � A tool used to predict the outcome of different types of monohybrid crosses is called a Punnett Square ◦ Example: TT x tt (Pure tall plants x Pure short plants) t t T T Tt Tt Result: 100% Heterozygous tall offspring Genotype = Tt Phenotype = tall
Incomplete Dominance � Incomplete Dominance occurs when offspring have a phenotype that is in between that of the two parents. RR Rr http: //smabiology. blogspot. com/2008_11_01_archive. html rr
� Punnett Squares for incomplete dominance are completed in the same manner. Except that the heterozygous individuals will have the blended phenotype. � Example: Pink Flower x Pink Flower Rr x Rr R RR Rr rr Results: 25% Red flowers 50% Pink flowers 25% White flowers Genotypic Ratio: 1 RR : 2 Rr : 1 rr Phenotypic Ratio: 1 Red : 2 Pink : 1 White
Codominance � Codominance occurs when both alleles in a pair are expressed but do not actually blend. � Human Blood Type is an example of codominance. � Three alleles (A, B and O) are involved in determining blood type. However, you still only inherit two (one from mom and one from dad) ◦ A and B are both dominant; O is recessive ◦ Type AB blood has one genotype: AB ◦ Type A blood has two possible genotypes: AA and AO ◦ Type B blood has two possible genotypes: BB and BO ◦ Type O blood has only one possible genotype: OO
� Dihybrid Cross --shows two traits at the same time. � Example: Rr. Yy x Rr. Yy (heterozygous round and yellow seeds) RY R = round r = wrinkled Y = yellow y = green Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy. RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy Results: 9 round/yellow 3 round/green 3 wrinkled/yellow 1 wrinkled/green