MENDELIAN LAWS OF HEREDITY, MONOHYBRID AND DIHYBRID CROSSES
Mendels Laws of Hereditya is a set of primary tenets that underlie much of genetics developed by Gregor Mendel in the latter part of the 19th century.a Mendel (1822-1884), an Austrian monk, was interested in understanding variances in plants, and between 1856 and 1863 cultivated and tested some 28,000 pea plants.
Mendel used peas...aThey reproduce sexuallyaThey have two distinct, male and female, sex cells called gametesaTheir traits are easy to isolate
Mendel crossed thema Fertilization - the uniting of male and female gametesa Cross - combining gametes from parents with different traits
What did Mendel find?a His experiments brought forth two generalizations which later became known as Mendels Laws of Heredity or Mendelian inheritance.
Rule of Dominancea The trait that is observed in the offspring is the dominant trait (uppercase)a The trait that disappears in the offspring is the recessive trait (lowercase)
MENDEL’S LAW OF INDEPENDENT ASSORTMENTa Also known as Mendels Second Lawa The most important principle of Mendels law of independent assortment is that the emergence of one trait will not affect the emergence of another.
MENDEL’S LAW OF SEGREGATIONa Alternative versions of genes account for variations in inherited characters. This is the concept of alleles. Alleles are different versions of genes that impart the same characteristic.a For each character, an organism inherits two genes, one from each parent. This means that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father.
a If the two alleles differ, then one, the dominant allele, is fully expressed in the organisms appearance; the other, the segregate during gamete production. This is the last part of Mendels generalization. The two alleles of the organism are separated into different gametes, ensuring variation.
Words to Knowa Allele: One of two or more forms a gene may take.a Dominant: An allele whose expression overpowers the effect of a second form of the same gene.a Gamete: A reproductive cell.a Heterozygous: A condition in which two alleles for a given gene are different from each other.a Homozygous: A condition in which two alleles for a given gene are the same.a Recessive: An allele whose effects are concealed in offspring by the dominant allele in the pair.
Dihybrid vs Monohybrida Dihybrid Cross - crossing parents who differ in two traits (AAEE with aaee)a Monohybrid Cross - crossing parents who differ in only one trait (AA with aa)
Example of a CrossParents: Tall round seeded plant (TTRR) X Dwarf wrinkled seeded plant (ttrr)Gametes: TR trF1: TtRr (Tall round seeded plants)When F1 self pollinated: F1 X F1TtRr(Tall round seeded plants) X TtRr(Tall round seeded plants)Gametes: TR Tr tR tr TR Tr tR tr
Results of Dihybrid Cross ExamplesGenotypea 1/16 will be homozygous dominant for both traits (TTRR) 2/16 will be homozygous dominant for height and heterozygous for shape (TTRr) 2/16 will be heterozygous for height and homozygous dominant for shape (TtRR) 1/16 will be homozygous dominant for height and homozygous recessive for shape (TTrr) 4/16 will be heterozygous for both traits (TtRr) 2/16 will be heterozygous for height and homozygous recessive for shape (Ttrr) 1/16 will be homozygous recessive for height and homozygous dominant for shape (ttRR) 2/16 will be homozygous recessive for height and heterozygous for shape (ttRr) 1/16 will be homozygous recessive for both traits (ttrr)a This is a 1:2:2:1:4:2:1:2:1 genotypic ratio
Phenotype 9/16 will be tall and round seeded 3/16 will be tall and wrinkled seeded 3/16 will be dwarf and round seeded 1/16 will be dwarf an wrinkled seededa This is a 9:3:3:1 phenotypic ratio.