MENDELIAN INHERITANCE Gregor Mendel proposed that heredity is passed on by what we now call genes.
GENOTYPES & PHENOTYPES • The genotype of an organism can be defined as the genes that it possesses, and is usually expressed as a combination of alleles. • The phenotype of the organism is the physical features, proteins produced that is determined by both the genotype and the environment.
DOMINANCE & RECESSIVENESS • The characteristic (or trait) itself is said to be dominant, when it is expresses when there is at least one copy of the allele present. • The characteristic (or trait) is said to be recessive when two copies must be present for it to be expressed.
GENOTYPES • In the trait of tongue rolling the dominant characteristic is represented by R and the recessive by r. • The genotypes of a roller could be • Rr • RR * The genotype of a non-roller would be • rr
C0- & INCOMPLETE DOMINANCE • Examples of co-dominance include: • The ABO blood group in humans. • The appearance of roan cattle. • Examples of incomplete dominance include: - Pink Snapdragons.
IMPORTANT TERMINOLOGY • HOMOZYGOUS • HETEROZYGOUS • CO-DOMINANT • INCOMPLETE DOMINANT • HYBRID • PURE BREEDING
MENDELIAN INHERITANCE • The example of tongue rolling where R represents the ability to roll and r represents the inability to roll. • Such a problem may be worded: - What are the F1 & F2 generation phenotypes of a monohybrid cross between homozygous dominant & homozygous recessive individuals?
TAKE A LOOK….. Parents RR + rr Gametes all R + all r Genotype F1 all Rr Phenotype F1 all tongue rollers Therefore F2 parents Rr + Rr Gametes (1/2 R + ½ r) + (1/2 R + ½ r)
MENDELIAN INHERITANCE….. • Genotype ratio 1:2:1 • Phenotype ratio 3:1 • The genotype ratio is the result of genes separating when gametes form & combing at fertilisation by chance. • This is the Principle of Segregation, or, Mendel’s First Law.
MENDEL’S SECOND LAW • When two different characteristics are considered, such as colour & texture of fruit, this is termed a dihybrid cross and leads to an explanation of Mendel’s 2nd law. • In this cross let G represent green fruit and g represent yellow and let H represent hairy fruit and h represent smooth.
Cross a homozygous dominant plant with a homozygous recessive plant Parents GGHH + gghh Gametes all GH + all gh F1 offspring all GgHh Parents for F2 GgHh + GgHh Gametes 1/4GH+1/4Gh+1/4gH+1/4gh
MENDEL’S SECOND LAW • This is explained by the fact that the two characteristics are independent of each other. • That is, they do not necessarily move to the same gamete in the same combinations every time meiosis occurs. • Mendel’s second law is the Principle of Independent Assortment.