Mendel’s Laws of Heredity

Gregor Mendel Austrian monk who worked with garden peas Did the first important studies of heredity – passing of characteristics from parent to offpring.

Mendel was the first to predict what traits would be passed from one generation to the next. Trait: Inherited characteristics We now know that traits are controlled by genes.

Most genes have two different forms or alleles. Ex. Gene for pea shape has 2 possible alleles: round (R) or wrinkled (r)

Mendel also determined that: Some alleles are dominant and others are recessive. (Principle of Dominance) If you have a dominant allele for a trait you will always express that form.

Dominant alleles shown with capital letters. (Ex. D, R, S) Recessive alleles shown with lowercase letters. (Ex. d,r,s)

An organism is homozygous for a trait if the two alleles are the same. Ex. TT or tt Heterozygous – the two alleles are different Ex. Tt, Ss, Cc

Homozygous dominant = DD Homozygous recessive = dd Heterozygous = Dd

Other Common Genetic Terms: Phenotype – physical characteristics Ex. Tall plants, green eyes Genotype – describes genetic makeup. Ex. TT, Tt, or tt

Geneticists use probability to predict the phenotypes and genotypes of offspring

Punnett Squares Grids that can be used to predict the possible gene combinations of offspring It shows probabilities of each genotype occuring not the actual results.

Mendel’s Principle of Segregation - When sex cells (gametes - egg and sperm) are made, the 2 alleles for each trait separate. The result: Each sex cell carries only one copy of a gene