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

Test Yourself Is it homozygous or heterozygous? Answers RR Tt Ff QQ

Test Yourself Homozygous Heterozygous RR Tt Ff QQ Is it homozygous or heterozygous? Answers RR Tt Ff QQ Homozygous Heterozygous

Homozygous dominant = DD Homozygous recessive = dd Heterozygous = Dd

Test Yourself Is it dominant or recessive? Hh rr GG

Test Yourself Hh rr GG Dominant (heterozygous) Homozygous recessive Is it dominant or recessive? Answers Hh rr GG Dominant (heterozygous) Homozygous recessive Homozygous dominant

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

Example Tall plants are dominant over short plants. What are the phenotypes? TT___tall____ Tt___tall_____ tt___short______ What are the genotypes? Tall___TT or Tt____ Short___tt____

Your turn In humans, brown eyes are dominant over green eyes. What are the possible phenotypes? BB_______ Bb_______ bb_______ What are the possible genotypes? Brown_______ Blue________

Check yourself! In humans, brown eyes are dominant over green eyes. What are the possible phenotypes? BB___brown____ Bb__brown_____ bb___blue____ What are the possible genotypes? Brown___BB or Bb____ Blue___bb_____

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.