Biology

Unit 4 Notes:Mendelian Genetics

(1) Gregor Mendel• Monk• Studied genetics of pea plants.

• He noticed that:– There were some traits which showed up all of

the time (dominant).– There were some traits which showed up some

times (recessive).– These traits are passed down from parents.– You can’t tell just by looking at an organism,

which genes it has.

(2) Dominant + Recessive Traits

• Dominant Trait:– These will ALWAYS SHOW UP if they are

present.– Representation Capital Letters

• Recessive Trait:– These will ONLY SHOW UP if the organism

inherited 2 together.– Representation Lower Case Letters

(3) Inheriting Traits• You inherit 2 alleles for every gene.

– Gene = Chunk of DNA.– Allele = Variation that chunk can come in.

• “Heterozygous” = You inherited 1-Dominant and 1-Recessive allele.

• “Homozygous” = You inherited 2 of the same alleles.– “Homozygous Dominant” = You inherited 2-Dominant

alleles.– “Homozygous Recessive” = You inherited 2-Recessive

alleles

(4) Ways to Describe Inheritance• Phenotype:

– Physical Description.– Examples:

• “White”• “Brown”• “White and Brown Spotted”

• Genotype:– Genetic Description.– Examples:

• AA or Homozygous Dominant• Aa or Heterozygous• aa or Homozygous Recessive

(5) What’s With These Letters?• The letters represent the alleles an organism can

inherit.

• You should choose letters with a different Capital + Lower Case appearance:– A and a– B and b– Bad Life Choices: S and s, X and x, C and c

• Actual Letters DO NOT MATTER:– Heterozygous needs to be: Capital + Lower– Homozygous Dominant needs to be: 2 Capital– Homozygous Recessive needs to be: 2 Lower

(6) What Are Punnett Squares?• Punnett Square = Tool used to make genetic

predictions.

• They do NOT tell you with certainty how many offspring will have which traits!!!

• Tells you probability.– “There is a ¼ or 25% chance that the offspring will….”

• SET-UP:– 1 Parent’s genotype on the top– Other Parent’s genotype on the side

(7) Setting Up Punnett Squares• 1 Parent’s Genotype on the Top

(doesn’t matter which one).• 1 Parent’s Genotype on the Side

(doesn’t matter which one).• 1 Letter per Box!

• For Example:– If one parent is AA and the other parent is aa…

Draw your Punnett Square!A A

a

a

(8) Interpreting Punnett Squares• Each Box = ¼ or 25% chance

• For Example:– 4/4 Boxes have Aa, so…– There is a 4/4 or 100% chance that the offspring will be

Heterozygous.– I could also say that there is a 100% chance that the

offspring will look dominant.

Draw your Punnett Square!A A

a

a

Aa Aa

Aa Aa