Chapter 3, section 1Mendel’s Work

The Basics• The passing of physical characteristics from

parents to offspring is called heredity.• These characteristics are called traits.• The study of heredity is called genetics.

Gregor Mendel• Studied how traits were passed from parent to

offspring in pea plants• He discovered that sometimes traits can seem to

disappear in offspring and reappear in later generations

• We now know this is due to some traits being dominant or recessive

Dominant and Recessive traits

• Genes are the factors that control a trait• Example: you have a gene that controls what eye color you

have

• Genes exist in pairs• Humans have about 20,000 genes, located on DNA,

which is organized into 23 pairs of chromosomes !

• Alleles are the different forms of the gene• Each gene has a pair of alleles

• One allele is inherited from the female parent one allele is inherited from the male parent

• We represent alleles with letters (AA, Aa, aa)

• Some alleles are dominant and others are recessive

• A dominant allele is one whose trait will ALWAYS show up if the allele is present• If you inherit a dominant allele from either your mom or

dad, you will have the dominant trait• We represent a dominant allele with capital letters (A)

• A recessive allele only shows up if both alleles are recessive

• You have to inherit a recessive trait from both mom AND dad to have the recessive trait

• We represent recessive alleles with lower case letters (a)

Practice• Will these people have the dominant or recessive

trait?• Tt• HH• Bb• nn• A frog inherits a dominant allele from its female parent

and recessive allele from its male parent. What trait will it have? Write what its alleles would be using the letter F.

• A purebred gene is one where both alleles are the same (AA or aa)• Also called homozygous

• A hybrid gene is one where there is one dominant and one recessive allele (Aa)• Will always show the dominant trait• Also called heterozygous