5.3 Mendelian Genetics

Mendel's laws

Gregor Mendel discovered that traits are passed by discrete factors (now alleles). His law of segregation says the two alleles for a trait separate into different gametes. His law of independent assortment says alleles for different traits are inherited independently.

Key vocabulary

The genotype is the genetic makeup; the phenotype is the observable trait. Homozygous = two identical alleles; heterozygous = two different alleles. A dominant allele masks a recessive one. A cross of two heterozygotes (Aa × Aa) gives a 3:1 phenotypic ratio, shown with a Punnett square.

Key terms

Allele

One of the alternative versions of a gene.

Dominant vs. recessive

A dominant allele masks a recessive allele when both are present.

Genotype vs. phenotype

Genotype = genetic makeup; phenotype = observable trait.

Homozygous vs. heterozygous

Homozygous = two identical alleles; heterozygous = two different alleles.

Law of segregation

The two alleles for a trait separate into different gametes.

Try an AP-style question

In pea plants, purple (P) is dominant to white (p). A purple plant crossed with a white plant produces 52 purple and 48 white offspring. What is the purple parent's genotype?

1. pp
2. Either PP or Pp
3. PP
4. Pp