Traits and Inheritance

that there must be two sets of instructions for each characteristic.

All of the first-generation plants showed the dominant trait.

However, they could give the recessive trait to their offspring.

Instructions for an inherited trait are called genes.

Offspring have two sets of genes—one from each parent.

The two sets of genes that parents give to offspring are

never exactly the same.

The same gene might have more than one version. The different versions of a gene are called alleles.

Alleles may be dominant or recessive. A trait for an organism is usually identified with two letters, one for each allele.

Dominant alleles are given capital letters (A).

Recessive alleles are given lowercase letters (a).

If a dominant allele is present, it will hide a recessive allele.

An organism can have a recessive trait only if it gets a recessive allele for that trait from both parents.

An organism’s genes affect its traits.

The appearance of an organism, or how it looks, is called its phenotype.

The phenotypes for flower color in Mendel’s pea plants were purple and white.

A genotype is the combination of alleles that an organism gets from its parents.

A plant with two dominant or two recessive alleles (PP, pp) is homozygous. Homo means “the same.”

A plant with one dominant allele and

one recessive allele (Pp) is heterozygous. Hetero means “different.”

The allele for purple flowers (P) in pea plants is dominant. The plant will have purple flowers even if it has only one P allele.

A Punnett square is used to predict the possible genotypes of offspring from certain parents. It can be used to show the alleles for any trait.

In a Punnett square, the alleles for one parent are written along the top of the square.

The alleles for the other parent are written along the side of the square.

The possible genotypes of offspring are found by combining the letters at the top and side of each square.