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Genetics with a Smile 1

Name _________________________________

Genetics with a Smile

Purpose

: To distinguish between dominant and recessive traits and make predictions

about possible outcomes of various genetic combinations of inherited characteristics

Background Information

: Genes are the material that control which traits are

expressed in an organism. There are two copies of each gene, one from the mother

and one from the father. These genes can take different forms called alleles. For

example, there is a gene for the height of a pea plant. The gene has two alleles, one for

tall and one for short. Genes are located on chromosomes in the nucleus of a cell.

Genes come in pairs and offspring inherit one copy of each gene from each parent.

A dominant allele is one whose trait always shows up, even when only one of the two

alleles is in the dominant form. A dominant allele is shown by a capital letter. A

recessive allele is one that is hidden when the other copy of the gene contains the

dominant allele. A recessive allele shows up only when there is no dominant allele

present.

When offspring inherit two dominant genes, (one dominant gene from each parent) they

are said to be homozygous dominant.

When offspring inherit two recessive genes, (one recessive gene from each parent) they

are said to be homozygous recessive.

When offspring inherit one dominant gene and one recessive gene, they are said to be

heterozygous dominant.

Some traits do not follow the dominant-recessive pattern. When an organism has two

different alleles for a gene that does not follow the pattern, it shows a trait that is a blend

of the traits represented by the two alleles. For example, the gene for the color of some

flowers has one allele for red and one for white. When both alleles are present, neither

is dominant, and the flower color is pink. This is called co – dominance or incomplete

dominance.

The probability of certain traits being shown can be figured by using Punnett Squares.

A Punnett Square is a chart that shows all the possible combinations of alleles that can

result when genes are crossed.

Procedure

Part A: Smiley Face Traits

(1) Obtain two coins from your teacher. Mark one coin with a “F” and the other with a

“M” to represent each of the parents. The parents are heterozygous for all the Smiley

Face traits.

An adaptation of an activity by T. Trimpe

Genetics Worksheets