Monohybrid Problems

(One trait being followed from one generation to the next.)

1. Polydactyly, having more than five digits, is a dominant allele. We can use any letter for our abbreviation, but we should use one which is easy to remember and which has different forms for upper and lower case. Let us use "F" to represent the dominant allele for Polydactyly. Five fingers is a recessive allele so we will abbreviate it "f1.

Two individuals who are heterozygous for Polydactyly marry. What is the phenotype and genotype of each parent? What is the chance that they will have a five-fingered child? a six-fingered child?

a. Father's phenotype_ Mother's phenotype_

b. Father's genotype_ Mother's genotype_

c. Father's sperm cells_or_

(Remember that sperm cells or ova can contain only one allele because they are haploid.)

One of the easiest ways to work genetic problems is to use a Punnett square. The square consists of boxes representing the offspring. The mother's potential ova are listed above the boxes, the father's potential sperm are listed down the left side. The boxes are then filled in by combining the sperm and egg alleles inside each box.

The dominant allele (upper case letter) is always written first, regardless of who donates it.

Punnett square

Notice that there is a 75% chance (3 out of 4) that a child will have six fingers (FF, Ff, and Ff). There is a 25% chance that a child will be five-fingered (ff).

2. A man who is heterozygous for Polydactyly marries a woman who has only five fingers on each hand. What is the probability of having these characteristics in their offspring?

Six fingers_ Five fingers_

3. In humans, curly hair is dominant over straight hair. If a straight-haired woman marries a heterozygous, curly-haired man, what is the chance that they will have a child who has straight hair?

Straight-haired child_

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