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Mendelian Genetics. April 2008. March 10, 2009 Mr. Bromwell. Drill – March 10, 2009. Aa BB cc Dd ee Ff GG. Which of the following genotypes are homozygous? heterozygous?. In peas, purple flower color is dominant to white flower color.

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Mendelian genetics

Mendelian Genetics

April 2008

March 10, 2009

Mr. Bromwell

Drill march 10 2009
Drill – March 10, 2009

Aa BB cc Ddee Ff GG

Which of the following genotypes are homozygous? heterozygous?

In peas, purple flower color is dominant to white flower color.

  • What would be the genotype of a plant with white flowers?

  • What would be the phenotype of a plant with the genotype ‘Pp’?

Which one of Mendel’s Laws explains why some traits in pea plants appear to ‘skip’ generations? Explain the law.

Using mendel s laws to predict
Using Mendel’s Laws to Predict

  • Geneticists can use Mendel’s Laws and the rules of probability to predict whether an allele (or alleles) will be passed from parent to offspring.

  • probability (n.) – the likelihood that an event will occur

  • Probability can be expressed as…

    • a fraction (1/2),

    • a percentage (50%),

    • or a ratio (1:1).

Making predictions dimples
Making Predictions - Dimples


Having dimples (depressions in the cheek when you smile) is a dominant trait controlled by a single gene.

Two parents have dimples and are heterozygous for the trait. What is the probability that they will have a child with dimples?

Making predictions dimples1
Making Predictions - Dimples


From the information in the problem you know that both parents have the heterozygous genotype ‘Dd’.

D = dominant allele (dimples)

d = recessive allele (no dimples)

From here there are two ways to solve the problem – using basic probability or using a Punnett square.

The Punnett square, developed by Sir Reginald Punnett in the early 1900s, is pictured at right.

♂ male gamete types

♀ female gamete types

A word about ratios
A Word About Ratios

A cross that involves two hybrids (heterozygous parents) for a single trait is called a monohybrid cross.

There are always three possible genotypes in a ratio of 1:2:1.

There are two possible phenotypes in a ratio of 3:1.

♂ male gamete types

♀ female gamete types

Pea plants provide practice problems
Pea Plants Provide Practice Problems

  • In peas, round-shaped peas are dominant to wrinkled peas. A farmer takes a pea plant that is heterozygous for round peas and breeds it with a pea plant that produces wrinkled peas. What is the probability that the offspring will produce round-shaped peas?

  • Tall pea plants are dominant to short pea plants. Two heterozygous tall pea plants are bred together. What is the probability that the offspring will be heterozygous like their parents?

  • Green pea pods are dominant to yellow pea pods. A farmer has two plants with green pods. He crosses them in the hopes of getting some plants with yellow pods? Is this possible? Explain our answer.


Biggs, Alton, et. al. Biology. New York: The McGraw Hill Companies, Inc., 2007.

Szeto, C. “Asian (Oriental) American girl with dimples” Wikimedia Commons. Jan 21, 2008. Retrieved Apr 6, 2008 from <>.