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Mendel and Punnett Squares

DAY 1

• MENDEL AND HIS PEAS

• HOW TO SET UP A MONOHYBRID PUNNETT SQUARE

The Father of Genetics • Gregor Mendel- known as the

father of genetics.

• He genetically crossed pea plants to determine how traits or characteristics were passed down.

• He started by looking at the inheritance patterns of a single character: a monohybrid cross

Mendel’s Conclusions 1. Passing traits to Offspring:

1. An Individual’s characteristics are controlled by factors (genes) that are passed from parents to their offspring

2. Dominant and Recessive Alleles:

1. Principle of Dominance: some alleles are dominant and some are recessive

Probability

•We cannot always know the exact outcome of a genetic cross but we can predict the possible outcomes using probability

• The best ways to predict the outcome of a genetic cross is to use a diagram known as a Punnett Square

How to Punnett Square 1. Write the genotypes that will serve at the parents of the cross

2. Determine what alleles would be found in all possible gametes

3. Draw a box with 4 squares. Write the genotypes of the parents on the left and top of the squares, 1 letter per square.

4. Fill in the table by combining the gametes’ genotypes

5. Determine the genotype and phenotype of each possible offspring

Examples

1. Cross a heterozygous male for Brown hair with a blond female. (Brown hair is dominant) What percentage of the offspring will be blond?

2. Cross 2 heterozygous parents. What are the genotypes and phenotypes possible?

Day 2

• Mendel’s new rules

• Dihybrid Crosses

Law of Independent Assortment • Genes for different traits can separate by

themselves during the formation of gametes.

Codominance

Incomplete Dominance

Multiple Alleles

Polygenic Traits

DiHybrid Crosses When two genes/factors are crossed

1. Write the genotypes of the parents

2. Determine what allele combinations could be made in all the possible gametes

3. Each parent can make 4 different types of gametes so the table needs to have 16 boxes. Write each gametes for the parents on the top and left side

4. Fill the boxes by combining the genotypes. Keep like letters together and write capital letters first.

5. Determine the genotypic and phenotypic ratios of the offspring.

Practice

• Cross two heterozygous yellow, round peas.

Short Cut The Multiplication Rule!

Sex-Linked Traits

• The genes located on the X and Y chromosomes show a pattern known as sex-linkage.

• Males – XY

• Females – XX

• In a male (XY) only the X chromosome must carry it to show the disease. (XhY)

• In females (XX), both X chromosomes must have the recessive allele. (XhXh)

Common Sex-Linked Traits In Humans:

• Colorblindness

• Hemophilia

• Muscular Dystrophy

Calico Cats:

Tricolored cats whose fur spots are controlled by the X chromosome. Most calico cats are female.

Normal color Vision

Monochromate Vision

NORMAL

RED/GREEN color blind

Color deficient does not read

Hemophilia:

Muscular Dystrophy

•In the US, 1 out of 3,000 males are born with the disease.

Punnett Square with X-Linked