Simple Mendelian Genetics

History of Genetics  Gregor Mendel was an Austrian monk who is essentially the founder of modern genetics.  Heredity had been known about for a very long time, but he was the first scientist to be able to explain how it worked.

 Mendel discovered two laws, Law of Segregation and the Law of Independent Assortment.  Law of Segregation tells us that each offspring has two alleles for a trait. One comes from the mother and one comes from the father.  Law of Independent Assortment tells us that each trait is independent of other traits. Meaning that traits can be inherited separately.

Genetics  Allele= Different forms of a gene. Each gene is represented by a letter.  Dominant allele= An allele that expresses its phenotype over other alleles for that trait. Represented by a capital letter.  Recessive allele= An allele that expresses its phenotype only when in a homozygous situation. Represented by a lower case letter.

 Homozygous= Two identical alleles for a trait.  Heterozygous= Two different alleles for a trait.  Dominant alleles phenotypes are expressed in homozygous or heterozygous genotypes.  Recessive alleles phenotypes are expressed only in homozygous genotypes.  Phenotype= The observable traits of an organism.  Genotype= The complete genetic makeup of an individual.

Punnett Squares  Punnett squares are useful tools for predicting genetic outcomes.  Look at board…this is an empty Punnett square.  The following are rules for using Punnett Squares:  1) Write out the parents genotypes.  2) Draw your Punnett Square.

 3) Place Parent 1’s genotype on the top of your square. One gene (letter) for each column.  4) Place Parent 2’s genotype on the side of your square. One gene (letter) for each row.  5) Fill in the Punnett Square.

 6) Write a phenotypic ratio for the predicted outcomes.  7) Write a genotypic ratio for the predicted outcomes.  8) Convert ratio’s to percentages if asked to do so.

 Punnett squares are useful when doctors are trying to predict the probability that an offspring may inherit a genetic condition.

EXAMPLE  In hamsters having long teeth is dominant to having short teeth. One parent is heterozygous for long teeth and the other parent is homozygous for short teeth. Construct a Punnett square and calculate the percentage of offspring that will have long teeth and the percentage that will have short teeth.