Introduction to Genetics
Mendel and Heredity
Key Concept: Mendel’s research showed that traits are inherited as discrete units.
Gregor Mendel’s Peas
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Mendel, born in 1822, worked with regular garden peas. The peas were true-breeding
(each pea plant produced offspring identical to itself.) Mendel crossed different types
of pea plants.
Genes and Dominance

Trait- a specific inherited characteristic that varies from one individual to another

Cross – the mating of two organisms

Hybrid- the offspring of crosses between parents with different traits

Genes- chemical factors that determine traits

Alleles- different forms of a gene (yellow or green pea color)

Some alleles are dominant and some are recessive. An organism with a
dominant allele for a particular trait will always exhibit that form of the trait.
An organism with a recessive allele for a particular trait will exhibit that form
only when the dominant allele for the trait is not present.
Mendel’s observations helped him form the “Law of Segregation.” There are two main parts.
1. Organisms inherit two copies of each gene (one from each parent.)
2. Only one copy of a gene goes into an organism’s gametes. The two copies of a gene
separate (or segregate) during gamete formation (meiosis.) Alleles pair up again when
gametes fuse during fertilization (forms a zygote.)
Traits, Genes, Alleles, and Probability
Key Concepts: Genes encode proteins that produce a diverse range of traits.
Genetics and Probability

Probability- the likelihood that a particular event will occur.

The principles of probability can be used to predict the outcomes of a genetic cross.
Punnett Squares

Punnett square- a diagram showing the gene combinations that might result from a
genetic cross

The genotype of one parent is put on the top of the square and the genotype of the
other parent is put on the side. The probable offspring are recorded in the middle.

Dominant alleles are given capital letters and recessive alleles are given lower-case
letters.

Homozygous- organism that has two identical alleles for a particular trait (both
dominant: TT, or both recessive: tt)

Heterozygous- organism that has two different alleles for a particular trait (one
dominant and one recessive: Tt) Heterozygous organisms are hybrid.

Genotype- genetic makeup: TT, tt, Tt

Phenotype- physical characteristics resulting from genotype: tall, short. Organisms
with different genotypes can have the same phenotype. If they have different
phenotypes, they must have different genotypes.
Probabilities Predict Averages

The larger the sample set, the more likely the numbers will get to the expected values.
(Not every family with two children has one boy and one girl, but the overall average
is 50/50, or a 1:1 ratio.)