# WHAT IS PROBABILITY? Punnett Squares & Probability.

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WHAT IS PROBABILITY? Punnett Squares & Probability

Probability What is probability?  The likelihood that a particular event will occur The principles of probability can be used to predict the outcomes of genetic crosses.

Punnett Squares The gene combinations that might result from a genetic cross can be determined by drawing a diagram

Section 11-2 Tt X Tt Cross Go to Section:

Allele combinations Homozygous: organisms that have two identical alleles = true-breeding for a particular trait  TT  tt Heterozygous: organisms that have two different alleles = hybrids for a particular trait  Tt

Phenotype: visible, physical characteristics  Tall or short Genotype: not visible, genetic makeup  Homozygous or heterozygous

Independent Assortment Does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color?

NO!!! The alleles for seed shape and seed color in pea plants do not influence each other’s inheritance

The Principle of Independent Assortment Genes for different traits can segregate independently during the formation of gametes.

Summary of Mendel’s Principles Inheritance is determined by genes passed from parents to offspring Some forms of genes are dominant and others are recessive Each offspring has two copies of a gene (alleles), one from each parent because they are segregated during gamete formation The allele for different genes usually segregate independently of one another

What will happen if some alleles are neither dominant nor recessive? What will happen if some traits are controlled by more than one gene?

Incomplete Dominance When one allele is not completely dominant over another  For example: 4 o’clock plant

Section 11-3 Figure 11-11 Incomplete Dominance in Four O’Clock Flowers Go to Section:

Section 11-3 Figure 11-11 Incomplete Dominance in Four O’Clock Flowers Go to Section:

Codominance Both alleles contribute to the phenotype of the organism  For example: roan

Multiple alleles There can be more than 2 alleles to determine phenotype. (An individual can not have more than 2 alleles for a trait, but there are more than 2 allele possible for that trait in the population)  For example: blood type

Polygenic traits Many traits are produced by the interaction of several genes  For example: eye color skin color, height

Environmental Effect Although genes are inherited, their expression can be modified by interactions with the environment.  For example: sun tanning, Himalayan rabbit