Transcript Punnett Squares & Probability

Punnett Squares & Probability
WHAT IS 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
Tt X Tt Cross
Section 11-2
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
Figure 11-11 Incomplete
Dominance in Four O’Clock Flowers
Section 11-3
Go to
Section:
Figure 11-11 Incomplete
Dominance in Four O’Clock Flowers
Section 11-3
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