Download Heredity Notes 2

Heredity Notes – Part II
Vocabulary continued:
Homozygous – when the alleles are identical for a
trait. (Purebred). Ex. TT or tt, SS or ss.
Heterozygous – when the alleles are different for the
trait. The dominant trait “wins”. Tt or Ss.
Phenotype – the physical appearance or visible trait.
(Uses words to describe the trait). Ex: Tall, short,
Square or round.
Genotype – the genetic makeup or allele combination
of the trait. (uses letters to describe the trait) Ex: TT,
tt, Ss or ss. (Can be either homozygous or
heterozygous)
Probability – the number that describes how likely it
is that an event 3will occur.
Probability and Genetics
A Punnett square is used to help predict traits. It is a
square used to show all of the possible combinations
of alleles that can result from a genetic cross. In a
genetic cross, the allele that each parent will pass on
to its offspring is based on probability.
 The top represents the alleles from the male
(father) and the side represents the alleles from
the female (mother).
Example of a cross:
R (round) is dominant over r (square).
In this cross, there is a 25% (1/4) chance that the
offspring will be RR or homozygous for round. There
is a 50% (2/4) chance that the offspring will be Rr or
heterozygous for round, and a 25% (1/4) chance that
the offspring will be rr or homozygous for square.