Bio NOTES: Traits and ProbabilityThe Punnett square is a grid system for predicting all possible genotypes resulting from a cross.

- The axes represent the possible gametes of each parent.

- The boxes show the possible genotypes of the offspring.

The Punnett square gives the ratio of possible genotypes and phenotypes.

homozygous dominant x

homozygous recessive=

all heterozygous

Monohybrid crosses examine the inheritance of only

one specific trait

heterozygous x

heterozygous=

1 homozygous dominant2 heterozygous

1 homozygous recessive

homozygous recessive x

heterozygous=

2 heterozygous2 homozygous recessive

genotype ratio – 1:1phenotype ratio – 1:1

genotype ratio – 1:2:1phenotype ratio – 3:1

Example: Two pea plants cross, one homozygous dominant yellow seeds and the other homozygous recessive green seeds.

What are the genotypes and phenotypes of the F1 generation?

Key: Y=_______ and y= _______P, Parents: ____ X ____

Results:F1 Genotypes:

F1 Phenotypes:

Ex. Continued: What offspring would result from an F1 cross?

F1, Parents: ____ X ____

Results:F2 Genotypes:

F2 Phenotypes:

An organisms phenotype is based on its genotype & its environment!!•A plant may have the GENES to grow tall, but not the sunlight.

Yy Yy

Yy Yy

Y Y

y

y

YY yyYellow Green

100% Yy

100% Yellow

Yy Yy

Y y

Y

y

YY

Yy

Yy

yy

25% YY, 50% Yy, 25%yy

75% Yellow, 25% green

Test Cross: A process that uses an organisms phenotype to determine its genotype. The unknown is crossed with an organism that is homozygous recessive (because it cannot hide any traits).

Monohybrid Crosses can be used in REVERSE to find the genotypes of an “unknown” parent.

Ex): A homozygous recessive bald cat had 4 kittens, two were bald like their mother and the other two were hairy.

What was the genotype of the father cat?

Key: H=_______ and h= _______Mom Cat Geno: ____Dad Geno: HH or Hh? _________________

Ex): In pea plants, tall plants are dominant to short. Tall plants could be TT or Tt.

Determine the genotype of an tall pea plant. ?? X t t _____ X t t ____X t t

Hh

hh hh

Tall Short

Tall Short

TallTall

TallTall

Hairy Baldhh

T tT Tt

t

TtTT

Hh

h hH

hHh

Hh Hh

Hhh h

H

H

Father is Hh!!

F1: 2 Bald

2 Hairy

t

t

Mendel’s dihybrid crosses led to his second law,the law of independent assortment.

The law of independent assortment states that allele pairs separate independently of each other during meiosis.

Mendel’s dihybrid crosses with heterozygous plants yielded a 9:3:3:1 phenotypic ratio.

A dihybrid cross involves two traits.

Dihybrid Cross

Heredity patterns can be calculated with probability.

• Probability is the likelihood that something will happen.• Probability predicts an average number of occurrences,

not an exact number of occurrences.

• Probability =number of ways a specific event can occur

number of total possible outcomes

• Probability applies to random events such as meiosis and fertilization.

Questions and Practice Problems:

1. An organisms physical characteristics are called its: _________2. An organisms genetic make up is called its: ____________3. Alternative forms of a gene are called: ___________4. Examples of allele alternatives are: Tall and _______ pea plants

Straight Hair & __________5. Indicate: Homozygous Dominant (HD), Homozygous Recessive (HR) or

Heterozygous (HE)AA=____ Rr=____ bb=____ E1E2=___ Xx=____

6. If green hair is dominant to white hair indicate the phenotypes below.GG = _____________ Gg=____________ gg=____________

7. Cross a homozygous dominant three-eyed alien with a homozygous recessive one-eyed alien. Determine the F1 genotypes & phenotypes.

Results:F1 Genotypes:

F1 Phenotypes:

Key: __=__________

__= __________

P, Parents: ____ X ____

TtTt

Tt Tt

PhenotypeGenotype

AllelesShort

Curly Hair

HD HE HR HE HE

Green Hair Green Hair White Hair

Tt

Three eyesOne eye

TT tt

100% Tt

100% Three eyes

T T

t

t