Patterns of Inheritance Patterns of Inheritance and Punnett Squaresand Punnett Squares

Homologous ChromosomesHomologous Chromosomes

One came from the mother, one from the father.One came from the mother, one from the father. Draw the following picture.Draw the following picture. The loci on the homologous chromosomes represent a The loci on the homologous chromosomes represent a

pair of alleles for a particular trait.pair of alleles for a particular trait.

Dominant and Recessive TraitsDominant and Recessive Traits

When an organism has two of the When an organism has two of the same alleles, it exhibits that same alleles, it exhibits that characteristic.characteristic.

Whether the alleles are dominant or Whether the alleles are dominant or recessive determines the trait recessive determines the trait expressed.expressed.

Dominant: represented by an Dominant: represented by an uppercase letter and it masks the uppercase letter and it masks the recessive allele.recessive allele.

Recessive: represented by a lowercase Recessive: represented by a lowercase letter; it is only expressed when no letter; it is only expressed when no dominant allele is present.dominant allele is present.

ExampleExample

Dark hair color is dominant; light hair color Dark hair color is dominant; light hair color is recessive.is recessive.

The hair could be described as HH (two The hair could be described as HH (two dominant alleles), Hh (one dominant, one dominant alleles), Hh (one dominant, one recessive allele), or hh (two recessive recessive allele), or hh (two recessive alleles).alleles).

So, HH and Hh would have dark hair and So, HH and Hh would have dark hair and hh would have light hair.hh would have light hair.

VocabularyVocabulary

HomoHomozygous (purebred): HH or hh; homo zygous (purebred): HH or hh; homo means same.means same.

HeteroHeterozygous (hybrid): Hh; hetero means zygous (hybrid): Hh; hetero means differentdifferent

Genotype: the possible alleles inherited from Genotype: the possible alleles inherited from the parentsthe parents Ex: HH, Hh, or hhEx: HH, Hh, or hh

Phenotype: the physical/outward expression of Phenotype: the physical/outward expression of the alleles.the alleles. Ex: dark hair or light hairEx: dark hair or light hair

Punnett SquaresPunnett Squares

A diagram used to identify possible A diagram used to identify possible combinations of dominant and recessive combinations of dominant and recessive alleles in offspring.alleles in offspring.

1.1. Divide square in 4 sectionsDivide square in 4 sections2.2. Write the alleles of 1 parent across the top (1 Write the alleles of 1 parent across the top (1

per square)per square)3.3. Write the alleles of the other parent down the Write the alleles of the other parent down the

side (1 per square)side (1 per square)4.4. In each box, combine the alleles of one parent In each box, combine the alleles of one parent

with the other parent.with the other parent.

Punnett Square ExamplePunnett Square Example

Black guinea pig Black guinea pig (dominant (dominant BB) BB) crossed with brown crossed with brown guinea pig (recessive guinea pig (recessive bb).bb).

Provides probability, Provides probability, not definite genotype.not definite genotype.

100% Bb100% Bb 100% black100% black

BbBb BbBb

BbBb BbBb

B B

b

b

Example 2Example 2 Two heterozygous purple Two heterozygous purple

plants, where purple is plants, where purple is dominant to white.dominant to white.

HETEROZYGOUS is the HETEROZYGOUS is the key word that lets you key word that lets you know one large, one know one large, one small letter small letter Pp Pp

Genotype:Genotype: PP = 25%PP = 25% Pp = 50 %Pp = 50 % pp = 25%pp = 25%

Phenotype:Phenotype: Purple = 75%Purple = 75% White = 25%White = 25%

PPPP PpPp

PpPp pppp

P p

p

P