Albinism is a disorder caused by a recessive allele.About 1 out of every 75 people carry the albino allele.

• Let’s say a mom & dad with normal pigmentation have 4 children.

• 3 children have normal pigmentation, but 1 child is albino.

Where did the albino trait come from?

What is the genotype of the albino child?

What are the genotypes of the parents?

The albino child’s genotype must be aa.

• Mom & dad are both heterozygous, Aa.

• The parents have normal pigmentation, but they are still carriers of the albino allele.

What are the phenotypic ratios?

What are the genotypic ratios?

Have you noticed?

• When both parents are heterozygous, there’s a phenotypic ratio of 3:1 in the offspring.

75% green 25% yellow

3:1 phenotypic ratio

75% purple25% white

Monohybrid Crosses

• So far, we have only followed the inheritance of 1 trait at a time.

Dihybrid Crosses

• We can also follow the inheritance of 2 traits at a time.

In this punnett square, we’re following 2 traits – seed

color & seed shape.

Remember – The Law of Independent Assortment

• The presence of 1 specific allele for a trait has no impact on the presence of a specific allele for the other trait.

You can have blue eyes, no matter what color hair you have.

You can have red hair, no matter what color

eyes you have.

How to Setup a Dihybrid Cross

• You’re following the inheritance of 2 traits at a time.– Tail Length: S = short, s = long– Fur Color: B = brown, b = white

• Your punnett square should have 16 squares.

• The possible gametes (egg & sperm) of the parents go on the outside of the punnett square.

Easy Method for Finding Gametes – FOILfirst, outside, inside, last

• Dad’s genotype is RrYy.

What are the possible sperm cells that can be produced?

• First – RY• Outside – Ry• Inside – rY • Last – rY

Finding the Possible Gametes

• Mom is heterozygous for both traits, SsBb. Her egg cells could contain:– SB, short tail & brown fur– Sb, short tail & white fur– sB, long tail & brown fur– sb, long tail & white fur

• Dad has the same genotype, so the results are the same for his sperm cells.

In guinea pig fur… B = black, b = brown, S = short, s = long

Let’s say both parents are heterozygous for both traits.

BbSs x BbSs

If both parents are heterozygous for both

traits, you will see a 9:3:3:1 phenotypic ratio.

BbSs x BbSs

There are 4 possible sperm & egg gametes: YR, Yr, yR, yr

This means there are 16 possible ways the gametes

can combine to make offspring.

These combinations produce 4 distinct phenotypes in a 9:3:3:1 ratio, when both

parents are heterozygous for both traits.

Both parents are heterozygous for both traits, YyRr.