Punnett Squares

• MENDEL LOOKED AT • Monohybrid crosses: ONE TRAIT• Dihybrid crosses: TWO TRAITS AT THE

SAME TIME

• MENDEL WANTED TO SEE IF TRAITS ARE INHERITED TOGETHER OR INDEPENDENTLY

PUNNETT SQUARES

• A QUICK WAY TO FIND THE GENOTYPES IN UPCOMING GENERATIONS

• 1ST DRAW A BIG SQUARE AND DIVIDE IT IN 4’S

PUNNETT SQUARES

CROSS TT X Tt

CONT’D

T T X T tTT TT

TT

tt

TT TT TT TT

TT tt TT tt

Practice with Punnett Squares

1.  A  round seeded plant (RR) is crossed with a wrinkle seeded plant (rr).  What are the phenotypes of the offspring?

Offspring phenotypes:all would be round

Punnett square practice

2.  If purple flowers are dominant to white flowers, two heterozygous purple flowered pea plants are crossed.  What are the phenotypes of their offspring and in what proportion? Use F for allele letter.

Ratio of phenotypes 3:4 purple1:4 white

Ratio of genotypes1:4 homozygous dominant2:4 heterozygous1:4 homozygous recessive

Punnett square practice

3.  A plant with green seeds (yy) is crossed with a heterozygous plant (Yy).  List the following:

• create a table to show a Punnett square

• ratio of phenotypes

• ratio of genotypes

In dragons...

Wings are a dominant trait (W), but some dragons are born wingless (w).

1. What are the chances (ratio or percent) that two heterozygous dragons have a baby that is wingless?

2. If a wingless dragon is crossed with one that is heterozygous, how many of its offspring will also be wingless?

Punnett square practice…

Why does the Punnett square work?

It all goes back to meiosis.. each side represents a sperm or egg.  The boxes simply give you the statistical chance that a certain sperm will fertilize a certain egg. 

Consider a pea plant that is both round and tall = RrTt

When this plant's cells go through MEIOSIS, the alleles segregate - each sperm receives a random combination...

These possible combinations include:

RT, Rt, rT, rt

Mendel's Law of Independent Assortment – Illustrated by the DIHYBRID cross

The second law describes the outcome of dihybrid (two character) crosses, or hybrid crosses involving more than one trait.

A dihybrid is an individual that is a double heterozygote (e.g., with the genotype RrYy - round seed, yellow seed).

What are the gametes that can be produced by this individual?

DIHYBRID CROSS

• MENDEL TOOK TWO TRUE BREEDING PLANTS FOR 2 DIFFERENT TRAITS (ROUND/WRINKLED SEEDS and YELLOW/GREEN SEEDS)

• 1ST GENERATION• WHAT WOULD THE OFFSPRING PHENOTYPES

BE IF HE CROSSED A TRUE BREEDING ROUND W/ A TRUE BREEDING WRINKLED (ROUND IS DOMINANT)

ALL THE OFFSPRING ARE ROUNDALL THE OFFSPRING ARE ROUND

DIHYBRID CROSS – 1ST

GENERATION CONT’D

• SO WHAT DO YOU THINK HAPPENED WHEN HE CROSSED TRUE BREEDING ROUND/YELLOW (RRYY) SEEDS WITH TRUE BREEDING WRINKLED/GREEN SEEDS (rryy)

ALL THE F1 WERE ROUND AND YELLOWALL THE F1 WERE ROUND AND YELLOW

DIHYBRID CROSS – 2ND

GENERATION• TOOK THE F1 PLANTS AND BRED THEM

TOGETHER (PHENOTYPE WAS ROUND/YELLOW X ROUND/YELLOW)

• 2ND GENERATION• FOUND ROUND/YELLOW - 9• FOUND ROUND/GREEN - 3• FOUND WRINKLED/YELLOW - 3• FOUND WRINKLED/GREEN - 1 ( 9 : 3 : 3 : 1 RATIO)

EXPLANATION OF 2ND

GENERATION• MENDEL CAME UP W/ 2ND LAW – THE

LAW OF INDEPENDENT ASSORTMENT• GENES FOR DIFFERENT TRAITS ARE

INHERITED INDEPENDENTLY FROM EACH OTHER

• THIS IS WHY MENDEL FOUND ALL THE DIFFERENT COMBONATIONS OF TRAITS

DIHYBRID CROSSES

• A LITTLE DIFFERENT

• HhGg X HhGg

• MUST FIND OUT ALL THE POSSIBLE ALLELE COMBINATIONS

• Must combine one g with one h

• huge is dominant to small, green is dominant to blue

H h G g X H h G g1. HG1. HG

2. Hg2. Hg

3. hG3. hG

4. hg4. hg

FOIL – FIRST, OUTSIDE, INSIDE, LASTFOIL – FIRST, OUTSIDE, INSIDE, LAST

BOTH PARENTS ARE THE SAME BOTH PARENTS ARE THE SAME

NOW LET’S DO A DIHYBRID

CROSS

H h G g X H h G gHGHG HgHg hGhG hghg

HGHG

HgHg

hGhG

hghg

HHGGHHGG HHGgHHGg HhGGHhGG HhGgHhGg

HHGgHHGg HHggHHgg HhGgHhGg HhggHhgg

HhGGHhGG HhGgHhGg hhGGhhGG hhGghhGg

HhGgHhGg HhggHhgg hhGghhGg hhgghhgg

WHAT ARE THE PHENOTYPIC

RATIO’S?

H h G g X H h G g (huge/green X huge/green)HGHG HgHg hGhG hghg

HGHG

HgHg

hGhG

hghg

HHGGHHGG HHGgHHGg HhGGHhGG HhGgHhGg

HHGgHHGg HHggHHgg HhGgHhGg HhggHhgg

HhGGHhGG HhGgHhGg hhGGhhGG hhGghhGg

HhGgHhGg HhggHhgg hhGghhGg hhgghhgg

•huge/green = 9•small/green = 3•huge/blue = 3•small/blue = 1

PROBABILITY

• WILL REAL LIFE FOLLOW THE RESULTS FROM A PUNNETT SQUARE?

• NO!!!!!! – A PUNNETT SQUARE ONLY SHOWS WHAT WILL PROBABLY OCCUR

• IT’S A LOT LIKE FLIPPING A COIN – YOU CAN ESTIMATE YOUR CHANCES OF GETTING HEADS, BUT REALITY DOESN’T ALWAYS FOLLOW PROBABILITY