Bell Work

1
Bell Work

• In pigeons, the allele B produces ash-red
  feathers. The allele b produces blue feathers.
  The B allele is dominant to the b allele.
• A pigeon with genotype Bb is crossed with a
  pigeon with genotype bb. What percent of the
  offspring are expected to have ash-red feathers?
• Justify your answer. You do not need to copy the
  question.

2
Dihybrid Crossa cross that shows the possible
offspring for two traits
Coat Texture R Rough r Smooth
Fur Color B Black b White
In this example, we will cross a heterozygous
individual with another heterozygous individual.
Their genotypes will be
BbRr x BbRr
3
Dihybrid Cross

• BbRr x BbRr
• Step 1 Find ALL possible gametes that can be
  made from each parent.
• Remember, each gamete must have one B and one R.

4
Dihybrid Cross

• BbRr x BbRr
• Possible gametes
• BR
• Br
• bR
• br

Step 2 Arrange all possible gametes for one
parent on the top of your Punnett Square and the
other parent on the side
5
Dihybrid Crossesa cross that shows the possible
offspring for two traits
BbRr x BbRr
Fur Color B Black b White
Coat Texture R Rough r Smooth
Step 3 Fill in the Punnett Square (find the
possible genotypes of the offspring)
6
Dihybrid Crossesa cross that shows the possible
offspring for two traits
BbRr x BbRr
Fur Color B Black b White
Coat Texture R Rough r Smooth
7
How many of the offspring would have a black,
rough coat? How many of the offspring would have
a black, smooth coat? How many of the offspring
would have a white, rough coat? How many of the
offspring would have a white, smooth coat?
Fur Color B Black b White
Coat Texture R Rough r Smooth
8
How many of the offspring would have black, rough
coat? How many of the offspring would have a
black, smooth coat? How many of the offspring
would have a white, rough coat? How many of the
offspring would have a white, smooth coat?
Phenotypic Ratio 9331
Fur Color B Black b White
Coat Texture R Rough r Smooth
9
Practice in your notes

• In pea plants, yellow seeds (Y) are dominant over
  green seeds (y), and rounded peas (R) are
  dominant over wrinkled peas (r).
• Cross (what do you have to draw?) a plant that is
  heterozygous for both traits with a plant that is
  homozygous recessive for both traits. Draw a
  Punnett square to show all possible offspring,
  and determine the genotypic and phenotypic ratios.

10
YyRr X yyrr
11
More Practice

• Work on the SpongeBob Dihybrid Cross worksheet.
  Turn it in at the end of class.

12
Mendelian Genetics ReviewGet with a
partner.Walk around the room and answer the
review questions with your partner.If you get
all the answers correct, you will get 1 extra
point on your quiz!
13

• What is a genotype? The genes (letters) that
  determine a phenotype
• A man is heterozygous for a trait. Will the
  dominant or recessive trait be expressed? Why?
  Dominant because dominant covers recessive
• What does it mean if a woman is homozygous
  recessive? Using the letter B, write her
  genotype. Both of her genes are recessive (bb)
• What is a phenotype? The physical characteristics
  of an organism

14

1. Long eyelashes (E) are dominant to short
   eyelashes (e). A heterozygous woman is crossed
   with a homozygous man. What is the percent chance
   their offspring will have short eyelashes? (Must
   draw a Punnett square) 0 (see board)
2. The offspring of two parents has a 100 chance of
   being homozygous recessive for blue eyes. If this
   is the case, what must the genotype be for both
   parents? bb
3. What does homozygous mean? Write an example.
   Homozygous means the same letters are present Ex.
   BB or bb

15

• What does heterozygous mean? Write an example.
  That the genes are different Ex. Bb
• What does it mean if a gene is expressed? The
  phenotype for that gene is seen (dominant)
• What does a dihybrid cross show? Genetic
  possibilities for 2 traits
• What was Mendels predicted genotypic ratio for a
  typical dihybrid cross? 9331
• Hazel eyes (H) are dominant to green eyes (h). A
  homozygous dominant man is crossed with a
  homozygous woman. What is the percent chance
  their offspring will have green eyes? (Must draw
  a Punnett square!) 100 (see board)