Chapter Three Section Assessment Questions sections 2 and 3
Section 2 page 89
1.a. Probability is a number that predicts the chance of an event happening.
b. If you know the parent’s alleles for a trait you also know the genotype of each parent. After drawing a
Punnett Square the allele for each parent’s gene is place in the appropriate spot outside of the Punnett Square.
The male’s allele’s are placed across the top, one over each box of the Punnett Square; the female’s are placed
down the left side, again one adjacent to each box. The probable offspring combinations are then combined in
each of the four squares.
c. The recessive allele is wrinkled seed shape and the dominant allele is round seed shape. Because the male or
female plant is not specified either genotype can be placed in the male’s position or the female’s position; this
will not affect the probable outcome established by the Punnett Square. The probability of wrinkled seeds will
be 2 out of 4 or 50%.
R
r
r
r
Rr
rr
Rr
rr
2.a. Genotype is the allele combination of genes. It is represented by a combination of capital and lowercase
letters. Phenotype is the physical appearance of the trait as established by the organism’s genotype. If you used
the last question as an example the two lowercase r’s (rr) would be the genotype of a pea plant that would have
the phenotype of wrinkled seeds.
b. If we again use the above example we see the genotype of Rr which tells us that the plant would have a
phenotype of round seeds because the dominant allele is present (R). Although the combination of RR is not
possible given the parents in that question the genotype RR would also produce a plant with round seeds as the
phenotype.
c. TT or Tt
3.a. Codominance is an inheritance pattern in which neither trait is recessive. If the allele for the trait is present
it will be present in the organism’s phenotype. An example would be the feathers of the chicken on page 89. In
this situation even though each chicken is homozygous for the trait of feather color, one white and one black,
the offspring will have a blended phenotype of white and black feathers.
b. If the chicken’s genotype is FB FW then the phenotype will be a combination of white and blended flowers.
Math Practice
4 and 5. Tall 13/25 or 52% short 12/25 or 48%
Chapter 3 Section 3
1.a. Sutton’s observations showed that the grasshopper’s body cells had twice as many chromosomes as its sex
cells.
b. When the fertilization occurs the number of chromosomes in the fertilized egg goes back up to 24
chromosomes.
c. Sutton’s observations support the chromosome theory of heredity because it shows the reason that each
offspring receives exactly half of its genetic material from each parent.
2.a. Meiosis is the formation of sex cells that have exactly half of the chromosomes that the body cells have.
b. Meiosis I resembles the stages of the cell cycle except during meiosis the except that the chromosomes don’t
split at the centromere resulting in each of the new cells having the same number of chromosomes as the parent
cell but in different combinations of alleles. During Meiosis II the steps again resemble the cell cycle except the
chromosomes don’t replicate at the beginning so that the end result is four cells with half the number of
chromosomes as the parent cell.
c. Sex cells don’t normally receive both chromosomes from a pair because the chromosome is not separated into
the sister chromatids until Meiosis II.
3.a. Genes are arranged like beads on a string. The individual bead represents the gene while the chromosome
represents the string of beads.
b. The order of genes on chromosome pairs stays the same however the alleles represented by the gene maybe
the dominant or recessive form of the gene.