Download Predicting Allele Combinations

REVIEW
CALIFORNIA CONTENT
STANDARD 2.g
Predicting Allele Combinations
STANDARD Students know how to predict possible combinations of alleles in a zygote
from the genetic makeup of the parents.
Read the summary and answer the questions on the next page.
During meiosis, cells go through two nuclear divisions to form genetically unique haploid
cells. If parental genotypes are known for a trait, you can use mathematics to predict how
likely it is that one of the offspring will inherit a specific allele.
STANDARD SET 2
Normal body cells are diploid: They contain two sets of homologous chromosomes.
Homologous chromosomes are chromosomes that are similar but not identical. They contain
the same sequence of genes, but each chromosome may have different alleles, or versions of
those genes. In organisms that reproduce sexually, individuals use the process of meiosis to
produce egg and sperm cells. Egg and sperm cells are haploid: They contain only one copy
of each chromosome.
During meiosis, homologous chromosomes are separated and placed in different sex cells,
or gametes. Each gamete contains only one allele for any particular trait. Because a diploid
organism has two alleles for each trait, the mathematics of probability states that one allele
will be found in half of the organism’s gametes and the other allele will be in the other half.
If the two alleles are the same, they are described as homozygous. If the two alleles are
different, they are described as heterozygous. If the makeup of an individual’s alleles for a
trait, its genotype, is known, we can predict how likely it will be that those alleles will appear
in its offspring.
Predicting Zygote Genotypes
Punnett squares are a convenient method of calculating genetic probabilities. The square is
a grid of boxes that represent all of the possible genotypes the progeny of a particular cross
may have. Parental alleles are placed by each box on the axes of the grid. In the diagram,
both parents have one R allele and one r allele. To determine the possible allele
combinations of their offspring, alleles from each parent are placed inside their
corresponding grid boxes.
Because chromosome segregation and fertilization are random, the
combinations of alleles in each box are equally likely. The number of squares
R
with each genetic combination corresponds to the ratio of that genotypes in the
offspring. In this example, one-fourth of the offspring would have RR; one-half of
r
the offspring would have Rr; and the remaining one-fourth would have rr.
Parent 2
Copyright © by McDougal Littell, a division of Houghton Mifflin Company
Allele Separation in Parents
Parent 1
R
r
RR
rR
Rr
rr
STANDARDS REVIEW AND PRACTICE 31
PRACTICE
CALIFORNIA CONTENT
STANDARD 2.g
Predicting Allele Combinations
DIRECTIONS: Choose the letter of the best answer.
(D) is dominant to the gene for
light wings (d). If a homozygous
dark moth (DD) is crossed with
a heterozygous dark moth (Dd),
which combination of alleles will
be most common among the
offspring?
T
t
T
TT
tT
t
X
tt
4
A DD
In the cross above, which
combination of alleles will be
found in the box marked X?
B Dd
A TT
C dd
B Tt
D DD and Dd are equally common.
C tt
2 The letters inside the grid boxes of
D TtTt
a Punnett square represent the
A genotypes of the parents.
5 In a Punnett square, the letters on
B phenotypes of the parents.
the outside of the grid represent
the
C genotypes of the offspring.
A parent’s alleles.
D phenotypes of the offspring.
B parent’s traits.
C offspring’s alleles.
3 The gene for brown eyes (B) is
dominant to the gene for blue eyes
(b). If a pair of individuals that are
heterozygous for brown eyes (Bb)
have children, what proportion
of their children will have at least
one B?
A 100%
B 75%
C 50%
D 25%
32 STANDARDS REVIEW AND PRACTICE
D offspring’s traits.
Copyright © by McDougal Littell, a division of Houghton Mifflin Company
STANDARD SET 2
1 In moths, the gene for dark wings