Heredity and Genetics*
Objectives

Solve problems of classical Mendelian
inheritance that involve:
o One factor crosses with complete
dominance
o One factor crosses with incomplete
dominance
o Two factor crosses with complete
dominance
o One factor crosses with sex-linkage
 Define the terms autosomal, sex-linked,
heterozygous, homozygous, hemizygous
Genetics Review
Last week in lab we learned that every
characteristic or trait has two forms, or two
alleles. Alleles are symbolized by small and
capital letters (e.g. B and b). Three
genotypes are possible: BB, Bb, and bb.
BB is called the homozygous dominant
phenotype while bb is the homozygous
recessive genotype. Bb is a heterozygous
genotype.
In producing the phenotype, or physical
expression of a genotype, dominant genes
mask the expression of non-dominant or
recessive genes. Dominant genes
symbolized with a capitalized alphabet
letter, while recessive genes are symbolized
by a lowercase letter. Thus, if B were the
symbol for brown eyes and b were the
symbol for blue eyes, the genotype Bb
would produce a phenotype of brown eyes.
The chromosome set of an organism can
be divided into two
subsets. The chromosomes involved with
sex determination ( X and Y) are
Name:
called the sex chromosomes. All the other
chromosomes are called autosomes. We will
begin working with genes on the autosomes
for the first part of this laboratory.
One Factor Crosses
Phenotypes from genotypes
For each of the following traits,
determine the phenotype from the genotype.
If the genotypes are not given, select letters
from the alphabet to symbolize genes and
then define the phenotypes from the three
possible genotypes.
Example:
Dimples (hollow marks on the face) are
inherited in an autosomal dominant mode.
GENOTYPE PHENOTYPE
DD
dimples
Dd
dimples
dd
no dimples
Widow’s peak hairline is dominant to
straight hairline.
GENOTYPE
PHENOTYPE
WW
Ww
ww
A bent little finger (versus “normal”
finger) is inherited in an autosomal
dominant mode.
GENOTYPE
PHENOTYPE
*Adapted from Kathleen Duncan, Foothill College, California
Bios 140 Biology Lab Manual
pg. 1
Purple flower color (versus white flower
color) in pea plants is inherited in an
autosomal dominant mode.
GENOTYPE
HhYy
Parent:
Gametes:
PHENOTYPE
Autosomal Dominant Inheritance
Forming Gametes
Gametes are haploid cells formed in
meiosis. In meiosis, the genes carried for a
characteristic segregate (separate) into
different gametes. Only one copy of each
gene goes into a gamete. For a two factor
parent (e.g. AaBb), where there are four
genes separating in meiosis, remember to
use the F.O.I.L. method to segregate them
up correctly. F.O.I.L stands for: Firsts,
Outers, Inners, Lasts.
genotypes, determine the kind of gametes
that can be produced.
Parent:
Genetics problems usually present you
with a set of parents and you are asked to
calculate the possible offspring from this
cross. It is customary to use a Punnett square
to bring together the gametes of one parent
with those of the other parent. The offspring
of any cross should have two genes per
characteristic, just like the parents.
The results of a cross can be expressed
as ratios, percentages, or fractions.
Example:
Cross two heterozygous purple flowers. Purple
flowers is dominant over white flowers.
P = purple flowers
p = white flowers
Aa
Parents:
Pp
x
Pp
Gametes:
P
Gametes:
Parent:
P
GG
Punnett
Square:
Gametes:
Parent:
p
bb
P
p
P
PP
Pp
p
Pp
pp
Gametes:
Parent:
Rr
Phenotypic ratio: 3 purple: 1 white
OR
75% purple, 25% white
OR
3/4 purple, 1/4 white
Gametes:
Parent:
Genotypic ratio: 1PP: 2Pp:1pp
AaBB
Gametes:
Bios 140 Biology Lab Manual
pg. 2
p
Solve the following genetics problems and
fill in the blanks.
In pea plants, tallness is dominant over
shortness.
1. Using the symbols T and t, define the
possible genotypes and corresponding
phenotypes.
GENOTYPE
Punnett Square:
a. What proportion of the offspring are tall?
%
b. What fraction of the offspring are
heterozygous?
c. What is the genotype of a short plant?
PHENOTYPE
d. If you cross a homozygous dominant
parent with a homozygous recessive parent,
what percentage of the offspring are
heterozygous? (Show all your work)
2. If you cross a short plant with a
heterozygous tall plant, what is the
genotypic ratio and the phenotypic ratio of
the offspring?
Parental phenotypes:
x
Parental genotypes:
x
Gametes:
%
Incomplete Dominance
Punnett Square:
There are some traits where neither
allele is dominant. In the heterozygote (e.g.
Rr), an intermediate phenotype results. This
type of inheritance is called incomplete
dominance.
Genotypic ratio:
Phenotypic ratio:
Solve the following genetics problems and
fill in the blanks.
3. Cross two heterozygous tall plants and
answer the questions that follow.
In snapdragon plants, one allele
produces red color and the other produces
no pigment (white).
1. Define symbols for the diploid genotypes:
Parental phenotypes:
x
Parental genotypes:
x
Red flowers: Pink flowers:
White flowers:
Gametes:
Bios 140 Biology Lab Manual
pg. 3
2. Cross a pink and a white flowered plant
and answer the following questions.
Parental phenotypes:
x
Parental genotypes:
x
characteristics. The main difference in these
crosses is how the gametes are made. Use
F.O.I.L!
 Solve the following genetics problems
and fill in the blanks.
Gametes:
In pea plants, tallness is completely
dominant over shortness and red flower
color is completely dominant over white
flower color.
Punnett Square:
1. Using the symbols T, t, R, and r, define
the genes and their phenotypes
a.
What is the expected fraction of redflowered plants?
b. What percentage of the offspring have
white flowers?
1. Cross two pink flowered plants and
determine the phenotypic and
genotypic ratios.
Parental phenotypes:
x
Parental genotypes:
x
GENE
PHENOTYPE
T=
t=
R=
r=
2. What are the genotypes and phenotypes
for a cross between two parents that are
both heterozygous for plant height and
flower color?
Parental
phenotypes:
x
Gametes:
Parental genotypes:
Punnett Square:
Gametes:
x
and
Punnett Square:
Genotypic ratio:
Phenotypic ratio:
Two factor Crosses
Two factor crosses follow the results of
crossing two parents with respect to two
continued
Genotypic ratio:
Bios 140 Biology Lab Manual
pg. 4
3. What is the genotype of a carrier?
Phenotypic ratio:
4. Cross a color-blind female with a male
with normal vision. Calculate the
percentages of offspring shown below.
a. What is the genotype of a short plant
with white flowers?
b. What fraction of the plants are tall with
red flowers?
Parental
phenotypes:
x
Parental genotypes:
x
Gametes:
c. What percentage of the plants are short
with red flowers?
Punnett Square:
Sex-linked traits
The X and the Y chromosomes are
associated with gender in humans. The
presence or absence of a Y chromosome
determines maleness in humans. Genes that
are carried on the X chromosome are said to
be X-linked. For these traits, women may
be homozygous or heterozygous, but men
can only be hemizygous. Hemizygous
refers to the condition where only one gene
is carried for a trait.
 Solve the following genetics problems
and fill in the blanks.
% of color blind children
% color blind males
% color blind females
1. Colorblindness is a sex-linked recessive
trait. Using the symbols XB, Xb, and Y, list
the five possible genotypes and their
corresponding phenotypes.
GENOTYPE
PHENOTYPE
(Vision and gender)
2. What is the phenotype of a carrier?
Bios 140 Biology Lab Manual
pg. 5
Further Questions
Inheritance
1. Huntington’s Disease, also called Huntington’s Chorea, is an autosomal dominant disease
that causes neurological problems that result in a gradual reduction of motor and mental
functioning, eventually leading to death. Symptoms usually begin around age 35. If a
person’s mother had this disease, what are the chances that the person would get the disease?
a. First write down any assumptions you are making about the mother and the father and their
genotypes (information is missing here so simply state some genotypes consistent with the
facts):
b. Write out the parental genotypes and determine the odds of a child exhibiting the disease.
Don’t forget that the gene causing the disease is dominant, not recessive (normal).
2. In cows, red fur (R) is incompletely dominant over white fur (r). The heterozygote is called a
roan cow (it’s actually pink). What are the genotypic and phenotypic ratios for a cross between a
red and a roan cow?
Bios 140 Biology Lab Manual
pg. 6
3. Freckles on your face and the ability to roll your tongue are both dominant traits. No freckles
and inability to roll the tongue are recessive traits. What kinds of offspring and in what
proportions could be expected from a person homozygous dominant for freckle and heterozygous
for tongue rolling and a person who is heterozygous for freckles and can’t roll their tongue?
Show all your work for full credit.
4. Hemophilia is an X-linked recessive disease where a person’s blood does not form clots after
an injury. A woman who is a carrier of hemophilia has children by a normal man. Set up this
cross and answer the following questions.
a. What percentage of their children are normal?
b. What percentage of their children are hemophiliac boys?
Bios 140 Biology Lab Manual
pg. 7