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Biology 30
Genes & Heredity
Your biological traits are controlled by genes, which are located on the
chromosomes that are found in every cell of your body. Only one copy of each
gene is on a chromosome. Genes are like books; they may or may not be read
by the chemical machinery of the cell.
Since you inherited half of your chromosomes from your mother and the other
half from your father, your traits are a result of the interactions of the genes
from both parents. Although you contain half the genes form each parent, your
genes and traits are uniquely your own.
There are over eight million
combinations possible from the 23 chromosomes you inherit from your mom and
23 you inherit from your dad. The passing of traits from parents to offspring is
called heredity.
Pioneer of Genetics: Gregor Mendel



People have been puzzled about heredity for years. Little progress in
solving the puzzle was made until Gregor Mendel, an Austrian monk,
conducted some simple but significant experiments in his monastery
garden a century ago (1822-1884)
Mendel’s work with garden peas not only explained the mechanism of
gene inheritance for plants, but also provided a basis for understanding
heredity
Why did Mendel work with the garden pea?
1)
2)

Mendel fertilized plants by cross-pollination rather than
self-fertilization. How did he do this?
Mendel’s Experiments
-
Crossed the pollen from a plant that produced round seeds with the
eggs of the one that produced wrinkled seeds, the offspring were
______________
Crossed the pollen from a plant that produced wrinkled seeds with
the eggs of the one that produced round seeds, the offspring were
______________
He repeated the procedure for other characteristics (eg. flower
colour). He discovered that one trait dominated another, whether
the sex cell came from a female or a male part of the plant.
Mendel reasoned that things called _____________ controlled the
traits of a plant. These factors were later termed _____________.
Dominant allele –
-
Recessive allele –
-
-
-
Mendel’s Laws of Heredity
Law of Unit Characters
Inheritance is governed by ___________ that exist in the individual and
are passed on to offspring. These factors (genes) occur in pairs, one
gene comes from the female and one gene comes from the male. The
alternate forms of the same genes are known as alleles.
Law of Dominance
One factor, or gene, masks the effect of another. This process is known
as the __________________________________________. The dominant
expression is seen and the recessive gene is not seen (remains hidden).
* Mendel’s First Law (Law of Segregation)
A pair of factors (genes) _______________ (separate), during the
formation of sex cells (meiosis). As a result, each parent can only
contribute one member (allele) of a pair of genes to their offspring.
Genetic Terms
Use Pages 589-560 to define the following terms:
Genotype –
What two genotypes could a tall-stem pea plant have?
Phenotype –
Homozygous (“purebred”) –
Heterozygous –
Alleles –
Hybrid -
Monohybrid Cross
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The combining of single contrasting traits:
eg. Crossing a tall plant (TT) with a short plant (tt)
A special chart called a _____________ square, helps geneticists organize
the results of a cross between the sex cells of two individuals
From a Punnett square, we can predict the genotypes & phenotypes of
the offspring
Examples:
Draw a Punnett square for a cross between a heterozygous round-seed plant and
a wrinkled-seed plant.
Phenotypic Ratio:
Genotypic Ratio:
Draw a Punnett square for a cross between two heterozygous round-seed plants.
Phenotypic Ratio:
Genotypic Ratio:
The flowers of a pea plant can be white (ww) or purple (WW/Ww). Draw a
Punnett square for the cross between a heterozygous purple flower with a white
flower.
Phenotypic Ratio:
Genotypic Ratio:
Do Genetics Problems - Single Trait Inheritance WS
Test Cross

A test cross is often performed to determine if a phenotypically dominant
individual is _________________ or __________________.
When would we need to know this?

i.e. A sheep farmer wants to ensure that all of his flock will have white
hair (black wool is brittle & hard to die). He chooses a white ram to mate
with the flock. What if the ram is heterozygous? ____________________

To ensure that the ram is not, a test cross can be performed to ensure
that the ram is homozygous for the white phenotype.

A test cross is always performed between the unknown genotype and a
______________________________ genotype.

Possible outcomes:
1) If 50% of the offspring are black and the other half white, the
unknown genotype must have been _______________________.
2) If 100% of the offspring are white, then the unknown genotype must
be _____________________________________.
Gene Interactions

There are three main types of gene interactions:
1) Multiple Alleles
 At this point, you know that a maximum of three genotypes can be
produced by one gene with two alleles. Two of the genotypes will be
__________________ for one of the two alleles, and the third genotype
will be ________________.
 There are many traits that are controlled by more than just two alleles.
 For example, eye colour in Drosophila is controlled by four possible alleles.
 The phenotypes and dominance hierarchy of Drosophila eye colour is:
wild type (red) > apricot > honey > white

When dealing with multiple alleles, it is no longer necessary to use upper
& lower case letters  ___________________ & ___________________
are used.
phenotype
wild type
apricot
honey
white
alleles
E1
E2
E3
E4
i.e. Predict the genotypes and phenotypes of the F1 generation from the
mating of wild type (E1E4) with apricot (E2E3).
 Inheritance in human blood groups is determined by one single gene, “I,”
which has three different alleles, only two of which can occur at the locus
at once. The alleles are responsible for producing antigens on the surface
of the red blood cells, which determines the blood group.
 Alleles A and B are co-dominant so that when they are both present, both
A and B antigens are produced. Both A and B are dominant to O.
i.e. Use the following information to answer the question below.
Blood Types (A, B, O, AB)
Allele IA – formation of blood factor A (antigen A)
Allele IB – formation of blood factor B (antigen B)
Allele I – no factors result
Genotype
IAIA or IAi
IBIB or IBi
ii
IAIB
Blood Type
A
B
O
AB
* the Rhesus factor will also play a role in blood typing (recall from Biology 20). Rh+ is dominant
over Rh- (we will cover this later in dihybrid crosses)
A mother with blood type A has a child with Blood type O. The father is blood
type B. Indicate the genotypes of the parents.
Do Questions #11,13,14,15,16,&17 on Page 606
2) Incomplete Dominance

In some heterozygotes, both alleles of a gene pair are expressed in the
phenotype. These alleles are said to be equally dominant. This lack of
dominance is known as ___________________________________.
i.e. The flower colour of the four o’clock plant (Mirabilis jalapa)
Red flower

White flower
i.e. Sickle cell anemia in humans.
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Caused by a gene mutation in the gene that directs the synthesis of the
________________ protein.
Hemoglobin is a molecule that carries oxygen in our blood.
When this happens, blood oxygen levels become lowered which causes
the hemoglobin to form needle like crystals inside the red blood cells.
This distorts the red blood shapes into “sickle” cells.
Let’s look at an example of how sickle cell anemia is an example of
incomplete dominance:
3) Codominance
 a form of incomplete dominance where both alleles are ___________

expressed.
The expression of one allele does not mask the expression of another.
i.e. A red bull crossed with a white cow = ____________ calf
The calf has intermingled white & red hair.
Sex Linked Inheritance

Sex linked traits are traits that are controlled by genes
on either the _____ or _____ chromosome.
So far, what do you know about sex chromosomes?

In addition to their role in determining sex, the sex chromosomes,
especially X chromosomes, have genes for many characters unrelated to
sex. We call these sex-linked alleles.

Female cells can differ from male cells in three ways:
1) Female cells contain _____ X chromosomes and males contain
____________________.
2) Female cells show dark spots of chromatin (called________________)
during interphase, male cells do not. Since females have two X
chromosomes, and males only have one, in every female cell one of the
two X chromosomes is ________________. This inactive X chromosome
is condensed tightly into a structure known as a ___________________.
3) The Y chromosome carries ____________ genes.
- There are very few genes on the Y chromosome that are common on
the X chromosome, and because of that, _________________ crossing
over may occur between an X and a Y.
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A male embryo does not differ from a female embryo until the
________________ week of pregnancy.
At this point, the “________________________” (TDF) gene on the
_____ chromosome is activated.
The TDF gene initiates the production of a protein that stimulates the
___________ to begin secreting male hormones.
i.e. Calico cats (Page 604)
Male cats tent to be black (XBY) or orange (X0Y). Female cats can be black
(XBXB), orange (X0X0) or calico (XBX0) – a mixture between black and orange.
Very few male cats can be calico – those who do carry a hidden X
chromosome, and are likely sterile.

We can also perform monohybrid crosses between sex linked traits.
i.e. Eye colour in Drosophila
-
Red eye color (R) is dominant to white (r).
Eye color is carried on the X chromosome.
Homozygous dominant female
Heterozygous female
Homozygous recessive female
XRXR (red)
XRXr (red)
XrXr (white)
Dominant male
Recessive male
XRY (red)
XrY (white)
Draw a Punnett square for a cross between a heterozygous female
Drosophila with a recessive male Drosophila. Calculate the phenotypic &
genotypic ratios.
Is it possible to get a white eyed female from crossing a white eyed
female with a red eyed male? Explain.
Pedigrees
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a chart or register showing a line of ancestors
circles represent females, squares represent males, solid circles & squares
represent those who have the trait being studied
horizontal lines between circles and squares represent
__________________
a vertical line joins parents and _____________________
pedigrees are often used to study sex-linked traits such as color-blindness
and hemophilia
PEDIGREE SUMMARY

this will help you determine the mode of inheritance of a trait in a
pedigree.
Autosomal Dominant
 must be in each generation
 affected individuals transmit to minimum ½ of their offspring
 males and females are equally affected
 cannot have carriers, they will be affected!
Autosomal Recessive
 may skip a generation
 affected offspring generally have normal (but heterozygous) parents
 male and female are equally affected
X-linked Dominant
 very likely to be observed in each generation
 females pass on to half of either sex
 no transmission from father to son (only daughters).
 cannot have carriers, they will be affected!
X-linked Recessive
 affect males more than females
 no transmission from father to son
 daughters of males are carriers
 females pass onto ½ sons
 affected females have affected fathers and carrier mothers
Probability
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
Probability is the likelihood of an event happening.
Probability can be expressed by the following formula:
Therefore, when a coin is tossed, there are two possibilities – heads or tails.
What are the chances of getting heads?
What is the probability of two coins being tossed and getting heads?
Coin 1 –
Coin 2 –
Product of the probabilities of two separate events:

The Product Rule –
i.e. What is the probability that a man with type AB blood and a woman with
type AB blood will have a baby girl with type B blood?

The Rule of Independent Events –
Dihybrid crosses

Mendel also studied two separate traits with a single cross by using the
same procedure he had used for studying single traits

the laws of genetics that apply for single-trait inheritance (monohybrid
cross), also apply for two trait inheritance (dihybrid cross)
i.e.
-
Mendel crossed a purebred yellow round pea with a purebred green
wrinkled pea
the pure breeding round is indicated by symbol RR, and the pure
breeding wrinkled is indicated by the recessive alleles rr
the pure breeding yellow is indicated by the symbols YY and the pure
breeding green is indicated by the symbols yy
the genotype for the yellow, round parent is RRYY and the genotype for
the green, wrinkled parent is rryy
P1: Purebred Yellow Round _____ X Purebred Green Wrinkled _____
-
The entire F1 generation is __________ & phenotypically __________
-
Mendel also discovered that when you cross the F1generations with one
another, you get an assortment of genotypes and phenotypes:
P2: Yellow Round _______
X
Yellow Round ________
-
the genotypes are very different, but it is the phenotypic ratio that is
important:
9/16 yellow round
3/16 yellow wrinkled
3/16 green round
1/16 green wrinkled
-
The phenotypic ratio is 9:3:3:1. This is the phenotypic ratio you
will find in all heterozygous dihybrid crosses.
-
This discovery led Mendel to developing his second law:
* Mendel’s Second Law (Law of Independent Assortment)
The two alleles for one gene segregate (assort), independently of the
alleles for other genes during gamete formation.
i.e.
In summer squash, white fruit color is dominant “W” and yellow fruit color is
recessive “w”. Another allele produces disc shaped fruit “S” while its
recessive allele “s” yields sphere-shaped fruit. If a pure breeding white disc
variety is crossed with a homozygous yellow sphere variety, the F1 are all
white disc hybrids. If members of the F1 generation are crossed, what would
be the expected phenotypic ratio in the F2 generation?
Rhesus Factor & Birth
P1 : Female Rh

×
Male Rh+
Baby is Rh+ because father is. Mother’s blood produces antibodies upon
birth, (since blood mixes at birth). First baby is okay.
Second pregnancy- mom’s antibodies can now move across the placenta
and cause baby’s RBC’s to clump (agglutinate) if second baby is also Rh+.
This decreases oxygen delivery in the baby – “blue baby.”
What can be done?

Mom can be given an injection of a drug that inhibits antibody production
immediately after delivery.
What happens if this is undetected?

Baby could be given a blood transfusion while in the womb. Fairly
uncommon.
R – dominant allele (Rh+)
r – recessive allele (Rh-)
Example: A woman homozygous for blood type A and heterozygous for
the rhesus allele, Rh+, has a child with a man with type O blood who is
Rh-. What is the probability that their child will have blood type A,
Rh+?
There are many techniques used in order to produce a specific genotype or
phenotype:
1) Selective Breeding: the crossing of desired traits from plants or
animals to produce offspring with both characteristics.
i.e. Development of canola in Canada
2) Inbreeding: the process by which breeding stock is drawn from a
limited number of individuals possessing desirable phenotypes
3) Hybridization – the blending of two parental traits to produce an
offspring that is different from both parents.
Other Gene Interactions
1) Many of the traits studied by Mendel are controlled by one gene
2) Some traits are regulated by more than one gene; many of your
characteristics are determined by several pairs of independent genes –
called ________________
eg.) skin color, eye color and height are polygenic characteristics
3) One trait controlled by more than two alleles = multiple alleles.
eg.) Drosophilia eye colour, blood types.
4) Genes that interfere with the expression of other genes are called
________________
eg.) The allele B produces a black coat color in dogs; b produces a brown
coat color. A second gene, W prevents the formation of pigment and w
does not prevent color. The genotype wwBb would be black, but the
genotype WwBb would appear white. The W allele masks the effect of
the B color gene. If wwBb is crossed with a WwBb, state the phenotypic
ratio.
Epistatic genes can sometimes be _______________
eg.) The “B” gene on a chromosome produces a pointed ear in horses,
and the “b” gene produces a floppy ear. On another chromosome, the
“A” gene allows ear growth, while “a” prevents it. The genotype “aa” is
lethal, and the horse won’t survive. Give the phenotypic ratios of a cross
between two fully heterozygous pointed-ear horses (BbAa).
5) _________________________________ occurs when two different
genotypes interact to produce a phenotype that neither is capable of
producing by itself
eg.) The allele R produces a rose comb in chickens; the allele P (on a
different chromosome) produces a pea comb. When the R and P alleles
are both present, they combine to produce a walnut comb. The absence
of rose and pea alleles results in an individual with a single comb.
Do Complementary Interaction WS on next page.
Chromosome Mapping and Gene Linkage

Mendel’s Second Law:

Genes that are found on the same chromosome sometimes do not sort
independently of one another and are called ___________ genes.
The sequence of genes on each chromosome pair should match each
other exactly.
Gene linkage reduces the chances for genetic recombination and
variety among the offspring.
Experimental data shows, however, that linked genes do segregate
on a regular basis. How can this happen?
Part of a chromosome holding many genes, may separate and switch
places with the matching part of the other chromosome in the
homologous pair. This is called ___________________.
Remember: Crossing over occurs during _______________ of meiosis.
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The distance between genes located on a chromosome is directly
related to the likelihood in which they will remain “linked” together.
The closer genes are to each other, the ________ likely they will
separate during crossing over – these are called
___________________.
Scientists use crossover frequencies on genes to determine their
positions on chromosomes.
i.e. if the crossover frequency of a gene is 5%, then the two genes
are 5 map units apart.

One map unit is defined as the distance between two points on a
chromosome where a crossover is likely to occur 1 percent of all
meiotic events.
P1 = PpVv × ppvv

Crossover frequency is determined by the following formula:
crossover % = number of recombinations x 100
total number of offspring

In the example above, there are 1000 flies total. Calculate the
crossover % in the fly population.

By using crossover frequencies, we can determine _______________.

Gene maps show the relative positions of genes located on a
chromosome.

Construction involves simultaneously monitoring recombination among
three or more genes that are located close together on the same
chromosome. Some genes do not code for traits that are easily visible
therefore ____________________ are often used.
Usually recessive genes that are easily observed in
offspring and can be used to identify other genes
found on the same chromosome.
Problem 1:
3 genes A, B, C
AB – 12%
CB – 7%
AC – 5%
Problem 2:
AB - 3%
BC - 28%
AC - 31%
Problem 3:
Genes
X
Y
Z
X
Y
Z
10
15
10
5
15
5
-
Do Chromosome Mapping WS
Do Mapping Chromosomes Case Study
Practice Questions
1. Twenty-five percent of the offspring of a certain pair of white sheep are
black. If the white is dominant to black, what was the genotype of the
parents?
2. Suppose two newborn babies were accidentally mixed up at the hospital, and
there was a question of which baby belonged to which parents. From the
following blood type, determine and explain which baby belongs to which
parents:
Baby 1
Baby 2
Mrs. Brown
Mr. Brown
Mrs. Smith
Mr. Smith
type O
type A
type B
type AB
type B
type B
3. In humans, wavy hair is dominant over straight hair. What possible
genotypes and phenotypes can occur in the offspring where one parent is
heterozygous for wavy hair and the other has straight hair?
4. In humans, the sex-linked gene for normal blood clotting, H is dominant to
the gene for hemophilia, h. A woman with normal blood clotting has four
children: a normal son, a hemophiliac son and two normal daughters. The
father has normal blood clotting. What is the probable genotype of each
member of the family?
5. In sheep, crosses between homozygous earless and homozygous normal
always result in an intermediate, small-eared condition. When a series of
matings are made between F1 rams and ewes, 79 small-eared, 42 earless,
and 37 normal progeny result. If one of the F2 small-eared individuals is
mated with a normal partner, what is the chance of getting an abnormal
lamb? an earless lamb?
6. Suppose that alien genetics mimics human genetics. Purple eyes, P, are
dominant to yellow eyes, p. Two purple-eyed aliens mate and produce six
offspring. Four of them have purple eyes and two have yellow eyes. What
are the genotypes of the parent? the phenotypes of the parents? What are
the genotypes of the offspring?
7. In a certain organism, one chromatid contains the genes A and b. Its
homologous chromatid contains a and B.
a) What combinations of genes would be found in gametes of this organism
if no crossing over occurs? Draw the chromatids as they would appear in
metaphase I and after meiosis II.
b) What combination of genes would be found in gametes if crossing over
does occur? Draw the chromatids as they would appear in metaphase I
and after meiosis II.
8. Bronze turkeys have at least one dominant allele R. Red turkeys are
homozygous for its recessive allele rr. Another dominant gene H produces
normal feathers and the recessive genotype hh produces feathers lacking
webbing, a condition termed “hairy”. In crosses between homozygous
bronze, hairy birds and homozygous red normal feathers, what proportion of
the F2 progeny will be:
a)
b)
c)
d)
e)
f)
g)
genotype Rrhh
phenotype bronze, hairy
genotype rrHH
phenotype red, normal feathers
genotype RrHh
phenotype bronze, normal feathered
genotype RRHh
9. A cross between two sweet pea plants produced 39 plants with pink flowers
(incomplete dominance), 18 with white flowers and 19 with red flowers.
What are the phenotypes of the parents?
10. A normal-visions man of blood group A marries a normal visioned woman of
blood group A. They have two children, a color-blind boy of blood group A
and a normal visioned girl of blood group O. What were the genotypes of the
parents?
11. A narrow reduced eye, B, is the dominant X linked condition in fruit flies. The
full red eye is produced by its recessive allele, b. A homozygous wild-type
female is mated to a reduced eye male. Determine the F1 and F2 genotypic
and phenotypic ratios.
12. In summer squash, white fruit color is dominant, W, and yellow fruit color is
recessive, w. A dominant allele at another locus, S, produces disc shaped
fruit, and its recessive allele, s, produces sphere-shaped fruit. If a true
breeding white disc variety is crossed with a homozygous yellow sphere
variety, the F1 are all white disc hybrids. If the F1 is allowed to mate at
random, what would be the phenotypic ratio expected in the F2 generation?
13. The shape of radishes may be long LL, round L’L’ or oval LL’. Color may be
red, RR, white R’R’ or purple RR’. If a long, white strain is crossed with a
round, red stem, what phenotypic ratios are expected in the F2 progeny?
14. Black coat cocker spaniels is determined by a dominant allele “B” and a
brown coat color by its’ recessive allele “b”. Solid pattern is determined by
the dominant allele of and independent chromosome, “S”, and spotted
pattern by its’ recessive allele “s”. A solid male is mated to a solid brown
female and produces a litter of six pups: two solid black, two solid brown, one
black and white, one brown and white. Determine the genotypes of the
parents
15. Normal leg size in cattle is produced by the homozygous genotype DD.
Short-legged cattle possess the heterozygous genotype, Dd. The
homozygous genotype dd is lethal, producing grossly deformed dead calves.
The presence of horns in cattle is governed by the recessive allele of another
gene, p, the polled condition (absence of horns) is produced by its dominant
allele, P. In matings between heterozygous polled, short legged cattle, what
is the phenotypic ratio expected in the ADULT progeny?
16. In Drosophila, S = normal body, s = sable body, W= normal wing and w =
miniature wing. In a cross between a heterozygous normal fly and a sablebodied, miniature winged fly the results were:
99
99
11
11
normal flies
sable-bodies, miniature wing
normal body, miniature wing
sable body, normal wing
How many map units separate the genes for body color and wing size?
17. A wild-type fruit fly (heterozygous for gray color and long wings) was mated
with a black fly with short wings (homozygous recessive). The offspring gave
the following distribution:
wild type: 778
black, short: 785
black, long: 158
gray, short: 162
What is the recombination frequency (crossover %), between the genes for
color and wing type?
18. Tay-Sachs disease is caused by a lethal autosomal recessive allele.
Individuals with Tay-Sachs first show disease symptoms at the age of 6
months and often dies around 4 years of age (if untreated). Examine the
pedigree below, where the shaded shapes show affected individuals.
I
II
1
1
2
2
3
4
5
III
1
2
3
4
5
6
Indicate the genotype of the following individuals:
a) I-2
b) II-1
c) II-5
d) III-6
19. Hemophilia is a recessive, sex-linked condition that used to be called the
“bleeder’s disease” because the blood of affected individuals takes a long
time to clot. Even common bruises can lead to severe bleeding in
hemophiliacs. Examine the pedigree below and answer the questions that
follow (the slashed marks represent the heterozygous state):
I
1
II
1
2
2
3
4
5
III
1
2
3
4
5
a) indicate the genotype of each individual using:
XHXH for a normal female
XHXh for a carrier female
XhXh for a hemophiliac female
XHY for a normal male
XhY for a hemophiliac male
b) why do more males show the hemophiliac phenotype than females?
c) Examine how female III-3 can have hemophilia.
20. For a series of breeding experiments, a linkage group composed of genes M,
N, O and P was found to show the following gene recombination frequencies,
expressed as percents of offspring. Construct a gene map, showing the
relative positions of each gene along the chromosome and indicate the
distances in map units.
Genes
M
N
O
P
M
3
4
11
N
3
7
14
O
4
7
7
P
11
14
7
(Interpret this chart as saying: The % crossover between genes M and N is 3%,
therefore these genes are 3 m.u. apart…)