(Assumption College Samutprakarn)

SCIENCE

Secondary 3

Dihybrid Crosses and Introduction to Pedigrees Worksheet

Introduction:
Dihybrid crosses are a fundamental concept in genetics that involves studying the inheritance
of two different traits simultaneously. Unlike monohybrid crosses, which focus on a single
trait, dihybrid crosses allow us to observe how two different genes are inherited together and
how they may interact. Understanding dihybrid crosses is crucial for predicting offspring
genotypes and phenotypes when two traits are involved, and it forms the basis for more
complex genetic analyses.

Task A: Punnett Square Practice

In rabbits, black fur (B) is dominant to brown fur (b), and floppy ears (F) are dominant to erect ears (f).
Complete the following Punnett square for a dihybrid cross between two heterozygous rabbits:

Task B: Understanding Dominance

Based on the Punnett square you completed in Task 1, determine the ratios of the offspring. Express your
answer as a ratio of Black/Floppy : Black/Erect : Brown/Floppy : Brown/Erect.

Genotypic ratio: ____ : ____ : ____ : ____ : ____ : ____ : ____ : ____ : ____

Phenotypic ratio: ____ : ____ : ____ : ____

Task C: Exploring Dominance

Indicate whether each statement is True or False. If false, briefly explain why.

1. ____ In a dihybrid cross, the alleles for different traits always segregate independently.

2. ____ The genotypic ratio in a dihybrid cross between two heterozygous individuals is always 9:3:3:1.

3. ____ Dihybrid crosses can only be performed with plants, not animals.

4. ____ The principle of independent assortment applies to genes located on different chromosomes.

5. ____ In a dihybrid cross, there are 16 possible genotypic combinations in the offspring.
วิชา / Subject … SCIENCE หน้า/Page ……
Task D: Genotype and Phenotype
In butterflies, orange wings (O) are dominant to white wings (o), and spotted pattern (S) is dominant to
solid pattern (s). A butterfly that is homozygous for both orange wings and spotted pattern is crossed with
a butterfly that has white wings and solid pattern.

1. What are the genotypes of the parent butterflies?

Parent 1: ________ Parent 2: ________

2. What will be the genotype and phenotype of all the F1 offspring?

Genotype: ________ Phenotype: ___________________________

3. If two F1 offspring are crossed, what will be the expected phenotypic ratio in the F2 generation?

_____ Orange/Spotted : _____ Orange/Solid : _____ White/Spotted : _____ White/Solid

Task E: Multiple Choice

Choose the best answer for each question.
1. In a dihybrid cross between AaBb × AaBb, what fraction of the offspring will be homozygous
recessive for both traits?
a) 1/16 b) 1/4 c) 9/16 d) 3/16
2. Which of the following genotypes from a dihybrid cross (AaBb × AaBb) will produce the "9" in a
9:3:3:1 ratio?
a) AAbb b) AaBb c) AABB d) aaBB
3. If a dihybrid cross results in a 1:1:1:1 ratio, what can you conclude about the parents?
a) Both parents are homozygous for both traits
b) One parent is homozygous dominant, the other is homozygous recessive for both traits
c) One parent is heterozygous for both traits, the other is homozygous recessive for both traits
d) Both parents are heterozygous for both traits

Task F: Matching
Match each term with its correct definition by writing the letter of the definition next to the term.
1. ____ Dihybrid cross
2. ____ Independent assortment
3. ____ Testcross
4. ____ Dominant allele
5. ____ Recessive allele
Definitions:
A. An allele that is masked when a dominant allele is present
B. The principle that genes for different traits are inherited independently of each other
C. A genetic cross involving two different genes, each with two alleles
D. An allele that is expressed in the phenotype when present
E. A cross between an individual with an unknown genotype and a homozygous recessive individual
 (Assumption College Samutprakarn)

SCIENCE

Secondary 3

Non-Mendelian Genetics
Introduction:
Gregor Mendel discovered that traits like height in pea plants follow simple dominant-
recessive patterns. But what happens when we look at flower colors, human blood types, or
certain genetic diseases? Not everything follows Mendel's simple rules!
What is Non-Dominance?
In Mendel's experiments, one allele was always dominant over another. For example:
Tall (T) was dominant over short (t)
TT = Tall, Tt = Tall, tt = Short
The heterozygote (Tt) looked exactly like the homozygous dominant (TT) – they have the
same phenotype.
But what if neither allele is dominant? This is called non-dominance or incomplete dominance.
Key Features of Non-Dominance:
Neither allele is fully dominant - they are equal in strength
The heterozygote shows a NEW phenotype - different from both parents
The phenotype appears blended or intermediate
The F₂ ratio is 1:2:1 (not Mendel's 3:1)
When two different alleles come together, neither one completely dominates. Instead, the
heterozygote shows a blended or intermediate phenotype.
I. Incomplete Dominance – Classic Example: Snapdragon Flowers
Red flowers (CR CR) × White flowers (CW CW)
F₁ offspring: ALL Pink flowers (CR CW)
Neither red nor white "wins" - they create something new! This happens because:
CR produces red pigment
CW produces no pigment (or white pigment)
CR CW produces HALF as much red pigment = Pink appearance
II. Co-Dominance – Classic Example: Human Blood Groups (antigen markers).
On the left and top side are shown possible antigen markers with the genes, coding them.
In the cells inside – blood types and genotypes, responsible for these blood types.
BLOOD TYPES A (IA) B (IB) 0 (i)
A (IA) Type A (IA IA) Type AB (IA IB) Type A (IA i)
B (IB) Type AB (IA IB) Type B (IB IB) Type B (IB i)
0 (i) Type A (IA i) Type B (IB i) Type O (ii)
The difference between co-dominance and incomplete dominance is the following: co-
dominant alleles are collectively dominant over another, recessive allele (in this case –
absence of antigens). Alleles A and B are incompletely dominant to one another, creating the
mixed phenotype as the result (blood type AB).
At the molecular level, incomplete dominance and co-dominance are actually the SAME
phenomenon!
III. DOMINANCE HIERARCHY:
When multiple alleles exist and show a ranking where each allele is dominant over
those below it in the hierarchy.
Example: Rabbit coat color
• C (fully colored coat) > CCH (chinchilla) > Ch (Himalayan) > c (albino)

Key Concepts:
• Incomplete/Non-Dominance: When heterozygotes show a blended/mixed phenotype
• (neither allele is fully dominant, creating an intermediate form).
• Co-dominance: When both alleles are expressed equally and simultaneously in
heterozygotes’ phenotypes.
• Dominance Hierarchy: When multiple alleles show a ranking (A > B > C > D)
• Multiple Alleles: When more than two alleles exist for a single gene in a population
• Blood Groups: Human ABO system showing co-dominance and multiple alleles
• Sickle Cell Anemia: A genetic disorder showing incomplete dominance
• Karyotype: A picture of all chromosomes in a cell, arranged in pairs
• Chromosomal Mutations: Changes in chromosome number or structure

Task A. Solve the following genetic problems and answer the questions:
1. Andalusian Chickens: Black feathers (FBFB) × White feathers (FWFW) = Blue feathers (FBFW)
a) If you bred two blue chickens, what colours could their offspring be?
______________________________________________________________
6)lue black white
, ,

b) Would you ever get brown chickens from this cross? Why?
No there is no
______________________________________________________________
, genes of down color
.

2. Human Hair Type: Straight hair (HSHS) × Curly hair (HCHC) = Wavy hair (HCHS)
a) What type of inheritance is this? _______________
In complete dominant

b) Can two wavy-haired parents have a straight-haired child?

,
because have alleles for straight hair .
______________________________________________________________
Yes
they
c) What is the chance of two wavy-haired parents having a curly-haired child? ____%

Task B: Punnett Square Practice - Multiple Patterns

A man with blood type A marries a woman with blood type B. Their child has blood type O.
Answer the questions:

23
1. What is the probability of a child with this blood type? ___% E
↑ IA i
2. What was the most probable blood group? _______
None

IB Ipi
3. Which offspring genotypes show dominance of one allele? In IB
(Blood AB) (BT B]
_______
-

With blood A and B

i
In i ii
4. What is the phenotypic ratio? ___
1 A : ___
M B : ___
1 AB : ___
1 O
(BT -

A) (BT -
O)
Key Differences from Single-Gene Disorders:
1. Inheritance: Usually NOT inherited (occur randomly)
2. Genes affected: Thousands (whole chromosome)
3. Predictability: Can't use simple Punnett squares
4. Detection: Need karyotype, not just blood test

Task D: Connecting All Concepts

Problem 1: Complete the Comparison Table

Feature Sickle Cell Blood Type AB Down Syndrome

Number of alleles 2 (H^A, H^S) 3 (1A , IB i) N/A

,

Co-dominance & chromosomal

Inheritance pattern Incomplete dominance
multiple alleles Shon disjunction)
Can be carrier? Yes No No

Inherited from parents? Yes Yes No

Detection method DNA Test Blood

typing Karyotype
Number of genes affected 1 1 Thousand

Problem 2: Critical Thinking

1. A couple has one child with sickle cell trait and another with Down syndrome. Are
these related?
o Answer: _____
No

Sickle cell single

genes multation ; Down syndons-whole
o Explain: _______________________________________________
= chromosome
addorm in altily
2. Can someone have both AB blood AND sickle cell disease?
o Yes
Answer: _____
o They arcontrol by different genes so a person can have a bath.
Explain: _______________________________________________
,

3. Why can we predict sickle cell with Punnett squares but not Down syndrome?
o Sickle cell fallow Mendelian inheritance , but Down syndown result
Answer: _______________________________________________
from randomchromosome
4. Which test detects which condition?
o Blood type (ABO)
Blood typing: _______________ hardisjuction.

o Sickle cell
DNA test: _______________
o Down syndome
Karyotype: _______________
 (Assumption College Samutprakarn)

SCIENCE
Secondary 3

Symbiosis and Relationships Between Living Things

Introduction:
Symbiosis is a close and long-term biological interaction between two different biological
organisms. These relationships can be categorized into different types based on how the
organisms interact and the benefits or drawbacks each experiences.

Key Types of Symbiotic Relationships:

1. Mutualism: Both organisms benefit from the relationship.
2. Commensalism: One organism benefits while the other is unaffected.
3. Parasitism: One organism (the parasite) benefits at the expense of the other (the host).
4. Competition: Both organisms are harmed by the relationship as they compete.
5. Predation: One organism (the predator) kills and eats the other (the prey).

Task A: Matching
Match each symbiotic relationship with its correct example:

1. Mutualism B
_____
2. Commensalism D
_____
3. Parasitism A
_____
4. Competition E
_____
5. Predation C
_____
A. A tick feeding on a dog's blood
B. Clownfish living in sea anemones
C. Lions hunting and eating zebras
D. Barnacles attached to a whale
-
E. Two plant species competing for sunlight in a forest

Task B: Fill in the Blanks

Use the words from the word bank to complete the sentences.
Word Bank: resources, benefits, harms, unaffected, predator, host, parasite, mutualism

1. In a mutalism
__________ relationship, both organisms gain something positive.

parasite
2. A __________ lives on or in another organism, gaining nourishment at the other's expense.
host
3. The organism that a parasite lives on is called the __________.
unaffected
4. In commensalism, one organism benefits while the other is __________.
5. Competition occurs when two species vie for the same __________.
Resource

predator kills and consumes the prey.

6. In a predator-prey relationship, the __________
วิชา / Subject … SCIENCE หน้า/Page ……
Task C: True or False
Indicate whether each statement is true or false. If false, briefly explain why.

1. All symbiotic relationships are beneficial to both organisms involved. (True/False)

It can lead to harm like parasitism

Why? __________________________________________________________________
,
.

2. Parasitism is always fatal to the host organism. (True/False)

Sometime parasite and host coexist for long period of time , like bacteria in gut.
Why? __________________________________________________________________

3. Commensalism can eventually evolve into mutualism. (True/False)

Why? __________________________________________________________________

4. Predation is not considered a form of symbiosis. (True/False)

It's considered
Why? __________________________________________________________________

5. Competition only occurs between different species, never within the same species. (True/False)

It can occour
eg lion fight
for food .
Why? __________________________________________________________________
,

Task D: Short Answer

Answer the following questions in 1-2 sentences.
1. How does mutualism differ from commensalism?

Mutation when commensalian

is both
organism benefit from relation
,
while is when only one benefit

2. Give an example of a parasitic relationship not mentioned in this worksheet:

Mosquito suck human Good

3. How might climate change affect symbiotic relationships in an ecosystem?

some animal speacher can become extinct and relation Otw . other might change .

Task E: Fill the table:

RELATIONSHIP BENEFICIAL NEUTRAL HARMFUL

S
Predation
BENEFICIAL Mutualism Commensalism
Parafishm

NEUTRAL Commensalism Neutralism Amensalism

S
Predation
HARMFUL
Amensalism
Parafishm competition
วิชา / Subject SCIENCE … หน้า/Page ……

Task F: Diagram Analysis

Study the following food web diagram and answer the questions below:

1. Identify one predator-prey relationship in this food web:

Fox eat snake.

2. How might the removal of one species affect the entire ecosystem?

If we remove fox(predator) the number of Kangaroo rate will increase and they will eat cactus.

3. Which organism(s) in this food web could be considered decomposers, and what role do they play?

Incest , they eat dead organismm.

Task G: Case Study

Read the following scenario and answer the questions:
The fig wasp and fig tree have a unique mutualistic relationship. The female wasp enters the fig fruit
to lay her eggs, simultaneously pollinating the fig flowers inside. When the eggs hatch, male wasps
mate with females and then dig tunnels for the females to escape. The males die inside the fig,
which provides nutrients to the developing fruit.

1. How does the fig tree benefit from this relationship?

.
1 Pollinate flower 2 .
Dead insect provide nutrient.
2. How do the fig wasps benefit?

They get place to lay egg

3. What might happen to fig trees if the wasp population drastically declined?
The flower will not be poliate

4. Is this an example of obligate or facultative mutualism? Explain your answer

obligate ,
because no other pollinate can do it.
 (Assumption College Samutprakarn)

SCIENCE

Secondary 3

Chromosomal Mutations and Related Syndromes

Introduction:
Chromosomal mutations are changes in the structure or number of chromosomes. These
mutations can lead to various genetic disorders. Some common types of chromosomal
mutations include deletions (loss of a chromosome segment), duplications (extra copies of a
segment), inversions (reversal of a segment's orientation), and translocations (exchange of
segments between chromosomes). Changes in chromosome number can result in conditions
like trisomy (an extra chromosome) or monosomy (a missing chromosome). This worksheet
will explore some of the syndromes caused by chromosomal mutations.
Key Terms and Concepts:
1. Chromosomal mutation: A change in chromosome structure or number
2. Aneuploidy: An abnormal number of chromosomes
3. Trisomy: The presence of an extra chromosome (2n+1)
4. Monosomy: The absence of one chromosome (2n-1)
5. Polyploidy: Having more than two complete sets of chromosomes
6. Deletion: Loss of a chromosome segment
7. Duplication: Extra copies of a chromosome segment
8. Inversion: Reversal of a chromosome segment's orientation
9. Translocation: Exchange of segments between chromosomes
10.Nondisjunction: Failure of chromosomes to separate properly during cell division

Task A: Multiple Choice Questions.

1. What type of chromosomal mutation results in an extra copy of a chromosome?
E
a) Deletion b) Translocation c) Inversion d) Trisomy
2. Down syndrome is caused by an extra copy of which chromosome?

-
a) Chromosome 18 b) Chromosome 21 c) X chromosome d) Y chromosome
3. Turner syndrome affects:

-
a) Only males b) Only females c) Both males and females equally d) Neither males nor females
4. Klinefelter syndrome is characterized by:
-
a) XXY chromosomes b) XO chromosomes c) XYY chromosomes d) XXX chromosomes
5. Which syndrome is caused by a missing X chromosome in females?
-
a) Down syndrome b) Klinefelter syndrome c) Turner syndrome d) Patau syndrome

Task B: Matching
Match each syndrome with its chromosomal characteristic:

1. Down syndrome ____ a) Missing one X chromosome (X_)

2. Turner syndrome ____
·
3. Klinefelter syndrome ____
b) Extra X chromosome in males (XXY)
c) Trisomy of chromosome 21
d
4. Edwards syndrome ____ d) Trisomy of chromosome 18
e
5. Patau syndrome ____ e) Trisomy of chromosome 13
วิชา / Subject … SCIENCE หน้า/Page ……
Task C: Fill in the Blanks
Use the words from the word bank to complete the sentences below:
Word bank: polyploidy, trisomy, monosomy, nondisjunction, mosaicism

Nondisjunction occurs when chromosomes fail to separate properly during cell division, often leading to
1. ____________
aneuploidy.

Polyploidy
2. ____________ is a condition where an organism has more than two complete sets of chromosomes.

trisomy
3. Down syndrome is an example of ____________, specifically of chromosome 21.
4. Turner syndrome is caused by ____________
monosome of the X chromosome.
5. ____________
Mac nosome is a condition where an individual has two or more genetically distinct cell populations
arising from a single fertilized egg.

Task D: Karyotype Analysis

How many chromosomes are present in
47
this karyotype? ____________________

Identify the type of aneuploidy shown in

trisomy
this karyotype. _____________________

What syndrome is associated with this

chromosomal abnormality?
Patay syndom
__________________________________

O Explain how this chromosomal mutation

likely occurred.
Multation during meiosis
__________________________________

List two potential physical characteristics

that might be observed in an individual
with this syndrome.

Extra finger and to

1) _________________________________________________________________________________

Cleft and Cleft palate

lip
2) _________________________________________________________________________________

Task E: True or False.

F -
1. ____ Down syndrome is caused by an extra copy of chromosome 18.
chromosome 21
Correction (if false): ________________________________________________
F S than in plants.
2. ____ Polyploidy is more common in animals
plant
Correction (if false): ________________________________________________
F A deletion mutation involves gaining extra
3. ____ / genetic material.
loss
Correction (if false): ________________________________________________
F
4. ____ All chromosomal mutations are harmful to the organism.
have no effect or even be denifical
Correction (if false): ________________________________________________
some
may
T
5. ____ Humans normally have 23 pairs of chromosomes.
Correction (if false): ________________________________________________
6. ____ Trisomy means having an extra copy of a particular chromosome.
Correction (if false): ________________________________________________