UNIT 3:
GENETICS
SUBJECT: BIOLOGY &GEOLOGY
Group: 4º ESO
Teacher: Irene Martínez Clares
Academic year: 2011-2012
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SUBJECT: BIOLOGY AND GEOLOGY
Name:________________________________________________________Date:______
UNIT 2: GENETICS (THEORY)
INDEX
0.- Introtroduction to Genetics.
1.- Alleles
2.- Multiple Alleles
3.- Genotypes and phenotypes
4.- Sex determination
5.- Dominant- recessive inheritance
6.- Pedigrees (Family trees)
7.- Test-cross
8.- Monohybrid cross
9.- Incomplete dominance (or co-domincance)
10.- Co-dominance and multiple alleles
11.- Antigens and antibodies in ABO blood types
12.- Complete dominance or dominant-recessive inheritance
13.- Sex-linked inheritance
0.- INTRODUCTION TO GENETICS
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Genetics is the study of heredity, that is, how characteristics such as eye colour are
inherited from parents to offspring.
Genes are the chemicals in the nuclei of cells that determine the characteristics that are
inherited. Each human cell has thousands of genes in the nucleus. Genes are made of
DNA (deoxyribonucleic acid).
Chromosomes are fine thread-like structures in the nucleus of all body cells. Genes
are situated on chromosomes.
Numbers of Human Chromosomes - Human body cells have 23 pairs or 46
chromosomes. Human gametes (sperm and egg) have only 23 chromosomes.
Human Sex Chromosomes - Of the 46 chromosomes in human body cells, 1 pair or 2
sex chromosomes only determine whether a person is male or female. Human females
have 2 X-shaped sex chromosomes (XX). Human males have 1 X-shaped and 1 Yshaped chromosome (XY).
Autosomes are the other 22 pairs or 44 chromosomes in human body cells.
1.- ALLELES


Genes are made of DNA. Genes help us to make proteins (e.g. muscle protein,
hormones, enzymes, antibodies, skin collagen, hair keratin) that will determine certain
characteristics we inherit.
For a specific characteristic (e.g. eye colour), there may be one or more types (e.g.
blue, brown) that may be determined by slightly different variations of the DNA in the
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gene. These are called alleles. For example, in eye colour, there may be brown eye
colour determined by a gene or allele B, or for blue eye colour, it is determined by a
slightly different gene or allele b.
2.- MULTIPLE ALLELES
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For a particular characteristic, there may be more than 2 alleles. For example, in Blood
Type, there are 3 alleles - A, B and O.)
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3.- GENOTYPES AND PHENOTYPES
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Each characteristic (e.g. eye colour) is determined by a pair of genes/alleles. One of
those genes came from the father via sperm, and the other came from the mother via
egg.
Genotype is the pair of genes/alleles written in letter form (e.g. BB).
Phenotype is the characteristic that appears as a result of that genotype (e.g. brown
eyes).
Example 1 - Eye Colour in Humans:
GENOTYPE
PHENOTYPE
BB
Brown-eyed person
Bb
Brown-eyed person
bb
Blue-eyed person
Example 2 - Height in Pea Plants:
GENOTYPE
PHENOTYPE
TT
Tall pea plant
Tt
Tall pea plant
tt
Short pea plant
Homozygous (or Purebred) - The genotype for the characteristic has the same
genes/alleles (e.g. BB, bb).
Heterozygous (or Hybrid) - The genotype for the characteristic has different
genes/alleles (e.g.Bb).
Wild-type - The most common phenotypes in a population are called the wild-type.
These may be dominant (e.g. brown eye colour) or recessive (e.g. green pea colour).
4.- SEX DETERMINATION
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Because the sperm or egg will contain only 1/2 of the genes and chromosomes of a
person, then each sperm could contain either an X or a Y chromosome. Each egg
contains either an X or the other X chromosome.
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To have a daughter, the sperm from the father and the egg from the mother must both
contain X chromosomes.
To have a son, the father's sperm must have a Y chromosome to join with the mother's
egg that has an X chromosome.
It is the father who determines the sex of a child.
5.- DOMINANT-RECESSIVE INHERITANCE
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Of the pair of genes/alleles for a characteristic, one may be dominant (or more strongly
inherited in the offspring), and the other may be recessive (or less strongly inherited in
the offspring).
Dominant genes/alleles are shown by capital letters (e.g. B, T).
Recessive genes/alleles are shown by small letters (e.g. b, t).
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Example1 - Eye Colour
Dad has purebred brown eyes (BB) and Mum has purebred blue eyes (bb). The
Punnett Square below shows the possible eye colours inherited by the children.
B
B
b
Bb
Bb
b
Bb
Bb
Possible genotypes of children = all Bb
Possible phenotypes of children = all brown-eyed children
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Example 2 - Eye Colour
Dad has heterozygous brown eyes (Bb) and Mum has blue eyes (bb). The possible eye
colours of the children will be ...
B
b
b
Bb
bb
b
Bb
bb
Possible genotypes = 2Bb : 2bb
= 1Bb : 1bb
Possible phenotypes = 2 brown : 2 blue
= 1 brown : 1 blue
This means that about 1/2 of the children will be brown-eyed and the other 1/2 will be blueeyed.
6.- PEDIGREES (FAMILY TREES)
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Pedigrees are drawn to examine a characteristic being studied (e.g. eye colour,
haemophilia).
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Symbols used are:
Male without characteristic being studied
Female without characteristic being studied
Male with characteristic being studied
Female with characteristic being studied
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Example of Pedigree
7.- TEST-CROSS
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If an individual has a dominant phenotype, it is not known what the exact genotype is.
For example with the phenotype of brown eye colour, the genotype could be BB or Bb.
To find out the genotype of a dominant phenotype, one must cross the individual with
the dominant phenotype (e.g. BB or Bb) with an individual with the recessive phenotype
(e.g. bb). If the offspring all have the dominant phenotype (e.g. brown eyes), then the
parent was pure-bred (e.g. BB). If the offspring have any with the recessive phenotype,
then the parent was hybrid (e.g. Bb).
8.- MONOHYBRID CROSS
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If both parents are hybrid or heterozygous for brown eye colour (both are Bb), the
possible eye colours of the children are ...
B
b
B
BB
Bb
b
Bb
bb
Possible genotypes = 1 BB : 2 Bb : 1 bb
Possible phenotypes = 3 brown : 1 blue
This means that 3/4 of the children will be brown-eyed, and 1/4 will be blue-eyed.
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9.- INCOMPLETE DOMINANCE (OR CO-DOMINANCE)
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Incomplete Dominance occurs where both genes/alleles are incompletely expressed
in the phenotype.
Example - 4 O'Clock Flowering Plants
GENOTYPE
PHENOTYPE
RR
Red flowers
WW
White flowers
RW
Pink flowers
If a red-flowering plant produced pollen that fertilised a white-flowering plant's egg, the possible
offspring would be ...
R
R
W
RW
RW
W
RW
RW
Possible genotypes = all RW
Possible phenotypes = all pink-flowering plants
10.- CO-DOMINANCE AND MULTIPLE ALLELES (AN EXAMPLE OF ABO BLOOD TYPES)
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Co-Dominance occurs when both genes/alleles in the genotype are equally dominant.
Multiple Alleles occurs when more than 2 genes/alleles determine a characteristic,
such as in ABO blood groups.
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Example of ABO Blood Types
There are 4 different blood types - A, B, AB and O.
BLOOD TYPE
OR PHENOTYPE
GENOTYPE
ANTIGEN
PRESENT
ANTIBODY
PRODUCED
A
AA or AO
A
Anti-B
B
BB or BO
B
Anti-A
AB
AB
A and B
none
O
OO
none
Anti-A and Anti-B
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Example 1 - Blood Types
Mum has blood type AB and Dad has blood type O. The possible blood types of the
children are ...
A
B
O
AO
BO
O
AO
BO
Possible genotypes = 1 AO : 1 BO
Possible phenotypes = 1 A : 1 B
1/2 the children will be A blood type, and the other 1/2 will be B blood type.
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Example 2 - Blood Types
Mum has A blood type and Dad has AB blood type. The possible children's blood types
are ...
First Possibility
A
A
A
AA
AA
B
AB
AB
Possible genotypes = 1 AA : 1 AB
Possible phenotypes = 1 A : 1 AB
1/2 the children will have blood type A, and the other 1/2 will have blood type AB.
Second Possibility
A
O
A
AA
AO
B
AB
BO
Possible genotypes = 1AA:1AO:1AB:1BO
Possible phenotypes = 2 A : 1 AB : 1 B
1/2 will have blood type A, 1/4 will have blood type AB, and 1/4 will have blood type B.
11.- ANTIGENS AND ANTIBODIES IN ABO BLOOD TYPES
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The blood type is so-called because the blood contains particular antigens - A,
A and B, or neither A nor B.
The body produces antibodies to neutralise any particle (e.g. bacteria, dust,
blood in transfusions) that it recognises as foreign. For example, if blood
contains Antigen A, then it will produce antibodies against B blood type
Antibody), because B antigens are foreign. Similarly, if blood type O contains
B, both
foreign
type A
(Anti-B
neither
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antigens A nor B, then a person with blood type O would produce anti-A and anti-B
antibodies.
Agglutination or 'Clumping' - If antigen-A came in contact with the antibody against it
(Anti-A), then the blood would clump or clot. This could occur in an incorrect blood
transfusion.
Universal Recipient - This is a person with blood type AB who can receive a blood
transfusion from any of the other blood types.
Universal Donor - This is a person with blood type O who can donate blood to any
other blood type.
12.- COMPLETE DOMINANCE OR DOMINANT-RECESSIVE INHERITANCE
(AN EXAMPLE OF RHESUS FACTOR IN ABO BLOOD TYPES)
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The ABO blood types are sub-divided into positive and negative types also, depending
on whether that blood type does or does not contain the Rhesus Factor.
If the Rhesus Factor is present, the genotype contains one or two R genes/alleles. If the
Rhesus Factor is absent, the genotype is rr.
Blood
Type
ABOAntigens
Present
Rhesus
Antigens
Present
ABO
Genotype
Rhesus
Genotype
A+
A
yes
Aa or AO
RR or Rr
A-
A
no
AA or AO
rr
B+
B
yes
BB or BO
RR or Rr
B-
B
no
BB or BO
rr
AB+
A and B
yes
AB
RR or Rr
AB-
A and B
no
AB
rr
O+
none
yes
OO
RR or Rr
O-
none
no
OO
rr
13.- SEX-LINKED INHERITANCE
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This is a form of inheritance where the gene/allele for the characteristic being studied is
on the X chromosome.
Diseases such as colour-blindness and haemophilia are inherited this way, and are
more common in males than females.
Alleles, Genotypes and Phenotypes for Haemophilia (Blood-Clotting Inability)
XH - allele for normal blood clotting
Xh - allele for haemophilia
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XHXH - genotype of normal female
XhXh - genotype of haemophiliac female
XHXh - genotype of carrier female (with normal blood-clotting ability, but who can pass
the defective gene to her children)
XHY - genotype of normal male
XhY - genotype of haemophiliac male
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Example 1 - Haemophilia
A man with normal blood-clotting ability (XHY) marries a woman who is a haemophiliac
(XhXh). The possible phenotypes of their children are ...
XH
Y
Xh
XHXh
XhY
Xh
XHXh
XhY
Possible genotypes = 1 XHXh : 1 XhY
Possible phenotypes
= 1 carrier female : 1 haemophiliac male
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Example 2 - Haemophilia
A normal woman (XHXH) marries a haemophiliac man (XhY). The possible genotypes
and phenotypes of the children are...
XH
XH
Xh
XHXh
XHXh
Y
XHY
XHY
Possible genotypes = 1 XHXh : 1XHY
Possible phenotypes
= 1 carrier female : 1 normal male
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Alleles, Genotypes and Phenotypes for Colour-blindness
Xc - allele for normal colour vision
Xc - allele for colour-blindness
Xc Xc - genotype of normal female
Xc Xc - genotype of colourblind female
Xc Xc - genotype of carrier female (with normal colour vision, but who can pass the
defective gene to her children)
Xc Y - genotype of normal male
Xc Y - genotype of colourblind male
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Example 3 - Colour Blindness
A male with normal vision (XcY) and a colourblind female (Xc Xc) have children. The
possible genotypes and phenotypes of the children are ...
Xc
Y
Xc
XcXc
XcY
Xc
XcXc
XcY
Possible genotypes = 1 Xc Xc : 1 Xc Y
Possible phenotype = 1 carrier female : 1 colourblind male
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Example 4 - Colour Blindness
A carrier female (Xc Xc ) marries a normal-visioned male (Xc Y). The possible
genotypes and phenotypes of the children are...
Xc
Xc
Xc
XcXc
XcXc
Y
XcY
XcY
Possible genotypes
= 1 XcXc: 1 XcXc: 1 XcY: 1 XcY
Possible phenotypes
=1 normal female: 1 carrier female: 1 normal male: 1 colourblind male
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SUBJECT: BIOLOGY AND GEOLOGY
UNIT 2: GENETICS – ACTIVITIES
INDEX
ACTIVITY 1. DOMINANT-RECESSIVE INHERITANCE PROBLEMS
ACTIVITY 2. GENES AND CHROMOSOMES CLOZE
ACTIVITY 3. A PEDIGREE OF POLYDACTYLY
ACTIVITY 4. GENETICS CROSSWORD
ACTIVITY 5. QUIZ: GENETICS
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ACTIVITY 1. DOMINANT-RECESSIVE INHERITANCE PROBLEMS
Name:________________________________________________________Date:______
1. Define: (a) gene (b) chromosome (c) genetics.
2. How many chromosomes are in: (a) human body cells such as skin and muscle (b) human
reproductive cells such as sperm and ova?
3. What are the sex chromosomes of a human male, and of a human female?
4. What is meant by the term " pure-bred"?
5. Distinguish between a genotype and a phenotype.
6. If two organisms have the same phenotype, does this mean they have the same genotype?
7. In peas, yellow colour is dominant to green. What will the colours of the offspring of: (a) a
homozygous yellow and a green pea plant (b) a heterozygous yellow and a green pea plant (c)
a heterozygous yellow and a homozygous yellow pea plant (d) two plants that are hybrid for
the yellow pea seed?
8. Could two brown-eyed parents have a blue-eyed child? Explain.
9. Could two blue-eyed parents have a brown-eyed child?
10. A blue-eyed man, both of whose parents were brown-eyed, marries a brown-eyed woman
whose father was blue-eyed and whose mother was brown-eyed. This man and this woman
have a blue-eyed child. What are the genotypes of all the individuals mentioned?
11. If two animals heterozygous for a single pair of genes are mated and have 200 offspring,
about how many have the dominant phenotype?
12. Mrs. Smith and Mrs. Jones had babies at the same maternity hospital at the same time.
Mrs. Smith took home a girl and named her Sue. Mrs. Jones took home a boy and named him
Jim. However she was sure she had a girl and brought suit against the hospital. Blood tests
showed that Mr. Jones had blood type O, Mrs. Jones was type AB, both Mr and Mrs Smith
were type B, Sue was type A and Jim was type O. Had a swap occurred?
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ACTIVITY 2. GENES AND CHROMOSOMES CLOZE
Name:________________________________________________________Date:______
Fill in the missing words:
Characteristics that are passed on from parents to their children are __________. When an
organism reproduces __________, both the parent and the offspring have the same
__________. When organisms reproduce sexually, the offspring inherit __________ the genes
and chromosomes from the __________ and the other half from the __________. The material
found in the nucleus of an egg and a sperm are __________ for the characteristics of the
offspring. This material in the __________ is known as chromosomes. All organisms have a
__________ number of chromosomes that determine all the characteristics of an organism.
Every body _________ in a human contains only __________ chromosomes (23 pairs). Each
sex cell, __________ or __________ contains 23 chromosomes. When the egg and the sperm
unite, the new fertilised cell (the zygote) has __________chromosomes. All humans have 23
pairs of chromosomes. The 23rd pair is known as the __________ chromosomes. In a male
human, the sex chromosomes are __________, and in a female human, the __________
chromosomes are XX. Because each sex cell contains only half the number of chromosomes
(1 of each __________), sperm cells contain either an X or a __________ chromosome. All
egg cells contain an __________ chromosome. The parent who determines the sex of the
child is the __________. The chance of having a daughter is the same as that of having a
__________.
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ACTIVITY 3. A PEDIGREE OF POLYDACTYLY
Name:________________________________________________________Date:______
The Klampett family have an inherited characteristic called polydactyly (extra fingers and toes).
The pedigree below shows affected family members as shaded shapes.
Examine the pedigree, and then answer the questions that follow.
Q1. What are the names of the children of Pa and Ma?
______________________________________________________________
Q2. How many sons and daughters does Billy have?
______________________________________________________________
Q3. How are Pete and Repete related?
______________________________________________________________
Q4. How many people in the pedigree have polydactyly?
______________________________________________________________
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Q5. Is the gene / allele for polydactyly dominant or recessive. Explain.
______________________________________________________________
______________________________________________________________
Q6. What are the genotypes for:
(a) Ma _______
(b) Angie_______
(c) Pa _______
Q7. If Jake married a woman with no family history of polydactyly, what is the
possibility of their children having polydactyly? (Show all working.)
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ACTIVITY 2. GENETICS CROSSWORD
Name:________________________________________________________Date:______
Across
1. Abbreviation of deoxyribonucleic acid.[3]
4. Colour blindness is a sex ____ hereditary
disease.[6]
6. Females have XX ___ chromosomes.[3]
5. Type of cross where both parents are hybrid for
a characteristic.[10]
9. Inherited chemical that is situated on a
chromosome.[4]
12. Less strongly inherited.[9]
10. Number of genes and chromosomes inherited
from each parent.[4]
11. Condition where the pair of alleles for a
characteristic are the same.[8]
16. Characteristic that is determined by a pair of
genes e.g. blue eye colour[9]
18. Shape used for a male in a pedigree.[6]
20. More strongly inherited.[8]
21. In a pedigree, a person affected with a disease
is shown by a ____ shape.[6]
22. Pair of alleles for a characteristic e.g. Bb.[8]
Down
2. Alternative form of a gene for a characteristic.[6]
3. Shape used for a female in a pedigree.[6]
7. Threadlike structures on which genes are
situated.[10]
8. Family tree in genetics.[8]
13. Study of heredity.[8]
14. Male reproductive cell.[5]
15. Type of genotype where the pair of alleles are
not the same.[6]
17. Scientific name for an egg.[4]
19. XY chromosomes indicate the sex
chromosomes for a ____.[4]
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ACTIVITY . QUIZ. GENETICS
Name:________________________________________________________Date:______
1. The hereditary units in cells are called:
A
genes
B
genetics
C
characteristics
2. How many chromosomes are contained in one human body cell?
A
50
B
46
C
23
3. How many chromosomes are contained in one egg or one sperm?
A
2
B
23
C
46
4. The human sex chromosomes are:
A
XX
B
XYY
C
XY
5. In a pedigree, what shape indicates males with the characteristic being studied?
A
shaded circle
B
unshaded circle
C
shaded square
6. An example of a genotype is:
A
Bb
B
purebred brown eyes
C
hybrid blue eye colour
7. When one characteristic is more strongly inherited than another characteristic, it is said to
be:
A
monohybrid
B
recessive
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C
dominant
8. If a purebred brown-eyed man marries a purebred blue-eyed woman, what colour is most
likely for their children's eye colour?
A
blue
B
brown
C
half and half
9. If both parents are hybrid for brown eye colour, the ratios of eye colours of their possible
offspring are:
A
all blue
B
3 brown:1 blue
C
1 brown:2 blue
10. In pea plants, tallness is dominant to dwarfness. What are the possible offspring of a
hybrid tall plant and a dwarf plant?
A
half tall and half dwarf
B
all tall
C
3/4 tall and 1/4 dwarf
11. In fruit flies, straight wing is dominant over curly wing. What are the possible offspring of
a homozygous or purebred straight winged fly and a curly winged fly?
A
half straight winged and half curly winged
B
all straight winged
C
insufficient information given
12. Albinism in humans is caused by a recessive allele. A normal couple have 4 children - 3
normal and 1 albino. What are the genotypes of the parents?
A
only one parent carries the albino allele
B
one parent is purebred for albinism and the other parent is hybrid
C
both hybrid for albinism
13. With reference to the question above, what is the chance that the next child will be
albino?
A
1/4
B
50%
C
nearly 100%
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14. In cats, short hair is dominant over long hair. What is the ratio of possible offspring if a
heterozygous or hybrid short-haired cat mates with a long-haired cat?
A
half and half
B
all long-haired
C
1 short-haired:1 long-haired
15. In chickens, black eye colour is dominant to blue eye colour. If a purebred black-eyed
rooster mated with a purebred blue-eyed hen, what is most likely for the next generation's eye
colour characteristics?
A
all blue-eyed
B
all black-eyed
C
insufficient information given
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