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GENETICS
FULYA KAHRAMAN
Who is Gregor Mendel?
Known as the father of
modern genetics
Conducted experiments with
pea plants to discover how
traits were passed on from
generation to generation
Identified the concept of
dominant and recessive traits
and several laws of heredity
Mendelian Genetics Vocab
Trait
A heritable feature such as flower color
Allele
Alternate versions of a gene that produce different phenotypes
Dominant Allele
An allele that is fully expressed in the phenotype of a heterozygote
Recessive Allele
An allele whose phenotype is not observed in a heterozygote
Homozygous
Having two identical alleles for a given gene
Heterozygous
Having two different alleles for a given gene
Carrier
An individual who is heterozygous with one normal allele and one
potentially harmful recessive allele. The individual is phenotypically
normal but can pass on the harmful allele
Mendelian Genetics Vocab
Genotype
The genetic makeup or a set of alleles of an organism (ie. Aa, AA, aa)
Phenotype
The physical traits which are determined by genotype (many phenotypes are
microscopic in the phenotype of a heterozygote)
True Breeding
Plants that-when self-pollinated- always produce the same phenotypic traits
(homozygous dominant or recessive)
Hyrbridization
The mating or crossing of two true-breeding varieties (true breeding parents- P
generation) Produces all heterozygous F1 generation offspring
Test Cross
Breeding an organism of unknown genotype (heterozygous or homozygous
dominant) with a homozygous recessive individual to determine the unknown
genotype. The Ratio of phenotypes in offspring determines unknown genotype
Monohybrid Cross
The cross between two heterozygotes (F1 generation) for a single trait (ie. Bb x Bb)
Dihybrid Cross
The cross between two heterozygotes (F1 generation) for two traits (ie. YyRr x
YyRr)
Mendelian Genetics Vocab cont.
Complete Dominance
The situation when the phenotypes of the heterozygote and the
dominant homozygote are indistinguishable
Incomplete Dominance
The situation in which the phenotype of heterozygotes is in between the
phenotypes of homozygous individuals for either allele
Codominance
The situation where the phenotypes of both alleles are exhibited in the
heterozygote
Multiple Alleles
When there are three or more alleles for a single gene (as in ABO blood
groups)
Polygenic Inheritance
An additive effect of two or more genes on a single phenotypic trait
Pedigree Analysis
Predicting the genotypes of individuals in a pedigree chart based on the
phenotypes of offspring
Brown coat (C); black eyes (E)
White coat (c); pink eyes (e)
Brown coat (C); black eyes (E)
White coat (c); pink eyes (e)
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
Meiosis
c
White
e
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Chromosomes of
the four gametes
ALTERATIONS OF
CHROMOSOME NUMBER
AND STRUCTURE
Copyright © 2009 Pearson Education, Inc.
8.19 A karyotype is a photographic inventory
of
an individual’s chromosomes
– A karyotype shows stained and magnified
versions of chromosomes
– Karyotypes are produced from dividing white blood
cells, stopped at metaphase
– Karyotypes allow observation of
– Homologous chromosome pairs
– Chromosome number
– Chromosome structure
Copyright © 2009 Pearson Education, Inc.
Packed red
and white blood
cells
Centrifuge
Blood
culture
1
Fluid
Hypotonic
solution
Packed red
and white blood
cells
Centrifuge
Blood
culture
2
1
Fluid
Hypotonic
solution
Packed red
and white blood
cells
Fixative
Stain
Centrifuge
Blood
culture
2
White
blood
cells
3
1
Fluid
4
Centromere
Sister
chromatids
Pair of homologous
chromosomes
5
8.20 CONNECTION: An extra copy of
chromosome 21 causes Down syndrome
– Trisomy 21 involves the inheritance of three
copies of chromosome 21
– Trisomy 21 is the most common human chromosome
abnormality
– An imbalance in chromosome number causes Down
syndrome, which is characterized by
– Characteristic facial features
– Susceptibility to disease
– Shortened life span
– Mental retardation
– Variation in characteristics
– The incidence increases with the age of the mother
Copyright © 2009 Pearson Education, Inc.
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
40
30
35
Age of mother
45
50
Nondisjunction
in meiosis I
Normal
meiosis II
Gametes
n+1
n+1
n–1
Number of chromosomes
n–1
Normal
meiosis I
Nondisjunction
in meiosis II
Gametes
n+1
n–1
n
Number of chromosomes
n
8.21 Accidents during meiosis can alter
chromosome number
– Nondisjunction is the failure of chromosomes or
chromatids to separate during meiosis
– During Meiosis I
– Both members of a homologous pair go to one pole
– During Meiosis II
– Both sister chromatids go to one pole
– Fertilization after nondisjunction yields zygotes
with altered numbers of chromosomes
Copyright © 2009 Pearson Education, Inc.
Example of Complete Dominance
If you cross Tall (TT) and Short (tt),
what do you expect?
All talls?
Some tall, some short?
All shorts?
Medium height?
Since T is completely
dominant, offspring
are all tall
Example of Incomplete Dominance
If you cross CRCR and CWCW,
what do you expect?
All red (CRCR)
All white? (CWCW)
Some red and some white?
All pink?
Some red, some white, some pink?
All pink, because the phenotype
is a mixture of both traits
What do you expect for the
F2 offspring?
Example of Codominance
If you cross STST and SRSR,
what do you expect?
All triangle spots?
All round spots?
Some triangle, some round?
No spots?
Both spots?
Both spots, because both traits
are expressed without mixing
STST
SRSR
STSR
What is the law of segregation?
Mendel’s first law- stating that Each allele in a
pair (diploid set) separates into a different
gamete (haploid) during gamete formation
*Offspring receive only one allele for a trait
What is the law of independent assortment?
Mendel’s second law
that Each pair of alleles
separates independently
during gamete
formation
(Means which allele of
Trait A a gamete
receives is not at all
related to which allele
of Trait B the gamete
receives)
This law only applies
when genes for two
traits are located on
different chromosomes.
The influence of the law of segregation and the
law of independence on gamete formation.
Both laws
mix up the
genes so
that each
gamete
receives
=
genetic
diversity
Punnet Square
Parent/Organism One
Parent/Organism Two
Used to examine
how one trait will
be passed down to
offspring
Use a Punnet
Square with
4 cells
Trait A
Allele 1
Trait A
Allele 1
Trait A
Allele 2
Trait A
Allele 2
Monohybrid Cross
Cross between two heterozygotes for a single trait
What would the genotypes of the parents be in a
monohybrid cross?
Aa x Aa
What would the genotypes
of the offspring be?
AA, Aa, and aa
What would the ratio of
phenotypes in the offspring be?
1:2:1
1 Homozygous Dominant: 2 Heterozygous: 1 Homozygous Recessive
Test Cross
Used to identify the unknown genotype of one
individual
Cross an unknown genotype with a
Homozygous recessive phenotype
Why homozygous recessive?
Because you know the genotype:
Two recessive alleles
Example of Test Cross
Captured Blue Mouse
B
B
b
Bb
Bb
b
Bb
Bb
Hom. Rec. White Mouse
Hom. Rec. White Mouse
In Bodine mice, blue fur is dominant and white fur is a
recessive trait. A captured mouse has blue fur. How would you
determine its genotype?
Mate the blue mouse with a white mouse
Possible genotypes and offspring:
Captured Blue Mouse
B
b
b
Bb
bb
b
Bb
bb
If the offspring were all blue (Bb), what is the genotype of the
original blue mouse?
Homozygous dominant (BB)
Two-Trait Punnet Squares
Used to
examine how
two nonlinked traits
are passed
down in
relation to
each other
Use a Punnet
Square with
16 cells
Dihyrbid
crosses use
16-cells; cross
between two
heterozygotes
Trait A,
allele 1
Trait B,
allele 1
Trait A,
allele 1
Trait B,
allele 1
Trait A,
allele 2
Trait B,
allele 1
Trait A,
allele 1
Trait B,
allele 2
Trait A,
allele 2
Trait B,
allele 2
Trait A,
allele 2
Trait B,
allele 1
Trait A,
allele 1
Trait B,
allele 2
Trait A,
allele 2
Trait B,
allele 2
How Dihybrid Crosses Work
Remember
dihybrid is a
cross between
two F1
individuals
(AaBb)
Combine the
traits like a
regular
4-celled
Punnet Square
Work from
left to right
Arrange the traits in pairs,
one B / b per cell
one E / e per cell Org. 2
Org. 1
How would you set up a dihybrid
cross between BbEe and BbEe?
How would the allele
arrangement look for
organism 1?
Organism 2?
Now solve the Punnet Square
How many different types
of offspring do we have?
Dominant B, Dominant E
BBEE, BbEE, BBEe, BbEe
Dominant B, Recessive e
Bbee, Bbee
Recessive b, Dominant E
bbEE, bbEe
Recessive b, Recessive e
bbee
Different Genotypes in Dihybrid Cross
How many different genotypes?
Try to find them.
Nine:
Homozygous dominant B and E
Homozygous dominant B, heterozygous E
Heterozygous B, homozygous dominant E
Heterozygous B, heterozygous E
Homozygous recessive B, homozygous dominant E
Homozygous recessive B, heterozygous E
Homozygous dominant B, homozygous recessive E
Heterozygous B, homozygous recessive E
Homozygous recessive B, homozygous recessive E
(BBEE)
(BBEe)
(BbEE)
(BbEe)
(bbEE)
(bbEe)
(BBee)
(Bbee)
(bbee)
(1)
(2)
(2)
(4)
(1)
(2)
(1)
(2)
(1)
Different Phenotypes in Dihybrid Cross
How many different phenotypes?
Four
1. Dominant B and Dominant E
2. Dominant B and Recessive e
3. Recessive b and Dominant E
4. Recessive b and recessive e
How many of each?
1. nine
2. three
3. three
4. one
Recombinants
Recombinants are the offspring that have a
different phenotype than their parents
If parents are XxHh and xxHh,
Recombinants are:
Xxhh
xxhh
Sex Linkage / Sex-Linked Traits
When the gene for a trait is on the X or Y chromosome
(sex chromosomes)
Sex traits express themselves more in one sex than the
other- often more in males than females.
In humans the term usually refers to X-linked
characters: genes located only on X chromosomes
Fathers can pass X-linked alleles to their daughters,
but not sons
Mothers can pass sex-linked alleles to both sons and
daughters
Dominant Sex-Linked Traits
Dominant gene on X chromosome
Affected males pass to all daughters and none
of their sons
Genotype= XAY
If the mother has an X-linked dominant trait
and is homozygous (XAXA) all children will be
affected
If mother is heterozygous (XAXa), there is a 50%
chance of each child being affected
Recessive Sex-Linked Traits
Gene located on the X chromosome
A female will express the phenotype only if she is homozygous recessive
XrXr
If a male receives the recessive allele from his mother he will express the
phenotype
XrY
Females can only inherit if the father is affected and mother is a carrier (hetero) or
affected (homo)
An affected female will pass the trait to all her sons
Daughters will be carriers if father is not affected
Males cannot be carriers (only have one X so either affected or not)
More males than females affected (males inherit affected X from mother)
Can skip generations
Examples:
Colour blindness
Haemophilia
Dominant Sex-Linked Trait Pedigree
Look for:
More males being affected
Affected males passing onto all daughters
(dominant) and none of his sons
Every affected person must have an affected
parent
Recessive Sex-Linked Trait Pedigree
Look for:
More males being affected
Affected female will pass onto all her sons
Affected male will pass to daughters who will
be a carrier (unless mother also affected)
Unaffected father and carrier mother can
produce affected son
Pedigree Analysis for Sex-Linked Trait
Dominant
or recessive?
Recessive,
III-4 does not
have trait
even though
father gave
her affected X
chromosome
Pedigree Analysis for Multiple Alleles
Who was the father of 2nd-1? Why?
1st-3, because 1st-1 has no B allele to give
If 2nd-3 marries a man with AB blood, what could their offspring
have?
If she is AA, offspring can be AA, AB
If she is AO, offspring can be AA, AB, AO, BO
Gene Linkage
Genes are on the same chromosome
Are usually inherited together
for example, if blue eyes and freckles are linked, if
your offspring have blue eyes, they will almost
always have freckles
Do these genes usually undergo independent
assortment?
No!
Under what circumstance would the genes be
inherited separately?
If there is crossing over that mixes up the genes
IB Exam Question
1. Define the terms gene and allele and
explain how they differ.
(4 marks)
gene is a heritable factor / unit of inheritance;
gene is composed of DNA;
gene controls a specific characteristic / codes for
a polypeptide/protein;
allele is a form of a gene;
alleles of a gene occupy the same gene
locus/same position on chromosome;
alleles differ (from each other) by one / a small
number of base(s) / base pair(s);
IB Exam Question
2. What is a difference between autosomes
and sex chromosomes?
(1 mark)
A.
Autosomes are not found in gametes but sex
chromosomes are.
B. Sex chromosomes are found in animal cells
and autosomes are found in plant cells.
C. Autosomes are diploid and sex
chromosomes are haploid.
D. Sex chromosomes determine gender and
autosomes do not.
Correct answer: D
IB Exam Question
3. Explain the relationship between Mendel’s
law of segregation and meiosis.
(3 marks)
law of segregation states that one half of the
alleles enter one gamete and the other half
enter the other gamete;
meiosis reduces the chromosome number by
half / diploid to haploid;
homologues carrying alleles separate (in
anaphase I);
end result is four cells, half with one
allele/homologue and the other half with the
other allele;
IB Exam Question
4. A parent organism of unknown genotype is mated in
a test cross. Half of the offspring have the same
phenotype as the parent. What can be concluded from
this result?
(1 mark)
A.
B.
C.
D.
The parent is heterozygous for the trait.
The trait being inherited is polygenic.
The parent is homozygous dominant for the
trait.
The parent is homozygous recessive for the
trait.
Correct answer: A
IB Exam Question
5. In peas the allele for round seed (R) is dominant over the
allele for wrinkled seed (r). The allele for yellow seed (Y) is
dominant over the allele for green seed (y).
If two pea plants with the genotypes YyRr and Yyrr are crossed
together, what ratio of phenotypes is expected in the offspring?
(1 mark)
A.
9 round yellow : 3 round green : 3 wrinkled yellow :
1 wrinkled green
B.
3 round yellow : 3 round green : 1 wrinkled yellow :
1 wrinkled green
C.
3 round yellow : 1 round green : 3 wrinkled yellow :
1 wrinkled green
D.
1 round yellow : 1 round green : 1 wrinkled yellow :
1 wrinkled green
Correct answer: C
IB Exam Question
6. In garden peas, the pairs of alleles coding for seed shape and
seed colour are unlinked. The allele for smooth seeds (S) is
dominant over the allele for wrinkled seeds (s). The allele for
yellow seeds (Y) is dominant over the allele for green seeds (y).
If a plant of genotype Ssyy is crossed with a plant of genotype
ssYy, which offspring are recombinants?
A.
B.
C.
D.
SsYy and Ssyy
SsYy and ssYy
SsYy and ssyy
Ssyy and ssYy
Correct answer: C
(1 mark)
IB Exam Question
7. A polygenic character is controlled by two genes
each with two alleles. How many different possible
genotypes are there for this character?
(1 mark)
A.
B.
C.
D.
2
4
9
16
Correct answer: C
I.e. BBEE, BbEE, BBEe, BbEe, BBee, Bbee,
bbEE, bbEe, bbee
IB Exam Question
8. A woman who is a carrier of hemophilia
marries a man who is not affected. What are the
possible genotypes of their children?
(1 mark)
A. XHXh, XHXH, XHY, XhY
B. XHXh, XHXH, XHYh, XHYH
C. XHXh, XhXh, XHYh, XhYh
D. XHXh, XhXh, XHY, XhY
Correct answer: A
woman can give normal or hemophilic X allele
man can only give normal X allele
trait is sex-linked, meaning nothing will be on
the Y
IB Exam Question
9. Two genes A and B are linked together as shown below.
If the genes are far enough apart such that crossing over
between the alleles occurs occasionally, which statement is
true of the gametes?
A.
All of the gametes will be Ab and aB.
B.
There will be 25 % Ab, 25 % aB, 25 % ab and 25 % AB.
C.
There will be approximately equal numbers of Ab and
ab gametes.
D.
The number of Ab gametes will be greater than the
number of ab gametes.
Correct answer: D
Because Ab is the undisturbed gamete, ab is when crossing
over occurs, which only happens occasionally
IB Exam Question
10. Using an example you have studied, explain a cross
between two linked genes, including the way in which
recombinants are produced.
(9 marks)
linked genes occur on the same chromosome / chromatid;
genes (tend to be) inherited together / not separated/do not segregate
independently;
non-Mendelian ratio / not 9:3:3:1 / 1:1:1:1;
real example of two linked genes;
Award [1] for each of the following examples of a cross between two linked genes.
key for alleles involved in the example of a cross;
homozygous parental genotypes and phenotypes shown;
F1 genotype and phenotype shown / double heterozygote genotype and
phenotype;
possible F2 genotypes and phenotypes shown;
recombinants identified;
recombinants due to crossing over;
in prophase I of meiosis;
diagram / explanation of mutual exchange of parts of chromatids during
crossing over;
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