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Theoretical genetics
SA Definitions Homework
Gene -
A unit or section of DNA that is inherited and encodes a particular
protein
Allele -
The form of a gene, in a diploid organism there will always be a pair
of alleles for each gene – one from each parent
Locus -
The location of a gene on a chromosome – the position is the locus
(plural loci)
Phenotype -
The appearance of a characteristic. How a genetic factor
manifests itself e.g. eye colour, disease
Genotype -
The genetic factors that determine a characteristic. The
combination of alleles that an organism possesses for a gene
Definitions
When one trait appears and the other does not it is said to be
Dominant - ‘dominant’ over the other
Recessive - When two copies of the allele are needed for the trait to appear
Codominant - When both characteristics appear together in the heterozygote
e.g. AB blood group
Homozygous Heterozygous -
when the alleles of a particular gene are identical.
when the alleles of a particular gene are different.
Diploid - a term applied to cells in which the nucleus contains 2
sets of chromosomes.
Haploid - a term referring to cells that contain only a single copy of
each chromosome eg the sex cells or gametes
New Definition
2 or more genes that are located on the same chromosome and are
Linkage - normally inherited together
Theoretical genetics
Learning objectives
Success criteria
 Understand the basics of
 Explain the terms allele,
theoretical genetics
locus, phenotype, genotype,
dominant, codominant and
recessive
 Use genetic diagrams to solve
problems
Inheritance
 Gregor Mendel (often called the father
of genetics).
 He investigated inheritance by crossing
varieties of pea plants that had
different characteristics.
 Monohybrid crosses – involving one
gene (same as GCSE!)
GCSE Recap
 Drawing monohybrid crosses
Predicting Outcomes of Genetic
Crosses
 Some characteristics are determined by only two genes
(one from the mother, one from the father).
 Draw a genetic diagram to predict the outcome
Boy
or
Girl?
Monohybrid
Cross
 Male: XY
Female: XX
 This means sperm produced will either have the X
gene or the Y gene
 Eggs produce will all have the X gene
Possible Eggs
Possible sperm
GCSE Task
 Have a go at, at least one question per section,
remember to write down the question number so you
can SA
Answers
1.
T
T
t
2.
TT
Tt
T
t
t
t
Tt
Tt
3.
Tt
tt
T
T
T
t
4.
Tt
Tt
T
TT
Tt
t
T
t
TT
Tt
t
Tt
tt
Tt
tt
Answers
5.
q
Q
q
6.
Q
q
Qq
qq
q
7.
Qq
qq
Q
Q
QQ
QQ
Qq
Qq
q
q
Q
Q
Qq
Qq
Qq
Qq
q
Q
q
Q
QQ
Qq
8.
Qq
qq
9.
R
Answers
R
r
10.
R
R
r
RR
Rr
Rr
rr
Answer = 25%
R
r
RR
Rr
RR
Rr
Answer = 0%
11.
r
R
r
12.
Rr
Rr
rr
rr
Answer = 50%
r
R
R
r
r
Rr
Rr
Rr
Rr
Answer = 0%
13.
A
a
Answers
a
a
14.
Aa
aa
Aa
aa
Answer = 50%
A
a
a
15.
A
Aa
Aa
Aa
Aa
Answer = 0%
B
B
b
16.
BB
Bb
Bb
bb
Answer = 25%
B
B
B
b
B
BB
BB
BB
BB
Answer = 0%
17.
B
B
Answers
B
b
19.
BB
BB
Bb
Bb
Answer = 100%
18. Someone who has a faulty
and a working allele, they do
not have the disease.
B
B
b
20.
b
BB
Bb
Bb
bb
Answer = 25%
B
b
b
B
Bb
Bb
Bb
Bb
Answer = 100%
A Level: Monohybrid Crosses
 Looking at one characteristic
 Shows what the offspring genotypes could be
 This first generation is known as F1
 Crossing this first generation for the second
generation would be known as F2
Steps to take when drawing a monohybrid cross
1. Choose one letter to represent
N normal wings
n – vestigial wings
Parents
genotypes
NN
Gametes
N
Genotypes for F2
generation
NN
Normal
Ratio of F2 generation
n
Nn
N
Gametes
nn
N
Genotypes for F1
generation
Nn
Phenotypes
x
Nn
n
Nn
Normal
3 : 1
n
N
Nn
n
Nn
Normal
nn
dominant and recessive alleles
2. Work out what the parental
genotypes are and then what gametes
they might produce
3. Work out what the offspring could
be (All heterozygous Nn)
4. Look at what the phenotypes are in
the F1 generation (All normal)
5. Second cross of offspring
6. Show gametes of F1 generation
7. Work out what the offspring could
be
8. Look at what the phenotypes are in
the F2 generation
9. Work out ratios
Vestigial
Same thing using punnett squares
1. Choose one letter to represent
N normal wings
n – vestigial wings
Parents
genotypes
Gametes
Genotypes for F1
generation
Genotypes for F2
Generation
Ratio of F2 generation
NN
N
x
nn
N
n
N
N
n
Nn Nn
n
Nn Nn
N
n
N
NN Nn
n
Nn
3 : 1
nn
n
dominant and recessive alleles
2. Work out what the parental
genotypes are and then what gametes
they might produce
3. Using a punnett square work out
what the offspring of the F1 generation
could be (All heterozygous Nn)
4. Look at what the phenotypes are in
the F1 generation (All normal)
5. Second cross of offspring
6. Using a punnett square work out
what the offspring of the F2 generation
could be
7. Look at what the phenotypes are in
the F2 generation
8. Work out ratios
Exam question
1.The ability to taste the chemical PTC is determined by a single
gene in humans with the ability to taste given by the dominant
allele T and inability to taste by the recessive allele t. Suppose
two heterozygous tasters (Tt) have a large family.
a. Predict the proportion of their children who will be tasters and
non-tasters. Use a Punnett square to illustrate how you make
these predictions.
b. What is the likelihood that their first child will be a taster?
c. What is the likelihood that their fourth child will be a taster?
1.The ability to taste the chemical PTC is determined by a single gene in
humans with the ability to taste given by the dominant allele T and
inability to taste by the recessive allele t. Suppose two heterozygous
tasters (Tt) have a large family.
a. Predict the proportion of their children who will be tasters and nontasters. Use a Punnett square to illustrate how you make these
predictions.
Heterozygous tasters
Tt X Tt
T
t
T t
TT Tt
Tt tt
75 % Taster and 25% non-tasters
b. What is the likelihood that their first child will be a taster?
75 %
c. What is the likelihood that their fourth child will be a taster?
75%
EXAM QUESTION
Q2. In certain trees, smooth bark is dominant over wrinkled.
a. Cross two trees that are heterozygous for smooth bark.
b. If there are 100 offspring produced, how many will have
wrinkled bark.
Trees : Allele Key
B = Smooth and b = wrinkled
Heterozygote X Heterozygote
Phenotype: Smooth X Smooth
Genotype:
Bb X Bb
Offspring Genotype:
Phenotype Ratio:
B
b
B
BB
Bb
b
Bb
bb
BB :2Bb : bb
3 Smooth: 1 Wrinkled
offspring : 75 Smooth : 25 Wrinkled
Monohybrid Crosses – try these situations. What will the
offspring be? Outline parental genotype, F1 genotype
and phenotype
Unattached and attached ear lobes. Unattached is
dominant. Heterozygous female and homozygous
recessive male.
2. Tall and short plants. Tall is dominant. Homozygous
dominant female and heterozygous male.
1.
Monohybrid Crosses – try these situations. What will the
offspring be?
Unattached and attached ear lobes. Unattached is dominant.
Heterozygous female and homozygous recessive male.
Parental Genotype: Ll x ll
F1 Genotype: Ll or ll
F1: Phenotype
L
l
l
Ll
ll
50% Unattached
l
Ll
ll
50% Attached
Tall and short plants. Tall is dominant. Homozygous dominant
female and heterozygous male.
Parental Genotype: PPxPp
P
P
F1 Genotype: PP or Pp
P
PP
PP
F1: Phenotype
p
Pp
Pp
100% Tall
GOLDEN RULES
 Homozygous dominant x homozygous recessive = all
heterozygous.
 Heterozygous x heterozygous = 3 : 1 ratio
 Heterozygous x homozygous recessive (test cross) = 1 : 1
Codominance
 What is codominance?
 When both alleles are present they both affect the
phenotype (equally dominant)
 For example: A & B in blood typing
Snapdragon Flowers
 An example of codominance: both alleles are
expressed in the phenotype when the plant is
heterozygous
 Alleles code for red or white
 When one of each allele is present then the flower is
pink
Snapdragon Flowers – more detail
Homozygous Red
 The 2 alleles for red flowers code for an enzyme which
catalyses red pigment formation, hence the flowers are
red
Homozygous White
 The 2 alleles for white flowers do not code for the
enzyme, so no pigment is produced, hence the flowers
are white
Heterozygous
 The one red allele codes for some pigment, but only
enough to make the flower pink, not red
Codominance: Punnet Squares
 Do not use one upper and one lower case letter, as we are
not talking about recessive and dominant alleles
 Use 2 different letters
Snapdragon Flowers
 Red flowers = CRCR
 White flowers = CWCW
 Pink flowers = CRCW
 TASK:
 Draw a punnett square for crossing a red flower with a white
flower.
 Once you find out the F1 generation, draw a punnett square
for crossing 2 of these plants to find out the F2 generation
Codominance: F1 Generation
 Crossing a red flower with a white flower
Male gametes
Female gametes
CR
CR
CW
CRCW
CRCW
CW
CRCW
CRCW
 All offspring are pink
Codominance: F2 Generation
Female gametes
 Crossing the F1 to find the F2 (Crossing 2 pink flowers)
Male gametes
CR
CW
CR
CRCR
CRCW
CW
CRCW
CWCW
 Offspring = 50% pink, 25% red, 25% white
Homework
 Research and prepare a leaflet on sickle cell anaemia.
It should include
 Causes at the genetic level
 Affect on protein structure
 Codominance
 Symptoms
 Worldwide distribution
 The link to malaria