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Genes and Heredity
Classical Genetics
Heredity
Genetics is the study of heredity.
Heredity – the passing of traits from parents to
offspring
Heredity ensures that you have characteristics similar to
your parents (but not exact copy)
Genes – units of instruction (located on chromosomes)
that produce or influence a specific trait in the offspring
(ie. Eye color)
Genome – a cell’s total hereditary endowment of DNA
Biological traits are controlled by genes of both
parents but are unique to individual
~8,000,000 combinations are possible between
23 chromosomes from each parent
Alleles – two or more alternate forms of a gene
like brown hair vs. blonde hair
Gene locus – location of gene on a
chromosome
Alleles
Gene Locus
History
Scientists noticed traits were passed on, but how?
Aristotle – suggested through blood (bloodlines)
Early Naturalists – believed in hybrids where species
result from breeding between other species
Buffon (1700’s) – suggested head and limbs came
from father, the rest from the mother
1800’s – a blending of traits from both parents
Very late 1800’s – microscopes allowed for meiosis to
be observed, there was speculation about chromosomal
involvement
Gregor Mendel
Austrian Monk
1822 – 1884
Worked with garden peas to explain gene
inheritance for plants and heredity
“Father of Genetics”
https://www.youtube.com/watch?v=Mehz7tCxjSE
How Mendel's pea plants helped us
understand genetics Ted Ed
Why Peas???
1) Have number of visible traits that are
explainable in one of two ways
–
–
–
Ex) green vs. yellow peas
Ex) tall vs. short plants
Ex) wrinkled vs. smooth peas
2) Garden peas are self-fertilizing and cross-fertilizing
Self-fertilization (Pollination) – pollen produced from
male stamens attaches to female pistil inside same
flower.
Cross-fertilization – pollen from male stamen attaches
to female pistil inside another flower.
Mendel used cross-fertilization (removed stamen and
transferred pollen from one plant to another plants pistil)
He combined male and female sex cells of different
plants
Mendel’s experiments involved the crossing of
pea plants with different traits to see what traits
the offspring would have
Mendel's Experiment
Parental Cross (P)
Round x Wrinkled Seed
First Filial Generation (F1)
All Round
Second Filial Generation (F2)
3:1 ratio of round to wrinkled
Mendel’s Thoughts…
Why did the F1 generation produce all round
seeds?
Why did the F2 generation produce more round
that wrinkled seeds?
Does it make a difference if the characteristic
came from the male or female plant?
In order to answer these questions, Mendel
repeated the procedure with other characteristics
Mendel’s Discoveries
Discovered that one trait dominates another (no
matter what characteristic came from the male or
female sex cell)
Mendel established the general principles of
genetics (later to become his laws of
Inheritance)
Mendel’s Laws of Inheritance
Law of Unit Characteristics: traits are
controlled by factors called genes which occur
in pairs (one from each parent). Ex) RR, Rr, rr
Law of Dominance: the dominant form of a
trait prevents or masks the expression of the
recessive form. The dominant form is
represented by capital letters (R) and the
recessive form by lower case letters (r)
Law of Segregation: during gamete formation
(meiosis), homologous pairs separate and
gametes have one of each homologue
Law of Independent Assortment: genes which
are located on separate chromosomes assort
(separate) independently during meiosis. One
gene does not effect where any other gene will
move. In other words, all dominant genes do not
end up in the same gamete.
NOTE: Just because a trait is dominant, it does not
necessarily mean the majority of a population will
express that phenotype
Most Dutch people are blue-eyed and blonde which are
both recessive traits.
Dominant genes – determine the expression of the
genetic trait in offspring (capital letter)
Recessive genes – overruled by dominant genes
(lower case letter)
*Normally the letters of genes follow the dominant gene
name (capital) and will stay the same for recessive
genes (lower case)
Genetic Terminology
Phenotype: the observed or displayed form of
the genes carried by an individual (what you
see); results from interaction between genes and
the environment.
– Ex. Round seeds or wrinkled seeds
Genotype – the combination of the genes that
are carried on the chromosomes for a given trait
– Ex. Round seeds = RR, Rr
– Wrinkled Seeds = rr
Heterozygous – refers to the genotype in which
the gene pair are different. Often called a
hybrid or carrier. They would express the
dominant phenotype
– Ex. Rr = Round
Homozygous – refers to the genotype in which
both genes are identical. Often called pure
bred.
– Ex. Homozygous dominant = RR
– Ex. Homozygous recessive = rr
RATIOS
In terms of possible offspring phenotypes and
genotypes, you may find it easier to write them
as a ratio
– Genotypic ratio: homo dominant : heterozygous :
homo recessive
– Ex. 9 RR, 3Rr, 1 rr  9:3:1
– Phenotypic ratio: Dominant: Recessive
– Ex. 3 Round, 1 wrinkled  3:1
Monohybrid Crosses
Genetics problems are sometimes complex, so
we simplify them using punnet squares
In a punnet square, the gametes from one
parent are arranged along one side of the
square and the gametes of the other parent are
arranged along a second side
With this, you can derive all possible
combinations of gametes
Monohybrid crosses (cross of a single trait) are
the easiest to study
Ex) a plant heterozygous for seed shape is
crossed with another plant which is also
heterozygous
Parents:
Rr x
Rr
Possible gametes:
R r
R r
Punnet Square
R
r
R
RR
Rr
r
Rr
rr
Genotypic Ratio: RR: Rr: rr = 1:2:1
Phenotypic Ratio: Round:wrinkled = 3:1
Thus, there is a 3 in 4 or 75% chance of
producing a round seed plant when two
heterozygous plants are crossed (or a 1 in 4
chance of producing a wrinkled seed plant)
Practice Examples
1) Using the trait for seed shape (round dominant
to wrinkled), determine the phenotypic and
genotypic ratios for the following crosses:
a) Homozygous dominant x homozygous dominant
b) Homozygous dominant x heterozygous
Practice Examples
c) Homozygous dominant x homozygous recessive
d) Heterozygous x homozygous recessive
e) Homozygous recessive x homozygous recessive
2) Determine the phenotypic and genotypic ratios
for a cross between a homozygous dominant red
flower and a recessive white flower
3) Determine the phenotypic and genotypic ratios
for a cross between a man who is heterozygous
for a skin pigment and an albino woman (no skin
pigment is a recessive trait)
4) Determine the phenotypic and genotypic ratios
for a cross between two plants which are
heterozygous for height (tall is dominant).
5) Determine the phenotypic and genotypic ratios
for both the F1 and F2 generations for a cross
between a homozygous tall plant and a short
plant
6) Determine the phenotypic and genotypic ratios
for a cross between a heterozygous hornless
cow and a horned bull. What are the chances of
producing a horned offspring?
Test Cross
Performed to determine the genotype of a
dominant phenotype (by looking at them we
cannot tell if they are homozygous or
heterozygous)
Test cross is always performed between the
unknown genotype and a homozygous recessive
genotype
If the unknown is heterozygous, half of the
offspring will be dominant and half will be
recessive
If the unknown is homozygous dominant, all the
offspring will be dominant
Ex. A farmer wants to know if his white sheep is
pure bred or a hybrid so he crosses it with a
black sheep
White sheep = W ___
Black sheep = ww
w
w
W
w
Ww ww
Ww ww
w
w
W W
Ww Ww
Ww Ww
Multiple Alleles
In Mendel’s experiments there were only two
possible alleles and the dominant allele
controlled the trait.
However, it is common for there to be more than
two alleles for a particular trait
This is called multiple alleles
Ex) Fruit Flies (drosophila) can have eyes colored red,
apricot, honey, and white (but it is only possible to have
two of these different genes at one time)
There is a dominance hierarchy
Red is dominant to apricot, is dominant to honey, is
dominant to white
We don’t use capital and lower-case letters. Instead we
use capital letters with superscript numbers to express
different genes and their combinations
E1 – Red
E2 – Apricot
E3 – Honey
E4 – White
Determine the phenotypic ratios from a cross
between a fly with wild-type eye color (E1E4) to
one with apricot eyes (E2E3)
Note: Wild-type refers to the ‘normal’ allele (the
one found in majority of wild populations (not
necessarily the most dominant though)
Incomplete Dominance
Sometimes hybrids have a blending of traits (not
found by Mendel)
When two traits are equally dominant, they
interact to produce a new phenotype – this is
known as incomplete dominance
Two types of incomplete dominance:
– Intermediate inheritance or non-dominance
– Codominance
Intermediate inheritance
Results when neither allele is dominant
Hybrid phenotype is a blend or mixture of two
phenotypes
Ex) Snapdragons – Red x White = Pink
This is also notated differently (with superscripts)
FRFR
x
FW FW
= FRFW
Determine the F1 phenotypic ratio from the
crossing of two pink snapdragons
Codominance
Results when both alleles are dominant
Hybrid expresses both phenotypes
Ex) Shorthorn cattle = Red bull x White Cow =
Roan Calf
Roan calf has intermingled white and red hair
Both genes expressed at same time
Also expressed in notation with superscript
HRHR
x
HWHW =
HRHW
If a roan shorthorn cow is crossed with a white
shorthorn bull, what is the probability that the
offspring will be roan?
Blood Types
Most familiar example of multiple alleles and
codominance in humans is blood type
Most common typing system divides individuals
into four classes: A, B, AB, and O
There are three alleles for this:
IA
IB
i
IA and IB are codominant
i is recessive to both of these
Phenotype
A
Genotype
IAIA, IAi
B
IBIB, IBi
AB
IAIB
O
ii
If one parent has blood type A and the other has
blood type B, determine the parental genotypes
if the offspring had the following blood types:
A) ½ AB, ½ B
B) all AB
If one parent has blood type A and the other has
blood type B, determine the parental genotypes
if the offspring had the following blood types:
C) ½ AB, ½ A
D) ¼ A, ¼ B, ¼ AB, ¼ O
Lethal Alleles
When the expression or phenotype of a gene
causes death, it is called a lethal allele
Lethal genes are usually recessive or mutant
genes with a low frequency of occurrences in
the population
Occasionally a fully dominant lethal allele can
form
Examples
Dihybrid Crosses
What’s the probability of producing a yellow if
two heterozygous plants (Yy) are crossed?
What’s the probability of producing a round seed
if two heterozygous plants (Rr) are crossed?
So what’s the probability of producing a yellow,
round seeded pea plant if two plants
heterozygous for both traits are crossed?
A cross that deals with two different traits is
called a dihybrid cross.
Dihybrid crosses can be solved with a larger 16
square punnet square
Mendel’s law of Independent Assortment was
created from his study of dihybrid crosses – this
law states that genes assort independently (one
gene does not influence the inheritance of
another when located on different
chromosomes)
Mendel cross-pollinated pure-breeding plants
that produced yellow, round seed coats with
pure-breeding plants that produced green,
wrinkled seed coats
–
–
–
–
Pure breeding round coat = RR
Pure breeding wrinkled coat = rr
Pure yellow = YY
Pure green = yy
Genotypes:
– Pure yellow, round parent = YYRR
– Pure green, wrinkled parent = yyrr
The F1 offspring produced from this cross are
heterozygous for both yellow and round
genotypes (YyRr)
In a dihybrid cross, there are 4 different possible
phenotypes
Ex) Cross YyRr x YyRr
– There are four possible gamete combinations
– Therefore our punnet square is no longer 2 x 2, but it
is 4 x 4
– Remember that seed coat gene is independent of
seed color
Probability
Probability = # of chances for event/Total # of
combinations
Two rules to remember:
1)
Rule of Independent events – Previous events do not
affect future events
Ex) Tossing two heads in a row – chances of tails is still 50%
2) Product Rule – Probability of independent events
occurring simultaneously is equal to the product of these
events occurring separately
Ex) Chances of tossing heads after two tosses is ½, but
chances of tossing heads three times in a row is:
½ x ½ x ½ = 1/8
You can calculate dihybrids by using two
monohybrids and using the product rule
Y
y
Y
YY
Yy
y
Yy
yy
R
r
R
RR
Rr
r
Rr
rr
Chances of getting yyrr is ¼ x ¼ = 1/16
Chances of getting YyRr is ½ x ½ = 1/4
You can also calculate dihybrids using a dihybrid
punnet square
Dihybrid Punnet square
YR
Yr
yR
yr
YR
YYRR
YYRr
YyRR
YyRr
Yr
YYRr
YYrr
YyRr
Yyrr
yR
YyRR
YyRr
yyRR
yyRr
yr
YyRr
Yyrr
yyRr
yyrr
YYRR – 1/16 (6.25%)
YYRr – 2/16 = 1/8 (12.5%)
YYrr – 1/16 (6.25%)
YyRR – 2/16 =1/8 (12.5%)
YyRr – 4/16 = 1/4 (25%)
Yyrr – 2/16 = 1/8 (12.5%)
yyRR – 1/16 (6.25%)
yyRr – 2/16 = 1/8 (12.5%)
Yyrr – 1/16 (6.25%)
Remember that all fractions should equal 16/16
when added together (or all percentages should
add to 100%)
Ratios for Dihybrid crosses
For Dihybrid crosses you MUST identify what the ratio
stands for!
Ex)
Genotype:
YYRR:YYRr:YYrr:YyRR:YyRr:Yyrr:yyRR:yyRr:yyrr
1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1 (all add to 16)
Phenotype:
Yellow,round:yellow,wrinkled:green, round:green, wrinkled
9 : 3 : 3: 1(all add to 16)
NOTE: A dihybrid cross of two parents
heterozygous for both traits will always give a
9:3:3:1 phenotypic ratio!
If n = number of traits, 2n = number of
phenotypes and 3n = number of genotypes
Examples
What are the gametes produced from the
following genotypes (You should list four for
each even if they are the same):
– BbRR
– BBrr
What is the probability of producing a tall, red
flowered plant if two plants heterozygous for
both traits are crossed (tall and red are both
dominant traits)?
What is the probability of having a blue eyed, blonde haired baby
if both parents are heterozygous for eye trait and one is
heterozygous for hair color and the other is blonde? (Reminder:
Blue eyes and blonde hair are recessive traits)
When a fruit fly of genotype Mm Nn Oo is mated to another fruit
fly of identical genotype what is the probability of the offspring
having the following genotypes:
a) MM Nn OO
b) mm NN Oo
c) Mm Nn Oo
Gene Interaction
Some traits are regulated by more than one
gene
These are called polygenic genes
– Ex) Skin color, eye color, height
Polygenic Characteristics
1) Epistatic gene interaction – action of one gene (or
set of genes) masks the expression of other, nonallelic genes
Ex) Dog coat
B = Black
b = brown
But:
W = prevents pigment forming
w = doesn’t prevent pigment forming
Therefore genotype wwBb = Black, but WwBb = white
*W is an epistatic gene (interferes with B)
Another example:
- at least 2 pairs of genes control eye color
- B = brown
- b = blue
- Another pair is
- A = pigment
- a = no pigment (albino)
- Gene pair aa is epistatic to B and b because it
produces no pigmented eyes
Epigenetics
http://www.youtube.com/watch?v=kp1bZEUgqVI
In humans, there is a dominant
allele that causes vitiligo, where
small-unpigmented spots
appear on the body.
Also, there is a recessive allele
for another gene that causes
albinism, which causes the
entire body to be unpigmented.
Vitiligo cannot be seen in
albinos
Using the Dog coat example:
Give the genotypes/phenotypes of the possible
offspring of the mating of bbAa x Bbaa
2) Complementary Interaction – produce an effect
(phenotype) that neither can produce by itself
Ex) Chicken combs
- R = Rose comb
- P = Pea comb (located on different chromosome)
- R + P = Walnut comb
- Absence of R + P = Single comb
* This is different from intermediate inheritance because
those were genes located on the same chromosomes
(alleles)
Enzymes
Enzyme – protein molecule that speeds up chemical
reaction in cells
Ex) Clover plant – production of cyanide (regulated
by two genes)
– G = provides info for production of enzyme 1 (glucoside)
– g = mutation – enzyme not produced
– H = provides info for production of enzyme 2 (turns
glucocide into cyanide)
– h = mutation – enzyme not produced
– GgHh will produce Cyanide. ggHH or GGhh will not.
3) Pleiotropic genes – affect more than one trait
Ex) mutated gene causing sickle cell anemia
(blood disorder)
- HbA – Normal Hemoglobin
- HbS – mutation – causes sickle cell anemia
Homozygous HbS hemoglobin can still carry
oxygen, but when it gives oxygen to the body,
the molecules become interlocked and change
the shape of red blood cells (sickle shape)
These blood cells do not pass through
capillaries, oxygen is not delivered, normal organ
function is impaired.
In addition to RBCs changing shape, sickle cell
people have other symptoms:
– Enlarged spleens, rheumatism, pneumonia, and
heart, kidney, lung, and muscle damage
Ex) Marfan’s syndrome
Inability to produce normal connective tissue due
to one gene
Symptoms: Eye, skeleton, cardiovascular
defects
http://www.youtube.com/watch?v=ab_B0lZqq6M
Breeding Patterns
Artificial selection (Selective Breeding) –
crossing of desired traits from plants or animals
to produce offspring with both characteristics
Used by farmers and ranchers for years
Improves domestic varieties of plants and
animals
– Ex) Canola that can germinate and grows quickly in
cold climates
Purebreds – genotypes are regulated by
inbreeding (homozygous traits). Similar
phenotypes are selected for breeding
Also, new varieties of plants or animals can be
created through hybridization – this is an attempt
to blend desirable but different traits
Heredity: Crash Course Biology #9
http://www.youtube.com/watch?v=CBezq1fFUEA
The Source of Heredity
The Chromosomal Theory
– Chromosomes carry genes (units of heredity
structure)
– Paired chromosomes segregate during meiosis.
Each gamete has ½ the number of chromosomes as
in a somatic cell
– Somatic cell – all cells except sex cells
– Chromosomes assort independently during meiosis
– Each chromosome contains many genes
– Autosomes – all body chromosomes EXCEPT sex
chromosomes
Morgan’s experiments
Thomas Hunt Morgan (1866 – 1945) – American
geneticist who used fruit flies (Drosophila
melanogaster) to study Mendel’s principles of
inheritance
Why Fruit Flies?
Rapid Reproduction
– >100 eggs/mating
– Reproduce soon after leaving eggs
– Good for genetics probability!
Short Life cycle (10 – 15 days)
– Study many generations in short period of time
Small size
– Many housed in one test tube with little food supply
Males easily distinguished from females
Discovered mutations linked to certain traits.
Supported the theory that genes responsible for
traits were located on the same chromosome
He found a white-eyed male among many redeyed offspring
– Conclusion: white-eyed trait = mutation
Mated white-eyed male with red-eyed female
and all F1 generation had red eyes (red
dominant). But when crossing two F1 hybrids for
F2 generation, he got ¾ red and ¼ white.
This was explained by Mendel, but he also found
that only males had white eyes (no females)
After further crosses, found females could
express the white mutation, but not in the F1 or
F2 generation
To explain this, Morgan studied cells from
salivary glands of Drosophila (8 chromosome)
He stained chromosomes and viewed under a
microscope
Found males had only 3 homologous
chromosome pairs (sex chromosomes not
identical) and females had 4
– XX = female
– XY = male
Conclusion: Because X and Y are not
completely homologous, they must contain
different genes (unlike autosomes)
Drosophila eye color located on X chromosome.
Because Y is not identical, it does not carry the
trait.
Sex linked Traits – controlled by genes located
on the sex chromosomes
Therefore:
– Red-eyed female = XRXR or XRXr
– White-eyed female = XrXr
– White-eyed male = XrY
**XR – Red gene dominant and located on X
chromosome
Punnet Square Explaination
XR
XR
Xr
XRXr
XRXr
Y
XRY
XRY
XR
Xr
XR
XRXR
XRXr
Y
XRY
XrY
F1
F2
Male offspring always inherit a X-linked trait from
their mother (father supplies Y chromosome
each time)
Notice the females can have 3 possible
genotypes and males only have 2 – Males
CANNOT be heterozygous for a sex-linked trait
(not a carrier – either has it or doesn’t)
To get a white-eyed female, a female with at
least one white gene must be crossed with a
white-eyed male
Sex Determination
Sex-linked genes are found in humans (pg 601)
– Ex) Red-green color blindness
– Determined by a recessive trait located on the X
chromosome
– Males colorblind more than females because
females need 2 recessive genes and males only
need one (other gene is Y)
– 10% of males experience color blindness compared
to only 0.4% of females
Examples
1) Colorblindness is inherited
as a sex-linked recessive
disease. An affected male
marries a heterozygous female.
– Draw the Punnett square of the
possible offspring
– What is the chance that they will
have an affected child?
– Could any of their daughters be
affected?
2) Suppose hairy ears is
inherited as a Y-linked trait. A
man with hairy ears marries a
woman with normal ears.
– Draw a punnett square of the
possible offspring
– Could any of their children have
hairy ears?
3) This disease is inherited as a sex-linked
dominant disease. An affected male marries a
homozygous recessive female.
– Draw a punnett square of the possible offspring
– What is the likelihood that any child they will
have will be affected
– Would the parents of the father (grandparents
of these offspring) have been affected?
– What is the likelihood that their male children
will be affected?
– What is the likelihood that their female children
will be affected?
Barr Bodies
Small dark spots of chromatin, located in the nuclei of
female mammalian cells (not found in male cells)
1961 – Mary Lyon – Proposed dark spot was a
dormant X chromosome
– One of the X chromosomes become inactive (but this
inactive chromosome varies between cells)
– This means that not all female cells are identical
(depending on which of the two X chromosomes is active)
– Heterozygous females who carry a lethal gene on their Xchromosome may escape disease because gene is only
active in 50% of cells.
Chimera Cats and Your Mom
https://www.youtube.com/watch?v=eSMQcy5Re
QQ&sns=em
Calico Cats Example
Male Cats = XBY – Black or XOY – orange
Female Cats = XBXB – Black or XOXO – Orange
or XBXO – black and orange patchwork
Areas which are orange – XO active; areas which
are black – XB active.
Males cannot be calico unless nondisjunction
occurs and an extra X chromosome appears
(XXY) – These cats are infertile.
TDF
Humans = 46 chromosomes
Female = 23 pairs homologous chromosomes
(22 autosomes, 2 X sex chromosomes)
Male = 22 pairs of homologous chromosomes
and one X and one Y sex chromosomes
Human X chromosome carries 100 – 200
different genetic traits
Y chromosome codes only for gender – this
gene is called TDF
TDF
TDF = Testes determining factor
TDF is not activated until the 6th or 7th week of
pregnancy therefore a male fetus is no different
from a female fetus until then
The testes develop in the body like ovaries until
TDF activated and then they descend
Testicular feminization syndrome
XY individual appear female
Positive test for male Y chromatin
Negative test for female chromatin
Syndrome caused by gene mutation on the X
chromosome that acts only in the XY zygotes
These individuals do not react to injections of
male sex hormones – therefore they don’t
display muscle development of males
Fragile X syndrome
A person with fragile X syndrome has a mutation
in the X chromosome. Negative test for female
chromatin
Symptoms of mental disability range depending
on whether the person is male (more severe) or
female (less severe)
Can be inherited from either the mother or the
father
Crossing over
Morgan discovered that specific genes are
located on specific chromosomes
– Ex) Eye color for Drosophila on X chromosomes
Remember: Two or more genes that are on
different chromosomes segregate independently
But what about genes on the same
chromosome?
Morgan concluded that the gene for wing shape
and gene for body color were located on the
same chromosome (They do not segregate
independently)
Linked Genes – located on the same
chromosome and tend to be transmitted together
Crossing-over can provide new combinations of
genes
– Occurs during synapsis of meiosis
– Single chromosome can change as it passes from
generation to generation
– Figure 21.9
Gametes with chromosomes that recombine
(through crossing over) have sections that are
maternal and other sections that are paternal
Example
A = wild-type body color
a = black body color
B = Straight wings
b = curled wings
AABB x aabb = Expect all AaBb
- But instead find some Aabb (wild-type, curled)
and aaBb (black, straight)
- This is due to crossing over
Frequency of Crossing over
Crossover % =
number of recombinations x 100
Total number of offspring
AABB x aabb
F1 – 282 – wild-type, straight wings (expected)
9 – black, straight (crossing over result)
9 – wild-type, curled (crossing over result)
= 18/300 x 100 = 6%
Mapping Chromosomes
Genes on the same chromosome segregate
together
Gene Marker – recessive traits that are
expressed in the recessive phenotype of an
organism. Markers used to identify other genes
on the same chromosome
Ex) Appearance of white eyes (recessive)
informs us of other genes located on the same
chromosome.
Crossing over alters gene linkages along a
chromosome. If new segments of DNA are
exchanged at marker sites, mapping is
impossible
Geneticists determine the position of the genes
by their crossing over frequency
– 1% frequency = genes close together
– 12% frequency = genes much farther apart
The greater the frequency, the greater the
map distance!
Crossover frequency of 5% means two genes
are 5 map units apart
16% frequency is much farther apart (16 map
units)
Special stains have been developed to create
bands of color used to identify positions of genes
along a chromosome.
Examples
Assume crossover frequency between gene A and B is
12%, between B and C is 7%, and between A and C is
5%. Create the genetic map.
– If we assume gene A is in the middle, then B – A and A – C is
equal to B – C
• 12 + 5 = 7 – Therefore A is not in the middle
– If we assume gene B is in the middle, then A – b and B – C is
equal to A – C.
• 12 + 7 = 5 – Therefore B is not in the middle
– If we assume gene C is in the middle, then A – C and C – B
is equal to A – B
• 5 + 7 = 12 – Therefore C is in the middle