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Genetics - the science of heredity
the wild-type, individuals with traits that are most common in nature
Theophrastus proposed that male flowers caused female flowers to ripen
Hippocrates speculated that "seeds" were produced by various body parts and transmitted to
offspring at the time of conception
Aristotle thought that male and female semen mixed at conception.
Pangenesis - pangenes (particles) travel from each part of the body to the egg or sperm and are
passed on to the next generation, furthermore changes that occur in parts of the body during an
organism's life can also be passed on this way.
The "blending" hypothesis states that hereditary materials contributed by the male and female
parents mix in forming the offspring in the way that blue and yellow paints mix to make green.
Gregor Mendel, developed the fundamental principles that would become the modern science of
genetics.
Mendel demonstrated that heritable properties are parceled out in discrete units, independently
inherited. These eventually were termed genes. In a paper published in 1866, Mendel correctly
argued that parents pass on to their offspring discrete heritable factors. Mendel stressed that the
heritable factors (now called genes) retain their individuality generation after generation.
In humans a typical body cell called a
somatic cell has 46 chromosomes. When we
examine these human chromosomes in
metaphase of mitosis, we see that each
duplicated chromosome has a twin that nearly
always is identical in length and centromere
position. So altogether we see 23 such
matched pairs.
This same pattern is seen in other
species which have different numbers of
chromosomes, but which are also generally in
matched pairs.
Here we see a human Karyotype (a
karyotype is the whole group of
characteristics that allows the identification of
a particular chromosomal set (i.e., the number
of chromosomes, relative size, position of
centromere, length of arms, secondary
constrictions, banding patterns and other
morphological characteristics) here we see a
karyotype of a male: The human karyotype
has 46 chromosomes, 44 + XY (in the male)
and 44 + XX (in the female). Of the 23 pairs of
chromosomes 22 are autosomes (found in
both males and females) and the other pair
of chromosomes, the sex chromosomes
determine a person's gender.
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For both autosomes and sex chromosomes we inherit one chromosome of the pair from
our mother and one from our father
The two chromosomes of such a pair are
called homologous chromosomes
because they both carry genes controlling
the same inherited characteristics. For
example if a gene for flower color is located
at a particular place, or locus (plural loci)
on one chromosome - within the narrow
lighter band in this figure for instance - then
the other chromosome of the homologous
pair also has a gene for flower color at that
locus. However, the two homologues may
have different versions of the flower-color
gene, perhaps, specifying different flower
colors.
Humans have between 100,000 and
300,000 genes with at estimated 3 billion
base pairs
For each inherited characteristic, an organism has two genes, one from each parent and these
genes may both be the same allele, or they may be different alleles, in the case of some of the
genes both alleles are the same (aa) whereas for others there are different alleles (Pp and Bb).
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Alleles of a gene reside at the same locus on homologous chromosomes.
Alleles are alternate forms of the gene that reside at the same gene loci on the pair of
chromosomes.
We use a capitol letter to denote a dominant gene, P and a lower case letter to denote a
recessive gene, a. If you inherit the same allele from each parent, then this is called
homozygous. PP is homozygous dominant and pp is homozygous recessive.
If you inherit a different allele from your mother and your father, then you are said to be
heterozygous at that gene loci.
Mendel's 4 hypotheses:
1. There are alternative forms of genes, which are the units that determine heritable traits. These
alternative forms are now called alleles. (an example is the gene for flower color in pea exists in
one form for purple an another for white)
2. For each inherited characteristic, an organism has two genes, one from each parent. These
genes may both be the same allele, or they may be different alleles.
3. A sperm or egg carries only one allele for each inherited trait, because allele pairs separate
(segregate) from each other during the production of gametes. When sperm and egg unite at
fertilization, each contributes its allele, restoring the paired condition in the offspring.
4. When the two genes of a pair are different alleles and one is fully expressed, while the other has
no noticeable effect on the organism's appearance, the alleles are called the dominant allele and
the recessive allele, respectively.
How did Mendel come to this
incredible series of hypotheses?
Between 1856 and 1863 Mendel
patiently cultivated and tested at least 28 000 pea plants,
carefully analyzing seven pairs of characteristics for
comparison, such as shape of seed, color of seed, tall
stemmed and short stemmed and tall plants and short
plants. Mendel was able to have strict control over matings
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because in nature pea plants selffertilize. This is due to the fact that the
male and female parts (the stamen
and pistil or carpel) are enclosed
within the petals and pollen will fall on
the stamen of the same flower. By
covering a flower with a small bag so
that no pollen from another plant could
reach the pistil (carpel) Mendel could
ensure a self-fertilization.
So Mendel would always knew the parentage of the new plants. Mendel worked on this for
several years, carefully collecting the seeds produced by the plants.
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The success Mendel had was not only due to
his experimental approach and choice of
organism, but also to the set of characteristics
he chose to study. In his experiments Mendel
followed seven characteristics, each of which
occurs in two distinct forms that is either tall
or short (never medium height) either white or
purple flowers (never intermediate color
etc…)
Mendel tested all 34 varieties of peas available to him through seed dealers. The garden
peas were planted and studied for eight years. Each character studied had two distinct forms, such
as tall or short plant height, or smooth or wrinkled seeds. Mendel's experiments used some 28,000
pea plants. Mendel worked with his plants until he was sure he had true-breeding varieties - that
is, varieties for which self-fertilization produced offspring all identical to the parent. For instance,
he identified a purple-flowered variety that, when self-fertilized produced offspring plants that all
had purple flowers.
He was now ready to ask what would happen when he crossed different varieties with each
other.
For example what offspring would result if plants with purple flowers and plants with white
flowers were cross-fertilized. The offspring of two different varieties are called hybrids, and the
cross-fertilization is called a hybridization, or more simply a cross. The parental plants are called
the P generation (P for parental), and their hybrid offspring are the F1 generation (F for filial). When
the F1 offspring self-fertilize or fertilize with each other their offspring (the next generation of plants)
is referred to as the F2 generation.
Mendel's contribution was unique because of his methodical approach to a definite problem,
use of clear-cut variables and application of mathematics (statistics) to the problem. Using pea
plants and statistical methods, Mendel was able to demonstrate that traits were passed from each
parent to their offspring through the inheritance of genes. Mendel performed lots of experiments
where he tracked the inheritance of a single characteristic, such as flower color. The results
Mendel obtained led him to formulate some important hypotheses about inheritance.
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A monohybrid cross between true-breeding
parents (the P generation) one pea plant
with white flowers and one with purple
flowers. Mendel discovered that F1 plants
(monohybrids) produced from this cross all
had purple flowers. These flowers were truly
purple, that is they weren't light purple as
would've been predicted by the blending
hypothesis. Did this mean the heritable factor
for white flowers was now lost as a result of
the hybridization?
To answer this Mendel bred F1
generation plants. Out of 929 plants,
Mendel found that 705 (about 3/4) and
purple flowers and 224 (about 1/4) had
white flowers, a ratio of about three
plants with purple flowers to one plant
with white flowers in the F2 generation.
Mendel concluded that the "heritable
factor for white flowers didn't disappear in
the F1 plants, but that only the purpleflower factor was affecting F1 flower
color. He also deduced that the F1 plants
must carry two factors, for the flowercolor characteristic, one for purple and
one for white. From these results and
many others and using and statistical
methods Mendel developed 4
hypotheses.
When parent plants differ in only a single characteristic, the hybridization is called a monohybrid
cross.
When a true breeding organism which has a pair of identical alleles for a characteristic it is called
homozygous.
An individual is heterozygous when it has two different alleles for a given gene.
When the two genes of a pair are different alleles and one is fully expressed, while the other has
no noticeable effect on the organism's appearance, the alleles are called the dominant allele and
the recessive allele, respectively.
The dominant allele then is the allele in a heterozygote which determines the phenotype with
respect to that particular gene.
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The recessive allele is the allele in a heterozygous individual that has no effect on the
phenotype.
The phenotype is of course the expressed traits of an individual, whereas the genotype is the
genetic makeup of the organism
And finally a Punett square is a diagram used in the study of inheritance to show the results of
random fertilization. In it we can see the possible combination of gametes. For example if T is
dominant over t and results in a tall plant (compared to a short plant) the following cross:
eggs
sperm
T
t
Tt
T
t
TT
Tt
tt
will result in 3 tall plants to 1 short plant (phenotypes), and 1 homozygous dominant genotype to 2
heterozygous, to 1 homozygous recessive genotype.
Because Mendel had a strong background in statistics, he understood that the segregation of
alleles during gamete formation and the re-forming of pairs at fertilization obey the laws of
probability - the same rules that apply to the tossing of coins, and the rolling of dice. Mendel also
understood the statistical nature of inheritance, he knew that he needed to obtain large samples
(that is count lots of offspring from his crosses) before he could begin to interpret the patterns of
inheritance he was seeing.
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How do the rules of probability apply to inheritance?
The probability scale ranges from 0 to 1. If something is certain to happen it has a
probability of 1, and an event that is certain not to happen has a probability of 0. The probability of
all possible outcomes must add up to 1. With a coin, the chance of tossing heads is 1/2, and the
chance of tossing tails is 1/2. In a standard deck of cards, the chance of drawing the jack of
diamonds is 1/52, and the chance of drawing any other card in the deck is 51/52.
For each toss of the coin, the
probability of heads is 1/2. That
means that the outcome of any
particular toss is unaffected by what
has happened on previous attempts.
In other words each toss is an
independent event.
So if we toss two coins
simultaneously, the outcome for each
coin is an independent event,
unaffected by the other coin. What is
the chance that both coins will land
heads up? The probability of such a
compound event is the product of
the separate probabilities of the
independent events - for the coins,
1/2 x 1/2 = 1/4. This is called the rule
of multiplication, and holds true for
genetics as well as coin tosses
If we look at a cross of two heterozygous
individuals , the probability that an egg will
have a c allele is 1/2 and the probability
that a sperm will have a c allele is 1/2 so
the probability that they will join to make a
cc genotype is 1/2 x 1/2 = 1/4.
Now let's consider how we can
calculate the probability of a heterozygous
offspring. As we can see in the Punnett
square there are two ways that this can
come about, that is the dominant allele
can be donated by the sperm and the
recessive by the female or the other way
around
.
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The probability that an event can occur in two or more alternative ways is the sum of the
separate probabilities of the different ways known as the rule of addition. Using this rule we can
calculate the probability of an F2 heterozygote as 1/4 + 1/4 =1/2.
Although many traits in human are controlled by several genes working together, there are several
that are known to be determined by simple dominant - recessive inheritance at one gene locus. If
we call the dominant allele of any such gene A, the dominant phenotype results from either the
Homozygous genotype AA or the heterozygous genotype Aa. Recessive genotypes will result
from the homozygous genotype aa.
Predicting outcomes using Punett squares: Procedure for solving Genetics Problems:
1. Read problem and record question(s) to be answered.
2. Decide on notation for gene and alleles and record decision.
3. Decide on the genotype for the two phenotypes.
4. Decide on genotypes of parents and record decision.
5. Decide on type and proportion of different gametes in both parents and record decision.
6. Make required genetic crosses using Punnet Square and record results of crosses.
Faye is an albino (a recessive trait) while Fred has normal pigmentation. However Fred's father
was an albino. What is the probability that Fay and Fred will have an albino child? What is the
probability that they will have a child that is a carrier for the albino allele?
Two questions asked:
 What is the probability of the recessive phenotype?
 What is the probability of the heterozygous genotype?
To solve problem one must first determine genotypes of parents.
 Since normal pigmentation is dominant, the gene will be represented by N
 The genotype(s) for a person with normal pigmentation are NN or Nn.
 The genotype for a person who is an albino is nn.
 Since Faye expresses the recessive trait, she must have the genotye nn.
 Since Fred expresses the dominant phenotype, he must have the genotype Nn or NN.
 Since his father expressed the recessive phenotype, his father must have the genotype nn.
Therefore, Fred must have received an n from his father, and therefore Fred's phenotype is Nn.
Faye produces eggs that all have the n allele; while Fred produces sperm, 50% of which have n
and 50% have N.
Fay's eggs
n
n
Fred's sperm
N
Nn
Nn
n
nn
nn
Therefore, the probability that Fred and Faye will have a child that is an albino is .50 and the
probability that they will have a child who is a carrier is also .50.
Practice questions:
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1. Tay Sachs is a recessive inherited disease affecting the brain. Children who have
inherited the disease die in early childhood. Clara and Robert have two children. Their third
child is born with Tay Sachs disease. What is the probability that their next child will have
Tay Sachs disease.
Parents must be carriers (Nn) if they have an affected child. The cross is between Nn and Nn.
Therefore, there is a .25 probability that their next child will have Tay sach's disease.
N
n
N
NN
Nn
n
Nn
nn
2. Steven has cystic fibrosis (a recessive inherited disease). Heather does not express the
disease and has been tested for cystic fibrosis. The tests have indicated she is not a carrier.
What is the probability that they will have a child who has cystic fibrosis?
Steven must have the genotype nn and Heather must have the genotype NN. The probability
that they will have a child who has cystic fibrosis is therefore 0.
N
n
N
Nn
Nn
N
Nn
Nn
In the context of humans the word dominant doesn't imply that a phenotype is either normal or
more common than a recessive phenotype; wild-type traits (that those traits that most common in
nature) aren't necessarily specified by dominant alleles.
In genetics when we refer to dominance it means that a heterozygote (Aa), carrying only one copy
of a dominant allele, displays the dominant phenotype. By contrast the phenotype of the recessive
allele is only seen in a homozygous recessive.
In fact recessive traits are more common in the population than dominant ones. For example, the
absence of freckles is more common than the presence of freckles, yet their presence is a
dominant trait.
dominant and recessive alleles, and his
So how do we know how certain human
principle of segregation.
traits are inherited? Obviously unlike
Mendel and his pea plants or a breeder with
budgies, geneticists who study humans
can't control the mating of their subjects.
Instead they have to study matings that
have already occurred. Suppose we're
interested in the inheritance of a type of
deafness that is inherited as a recessive
trait. First you'd have to collect as much
information as possible about a family's
history for that trait. Then you'd have
assemble this information into a family tree
or a pedigree. Finally in order to analyze
the pedigree you'd use Mendel's concept of
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Once phenotypic data is collected from several generations and the pedigree is drawn, careful
analysis will allow you to determine whether the trait is dominant or recessive. Here are some rules
to follow.
For those traits exhibiting dominant gene action: Affected individuals have at least one
affected parent the phenotype generally appears every generation. Two unaffected parents only
have unaffected offspring
For those traits exhibiting recessive gene action:
Unaffected parents can have affected offspring. Both males and females are affected.
Dominant Pedigree:
affected individuals have at least one affected
parent
the phenotype generally appears every
generation
two unaffected parents only have unaffected
offspring
Recessive Pedigree:
Unaffected parents can have affected
offspring
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Both males and females affected
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What are some of the single-gene human traits known to be dictated by dominant
alleles? Widow’s peaks, dimples, Freckles, detached earlobes.
Lets say for instance your maternal grandmother does not have a widow’s
peak and her husband (your maternal grandfather) does but his mother didn’t
(your great grandmother) making him a heterozygote. Your father’s two brothers
(your uncles) didn’t have a widow’s peak, but his sister did Thus 50% of the
children had a widow’s peak:
Punett square:
Grandmother
w
w
Grandfather
W
Ww Ww
w
ww
ww
Your father (who is a heterozygote) marries a woman does not have widow’s
peak (and is thus a homozygote). You have one your sister who does not have a
widow’s peak. What is the chance that if your parents had another child he or she
would have a widow’s peak?
Punett square:
mother
w
w
father
W
Ww Ww
w
ww
ww
There is a 50% chance your sibling would have a widow’s peak.