Download 18.1 The Basis of Heredity Genetics: is the study of heredity Heredity

18.1 The Basis of Heredity
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Genetics: is the study of heredity
Heredity: is the passing of traits from parents to offspring
E.g. blue eyes
Traits: are what you see and are determined by genes
Genes: are specific segments of DNA that code for traits
During reproduction the parents genes are transmitted to the next
generation
Gregor Mendel –(Pioneer of Genetics)
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Studied inheritance
Was an Austrian monk
tracked and recorded the transmission of seven visible traits in peas
Why Mendel Studied Peas:
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characteristics are expressing in one of two forms
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the species reproduces easily
Review
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Self pollination: occurs when the pollen from the anther is transferred to the
pistil of the same flower
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Crosspollination: occurs when the pollen from one plant is transferred to the
pistil of anther plant
Self
Pollination
Cross Pollination
Mendel’s Experiment
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He began with plants that had known traits
The plants he used were true-breeding …….“pure varieties”
Each plant was true breeding for a different trait
(E.g. round seeds & wrinkled seeds)
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True breeding: means that when the plant self-pollinate all the off spring
show the same traits as the parent plant
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E.g. a plant with white flowers self pollinates to produce a plant with white
flowers
the first cross was called the parental generation, or P generation
The offspring of this cross was called the first filial generation, or the F1
generation
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Filial comes from the Latin word for son
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The next generation was the F2 and so on
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He cross pollinated the P generation
RR x rr (Crossing of P generation)
Genotype
(the genes the plant has)
 The off spring (F1 generation) produced were hybrids (Rr)
 The hybrids have two different alleles (forms of a
gene). one for Round and the other is for wrinkled
 The F1 generation self pollinated and produced the F2 generation (second
filial generation)
X
R
Crossing of F1 generation
r
R
RR
r
Rr
F2 Generation Produced
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3 are Round
1 is wrinkled
Rr
rr
The Principle of Dominance
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Mendel crossed a pea plant that produces round seeds with a pea plant that
produces wrinkled seed
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All the offspring (F1 generation) that this cross produced had round seeds
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Mendel believed:
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that traits were determined by factors
the round factor was dominant
We now know:
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that those factors are genes
Genes determine traits
Genes can have alternate forms
We call alternate forms of a gene an allele
The alleles for seed shape were round or wrinkled
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Dominant Alleles
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are Alleles that code for dominant traits
you only need one copy (dominant allele) for the trait to show
it is indicated by an uppercase italic letter: R
E.g. RR or Rr codes for round seeds
Recessive Alleles
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are Alleles that code for recessive traits
you need two copies (recessive alleles) for the trait to show
It is indicated a lowercase italic letter: r
E.g. rr for wrinkled seeds
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Mendel’s Principle of Segregation
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Mendel let the F1 plants self fertilize
X
R
Crossing of F1 generation
r
R
RR
r
Rr
F2 Generation Produced
3 are Round
Rr
rr
1 is wrinkled
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75% of the F2 generation had round seeds (RR, Rr, Rr)
25 % of the F2 generation had wrinkled seeds (rr)
For seed shape (the phenotype) there was a 3:1 ratio of round to wrinkled
Mendel concluded that:
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the two copies of a gene in a gene pair undergo segregation
(separate) when sex cells are formed
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Segregation occurs in anaphase I when homologous chromosomes separate
As a result each gamete contains only one member of a gene pair
Example:
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If a plant had round seeds at least one of the alleles is the dominant allele
for round seed shape
 E.g. Rr
It could also have two alleles for round seed shape
 E.g. RR
If a plant has wrinkled seeds it has two copies of the recessive alleles
 E.g. rr
If an individual has (RR);
 all of the gametes will carry the same allele which is R
If an individual has (Rr),
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then the gametes could receive either allele
50% of the gametes will have R
50% of the gametes will have r
Homozygous Verses Heterozygous
Organisms can be homozygous or heterozygous for a trait
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Homozygous:
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Occurs when they have two alleles that are the same in a gene pair
E.g. RR & rr
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RR is homozygous dominant
rr is homozygous recessive
Heterozygous
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Occurs when they have different alleles in a gene pair
E.g. Rr
Genotype Verses Phenotype
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Genotype: is the genes that an organism has
 E.g. RR
Phenotype: are the traits you see
 E.g. Round Seeds
Try the following
Page 600 #1-5
18.2 Probability and Inheritance of Single Traits
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Mendel:
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kept track of the offspring that inherited the dominant and recessive trait
He showed that when the heterozygous plant (F2 generation) self
pollinated, 75 % showed the dominant phenotype and 25 % showed the
recessive phenotype (3:1) ratio
he concluded that each gamete produced by a heterozygous individual
has an equal chance of getting the R allele or r allele ………50/50 chance
 occurs during segregation
Probability
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is a measure of the likelihood that the outcome will occur
It is expressed as a fraction, decimal or percent
It is calculated using the formula:
P = number of ways that a given outcome can occur
Total number of possible outcomes
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Example 1
What is the probability of getting heads in a coin toss?
Solution
The total number of possible outcomes = 2……. heads or tails
The number of ways that the outcome can occur = 1
P=1
2
Therefore the probability is 0.50 or 50%
Example 2
What is the probability of tossing heads 6 times in a row?
Solution
Tossing it once
P =½
Tossing it 6 times:
P= ½ x ½ x ½ x ½ x ½ x ½ = 1/64
Punnett Square
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is a chart s used to predict offspring’s phenotypes
It allows us to determine the expected ratio of genotypes and phenotypes
for a cross
It also allows us to calculate the probability of that particular genotype or
phenotype will occur
`
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Test Crosses
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It is Frequently performed by breeders
a cross that is performed with a homozygous recessive genotype
It is used to determine whether an individual with the dominant trait is
heterozygous( Rr) or homozygous (RR)
The phenotypes of the F1 generation reveal whether an individual is
homozygous or heterozygous
R
r
r
r
Rr
rr
Rr
rr
Notice that the recessive trait shows
when the organism is heterozygous
for the dominant trait
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It is the simplest way of determining the genotype of an individual showing
the dominant trait (E.g. are they RR or Rr)
Working backwards, it can be used to determine the parents genotype and
phenotype if you know the phenotypes of the offspring
Try the following
Page 602, #’s 1-2
Page 604, #’s 3-4
Page 604, #’s 1-3
18.3 Pedigree Charts
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shows the transmission of an inherited disease among members of a
family
it is a tool that is used to solve genetic problems
is often used by genetic counsellors
It is useful; when it is not possible to perform crosses (humans)
It is like a family tree
the inheritance of a trait can be traced from parents to offspring (throughout
generations)
It shows the family relationship among individuals
Setting it up:
Symbols identify the gender of each individual
Color identifies whether they have the trait
Half coloured means the person is heterozygous for an autosomal
recessive
In each pedigree diagram:
 Generations are numbered using roman numerals
 Individuals within each generation are numbered using Arabic
numbers
 Males are represented by squares
 Females are represented by circles
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These charts can be used to advise individuals with genetic disorders in their
Family
It can tell them the chances of their offspring inheriting a disorder
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Try the following:
Page 606, # 1
Page 607, #’s 1-3
18.4 Other Patterns of Inheritance
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The traits that Mendel studied showed little variability
Many traits show more than just two alternate forms
These traits will not have the predicted 3:1 phenotypic ratio that Mendel
found with his monohybrid cross
Pleiotrophic Genes
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Affect many different characteristics
Will not have the 3:1 ratio
E.g. The gene that causes the blood disorder sickle cell anemia
Normal hemoglobin allele is HbA
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Sickle cell anemia:
 is a recessive disorder
 You need two copies of the mutated allele HbS
This mutation produces hemoglobin molecules that are abnormal in shape
The abnormal hemoglobin interlocks and changes the shape of red blood
cells to a sickle shape
sickle shape red blood cells can not pass through capillaries and deliver
oxygen to cells
People suffer from fatigue, weakness, enlarged spleen, rheumatism and
pneumonia
They also show signs of heart, kidney, lung and muscle damage
Multiple Alleles
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A trait is controlled by more than two alleles
It will not have the 3:1 ratio
E.g. Fruit flies have many different eye colors: wild type, apricot, honey and white
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The most commonly seen trait is called the wild type, and the allele is the
wild type allele
The non –wild type traits are said to be mutant, and the alleles are the
mutant alleles
In multiple alleles there is a hierarchy of dominance
The alleles are written in a series from most dominant to the recessive
 wild type> apricot > honey> white
Wild type >( is dominant to) apricot >(is dominant to) honey >( is dominant to)
white
Wild type is dominant to all (apricot, honey and white)
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E1> E2>E3>E4
E1(wild type allele) ,E2 (apricot), E3 (honey) ,E4 (white)
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An upper case letter E is used to symbolize the gene for the eye color
Superscript #’s are used to indicate the hierarchy
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Phenotype
Allele
Symbol
Possible
genotype(s)
Dominant
Over
wild type
E1
E1E1, E1E2,
E1E3, E1E4
apricot,
honey,
white
apricot
E2
E2E2, E2E3,
E2E4
honey,
white
honey
E3
E3E3, E3E4
white
white
E4
E4E4
E.g. Cross an apricot fly with the genotype E2E3 and a wild type that has the genotype
E1E4
Incomplete Dominance
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Occurs when two alleles in the heterozygous interact to produce an
intermediate phenotype
both alleles are equally expressed… but at half the level
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A monohybrid cross (heterozygous) produces 1:2:1 ratio….. it does not produce
Mendelian ratios (3:1 ratio)
Example
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A homozygous red snap dragon (CRCR) crossed with a homozygous white
snap dragon (CWCW)
All the F1 generation is all heterozygous and pink (CRCW)
CRCW X CRCW
(crossing the F1 Generation)
F2 generation has a ratio of 1:2: 1
(I red , to 2 pink, to 1 white)… no Mendelian ratio
Codominance
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Both alleles are fully expressed when an individual is heterozygous
e.g. coat color in shorthorn cattle shows codominance
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Red coats are composed of all red hair (WrWr)
White coats are composed of all white hair (WwWw)
Crossing a red shorthorn and a white shorthorn produces a calve that is
roan
WrWr X WwWw
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A roan has a coat is a mixture of white and red hairs
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Some follicles produce red hair and others produce white
hair
Human Blood Type
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Has multiple alleles IA, IB, i
Shows codominance
 E.g. IA, IB are codominant……. and both are dominant over I
There are 4 different blood types:
 Type A……. (IA IA, or IA i)
 Type B…… (IB IB, or IB i)
 Type AB…. .(IA IB)
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Type O ……(ii)
Try the Case Study Page 611
Environment and Phenotype
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Just because you inherit a gene it does not mean that it is expressed
The environment can influence gene expression
Himalayan rabbits have black fur when they are raised in cold temperatures and
white fur when raised in warm
Try the following:
Page 612 #’s1-3
18.5 Dihybrid Crosses and Polygenic Traits
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A dihybrid cross is a cross between individuals with two traits
These traits are independent of one another and are present in alternate
forms
Mendel performed dihybrid crosses with peas
He crossed plants that were homozygous for two dominant traits (Yellow
and round) with plants that were homozygous for two recessive traits
(green and wrinkled)
YYRR X yyrr
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All the F1 generation were heterozygous for seed color and seed shape
(Yellow and round….YyRr)
Evidence Of Independent Assortment
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Mendel explained these results by proposing that Genes located on different
chromosomes assort independently during meiosis
This is the Law of Independent Assortment…… Mendel’s second Law
The law holds true as long as the traits are located on different chromosomes
During meiosis homologous chromosomes move to opposite poles and each
gamete receives 2 chromosomes:
 One chromosome carries the gene for seed shape
 One chromosome carries the gene for seed colour
According to the Law of Segregation both of these genes segregate during
meiosis
The allele for yellow seed segregates from the allele for green seeds
The allele for wrinkled seeds segregates from the allele for round seeds
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To create a punnett square for a dihybrid:
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Include one allele for both of the genes in the possible gametes
Assume that the genes for seed shape and seed colour are inherited
independently
Example
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Mendel crossed YYrr Yellow and wrinkled with yyRR green and round
YYrr X yyRR (Parental generation)
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the gametes produced were Yr & yR
all the F1 members were YyRr (yellow and round)
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YyRr X YyRr (F1 self pollinated)
The gametes produced were:
YR, Yr, yR, yR
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The F2 had the following results:
9/16 yellow round seeds……(YYRR, YyRR, YYRr, YyRr)
3/16 green round seeds……(yyRR, yyRr)
3/16 yellow wrinkled seeds….. (YYrr, Yyrr)
1/16 green wrinkled seeds….(yyrr)
A phenotypic ratio of 9:3:3:1
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Probability in Dihybrid Cross
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Use the same formula as before:
P = number of ways that a given outcome can occur
total number of possible outcomes
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Probability can help us predict the likelihood that two traits on different
chromosomes will be inherited together
Calculate the probability of the first trait being inherited and do the same for
the second trait
Then multiple your fractions together
See sample exercise 1 Page 18.5
Do exercises page 616
Selective Breeding
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Involves identifying individuals with desirable traits and using them as
parents to reproduce
Over time with each generation the traits become more common
Aboriginal farmers used this to develop many useful crop plants such as:
 rust-resistant wheat
 sweet full kernel corn
 canola
Inbreeding
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Involves selecting animals with similar phenotypes for breeding
Used to create purebreds
Can cause problems because often the animals are closely related
The problem is the undesirable traits occur more often ……arthritis in hips..
or aggression in dogs
Examples :
 Irish Setters: to obtain the long facial features
 Bull terrier: bred for fighting
Hybridization
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Is the opposite of inbreeding
Can be used to develop new varieties of plants and animals
breeds plants and animals with very different traits
The hybrids tend to be more vigorous then either parent
Example:
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Cross two plants AA and BB to produce a hybrid AB
Cross two more plants CC and DD to produce a hybrid CD
AB and CD are then crossed to produce a hybrid ABCD
Polygenic Traits
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They are traits that are determined by more than one gene
 Eye colour is determined by the interaction of 3 genes found on different
chromosomes
 E.g. eye colour, skin colour and height
 Have more variability in a population than those determine by a single gene
 Sometimes the interaction these genes produce a phenotype that neither
could produce by itself
 Each of these genes can have:
 multiple alleles
 show incomplete dominance
 Show co-dominance
 be affected by the environment
E.g. height can be affected by nutrition
Epistatic Genes
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Are genes that interfere with the expression of another gene
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They mask a genes affect (hide it)
E.g. coat colour in dogs
 The allele B produces a black coat
 The allele b produces a brown coat
 w gives pigment
 W gives no pigment
wwBb or wwBB is black
wwbb is brown
Ww Bb, WwBB or Wwbb is all white
The allele W prevents the formation of pigment….. meaning the coat will
appear white
Read Drought –tolerant and Salt-tolerant plants; pages 618-619
Page 619, #’s 1-4
Page 616 #1
Review Page623 #’s 1-21, 22-29
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