Download The Genetic Basis of Inheritance

The Genetic Basis of Inheritance
From Chromosomes to Genes
Chromosomal Basis of Inheritance

Traits are transmitted by chromosomes
which contain units of heredity called
genes

Genes are formed from DNA
Mendel and the History of Genetics
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Gregor Mendel
Born 1822
Began his work at age
21
A monk and school
teacher interested in
plant breeding
Studied pea plants
Discovered the basic
principles of heredity
Conclusions Mendel made:
1.
2.
3.
Each trait has different forms called
alleles
Each trait is controlled by 2 alleles that
can be dominant (what appears) or
recessive (what gets hidden by the
dominant)
An organism is homozygous if the 2
alleles are the same and heterozygous if
the 2 alleles are different
Allele #1
Aa
Dominant
Allele #2
Recessive
Homozygous
Heterozygous
AA , aa
Aa
Pure Lines
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Mendel recognized the 2 contrasting forms as
distinct varieties, or purebred
In a purebred, the offspring have same trait as the
parents
‘Tall’ always produced ‘tall,’ etc.
Called parental pure lines "P1" generation
First generation of offspring are F1 (for filial), then
F2 and so on
Hybrid = An offspring produced by breeding 2
different purebreds
What happens if you cross 2 pure lines?
Complete Dominance
Mendel crossed the two pure lines for each
trait.
 These were the P1 generation.
 All offspring of these crosses (F1) had the
trait of only one parent
 The trait of the other parent disappeared in
the F1 generation (but reappeared in F2)
 Mendel hypothesized that there were 2
factors for each trait
 Mendel called 1 factor dominant because
it prevailed (covered up the other)
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Dominant vs. Recessive Traits
Mendel crossed F1 plants with other F1 plants,
producing the F2 generation
 Not only did the recessive trait reappear in the F2
generation, but in a consistent proportion:

◦ 1/4 showed the recessive trait (ratio 3:1)
Mendel referred to the factor that was hidden in
the F1 as recessive
 If an offspring carries 2 dominant or one dominant
and one recessive factor, the offspring will appear
to have the dominant trait.
 If the offspring carries 2 recessive factors, it will
appear to have the recessive trait
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The Experiment
P Generation:
◦ Cross 2 pure lines –
◦ One white, one purple
 F1 are all purple
◦ Purple is dominant
 Cross F1 x F1 (all
purple)
◦ ¼ F2 offspring are
white
◦ White is recessive
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PRESS PAUSE,THINK,
AND SHARE
Think about the following…
1) What are sections of DNA that
contain heredity information ?
2) How does a purebred differ
from a hybrid?
3) In a cross that displays
completed dominance, if an
offspring carries 1 dominant
factor and 1 recessive factor,
which trait will the offspring
have?
Share with your team what you
think.
What did you discuss?
What are sections of DNA that contain heredity
information ?
genes
1) How does a purebred differ from a hybrid?
A purebred the offspring always has the same traits as the
parent
A hybrid is the result of breeding two different purebreds.
1) In a cross that displays completed dominance, if an
offspring carries 1 dominant factor and 1 recessive
factor, which trait will the offspring have?
The dominant trait
1)
Press Pause, Think, and Write

On your vocabulary
sheet define the
following
Gene
2) Purebred
3) Hybrid
4) Complete
dominance
5) Dominant
6) Recessive
1)
Vocabulary
1.
2.
3.
Gene- a section of a DNA
molecule that controls a trait
Purebred- Has two identical alleles
for same trait
Hybrid- Has two different alleles
for same trait
Vocabulary
4.
5.
6.
Complete dominance- the effects of
one allele dominating over another
Dominant- trait will show. Capital letter
Recessive- only see this trait if two
alleles are present. Lower case letter.
Principle of Segregation
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Members of each pair of genes separate, or
segregate, when gametes are formed.
◦ Recall what we now know happens to
chromosomes during meiosis
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From Mendel:
◦ For each characteristic an individual carries 2
‘factors’
◦ Each parent contributes one of its 2 factors to
each offspring.
◦ Chances of contributing either factor are equal.
Genes and Alleles
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We now know that the heredity units are genes.
The separate forms of a gene that Mendel called
‘factors’ = alleles
One allele in a pair may prevent the other from
being expressed
If an offspring carries 2 dominant, or one
dominant and one recessive allele, the offspring
will appear to have the dominant trait.
If an offspring carries 2 recessive alleles, it will
appear to have the recessive trait.
Each individual carries one copy (allele) of a gene on
the chromosome from their mother, and a second copy
on the homologous chromosome from their father.
Representing Genes & Alleles
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Generally the capitalized first letter of the dominant
trait is used to represent the dominant allele
The recessive allele is then lower case of the same
letter (e.g. T for tall, so t for short)
Each characteristic can be represented by a pair of
letters representing the genes
Some multi-allele systems (more than 2 possible
forms of the trait) use different conventions
Traits carried on sex chromosomes are written as a
superscript of the X or Y chromosome
Genotype vs. Phenotype
Punnett Squares
A way to visualize crosses
 Punnett square can be used
to determine probability of
different genotypes or
phenotypes
 Each box contains a
possible combination of
alleles for offspring
 Punnett square can be used
to determine probability of
different genotypes or
phenotypes
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Determining Genotype
If you know the phenotype, is it
possible to determine the genotype?
 If an organism shows the recessive
trait, you know the genotype.
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◦ It must be homozygous recessive (tt)
What if it shows the dominant trait?
 If the dominant trait is expressed there
are 2 possibilities:
 homozygous dominant ( TT)
 or heterozygous (Tt)
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PRESS PAUSE,THINK,
AND SHARE
Think about the following…
1)
2)
3)
4)
What does the principle
of segregation state?
What is an allele?
What is the difference
between a genotype and
a phenotype?
Define homozygous and
heterozygous.
Share with your team what you
think.
What did you discuss?
What does the principle of segregation state?
Members of each pair of genes separate when gametes are
What is an allele?
Different representations of a gene
What is the difference between a genotype and a phenotype?
Genotype is the representation of the alleles; ex: BB or bb
Phenotype is the physical representation of the trait; ex: Black or white
Define homozygous and heterozygous.
Homozygous is when both alleles are the same: ex: BB or bb
Heterozygous is when you have one dominant and one recessive
allele; ex: Bb
Press Pause, Think, and Write
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7)
8)
9)
10)
11)
•
On your vocabulary
sheet define the
following
Allele
Genotype
Phenotype
Homozygous
Heterozygous
In your composition
notebook fill in what the
Principle of Segregation
States
Incomplete Dominance

In all of Mendel's cases, 1 allele was clearly
dominant
◦ This is not always the case
Some alleles show incomplete dominance
 Heterozygotes express traits that are a blend of
the phenotypes of the 2 alleles
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◦ red and white flowers make pink
Still obey the law of segregation
 Only difference is phenotype of homozygous
dominant individual is different from phenotype
of heterozygous individual
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Demonstrating Incomplete Dominance
Co-Dominance
Occurs when both
alleles for a gene
are expressed in a
heterozygote
 Example – Red &
white produce roan
color in some
animals because
both genes are
expressed
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Multi-allele Systems
Some traits are the result of more than 2
possible alleles at a locus
 The ABO blood group is an important example
 3 possible alleles:
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◦ IA - produces A antigen;
anti-B antibodies in blood serum
◦ IB - produces B antigen;
anti-A antibodies in serum
◦ i - produces no antigen;
anti-A & anti-B antibodies in serum
 IA
& IB are co-dominant alleles
 i is recessive
The ABO Blood System
Blood Donation
Blood Type
Genotype(s)
Receive
From
Donate To
A
AA, AO
A or O
A or AB
B
BB, BO
B or O
B or AB
AB
AB
A, B, AB, or O
AB
O
OO
O
A, B,AB, O
Type AB blood is known as the universal accepter
Type O Blood is known as the universal donor
PRESS PAUSE,THINK,
AND SHARE
Think about the following…
1) How is incomplete dominance
different from co-dominance?
2) What is an example of a trait
that is the result of multi-allele
systems?
3) Which blood type is
homozygous recessive?
Share with your team what you
think.
What did you discuss?
How is incomplete dominance different from co-dominance?
In incomplete dominance the heterozygous phenotype is a blend of the
dominant and recessive trait; Ex: BB=black, bb= white, Bb= gray
In co-dominance the heterozygous phenotype shows both the
dominant and recessive trait at the same time; Ex: BB=black, bb= white,
Bb= black and white stripped
What is an example of a trait that is the result of multi-allele systems?
Blood type
Which blood type is homozygous recessive?
O blood
Press Pause, Think, and Write
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On your vocabulary
sheet define the
following
12) Incomplete
dominance
13) Co-dominance
14) Multi-allele
system
Sex Linkage
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Autosomal trait
◦ a gene carried on a non-sex chromosome & present in 2
copies
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Sex linked trait
◦ a gene carried on a sex chromosome that is present in
both sexes; one copy in one sex, 2 in the other
◦ X chromosome in mammals
◦ Z chromosome in birds (one copy in females, 2 in males)
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Y- linked trait
◦ a gene carried on the Y chromosome in humans; very
rare
Showing Sex Linkage
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Symbols are written as superscript of the sex
chromosome:
◦ Xa - X chromosome carrying the recessive allele
◦ XA – X chromosome carrying the dominant allele
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Suspect sex linkage when the ratios of
phenotypes are different in males and
females
Sex-linked Trait Example:
Colorblindness: a recessive disease
How many females will
be colorblind?
 How many females will
be carriers?
 How many males will be
colorblind?
 How many will be
carriers?
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Showing Sex Linkage
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Symbols are written as superscript of the sex
chromosome:
◦ Xa - X chromosome carrying the recessive allele
◦ XA – X chromosome carrying the dominant allele
◦ No superscript is used for the normal or wild type
allele
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Suspect sex linkage when the ratios of
phenotypes are different in males and
females
Pedigrees
Used to determine mode of inheritance when few
individuals, but several generations are involved
 Assume genetic trait discussed is rare, so
individuals marrying into the family are not
assumed to carry the trait
 Symbols:
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O female
shaded = affected
 male partial shading = carrier
Pedigree Analysis
Write: In your composition
notebook glue in the pedigree
notes. Try to answer the questions
relating to the pedigree.
Polygenic Inheritance
Most traits are not
limited to 2
possibilities, yes or no
 Most traits are a
continuum
(quantitative)
 Examples: height,
skin color
 2 or more genes act
additively on a trait
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Law of Independent Assortment
Mendel showed that traits produced by dominant
factors do not necessarily appear together
 2 or more pairs of genes segregate independently
of one another during the formation of gametes
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◦ Genes are distributed to gametes independently
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Genes for different traits are inherrited seperatly from
eachother.(Ex. The color of you hair is controled by a
different allele than the trait for your eye color)
Dihybrid Crosses
Involve 2 genes
 Can demonstrate independent assortment
 Cross homzygous dominant for 2 alleles with
homozygous recessive for 2 alleles:
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◦ Y = yellow
◦ R = round
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YYRR x yyrr can produce:
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◦
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Yellow/ round
Yellow/ wrinkled
Green/ round
Green wrinkled
Produces 9:3:3:1 ratio
Diagramming a Dihybrid Cross