Section 5 - Inheritance
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Transcript of Section 5 - Inheritance
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SECTION 5 - INHERITANCENational 4 & 5 – Multicellular Organisms
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Why are we so similar yet different?• We all belong to the same species• We are similar in many ways• But, we show a great deal of variation • - continuous (e.g. height, weight)• - discrete (e.g. eye colour, blood type)• These similarities and differences are mainly determined
by our genes
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Learning Outcomes• By the end of this section I will be able to • - identify how genes determine characteristics• - map out patterns of inheritance, including family trees• - identify phenotypes and genotypes using punnet
squares• - define dominant and recessive characteristics, and
identify homozygous and heterozygous individuals
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Inherited Characteristics• Our characteristics are determined
by genetic information• E.g. hair colour, eye colour, tongue-
rolling• Each parent passes on 1 piece of
information for a certain characteristic
• The pieces of information from each parent may be the same or different
• A family tree can show how characteristics pass on through several generations
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Phenotype• An organisms appearance resulting from genetic
information received from parents• E.g• - wing shape – wild-type, weak, strong • - flower colour – red, white, purple• - eye colour – green, blue, brown
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Phenotype and genotypeThe overall appearance of an organism depends on two things:
The full set of genes of an organism is called its genotype.
All the observable characteristics of an organism are called its phenotype.
1. its genes (inherited characteristics)
2. the effects of the environment in which it lives.
An organism’s phenotype therefore depends on its genotype plus environmental effects.
phenotype = genotype + environmental effects
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Genes & Alleles• Each cell has two sets of chromosomes• One set from each parent• Each chromosome is made up of units called
genes• Each gene contains information for a particular
characteristic• Each gene normally has at least 2 different
forms• e.g. flower colour could be purple or white• These different forms are called alleles
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Inheritance studies• Show the inheritance patterns of certain
characteristics• Usually done with an easily-bred
species • e.g pea plants• Specific characteristics chosen for study
• e.g. flower colour• When only 1 characteristic is examined,
it is said to be a monohybrid cross• Parents (P) are bred to produce the first
filial generation of offspring (F1)• The F1 generation are then interbred to
produce the second filial generation (F2)• Phenotypes are observed to show how
characteristics are passed on
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Dominant/Recessive• F1 generation often shows only
one characteristic coming through• This characteristic is said to be
dominant• E.g. purple flower• The other characteristic is often
hidden• It is said to be recessive• e.g white flower• BUT, in the F2 both
characteristics often appear• Why?
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Homozygous alleles If the alleles for a characteristic are the same, the organism is said to be homozygous for that characteristic.
What colour eyes will these homozygous pairs of alleles produce?
allele forbrown eyes
allele forbrown eyes
allele forblue eyes
allele forblue eyes
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Heterozygous alleles
The characteristic expressed by heterozygous alleles will depend on which allele is dominant and which allele is recessive.
If the alleles for a characteristic are different, the organism is said to be heterozygous for that characteristic.
What colour eyes will this heterozygous pair of alleles produce?
allele forbrown eyes
allele forblue eyes
?
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What eye colour?The allele for brown eyes is dominant over the allele for blue eyes.
The individual will have brown eyes, because the allele for brown eyes masks the allele for blue eyes.
allele forbrown eyes
allele forblue eyes
So, what colour will the eyes be of an individual who has both alleles for eye colour?
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Inheritance terms
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B is the gene for brown eyes b is the gene for blue eyes
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbBody cell in father
with a pair of genes for brown eyes
Body cell in mother with a pair of genes for blue eyes
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbBody cell in father
with a pair of genes for brown eyes
Body cell in mother with a pair of genes for blue eyes
Gametes
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbbody cell in father
with a pair of genes for brown eyes
body cell in mother with a pair of genes for blue eyes
Gametes
B Beach sperm has a gene for brown eyes
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbbody cell in father
with a pair of genes for brown eyes
body cell in mother with a pair of genes for blue eyes
Gametes
B Beach sperm has a gene for brown eyes
b b each egg has a gene for blue eyes
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbbody cell in father
with a pair of genes for brown eyes
body cell in mother with a pair of genes for blue eyes
Gametes
B Beach sperm has a gene for brown eyes
b b each egg has a gene for blue eyes
At fertilizationThere are 4 possible combinations of sperm and egg
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbbody cell in father
with a pair of genes for brown eyes
body cell in mother with a pair of genes for blue eyes
Gametes
B Beach sperm has a gene for brown eyes
b b each egg has a gene for blue eyes
At fertilizationThere are 4 possible combinations of sperm and egg
B
B
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B is the gene for brown eyes b is the gene for blue eyes
ParentsBB bbbody cell in father
with a pair of genes for brown eyes
body cell in mother with a pair of genes for blue eyes
Gametes
B Beach sperm has a gene for brown eyes
b b each egg has a gene for blue eyes
At fertilizationThere are 4 possible combinations of sperm and egg
Bb BbBb Bb
B
B
b b
All the children of this F1 generation have genotype Bb and phenotype brown eyes
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Parents (F1)father with brown eyes
mother with brown eyes
Gametes
At fertilization
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Parents (F1)Bb Bbfather with brown
eyesmother with brown eyes
Gametes
At fertilization
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Parents (F1)Bb Bbfather with brown
eyesmother with brown eyes
Gametes
B b B b
At fertilization
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Parents (F1)Bb Bbfather with brown
eyesmother with brown eyes
Gametes
B b B b
At fertilization
B
b
B b
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Parents (F1)Bb Bbfather with brown
eyesmother with brown eyes
Gametes
B b B b
At fertilization
BB BbBb bb
B
b
B b
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Parents (F1)Bb Bbfather with brown
eyesmother with brown eyes
Gametes
B b B b
At fertilization
BB BbBb bb
B
b
B b
A child who inherits the genes BB will have brown eyes
A child who inherits the genes Bb will have brown eyes
A child who inherits the genes bb will have blue eyes
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Monohybrid Cross• In this example purple is dominant to white• P = purple• p = white• Let’s assume that both parents are
homozygous (true breeding)• One is PP (purple), the other pp (white)• The F1 offspring would gain a purple allele (P)
and a white allele (p)• - therefore only purple flowers in the F1
generation (Pp)• When the F1 interbreed they could put forward
either a purple (P) allele • - or a white (p) allele • A punnet square allows us to map out the
possible combinations in the F2
• We discover that the numbers produced are 3 purple:1white
• This is called the phenotypic ratio
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• Green flower (G) is dominant
• Yellow (g) is recessive• All the F1 generation have
the Gg genotype• They are all therefore green• Then the F1 plants are
crossed• The results of the F2 are:• 1GG:2Gg:1gg• - this is called the
genotypic ratio• - 3 green:1 yellow• - this is the phenotypic
ratio
1 GG = green2 Gg = green1 gg = yellow
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Expected vs Observed ratio• With a monohybrid cross, a 3:1 ratio would always be expected in the F2
generation• However, there is often a difference between the expected and the
observed results
This is because fertilisation is a random process, involving an element of chanceA punnet square only shows the likely outcomes, not what will actually occurIn real life, if you toss a coin 20 times, you would expect 10 tails:10 heads – rarely occurs
Generation Green YellowParents (P) 6 (GG) 6 (gg)
F1 198 (Gg)
F1 cross Gg x Gg
F2 147 53
Using the previous example the 200 offspring from the F2
generation doesn’t exactly match a 3:1 ratio
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Using a test-cross
To help identify it’s genotype, it is crossed with a white flower(a white flower can only have a ppgenotype)
In this example purple (P) is dominant to white (p)
We have a flower that is purple, but don’t know if it is homozygous (PP) or heterozygous (Pp)
The offspring produced should prove what the unknown genotype is
On occasion, an organisms genotype may be uncertain, but needs to be identified.
A test-cross is used to prove an unknown genotype
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For example, when a red snapdragon plant is crossed with a white snapdragon plant, all the offspring flowers are pink.
What is incomplete dominance?Sometimes two different alleles are neither fully dominant or recessive to each other.
In heterozygous individuals, this creates a phenotype that is a mix of the other two. This is called incomplete dominance.
- because both the red and white alleles are expressed.
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What is co-dominance?The human blood group system is controlled by three alleles: A, B and o.
In heterozygous individuals who have A and B alleles, both are fully expressed
This is called co-dominance.
A and B are dominant while o is recessive.
- creating an extra phenotype (AB)
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Co-dominance in humans
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The life and work of Gregor Mendel
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Mendel’s experimentsOver seven years, Mendel experimented on more than 28,000 pea plants! Why were his experiments so successful?
Pea plants grow quickly.
Pea plants are available in pure-breeding (homozygous) strains.
Many pea plant characteristics show discrete variation; they are either one form or another.
This means that their phenotypes are easily distinguishable.
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What are sex chromosomes?Humans cells contain one pair of sex chromosomes, which control gender.
Males have one X andone Y chromosome (XY).
Y chromosomes are small and contain 78 genesX chromosomes are larger and contain 900–1,200 genes.
Because females can only produce X gametes, it is the sperm that determine the sex of the offspring.
X chromosome
Y chromosome
Females have two X chromosomes (XX).
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Boy or girl?
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Multiple-choice quiz
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Anagrams
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Glossary (1/4)acquired – A characteristic of an organism that depends on environmental
factors.
allele – One version of a gene, found at a specific location along a chromosome.
carrier – An individual with a recessive allele, whose effect is masked by a dominant allele.
characteristic – A specific feature of an organism, such as eye colour.
co-dominance – A situation where two alleles are equally dominant.
continuous – Variation represented by a continuous range of values and which can be measured.
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Glossary (2/4)discontinuous – Variation represented by discrete categories.
dominant – An allele that is always expressed, even if the cell only contains one copy.
gene – The unit of inheritance.
genotype – The full set of genes of an organism.
heterozygous – Having two different alleles of a specific gene.
homologous chromosomes – A matched pair of chromosomes that carry genes for the same characteristics.
homozygous – Having two identical alleles of a specific gene.
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Glossary (3/4)incomplete dominance – A situation where two alleles are both
partially expressed, producing an intermediate phenotype.
inherited – A characteristic of an organism that depends on its genes.
monohybrid cross – A cross in which one pair of characteristics is studied.
phenotype – All the observable characteristics of an organism.
recessive – An allele that is only expressed if two versions of it are present in a cell.
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Glossary (4/4)
test cross – A situation where an individual with an unknown genotype is bred with a homozygous recessive individual to reveal the unknown genotype.
variation – The difference between individuals within a population.