1. Genes, Inheritance and Selection
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Genes, Inheritance and Selection
OCR Gateway 2016 Biology topic 5
W Richards
The Weald School

2. B5.1 Inheritance
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3. Section of a chromosome:
Genome and Phenotype
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Genetic information is stored by genes which are arranged on chromosomes:
Section of a chromosome:
Genes for eye colour
Genes for blood group:
Genes for hair colour
Definitions of key terms:
The “genome” is the entire genetic material of an organism.
The “phenotype” of an organism is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment.

4. Examples of Continuous and Discontinuous Variation
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Examples of Continuous and Discontinuous Variation
No. of people with this eye colour
Brown
Blue
Green
No. of people in this height range
Below 140cm cm cm cm cm cm
Above 165cm
Which table shows continuous data and which one shows discontinuous data?

5. Variation
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Variation is a name given to differences in a species, e.g. dogs:
Variation in any species is due to each animal having different genetic information in their cells. These variations arise from mutations and these mutations can:
Have no effect on phenotype
Influence phenotype
Determine phenotype

6. Genetic mutations (Bio HT only)
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Here’s a sequence of bases in DNA: A T C G
Q. What happens if this sequence is changed?
For example, what if a mutation caused this to happen? A T C G A
Genetic variation may influence phenotype by:
In coding DNA, in can alter the activity of a protein by changing the shape of the active site
In non-coding DNA in can alter how genes are expressed by stopping transcription of mRNA

7. Sexual vs. Asexual reproduction
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Sexual reproduction:
2 parents are needed
Offspring will have “pairs” of chromosomes
This will cause genetic variation
Asexual reproduction:
Only 1 parent needed
Offspring are GENETICALLY IDENTICAL to parent (“clones”)
“Snuppy” – the first cloned dog (Aug 05)

8. Sexual vs asexual reproduction (Bio only)
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Advantages of sexual reproduction
Advantages of asexual reproduction
No need to find a mate
Increased variation, which may cause an evolutionary advantage
Which form of reproduction is better?
More time and energy-efficient
Faster than sexual reproduction
Selective breeding can be used to increase food production
Produces a clone of a good animal

9. Cell growth revision - Mitosis
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Each daughter cell has the same number of chromosomes and genetic information as the parent – a “diploid cell”.

10. Cell growth 2 - Meiosis
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Each daughter cell has half the number of chromosomes of the parent – a genetically different haploid gamete.

11. Mitosis vs. Meiosis Mitosis: Meiosis:
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Mitosis:
Used for growth and repair of cells
Used in asexual reproduction
Cells with identical number of chromosomes and genetic information are produced (“clones”)
Meiosis:
Used to produce haploid gametes for sexual reproduction
Each daughter cell has half the number of chromosomes of the parent
During meiosis copies of the genetic information are made and then the cell divides twice to form four daughter cells.

12. Fusing gametes during Fertilisation
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Egg
Sperm
46 chromosomes in a fertilised egg
23 chromosomes in here
Fertilisation
23 chromosomes in here

13. Basic genetics - Boy or Girl?
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Note that the Y chromosome is “dominant” and the X is “recessive”. The Y chromosome dictates the development of testes. X X Y
“Allele”
“Phenotype” XX XY
Girl
Boy
Heterozygous
Homozygous

14. Boy or Girl? Mother Son Daughter Father
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Mother
During sexual reproduction, children inherit two alleles of each gene (one from each parent).
Son
Daughter
Father

15. Genetic Diagrams
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Here’s what happens (genetically) when an egg is fertilised:
Mother
Father xx xy
Equal (50%) chance of being a boy or a girl x x y xx xy xx xy

16. x y Genetic Diagrams Here’s another way of drawing it: Father Mother
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Here’s another way of drawing it: x y
Father
Mother

17. Key words Genotype Phenotype Allele Dominant Recessive Homozygous
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Genotype
Phenotype
Allele
Dominant
Recessive
Homozygous
Heterozygous
This allele determines the development of a characteristic
The characteristic caused by the genotype
This allele will determine a characteristic only if there are no dominant ones
This word refers to a pair of chromosomes being made of two different alleles of a gene
The genetic make up in a nucleus
This word refers to a pair of chromosomes being made of two of the same alleles of a gene
An alternative form of a gene

18. Eye colour
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In eye colour the brown eye allele is dominant, so we call it B, and the blue eye is recessive, so we call it b: BB Bb bb
Homozygous brown-eyed parent
Heterozygous brown-eyed parent
Blue-eyed parent
What would the offspring have?

19. X X Eye colour BB bb Bb Bb (FOIL)
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Example 1: A homozygous brown-eyed parent and a blue-eyed parent:
Example 2: 2 heterozygous brown-eyed parents BB bb Bb Bb X X
Parents: B b B b B b
Gametes:
(FOIL) Bb Bb Bb Bb BB Bb bB bb
Offspring:
All offspring have brown eyes
25% chance of blue eyes

20. Eye colour
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Example 3: A heterozygous brown-eyed father and a blue-eyed mother: Bb bb
Equal (50%) chance of being either brown eyed or blue eyed. B b b Bb Bb bb bb
Note – in reality, characteristics like this are usually depend on the instructions of multiple genes and other parts of the genome.

21. Another method – the “Punnett square”
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Example 3: A heterozygous brown-eyed father and a blue-eyed mother: B b B b Bb bb
Father
Mother

22. Example questions
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1) In mice, white fur is dominant. What type of offspring would you expect from a cross between a heterozygous individual and one with grey fur? Explain your answer with a genetic diagram.
2) A homozygous long-tailed cat is crossed with a homozygous short-tailed cat and produces a litter of 9 long-tailed kittens. Show the probable offspring which would be produced if two of these kittens were mated and describe the characteristics of the offspring (hint: work out the kitten’s genotype first).

23. X Modern Genetics Mendel’s experiment:
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Guten tag! My name is Gregor Mendel. I am the father of modern genetics because of the work I did on pea plants in 1865…
Mendel’s experiment: X
Take two plants; one which is pure-bred for tallness and one pure-bred for shortness, and cross them:

24. Now cross two of these plants…
Modern Genetics
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All the plants produced were tall.
Now cross two of these plants…
3 out of every 4 plants were tall, leading Mendel to hypothesise that “for every characteristic there must be two determiners”

25. Modern Genetics
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Achtung! Unfortunately, nobody knew about chromosomes or genes when I published my findings so no one believed me until after my death, when more powerful microscopes were available.
Mendel’s work illustrates how scientists develop ideas that account for the data they have collected. We now know that features of organisms (“phenotypic features”) are the result of multiple genes rather than a single gene.

26. B5.2 Natural Selection and Evolution
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27. Genetic Variation
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The world is populated by millions of different species of animals and plants with HUGE genetic variation…

28. Classification
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How would you construct a key to classify these organisms?

29. Classifying organisms
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All organisms are classified into groups. For example:
Organism
What is the main difference between these?
“Kingdoms”
Plants
Animals
Vertebrates
Invertebrates
Amphibians
Birds
Reptiles
Fish
Mammals
Notice that the number of similarities increases as you go down this tree
“Species”
Dogs
Cats

30. Why use classification?
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Wasp – “vespula germanica”
Human – “homo sapien”
Dog – “Canis lupus familiaris”
Carolus Linnaeus,
I invented the modern system of naming species. I did this so that species would have the same name in every language and so that we would have a greater ability to understand different species and how they have evolved.

31. Molecular Phylogenetics
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Modern DNA research shows that all forms of life share a lot of their DNA. This is used by scientists to help them classify species – “molecular phylogenetics”.
98.8% shared DNA
85% shared DNA

32. Evolution
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Evolution is the slow change in organisms that happens over a long period of time. All life on Earth has evolved from simple life forms that existed around 3 billion years ago. It happens through a process called “natural selection”, which basically says this:
Charles Darwin,
1) Different species show variation due to mutations
2) The “better adapted” ones survive
3) They then have kids who also have the “better” phenotypes

33. Evidence for Evolution
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Fossil records, showing the development of an organism over a long period of time
Antibiotic-resistant bacteria, giving evidence for natural selection
Now that the mechanism of natural selection has been understood and with evidence like fossils and antibiotic-resistant bacteria, the theory of evolution is widely accepted.

34. Evolution
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Charles Darwin,
As I said, evolution is the slow change in organisms that happens over a long period of time. Who did I work with?
Alfred Wallace
I also worked on a theory of evolution and came up with ideas about natural selection. I also realised that warning colours are produced by some species (e.g. Butterflies) – an example of a beneficial adaptation that had evolved through natural selection.

35. The “Evolution Tree”
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Ideas about evolution have impacted modern biology. For example, consider the evolution tree:
Family Hominidae (Great Apes)
Family Hylobatidae (Lesser Apes)
Subfamily Hominidae
Subfamily Ponginae
Tribe Homini
Tribe Panini
Tribe Gorillini
Humans
Chimpanzees
Gorillas
Orangutans
Gibbons
The theory is that we have descended from the same ancestors. Why is that important?

36. Using ideas about Evolution
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Here are some ways in which theories of modern evolution have impacted modern biology:
Classification is now based on evolutionary relationships
We know that bacteria can develop resistance to antibiotics and can therefore understand the need to develop new antibiotics and “finish the course”
We understand the need to preserve species, e.g. the Millennium Seed bank