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Life processes – Mendelian Genetics
Introduction
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Since 8000 BC plants with desirable characteristics have been selected for
Gregor Mendel was a contemporary of Darwin and published a paper on genetics in 1865
however it went unnoticed until 1900
- He asked the right question
- He followed correct scientific method and kept meticulous lab books
- He tested the hypothesis with the correct organism, the pea plant
- Produces many peas per cross (large sample size)
- Short generation time
- Easy to self- and cross- fertilise
- Expresses discrete traits
- Mendel made true breeding lines where from one generation to the next the
organisms all appeared the same and were therefore homozygous (both copies of
each gene at a given locus are the same)

Mendel’s 1st and 2nd laws, epistasis, co-dominance and lethal alleles
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Laboratory applications of Mendelian genetics
- Studying obesity in mice
- Have a normal and obese line which you have shown are pure breeding lines
- Obese mouse and normal sibling
- To test the action of the POMC- gene you cross the obese mouse with the
wild type, then you can tell if the obesity gene is dominant or recessive by
the offspring
- To test if a new allele is recessive or dominant
- Show phenotype is true breeding
- Cross new allele with wild type
F2
- F1 individuals new phenotype -> new allele dominant
- F1 individuals wild type -> new allele recessive
- F2 can be used to prove the gene is recessive, by
crossing the offspring
- Using mice to find new alleles or genes affecting obesity
- Chemical mutagen (e
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mutagenise like EMS) causes random mutations
- Isolation and mapping provides insight into human disease
- You can test if the mutants are recessive and if its only one gene
causing the mutated phenotype
- Test hypothesis that these are recessive alleles of the same gene

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Punnet square of F2 generation

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Test hypothesis that these are recessive alleles of different genes

9:3:3:1 ratio ( Wild:ob1:ob2:ob1ob2)

Mendels 2 Law
- Each character is independently inherited during gamete formation
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Co-dominance (heterozygote expresses both forms)
- Offspring can look like both parents at once
- Both alleles contribute to the phenotype of the organism by showing up
simultaneously (at the same time) in heterozygous individuals

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In cattle and horses, if you cross a pure red (RR) with a pure white (WW), you get
(RW) which produces the color roan
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In this case the loci are said to be in linkage
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Genetic loci physically close to one another on the same chromosome tend to stay
together during meiosis, and are therefore genetically linked
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Even though loci may be physically linked on a chromosome, because crossovers
occur, before the chromosomes segregate during meiosis, alleles on the same
chromosome can be separated and go to different daughter cells
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Alleles for genes on different chromosomes are not linked, due to independent
assortment of chromosomes during meiosis
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Consequences of meiosis
- Recombination: movement of genetic material between chromosomes (Prophase 1)
- Segregation: Even distribution of chromosomes during meiosis
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- Add up no
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of progeny
- Put into equation
- Numbers don’t always add up due to double crossing over
Was Mendel just lucky?
- Never described linkage, although good chance some of his traits were on the same
chromosome (only seven chromosomes in peas)
- Also: his data match too closely his expected ratios (for the sample size)…
- Probably had over-helpful assistants…

Hotspots and further complications of Linkage
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Hotspots
- Chance of chiasma (cross over) formation is not a simple function of physical
distance
- Lots of crossing over in some chromosomal regions, very few in others
- There are many recombination hotspots (e
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25,000 in humans); usually very
localised
- Recombination doesn’t occur uniformly
Further complications of linkage
- Sex linkage
- Allosomes are normal chromosomes
- Autosomes are sex chromosomes

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In addition to their role in determining sex, the sex chromosomes have
genes for several characters
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Morgan discovered sex linkage by crossing homozygote red eye female fruit flies and
homozygote male white eye fruit files (where red is dominant over white) results in
all red eyed F1 fruit flies
F2 should produce a 3:1 ratio but this wasn’t observed

Morgan tried the cross the other way round, i
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white-eyed female x red-eyed male
- Result: All red-eyed females and all white-eyed male
- This confirmed what Morgan suspected, the gene for eye colour is linked to
the X chromosome
X-linked genes
- In sex linked characteristics the reciprocal crosses do not give the same
results
- For X-linked genes fathers do not pass the mutant allele onto their sons
- For X-linked genes fathers pass the mutant allele onto their daughters who
are carriers
- Carrier mothers may pass the allele onto their sons (50% chance)
- Females showing the trait for an X-linked mutant allele can exist but they
are rare
Colour blindness is an example of a X-linked trait
- Colour blindness is not blindness; it is the inability to tell certain hues from
certain other hues
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- If a male receives the Xn allele he will have impaired colour vision,
whereas a female with XNXn will not
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Within the UK population, on average 1 in 12 males have some form
of colour blindness
Hence 1 in 144 females should also have some form of colour
blindness
X-inactivation: The inactivation of one of the two copies of the X
chromosome in females

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BUT since only one of a woman's X chromosomes works, why aren't
more women colour blind?
- Reason: The decision on which X chromosome is inactivated is made
on a cell by cell basis when the embryo consists of a few hundred
cells
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- The human eye comes from several different cells, hence some will
have one X-chromosome inactivated while the rest have the other
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- Antihaemophilic factors
- The blood clotting reaction is an enzyme cascade involving Factors VIII, XII,
XI, IX, X and II
- Each of these enzymes are proteases that cut the next protein in line
- Other factors including proteins like Factor VIII are essential as coenzymes
- About 85% of haemophiliacs suffer from classic haemophilia (1 male in 10
000)
- They cannot produce factor VIII
- The rest show Christmas disease where they cannot make factor IX
- The genes for both forms of haemophilia are sex linked
- Haemophiliacs do clot their blood slowly because there is an alternative
pathway via thromboplastin
- Hemophillia in European royal families
- Disorder is lethal or debilitating prior to reproductive maturation
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- Hemizygous males were affected in Russian, German, Spanish royal
families, trait not passed on in British royal family
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