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You and your genesYou and your genes

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B1.1 What are genes?B1.1 What are genes?

21/04/23Genes, Chromosomes and Genes, Chromosomes and DNADNA

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Some facts:

- Genes are sections of very long DNA molecules that make up chromosomes in the nuclei of cells

- The DNA contains instructions on how the cell should work

- Genes control the development of characteristics (“it’s in the genes”) by issuing instructions to the cell to produce certain proteins

- These proteins are either structural (used for cell growth and repair, e.g. collagen) or enzymes (used for speeding up reactions, e.g. amylase)

- Some characteristics (e.g. eye colour) are controlled by several genes working together

How genes workHow genes work

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VariationVariation“Variation” is the name given to differences between individuals of the SAME species.

Variation is due to GENETIC or ENVIRONMENTAL causes. For example, consider dogs:

1) Ways in which they are the same:

2) Ways in which they are different:

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Environmental differencesEnvironmental differencesSome of this variation is due to our parents, but some of it is due to our upbringing and the environment in which we live – this is called “Environmental variation”.

Variation due to inheritance only

Variation due to environment only

Variation due to a bit of both

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B1.2 “It’s in your genes”B1.2 “It’s in your genes”

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Sexual ReproductionSexual Reproduction

The human egg and sperm cell (“GAMETES”) contain 23 chromosomes each.

When fertilisation happens the gametes fuse together to make a single cell called a ZYGOTE. The zygote has 46 chromosomes (23 pairs) and contains information from each parent.

We have similar characteristics to our parents due to genetic information being passed down in genes through gametes:

21/04/23Sexual vs. Asexual Sexual vs. Asexual reproductionreproduction

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)

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Boy or Girl?Boy or Girl?

X Y X

XX XYGirl Boy

“Allele”

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Father

Mother

Son

Daughter

Boy or Girl?Boy or Girl?

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Key wordsKey wordsGenotype

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

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AndrogenAndrogenAndrogen is a hormone that controls the development of male reproductive organs:

Step 1: An egg is fertilised by an X chromosome and a Y chromosome

Step 2: Testes start to develop due to the presence of a Y chromosome

Step 3: The testes start producing androgen

Step 4: Androgen causes male reproductive organs to grow

Sometimes the Y chromosome is present but androgen is not detected. This causes the development of female reproductive organs but the individual is genetically a male and unable to reproduce.

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Eye colourEye colour

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:

bbBB Bb

Homozygous brown-eyed

parent

Heterozygous brown-eyed

parent

Blue-eyed parent

What would the offspring have?

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Eye colourEye colourExample 1: A homozygous brown-eyed parent and a

blue-eyed parent:

Example 2: 2 heterozygous brown-

eyed parents

BB bbX Bb BbXParents:

Gametes:

Offspring: Bb Bb BbBb BB Bb bbbB

B B bb B bB b

(FOIL)

All offspring have brown eyes

25% chance of blue eyes

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Eye colourEye colour

Example 3: A heterozygous brown-eyed father and a blue-eyed mother:

Bb

Bb Bb bbbb

bb

b bB b

Equal (50%) chance of being either brown eyed or blue eyed.

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B b

b

b

Another method – the “Punnett Another method – the “Punnett square”square”

Example 3: A heterozygous brown-eyed father and a blue-eyed mother:

B b

b Bb bb

b Bb bb

Father

Mother

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B1.3 Inherited diseasesB1.3 Inherited diseases

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Example questionsExample questions

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).

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Inherited diseasesInherited diseases1) Cystic fibrosis – a disease that causes thick and sticky mucus to coat the lungs, gut and pancreas, making breathing and digestion difficult. It’s caused by faulty recessive alleles:

2) Huntingdon's disease – a disease of the nervous system that causes shaking, memory loss, mood changes and eventually dementia. It’s caused by a faulty dominant allele:

3) Sickle cell anaemia – a disease that alters the shape of red blood cells, thereby reducing their oxygen capacity, causing weakness and anaemia. It’s caused by recessive alleles:

Ff FfX

Cc ccX

Ss SsX

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Family Pedigree ChartsFamily Pedigree Charts21/04/23

Consider the following chart of the offspring and grandchildren between two sickle-cell anaemia carriers:

Key:

= male

= female

= S allele

= s allele

Q. Describe the genotype and the phenotype of each of the grandchildren. Also, which member of

this family has got sickle-cell anaemia?

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Genetic testingGenetic testingIt is now possible to test individuals before they are born for any faulty alleles. There are two main methods:

1) Amniocentesis testing:

- Done at 14-16 weeks

- 0.5% chance of miscarriage

- Small chance of infection

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Genetic testingGenetic testing2) Chorionic villi testing:

- Done at 8-10 weeks

- 2% chance of miscarriage

- Virtually no chance of infection

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Genetic testingGenetic testingPossible outcomes:

Outcome Test result Reality

True positive Fetus has the disorder

Fetus has the disorder

True negative Fetus does not have the disorder

Fetus does not have the disorder

False positive Fetus has the disorder

Fetus does not have the disorder

False negative Fetus does not have the disorder

Fetus has the disorder

21/04/23The Ethics of Genetic The Ethics of Genetic TestingTesting

Genetic testing

Aborting deformed children

Predicting future

diseases

Eliminating genetic diseases

Genetic information/

profiles stored on computer

Deciding whether or not

to have children based on the

results

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Embryo selectionEmbryo selectionAnother way of preventing babies born with genetic disorders is embryo selection.

Basically, embryos are harvested from the mother and fertilised in a lab by the father’s sperm (IVF). Healthy embryos are then impmanted back into the mother. This procedure is called pre-implantation genetic diagnosis (PGD).

Genetically engineered people in TV:

Mutant X, a team of genetically engineered mutants.

Khan, from Star Trek 2: The Wrath of Khan

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B1.4 How is a clone made?B1.4 How is a clone made?

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Cloning PlantsCloning PlantsPlants can reproduce ASEXUALLY. The offspring are genetically ________ to the parent plant and are called _________. The only variation between then will be due to environmental factors. Two examples:

1) This spider plant has grown a rooting side branch (“stolon”) which will eventually become __________.

2) A gardener has taken cuttings of this plant (which probably has good characteristics) and is growing them in a ____ atmosphere until the ____ develop.

Words – clones, damp, independent, roots, identical

21/04/2321/04/23Cloning Plants by tissue Cloning Plants by tissue cultureculture

1) Scrape off a few cells from the desired plant

2) Place the scrapings in hormones and nutrients

3) 2 weeks later you should have lots of genetically identical plants

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Cloning Animals 1Cloning Animals 1

Here’s a zygote in its early stages before it turns into an embryo:

Normally, this embryo would grow into one adult. However, the zygote could split into two embryos before maturing, and this causes identical twins with the same genetic information:

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Cloning Animals 2Cloning Animals 2

Host mother

Clone

Animals can be cloned by taking the nucleus from an adult body cell and transferring it to an empty, unfertilised egg:

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Stem cell researchStem cell researchStem cells are cells that have not yet specialised:

Egg and

sperm

Embryo

Cloned embryo

s

These stem cells have the potential to develop into any kind of cell. The rest of the embryo is destroyed. Most of these embryos come from unused IVF treatments.

The ethical issue:

Should these embryos be treated as humans?