Dwarfism

Dwarfism

Can be caused by over 300 different things!

A form of dwarfism called Achondroplasia is caused by a mutation to a gene (FGFR3) that regulates bone growth.

The mutation makes the FGFR3 gene too aggressive, which stops cartilage being turned to bone so limbs don’t grow properly.

GENE-GENE INTERACTIONS

Gene-gene Interactions

Most traits are actually controlled by more than one gene

Epistasis: when the expression of one gene affects the expression of another

E.g: Biochemical pathway with 2 steps controlled by 2 genes

A B C

Gene 1 Gene 2

Enzyme 1 Enzyme 2

Epistasis

Collaboration

Supplementary Genes

Complementary Genes

CollaborationTwo genes influence the same trait, and interact to produce phenotypes that could not result from the action of either gene alone

The genes interact to produce FOUR possible phenotypes

Combs in chickens:Genes P and R control combP_R_ WalnutppR_ RoseP_rr Peapprr Single

Complete a punnet square for a cross between two chickens PpRr x PpRr•How many offspring have Walnut combs?

9•How many have pea?3• How many have rose?3• How many have single1

So the typical

ratio for Collaborative Genes is

9:3:3:1

Epistasis

Collaboration

Supplementary Genes

Complementary Genes

Supplementary GenesWhen one gene masks or alters the effect of another gene

There are THREE possible phenotypes

In mice, coat colour is controlled by gene A and gene B

Gene A makes melanin for coat colour

Gene B deposits the melanin

For black coat colour, a dominant A allele is needed (causes lots of melanin produced) AND a dominant B is needed to deposit the melanin (A_B_)

For brown coat colour, no dominant A allele is present (causes only a small amount of melanin produced) AND a dominant B is needed to deposit the melanin (aaB_)

If no dominant B allele is present, the melanin is not deposited and so the mouse will be white (_ _ bb)

Complete a punnet square for a cross between two mice AaBb x AaBb•How many offspring are Black?9•How many are Brown?3•How many are White?4

So the typical ratio for Supplementary Genes is 9:3:4

Epistasis

Collaboration

Supplementary Genes

Complementary Genes

Complimentary GenesSome genes can only be expressed in the presence of other genes.

These genes are said to be complimentary; both genes need a dominant allele for the final phenotype to be produced.

There are TWO possible phenotypes

Sweet pea flower colour involves gene P and gene C. For the flower to be purple, dominant alleles for BOTH genes must be present. Otherwise the flower will be white.

Flower Colour

Colourless starting chemical

Gene P Gene C

Enzyme A Enzyme BColourless (white) intermediate

PURPLE PRODUCT

Purple: White:P_C_ P_cc, ppC_, ppcc

Complete a punnet square for a cross between two flowers PpCc x PpCc

•How many offspring are Purple?

9•How many are White?7

So the typical ratio for Complementary

Genes is 9:7

Worksheet

RECAPMatch the definition with the type of gene interaction…Collaboration Genes that can only be

expressed in the presence of other genes (e.g. sweet pea flower colour)

Supplementary Genes

Two genes influence the same trait, and interact to produce phenotypes that could not result from the action of either gene alone (e.g. chickens combs)

Complementary Genes

When one gene masks or alters the effect of another gene (e.g. mouse coat colour)

SEX LINKED GENES

What do you remember?•The human sex chromosomes are….

•The genotype of a female is…..

•The genotype of a male is….

•Is it the same for all species??

X and Y

XX << This is called Homogametic (2 of the same)

XY << This is called heterogametic (2 different)

Nope!! In birds, butterflies and moths, the female is heterogametic and the male is homogametic. And in some insects (grasshoppers) there is no Y chromosome at all, so: XO = male and XX = female.

A sex linked gene is….A gene that is only found on

only one of the sex chromosomes – either the X or

the Y

Which sex chromosome (X or Y) do you think has the

most sex linked genes and why?

These are your sex chromosomes:

X A good way to visualise sex-linked genes is to think of the Y as being like an X with on bottom cut off. Imagine the sex-linked genes are the ones that are carried on the the bottom right hand bar of the X, which the Y is missing.

Sex linked genes generally show up more in one sex than the other

What are some examples of conditions or characteristics that show up more regularly in females or males?

•Red-green colour blindness•Haemophilia (blood disorder where clotting factors aren’t produced so blood can’t clot properly)

•All tortoiseshell cats are female!

Red Green Colour Blindness

X – Linked Recessive Disorder.

XcXc or XcY to have the disorder

Haemophillia

A disease that impairs the body’s ability to form blood clots

X-linked recessive disorder.

XhXh or XhY to have the disorder

Tortoiseshell Cats – They’re all Girls!

Red-Green Colour BlindnessAn X-linked RECESSIVE disorder

Who gets it more, men or women?

Colour blindness is an X-linked recessive disorder. If it’s found on the X chromosome, why do you think that more males get it than females?

If the trait is recessive, males only need to receive one recessive allele to show the trait because they have nothing to maskit. Females need two recessive alleles toreceive the trait, so they are less likely to show it than males.

Checkpoint:

If an X-linked gene for a disorder is recessive, why is it more common for males to get it than females?

Either write in in your book or explain it to the person next to you.

In humans, red-green colour blindness is a sex linked trait. Having the disorder (Xc) is recessive to not having the disorder (XC)

What are the sexes and phenotypes (colour blind, normal, normal but carrier) of humans with with following genotypes?

XCXc

XCYXcXc

XCXC

XcY

Female, Normal but carrierMale, NormalFemale, colour blindFemale, Normal

Male, colour blind

Red-Green colour blindness is an X linked recessive trait (c)

A woman with normal vision but who is a carrier of the red-green colour blindness gene marries a man with normal vision. Give the predicted genotype and phenotype of their children.

Woman (carrier) x Man (normal)

XCXc XCYGametes:

XC Xc

Don’t put anything above the Y! It’s genetically empty!

XC Y

XC Xc

XC

Y

Genotype: 25% XCXC 25% XCXc, 25% XCY, 25% XcY

Phenotype: 25% girl normal vision 25% girl normal but

carrier 25% boy normal vision 25% boy with colour

blindness

Genotype:

XCXC XCXc

XCY XcY

Haemophillia is an X linked recessive trait (h)

A woman with normal blood but who is a carrier of the haemophillia gene marries a man who has haemophillia. Give the predicted genotype and phenotype of their children.

Woman (carrier) x Man (has haemophillia)

XHXh XhYGametes:

XH Xh

Don’t put anything above the Y! It’s genetically empty!

Xh Y

XH Xh

Xh XHXh XhXh

Y XHY XhY

Genotype: 25% XHXh, 25% XHXh, 25% XHY, 25% XhY

Phenotype: 25% girl normal but carrier

25% girl with haemophillia

25% boy with haemophillia

25% boy normal

Genotype:

Worksheet…If you’ve finished..

A normal sighted woman whose father was colour blind marries a man with normal vision. Give the genotypes and phenotypes, plus their ratios, that could be expected among the children of such a marriage

Test Yourself:•What is a sex-linked Gene?A gene found on only ONE of sex chromosomes

•Why can’t a male be a carrier for an X-linked recessive trait like colour blindness?

Because he only has ONE copy of the allele, so he will either HAVE the disorder, or NOT HAVE the disorder

LINKED GENES

Where are genes located?

CHROMOSOMES!

Genes are arranged in a linear order on the chromosome, so one chromosome has many genes.

How are genes arranged on a chromosome??

Linked Genes are:Genes that are located on the same chromosome.

A B written AB

a b a b

OR

A b written Ab

a B aB

Genes A and B are LINKED

Linked Genes result in fewer possible

combinations of alleles in offspring…

If genes are located on different chromosomes, each allele will separate during meiosis (in this case, the A allele will be able to separate from the B allele) so that new combinations can be produced.

Normally…

But if the genes are linked, then the alleles don’t separate – and so the traits are inherited together.

Checkpoint:Why do linked-genes result in fewer combinations of alleles in offspring?Because when genes are linked, the alleles aren’t separated during meiosis so they move into the gametes together.

This means that the inheritance patterns we would expect to see aren’t always seen….

Flies have different phenotypes for wings and body colour.They can have:

Grey body (B) Normal wings (W)Black body (b) Curly wings (w)

What genotype and phenotype ratios would you expect if a fly heterozygous for body colour and wing type mated with a fly with a black body and curly wings?

1 BbWw : 1 bbWw : 1 Bbww : 1 bbww1 Grey body, normal wings : 1 black body normal wings : 1 grey body curly wings : 1 black body curly wings.

BW Bw bW bw

bw

BUT when the cross was actually done, the ratios that were observed were very different….

Grey Body, Normal wings

Black Body, Curly wings

Grey body, curly wings

Black body, normal wings

What connection can you make between the parental phenotypes and the phenotype ratio of the offspring??Much higher ratio of offspring that are of the parental type

BW bw Bw bW

bw

Why are most offspring of the parental type??Think back to this….If the genes are linked, then the alleles don’t separate – and so the traits are inherited together.

What can you remember about linked genes?

A recap:Linked genes are genes that are found on the same chromosome.

Linkage results in fewer genetic combinations of alleles in offspring, because traits that are linked tend to be inherited together Most offspring are of the parental type

But why were there some flies that were a mixture of the parents phenotypes??

•What happens during meiosis to reshuffle alleles?

•CROSSING OVER!!

•When crossing over occurs between linked alleles, then they will be separated into different gametes and so you get different combinations of alleles together (recombinants)

Practice…Red Hair - b Brown hair - BFreckles - f No Freckles - FA woman with red hair and freckles (bbff) marries a man with brown hair and no freckles (BbFf). They have 12 children.6 have red hair and freckles, 5 have brown hair and no freckles and 1 has red hair and no freckles.

1. Is this the ratio you would expect from this cross? Support your answer with a punnet square, and state the expected ratio.

2. Explain in detail a possible reason why the expected ratio is not seen.

3. Explain how one of the children had red hair and no freckles

Nature vs Nurture

One of the great debates in life is whether you are the way you are because of your genes or because of the environment you were brought up in.What do you think?? Does nature or nurture have the biggest effect??

Its very difficult to determine this in reality, we would need about 1000 sets of identical twins, separated at birth, exposed to different environments and then compared later in life.

Research…There are many things in the environment that can influence genes, such as:• Drugs (alcohol, cigarettes, medications)• Nutrition• Temperature• Ionising Radiation• Polluting Chemicals

DrugsWhen taken by pregnant woman these can affect the unborn child.

Thalidomide•This was a mild sedative, given to mothers for morning sickness.

•This drug caused severe deformities, particularly unformed limbs, in unborn babies.

Thalidomide Babies

Alcohol and Cigarettes• Babies of smokers have low birth weights

• Alcohol can have a very bad effect on a child, leading to foetal alcohol syndrome.

• This can be identified by certain facial traits, and an unusual behaviour pattern that seems to fail to connect actions with consequences.

• The babies of chronic alcoholics are often severely retarded in their development, leading to:

deformed joints imperfectly formed heart lowered IQ.

FAS

Starvation or Malnutrition

•This can effect the proper development of both plants and animals.

•The effect of severe malnutrition in humans is stunted growth and mental retardation, especially if it is due to a protein deficiency.

•Malnutrition has an especially bad effect on the brain if it occurs under the age of about 8.

Starvation or Malnutrition

• In many third world countries whole populations are affected.

•This will lead to a future population of mentally slow adults.

• In these circumstances genes do not get a chance to show through.

Temperature• This can affect enzymes and fur colour.• In Himalayan rabbits and Siamese cats, the fur colour can be affected by temperature.

• These animals are normally light-coloured with black feet, ears, nose and tail

All cats are born with a mutation to the enzyme in charge of depositing melanin in the fur. The mutated enzyme is heat-sensitive; it fails to work at normal body temperature, but becomes active in cooler areas of the skin. The extremities are colder than the rest of the body, which results in dark colouration in the face, ears, feet and tail.

Temperature

• If an area of the rabbit is shaved and a cool pack is placed on that area, the low temperature causes the growth of a patch of black hair.

• Coldness affects the expression of the gene.

Temperature

The temperature that certain reptile eggs are incubated in can determine the sex of the offspring.

Temperature

• In the fruit fly Drosophila, there is a condition that causes the wings to curl.

• If flies with this gene are raised at a temperature of 25ºC, the wings curl.

• If the flies are raised at a temperature of around 16ºC, the trait appears only rarely; most of the flies have straight wings.

• If their offspring are raised at 25ºC, the curly wings reappear.

Ionising Radiation

Ionising Radiation causes changes to the DNA sequence (mutations), which has a huge affect on genes…• A large increase in the incidence of thyroid cancer

has occurred among people who were young children and adolescents at the time of the Chernobyl accident

• Also increases in cardiovascular disease, leukaemia and other cancers

• If mutations from radiation occur in gametes, problems can be passed to the next generation and stillbirths, cancer and death can occur.

Polluting Chemicals in the Environment

• The chemical 2.4.D was part of agent orange, used in the Vietnam war to defoliate the forests.

• It was full of a toxic substance called dioxin, which got into the bloodstream of those exposed and caused mutations to their DNA sequence and chromosomes

• Many died

• Caused miscarriages, stillbirths and birth defects in babies of those exposed - Cleft palate, mental disabilities, hernias, facial distortion, limb distortion and extra fingers and toes!!

Light

• Lack of light can prevent the formation of chlorophyll or development of seedlings.

• Light also affects skin colour. Some people can tan, as the sun affects the deposition of melanin.