J Corley FDeg

Session aims To understand about:

Monohybrid genes

Multiple alleles

Co-dominance

Epistasis

Sex linked traits

Pleiotrophy

To be able to perform crosses, to predict theoretical outcomes of litters

Recap – define the following Gene

Genome / DNA

Chromosome

Allele

Locus

Dominant

Recessive

Heterozygote

Homozygote

Monohybrid cross

Dihybrid cross

Co dominance

History Felis catus, generally believed to have been domesticated in

Egypt over 3500 years ago.

Ancestor likely to be the African wild cat (Felis libyca)

European wild cat (Felis silvestris) contributed to genetics of domestic cats by cross breeding with the African wild cat.

markingsAnimals Famous For Their Unusual Fur Markings

Simple genetics - monohybrid

Monohybrid crosses – 1 gene Shorthaired allele, L, is dominant to the longhaired

allele, l

Genotype of a longhaired cat is ll.

Homozygous recessive (both alleles are the same).

In a heterozygote (Ll) the the shorthaired allele dominates over the effect of the recessive longhairedallele.

Can infer the genotype of a shorthaired cat by observing phenotypes of its offspring from mating with a longhaired cat = a test cross -

Test cross Do the cross now between a long haired cat, and a

short hair cat

Possible genotypes for the long haired parent?

Possible genotypes for the short haired parent?

Simple geneticsof the shorthaired of unknown genotype to a homozygous recessive longhaired.

Tester Possible genotypes of shorthaired phenotype

phenotype genotype Ll ll

longhaired ll 50% longhaired: 50% shorthaired

100% longhaired

MULTIPLE ALLELES - BLACK Black gene has three alleles, controlling density of

eumelanin granules in the hair.

Seen in all-black cats, black stripes on a tabby, and the dark ear-tips, feet and tails (points) of seal point Siamese.

BLACK Black allele, B, is dominant, produces black (actually

super-dark brown) coat. Black = B_

Dark-brown allele, b, reduces black to a dark brown, chocolate colour. Dark Brown = b_

Light-brown allele, bl, reduces melanin density even more, giving a medium brown coat (blbl cinnamon).

Dominance hierarchy is B > b > bl

Task What are the offspring of a hetero black cat carrying

brown and a homo chocolate brown cat?

What are the offspring of a hetero chocolate brown and a cinnamon cat?

What are the offspring of a hetero black cat carrying brown and a cinnamon cat?

DILUTE Dilute locus has two alleles that affect distribution of

pigment granules in the fur.

Dominant dense allele, D, produces dense colour

Recessive dilute allele (d) causes clumping of pigment granules in the hair shaft, leaving large areas between clumps with no granules.

Open areas cause colour dilution

Effect of black (eumelanin), orange (phaeomelanin), and dilute genes on cat coat colors.

Co dominance Dominance is a continuum

Runs from completely dominant (e.g. the shorthairedallele, which completely masks the recessive longhaired allele) to codominant, where the heterozygote doesn’t look like either of the homozygotes.

Co dominance - Cremello 1 dilution

Co dominance - Cremello 2 dilutions

What is a white animal? White coat and grey skin, any colour eyes = grey

White coat and pink skin, dark eyes = white

White coat and pink skin and pink eyes = albino

CO DOMINANCE - ALBINO

Two alleles at the albino locus, pointed, cs, and sepia, cb

Both alleles are recessive to the full-color allele, C, but are codominant with each other.

The homozygous genotype cscs reduces pigment expression across most of the animal.

CO DOMINANCE - ALBINO Reduced pigment production in the eyes causes bright

blue eyes

Reduced pigment density in the hairs turns the coat from black/brown to a light beige with dark brown points in the classic Siamese pattern

CO DOMINANCE - ALBINO Homozygous cb cb genotype has a smaller reduction in

pigment production, turning a black coat to very dark brown with green or green-gold eyes. (Burmese)

CO DOMINANCE - ALBINO Heterozygote cb cs gives a combined phenotype called

Tonkinese - a Siamese-patterned coat with darker base body color and turquoise (aquamarine) eyes.

CO DOMINANCE - ALBINO All three alleles (C, cs, and cb) are dominant to the

very rare albino alleles, c and ca, which when homozygous produce white cats with either pale blue eyes (caca) or unpigmented pink eyes (cc).

Dominance hierarchy at the albino locus is: C > cb = cs

> ca > c.

Task What do you expect to get when you cross:

A pointed Siamese cat with a Burmese cat?

A Burmese cat with a Tonkinese cat?

A Siamese cat with a Tonkinese cat?

Genotype by Environment Interaction The Siamese allele, cs, causes temperature-sensitive

pigment expression

Allele produces temperature-sensitive tyrosine gene that is inactive at the cats core body temperature, leaving a light brown background.

At the cooler extremities, the enzyme is active, producing pigment, so forming dark points

Genotype by Environment Interaction Siamese house cats living in warm homes tend to be

lighter than outdoor cats, which also get darker when it’s cold

The cb allele also temperature sensitive, less so than the cs allele, so produces a darker coat.

Epistasis: Gene masking another gene The orange gene has two alleles: non- orange and

orange.

Non-orange allele, o, is recessive and allows full expression of the black locus.

Epistasis: Gene masking another gene Dominant orange allele, O, influences expression of

the black and agouti loci - produces red/orange phaeomelanin instead of black/brown eumelanin.

Masks the effect of the black gene by converting a black or brown coat to orange.

Task What are the expected offspring of crossing:

A homo orange female with a hetero orange male?

A hetero orange female with a black male?

Sex-linked Traits The orange gene is carried on the X chromosome, so is

sex-linked.

Male cats, can only be black or orange

Females can be black, orange or tortoiseshell.

Males are normally XY (heterogametic), so only one X-chromosome (unless they are Jake, Harry or Eddie).

Sex-linked Traits If a male carries the orange allele he will be orange (OY).

Females are XX, so have two X-chromosomes (homogametic).

If both chromosomes carry the orange allele, she will be orange.

If she is heterozygous (Oo), she will be a patchwork of orange and black patches

Sex-linked Traits So the amount of colour produced in female cells is the

same as in male cells, one X-chromosome is inactivated in every cell in the female embryo.

For a heterozygous female, some cells produce phaeomelanin (the active X-chromosome contains the O allele) and others eumelanin (the active X-chromosome contains the o allele).

Which X-chromosome is inactivated is totally random, producing random tortoiseshell patterns

Task What are the expected offspring of :

Crossing a black female with an orange male

Crossing a tortoiseshell female with a black male?

Multiple alleles Controlled by more than one gene.

Cat colour is controlled by multiple genes.

Can do a test cross to work out the genotype

Multiple alleles - Agouti Other genes with dominant alleles are:

the agouti gene - controls colour expression along the length of each shaft of hair

the dilute genes, which also influence coat color.

AGOUTI Hairs with more than one colour band on the hair shaft

Produce a ticked / agouti coat.

Typical colour animals (mice, squirrels, rabbits, wolves etc)

Thought important to crypticity (ability to blend into the background).

Determined by the dominant agouti allele, A.

Non-agouti cats are unbanded, with a solid coloured coat, if homozygous for the non-agouti allele (aa) at the agouti locus

Though it looks ‘brown’ it is technically brown and black banded fur

How does it work? The ‘agouti gene’ controls where and how brown and

black pigments are set into hair.

Need to look at how colour is formed in mammal hair.

Melanocytes At the base of each hair follicle is a melanocyte cell

Produces pigment and inserts it into the growing hair

Hair follicle

Hair

Melanocyte

Two types of pigment Melanocytes make two types of pigment

Eumelanin (Browns and blacks)

Phaeomelanin (Reds and yellows)

Each relies on a series of pathways before it gets to its final ‘colour’

Pathways Melanocyte initially produced ‘Tyrosine’ which is

colourless

Tyrosine is converted into 5,6 dihydroxyindole, which is brown

5,6 dihydroxyindole then converted into eumelanin which is black

Similar process happens with phaeomelanin to produce red/yellow/browns

Agouti Gene in action As each step produces a different colour, if

any of those steps are disrupted, or broken, then the fur contain different colours

The agouti gene controls whether certain pathways are on or off.

In the animals seen earlier, the banded fur is caused by the agouti gene switching the final brown to black pathway on and off as the hair grows

Tyrosine

5,6 dihydroxyindole

Eumelanin

Tyrosine 5,6 dihydroxyindole

Eumelanin

Tyrosine 5,6 dihydroxyindole

Eumelanin

Task What are the offspring of a homo agouti and a hetero

agouti rabbit?

What are the offspring of two hetero agouti rabbits?

What are the offspring of a hetero agouti rabbit and a solid coloured rabbit?

MULTIPLE ALLELES - TABBY Causes banded (ticked) hairs to alternate with stripes,

blotches, or spots of solid coloured hairs, forming a stripy pattern

Two common striping patterns are mackerel (parallel stripes) or classic (thick stripes or whorls, creating a blotched pattern).

Tabby patterns

TABBY The stripe pattern is produced by the dominant tabby

allele, T. (TT, Ttb )

The recessive blotched allele, tb, produces the classic / blotched pattern (tbtb )

Abyssinian (Ta) also called ‘ticked’ has faint striping on the face or tail, and sometimes a dark stripe down the back.

Dominance hierarchy is Ta > T > tb.

Tabby cats have:• M on forehead.

• Thin pencil lines on face.

• Black "eyeliner" appearance and white or pale fur around eyeliner.

• Pigmented lips and paws.

• A pink nose outlined in darker pigment

• Torso, leg, and tail banding. (Torso banding not in the ticked tabby.)

Task What offspring will you get from crossing a blotched

tabby with a ticked tabby – all possibilities

What offspring will you get from crossing a mackerel tabby with a ticked tabby – all possibilities

Effects of agouti and tabby on the black, dilute and orange gene effects.

Pleiotropy When a gene affects more than one trait

Most coat color genes have pleiotropic effects on eye color.

Two genes have a pleiotropic effect on coat colour, eye colour and hearing – dominant white, and piebald spotting

Dominant white Dominant white allele, W, overrides all other genes for

pigmentation, producing a white coat and blue eyes.

Epistatic to all other coat color genes.

Other genes for colour and pattern present, but hidden as the dominant white mutation blocks production of melanin by melanocytes.

The cochlea in the ear contains a band of melanocytes that regulate ion balance, necessary for transmission of electrical signals, stimulated by vibration of the hair cells in the cochlea.

If the ion balance isn’t maintained, signal transmission to the brain ceases a few days after birth, causing permanent deafness.

So dominant white locus is pleiotrophic for coat colour and hearing.

Dominant white

Piebald spotting Very common.

Can occur with any coat colour.

Spotted allele, S, creates white spots, the s allele doesn’t

So homozygote, ss, has no white spots.

Heterozygote, Ss, has restricted areas of white spotting; usually the feet, nose, chest, and belly.

SS homozygote has white regions covering more than half the body.

The white area is the spot, so a spotted (SS) cat can be completely white!

Usually follow a regular progression.

Piebald spotting

Piebald spotting

Piebald spotting Least spotting = small spots on the breast and belly.

Increased spotting = covers entire belly, the neck, chin and front feet.

Most spotting have spots up the sides, over the back and onto the head.

Tail is the last area to have white spots.

Several genes modify the action of the spotting gene to produce the continuum of patterns seen in cats

Piebald spotting Spotted allele, S, also disturbs migration of

melanocytes in embryo development.

White spots are areas lacking melanocytes.

Spotted cat shows the same pleiotropy as the dominant-white gene.

If spot is over the eye, it will be blue, so spotted cats may be blue-eyed or odd-eyed.

If ear is in the spot, cat will be deaf in that ear.

If spot covers the eye and ear, an odd-eyed cat will be deaf on the blue-eyed side.

Piebald spotting

Session aims To understand about:

Monohybrid genes

Multiple alleles

Co-dominance

Epistasis

Sex linked traits

Pleiotrophy

To be able to perform crosses, to predict theoretical outcomes of litters