Non-Mendelian Genetics

Learn more about non-Mendelian genetics watch and take notes: http://www.bozemanscience.com/030-advanced-genetic/

Today, we know that genes often do not

follow Mendel’s Laws all the time! There are

exceptions…..

• Incomplete dominance

• Co-dominance

• Epistatic/blocked Trait

• Polygenics

• Sex linkage

• Environmental effects

Incomplete dominance

• Blending of traits. Phenotype

of offspring falls somewhere

between each parent, when

genotype is heterozygotic.

• RED Flower x WHITE Flower

---> PINK Flower

• It's like mixing paints, red +

white will make pink. Red

doesn't totally block

(dominate) the white, but

blends.

• R = allele for red flowers

• W = allele for white flowers

• red x white ---> pink

• RR x WW ---> 100% RW

Co-dominance

• Characterized by the

expression of both

alleles. Even the

recessive allele affects

the organism.

• These offspring are

called roans in animals.

EX: Red horses (RR)

crossed with white

horses (rr) produce Rr

roans that are both red

and white haired.

EX: Blood type - A

and B are both

dominant!

• IAIA or I

Ai= A type

• IAIB = AB type

• IBIB or I

Bi = B type

• ii = O type

On your own, determine which type the

following are:

• Cream colored bull crossed with a red cow

produces a spotted/roan offspring:

Is this example co-dominance or

incomplete dominance?

• Dark blue parakeet crossed with a light sky

blue parakeet=medium cobalt blue

parakeet

Mosaic trait

• The genotype of a cell or organism is the collective term for all

of its genetic material/DNA. In genetics, the terms mosaic and

mosaicism are used to describe cell populations which have two

different genotypes.

• Some cells in the body are genetically different from the rest.

Fairly common phenomenon in plants, and can also occur in

animals and humans.

Ex: Heterochromia iridum

Polygenic traits

Characterized by many genes that are needed to

affect a single trait.

Examples: eye color is controlled by at least 3 genes

working together to produce one color. Brown,

blue, green combinations can produce hazel eyes.

Height and hair/fur color are also polygenic.

Eye color

Hair color

How Polygenic traits Work

In animal fur and skin color, at least 5 genes control

the phenotype!!

EX: In mice and other animals, “A” genes are for

amount of color , “B” genes for main pigment

color, C genes for control of color expression, the

gene for shading is (DD), and one gene for spots

(SS).

Fur color

How Polygenic traits Work

Total genotype for Fur or skin color---AaBbCcDdSs!!WOW!!

You now have a mouse or even reptile with overall black

coloring and silver shading with lots of spots!

A punnett square to show crosses would be huge (over 64

boxes), so just show a parental cross as the example:

AaBbCcDdSs x AaBbCcDdSs

*Fun fact! Did you know that polar bear

fur is colorless?

Skin color

Epistatic/Blocked Trait

Epistasis occurs in polygenic genes. Ex: Albinism. If an

organisms carries the homozygous copies of albino

gene (allele choices are usually cc or aa) no matter

what combination of genes that organism has for color,

the organism will be albino. B=black coat and aa=

albino So a mouse with a

genotype of BBGGaa will have

NO color even though it carries

dominant black trait!

Polygenic/Albino Trait (Epistatic)

Albinism

Why are albino eyes red? • There are lots of blood vessels in the back of a

colorless eye that show through.

Other Epistatic/Blocked Trait in fur

Sex linked traits

First discovered in fruit fly crosses in laboratory

Sex linked traits

• Some genes are on the

Y chromosome

• Ex: Ear Hair

• Those carried on the X

are more abundant

• EX: Muscular Dystrophy

Characterized by genes carried on the X or Y

chromosomes.

• EX: Hemophilia

• XhX

h or X

hY=

hemophilia.

• Females“carry” the gene

on X chromosome, but

won’t express/show it

because they have the

extra “healthy” dominant

copy on other X

chromosome

• Males CAN’T be carriers

of an X-linked gene

because they always

show/express the trait!

Sex-linked or “X-linked” traits

Ex: Red/Green

Colorblindness

Sex-linked or

“X-linked”

traits

Cross a color-blind male and a

carrier female? How many of their

children will be colorblind?

Xc X

Xc

Y

XcXc XcX

XcY XY

Cross a color-blind male and a non-

carrier female? How many of their

children will be colorblind?

Environmental effects on phenotype

Phenotypes can change as genes are exposed to

external stimuli, like temperature.

Example: Temperature dependent sex determination-

Crocodile eggs will hatch females if incubation temp

is below 85 degrees or an extremely high

temperature and males develop only in

intermediate temp range

Environmental effects on phenotype

Himalayan point colors

in cats, rabbits and

mice—fur color

changes with body

temperature. When

animals get below a

certain temperature,

they turn darker!! (the

baby rabbit on left

was left at a “chilly”

70 degrees.

Pedigrees Pedigrees are a visual representation of traits that are passed on through generations.

• Females=circles

• Males=squares.

• Individuals that express the “disorder” are completely shaded in.

• Carrier individuals for a recessive trait are partially shaded.

For example: Cystic Fibrosis is a recessive trait that will only be expressed when two copies of the recessive allele are inherited.

The square “son” in the F2 generation of this picture inherited both recessive alleles.

Allele Interactions

Eye Color & Height

Heterozygous phenotype

Dominant homozygous phenotype

Homozygous recessive

phenotype

Co-dominance

Expression

Both Traits

AB blood types

Polygenic Traits

Many genes

Range

Incomplete Dominance