Complex Patterns of Inheritance

Incomplete dominance

• Incomplete dominance is a type of inheritance in which both alleles for a specific trait are expressed. This results in a combined phenotype (expressed physical trait).

Incomplete Dominance• For example, if you cross

pollinate red and white snapdragon plants, both colors are expressed in the offspring. The resulting offspring are pink.

• It's like mixing paints, red + white will make pink.  Red doesn't totally block (dominate) the white, instead there is incomplete dominance, and we end up with something in-between.

Incomplete Dominance

• The pink flower would be a hybrid. A hybrid shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype. 

• With incomplete dominance we get a blending of the dominant & recessive traits so that the third phenotype is something in the middle (red x white = pink).

Cross red flowers with white flowers – show genotypes

Fr Fr

Fw

Fw

FrFwFrFw

FrFw FrFw

Cross red flowers with white flowers – show phenotypes

Fr Fr

Fw

Fw

FrFw FrFw

FrFw FrFw

Cross a pink flower with another pink flower – you can see 50% of the offspring will also be pink. What about the rest?

FrFrFr

FwFw

Genotypic and Phenotypic ratios

• What is the genotypic ratio?

• 1FrFr: 2FrFw:1FwFw

• What is the phenotypic ratio?

• 1red:2pink:1white

Codominance• Is similar to Incomplete dominance but don’t

get them confused!!!• In Incomplete dominance we see a NEW

phenotype which is a blend of the 2 parents. (like mixing red and white paint to get pink).

• In co-dominance we see both the parental alleles expressed not mixed.

• Think of a reddish cow and a white bull. The offspring would not be pink. It might have both red and white hairs.

Co-dominance

• This type of colouring is called roan fur in cattle. 

• Cattle can be red (RR = all red hairs), white (WW = all white hairs),

• or roan (RW = red & white hairs together).• ( of course cattle can be other colours too-

but we’re not worrying about that right now)

Co-dominance Cross a red cow with a white bull

Co-dominance Cross 2 roan cattle

Phenotypic and Genotypic ratios

• In the last example what would be the genotypic ratio?

• 1RR:2RW:1WW• What would be the

phenotypic ratio? • 1Red:2Roan:1White

Co-Dominance – Human Blood Type

• In human blood type, type A and type B are called co-dominant. Neither dominates the other. That means that if an offspring inherits one allele for A and another for B then offspring will have type AB blood.

However, both A&B are dominant over type O which is recessive.

Co-Dominance – Human Blood Type

• A person can have one of 4 blood types

TYPE A - dominant

TYPE B -dominant

TYPE AB – neither dominates the other

TYPE O -recessive

Here is how to write the Phenotype and Genotypes

• Note that conventionally the letter “I” is used in genetics to denote blood type.

• A capital “I” with the superscript a or b is used as they are both a dominant allele.

• A lowercase “i” is used for type O which is recessive

Cross Homozygous type A blood with Homozygous type B blood

Cross Heterozygous type A blood with Homozygous type B blood

What is the phenotypic ratio?

2AB:2B

Cross Heterozygous type A blood with Heterozygous type B blood

What is the phenotypic ratio?

1AB:1B:1A:1O

Cross type O blood with Heterozygous type B blood

What is the phenotypic ratio?

2B:2O

Cross type O blood with type AB blood

What is the phenotypic ratio?

2A:2B

Multiple AllelesRhesusAnother characteristic of blood is Rhesus factor or Rh factor. Someone either has or does not have the Rh factor on the surface of their red blood cells. This is indicated as + or -.. Type O+ blood is most common in most parts of the world. Rh factor is named after the rhesus monkey where the factor was first identified.

Colour of Rabbits

• In rabbits 4 alleles can code for coat colour

• C (dominant codes for full colour –black)

• cch (chinchilla which is dominant to ch

• ch (Himalayan which is dominant over c)

• and c (recessive –codes for white or albino)

Epistasis

• Epistasis - the interaction between two or more genes to control a single phenotype. Epistasis takes place when the action of one gene is modified by one or several other genes, which are sometimes called modifier genes. The gene whose phenotype is expressed is said to be epistatic

• BB = black Lab, no chocolate gene Bb = black Lab, carries chocolate gene bb = chocolate Lab, no black gene

• Yellow is produced by the presence of a recessive epistatic gene which has the effect of masking the the black or chocolate genes.EE = no yellow gene Ee = yellow carrier but appears either black or chocolate ee = yellow Lab

So....

• EEBB =Basic Black (BB)EEBb =Black that carries Choc. (Bc)

So....

• EeBB =Black that carries Yellow (By)EeBb =Black that carries Yellow and chocolate (Byc)

So....

• eeBB =Yellow (Yy) [does not carry chocolate]eeBb =Yellow that carries Chocolate (Yc)

• eebb =yellow ~ No Black Pigment (NBP)

So....

• EEbb =Chocolate (CC) [does not carry yellow]Eebb =Chocolate that carries yellow (Cy)

Practice – what colour is each dog?

EEBB

EebbEeBb

EEBbeeBBEEbb

EeBBeeBb

Practice

EEBB

EebbEeBb

EEBbeeBBEEbb

EeBBeeBb

Sex determination (click)

• There are 2 types of sex chromosomes X and Y

• Females have double xx

• Males have XY• Note the punnett

squares shows why sex is generally pretty 50-50

Dosage Compensation

• Dosage compensation is a genetic regulatory mechanism which operates to equalize the phenotypic expression of characteristics determined by genes on the X chromosome so that they are equally expressed in the human XY male and the XX female. ..

Dosage Compensation Continued

• calico cats are always female

• (almost true, some XXY males have been calico)

• one X chromosome carries the gene for black coat color

• the other X chromosome carries the gene for yellow coat color

Dosage Compensation Continued

• in 64-cell embryos, one of each pair of X chromosomes and its genes are randomly silenced

• daughter cells inherit active or inactive X chromosomes, creating a cat with patches of coat color

Sex Linked Traits

• (also called x linked traits)

• Since males have only one X chromosome they are affected by sex linked traits more often than females would be.

Colour Blindness

X-linked trait – Colour Blindness

Father is colour blind mother is a carrier

X-linked trait – Colour Blindness

Father is normal mother is a carrier

X-linked trait – Colour Blindness

Father is colour blind mother is normal

Hemophilia

Polygenic Traits

• Since Mendel's time, our knowledge of the mechanisms of genetic inheritance has grown immensely.  It is now understood that inheriting one allele can, at times, increase the chance of inheriting another or can affect how and when a trait is expressed in an individual's phenotype.  Likewise, there are degrees of dominance and recessiveness with some traits.  The simple rules of Mendelian inheritance do not apply in these and other exceptions.

Polygenic Traits

• Some traits are determined by the combined effect of more than one pair of genes.  These are referred to as polygenic , or continuous, traits.  An example of this is height.    Human skin, hair, and eye colour are also polygenic traits because they are influenced by more than one allele at different loci.  The result is the perception of continuous gradation in the expression of these traits.

1.

• In chapter 10 we just looked at simple dominant/ recessive problems – in this chapter we are looking at complex inheritance involving co-dominance, incomplete dominance and sex-linked traits

2.

• Epistasis is when one allele masks or hides the expression of another allele.

• It differs from dominance in that a recessive allele could potentially mask a dominant allele of another gene pair (as in the case of the e allele making the B in Labradors )

3.

• If the daughter has type O then she had to receive a recessive allele from each parent so both parents have to be heterozygous. One is Iai the other is Ibi

4.

• Since twin are genetically identical scientists can hypothesize that traits that are similar are inherited and traits that are different are likely the result of environmental factors.