NOTES: 11.3Exceptions to Mendelian Genetics!

Beyond Dominant and Recessive Alleles

● Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles ORmultiple genes.

● Examples of genes that are different than being totally “Dominant” or “Recessive:”

1. Incomplete dominance

2. Codominance

3. Multiple Alleles

4. Polygenic Traits

5. Environmental Influences

6. Sex-Linked Inheritance

● One allele is NOT completely dominant over another. -The heterozygous phenotype is somewhere

between the 2 homozygous phenotypes .What does this mean?

● Mendel crossed a homozygous red plant with a homozygous white plant.

● What do you think would be the expected results?...

R = RED R’ = white

● P: RR x R’R’

● F1: what is the F1 generation

going to look like

(phenotype)?

● F2: what is the F2

generation going to look like

(phenotype)?

Do the crosses now in your notes

R = Red R’ = White

• P: RR x R’R’

• F1: all RR’ (all pink)

• F2: 1 Red: 2 Pink: 1 White

**notice the ratio for incomplete

dominance 1:2:1

R R

R’

R’

RR’ RR’

RR’ RR’

Which allele is dominant in

pink offspring?……….neither

R R’

R

R’

RR RR’red pink

RR’ R’R’pink white

● Definition: BOTH alleles for a trait contribute to the phenotype of the organism.

● Examples:

-The alleles for red (RR) and white (WW) hair in cattle are co-dominant. Cattle with both alleles have brown/white

patterning or roan (RW).

-In certain varieties of chickens the alleles for black and white feathers are co-dominant. Chickens with both alleles appear speckled.

What is the difference between incomplete

dominance and codominance?

• Incomplete dominance = heterozygous

phenotype is somewhere in between the 2

homozygous phenotypes.

• For example, in (RR’), the R’ allele is not

active, but R cannot produce its full effect

when it is combined with R’.RR = red

RR’ = pink

R’R’ = white

1:2:1 ratio for F2 generation

What is the difference between incomplete

dominance and codominance?

• Codominance = heterozygous phenotype

has characteristics of both alleles for that

trait. …

– BOTH alleles are active and are expressed

together (both act like dominant genes).

• For example, cross between red hair (RR)

and white hair (WW), the calf will be roan

(RW) both red and white hairs.

RR = red WW = white RW= red & white

● Definition: Genes with more than two

alleles

● Remember: YOU only inherit TWO

alleles (one from mom, one from dad)

● Example 1:

-in rabbits, coat color is determined by a

single gene with four alleles.

Awww…..

Multiple Alleles…

● Example 2: Human Blood Types:

3 alleles (IA, IB, i)

-Phenotypically Type A Blood (genotype = IAIA or IAi)

-Phenotypically Type B Blood (genotype = IBIB or IBi)

-Phenotypically Type AB Blood (genotype = IAIB)

-Phenotypically Type O Blood (genotype = i i)

● Traits that are controlled by two or more genes

● Examples:

– Stem length in some plants;

– Eye color in fruit flies is controlled by three genes;

– Human skin color is controlled by more than 4 different genes;

– Shows a wide range of phenotypes as result

Example: STEM LENGTH

● suppose stem length in a plant is controlled by 3 different genes: A, B, and C

● each diploid plant has 2 alleles for each gene (e.g. AaBBcc OR aaBbCc, etc.)

Example: STEM LENGTH

● a plant homozygous for short alleles for all 3 genes (aabbcc) might grow to 4 cm

● a plant homozygous for TALL alleles for all 3 genes (AABBCC) might grow to 16 cm

Example: STEM LENGTH

● the difference in heights is 12 cm (or, 2 cm per each of the 6 tall alleles)…

● you could say that each “uppercase” allele contributes 2 cm to the total plant height…

SO, predict the phenotypes for the following genotypes:

AaBbCc:

AabbCc:

AABBCc:

Example: STEM LENGTH

● the difference in heights is 12 cm (or, 2 cm per each of the 6 tall alleles)…

● you could say that each “uppercase” allele contributes 2 cm to the total plant height…

SO, predict the phenotypes for the following genotypes:

AaBbCc: 10 cm

AabbCc: 8 cm

AABBCc: 14 cm

Example: STEM LENGTH

● so, if you crossed a TALL 16 cm plant

(AABBCC) with a short 4 cm plant (aabbcc),

all of the F1 plants would be:

Genotype: AaBbCc

Phenotype:

intermediate height (10 cm)

Example: STEM LENGTH

● THEN, if you let 2 F1 plants cross, you would

see a broad range of heights in the F2

● if you counted the different phenotypes, they

could be represented with a “bell curve” – a

typical pattern see with POLYGENIC

INHERITANCE!

• Human skin color is controlled by 4 different genes

• Dark skinned people have “uppercase” alleles that code for melanin at all gene positions for skin color.

• Lighter skinned people have few gene positions with alleles that code for melanin (in other words, they have more “lower case” alleles for those genes)

5) Environmental Influences:

● as an organism develops, many factors can

influence how the gene is expressed, OR

even whether the gene is expressed at all

● influences can be EXTERNAL or INTERNAL

EXTERNAL INFLUENCES:

Examples:

Temperature

Nutrition

Light (e.g. shade or sunlight for

plant leaf size)

Chemicals / pH

Infectious agents

INTERNAL INFLUENCES:

● the internal environments of males and

females are different because of hormones

and structural differences

● Examples:

-horn size in mountain sheep

-male-pattern baldness in humans

-feather color in peacocks

INTERNAL INFLUENCES:

● could also include AGE (although the effects

of age on gene expression are not well

understood)

SEX DETERMINATION: (CH 14)

● RECALL: in humans, the diploid # of

chromosomes is 46 (23 pairs)

● of the 23 pairs, 22 are AUTOSOMES, and

the 23rd pair represents the SEX

CHROMOSOMES

● human females: XX

● human males: XY

SEX DETERMINATION:

● Males (XY) can produce 2 kinds of gametes:

sperm cells carrying X

sperm cells carrying Y

● Females (XX) will only produce “X” gametes

SEX DETERMINATION:

● so the odds of having a boy or girl are always

50/50:

6) SEX-LINKED INHERITANCE:

(CH 14)

● SEX-LINKED TRAITS = traits controlled by

genes located on sex chromosomes

● the alleles are written as superscripts of the X

and Y chromosome

● Y-linked traits are passed only from male to

male

● since males only have 1 X chromosome, if

there is a gene on the X chromosome, males

only get 1 copy

6) SEX-LINKED INHERITANCE:

Example: eye color in fruit flies

-the gene for eye color is on the X chromosome

-RED eyes are dominant: XR

-white eyes are recessive: Xr

CROSS #1:

homozygous red-eyed female

X

white-eyed male

**change in

your notes!

CROSS #1:

Female genotype: XR XR

Male genotype: Xr Y

PUNNETT SQUARE:

XR XR

Xr

Y

PUNNETT SQUARE:

XR XR

Xr XR Xr XR Xr

Y XR Y XR Y

CROSS #1:

Offspring genotype ratio:

2 XR Xr : 2 XR Y

Offspring phenotype ratio:

2 red-eyed females : 2 red-eyed males

CROSS #2:

heterozygous red-eyed female

X

red-eyed male

CROSS #2:

Female genotype: XR Xr

Male genotype: XR Y

PUNNETT SQUARE:

XR Xr

XR

Y

PUNNETT SQUARE:

XR Xr

XR XR XR XR Xr

Y XR Y Xr Y

CROSS #2:

Offspring genotype ratio:

1 XR XR : 1 XR Xr : 1 XR Y : 1 Xr Y

Offspring phenotype ratio:

2 red-eyed females :

1 red-eyed male :

1 WHITE-EYED MALE