Non-Mendelian Genetics

When Heredity Follows Different Rules

1. Incomplete Dominance – the phenotype of heterozygous individuals is intermediate to the homozygotes

a) NEITHER allele is dominant over the other so they MIX together

2. Example:a) Homozygous R snap-dragons are RED (RR)b) Homozygous R’ snap-dragons are WHITE (R’R’)c) A cross between these two parents will result in all heterozygous

snap-dragons, which are PINK (RR’)d) A cross between two pink heterozygotes results in a 1:2:1 ratio

of phenotypes

Incomplete Dominance

Complex Genetics – Incomplete Dominance

1. The dominant allele is not completely dominant over the allele.

2. Result: a blend of the 2 forms of the trait.

1. R – red flower2. R’ – white flower

3. RR – red4. R’R’ – white5. RR’ - pink!

RR R’R’

RR’RR’RR’RR’

RR

R’R’

RR’ R

R’

R R’

RR RR’

RR’ R’R’

P

(All pink)

Codominance1. Codominance – BOTH alleles are dominant so BOTH

phenotypes are present in heterozygous individualsa) Alleles do NOT MIX; both are equally present

2. Example:a) Homozygous B chickens are BLACK (BB)b) Homozygous W chickens are WHITE (WW)c) Heterozygous chickens are BLACK AND WHITE (BW) in

a checkered patternd) A cross between two checkered heterozygous chickens

results in a 1:2:1 ratio of phenotypes

Complex Genetics – Co-Dominance1. There are 2

dominant alleles for the trait.

2. Result: Both show up equally.

1. Ex. B – black feathers

2. W – white feathers

3. BB – black feathered chicken

4. WW – white feathered chicken

5. BW – black & white checkered feathered chicken!

Both are dominant allels and will display in the offspringWW X BB=BW

BB

WW

BW B

W

B W

BB BW

BW WW

P

(All checkered)

Sex Determination1. Humans have a diploid number of 46 chromosomes in 23

pairs

2. 22 of those pairs are called AUTOSOMES and are the same between males and females

3. The 23rd pair are the SEX CHROMOSOMES and are indicated by the letters X and Y

4. Males – XY5. Females - XX

Y = PENIS !The sperm (father) always determines the sex of the offspring

50%

Sex Chromosom

es

Sperm

Egg

XY

XXXX

XY

XX

XYXX

XY

Sex-linked Inheritance1. Sex-linked traits – traits which are controlled by a gene on the X

chromosomea) Inherited differently in males and females

2. The Y chromosome does not carry any genes to override the homologous gene on the X chromosome

a) Males have the trait more often because they don’t get a second X to override a recessive gene on the first X chromosome (males cannot be carriers)

b) Females still need TWO copies of the recessive gene to exhibit the recessive phenotype (females can be carriers)

3. Colorblindness4. Hemophilia – bleeding disorder causing your blood not to clot

properly

X-Linked Traits1. The allele is attached

to the X chromosome.

2. Can only be passed from Mother to son, Mother to daughter, or father to daughter.

X-linked genes means…1. Girls can be normal or

Carriers but Boys will be AFFECTED the most.

X Y

X XX XY

X XX XY

XH = normal visionXh = colorblind

Y = no genesXH

Xh

XH

Yx XH

Xh

XH

Yx

XH

XHNormal Female

Xh

XHFemale Carrier

YXHNormal Male

YXhColorblind Male

Female Carrier Normal Male

Hemophilia X-Linked

Lacks clotting in the blood

Menkes DiseaseX-linked gene causes kinky hair and mental disorders