Variations on Mendel’s principles

Incomplete dominanceCo-dominanceMultiple allelesSex-linked alleles

2

Incomplete DominanceIncomplete Dominance

F1 hybrids F1 hybrids have an appearance somewhat in in betweenbetween (intermediate ) the phenotypes phenotypes of the two parental varieties.

Example:Example: snapdragons (flower)snapdragons (flower) red (RR) x white (WW)

RR = red flowerRR = red flower WW = white flower

3

R

R

W W

Incomplete DominanceIncomplete Dominance

4

RWRW

RWRW

RWRW

RWRW

RR

RR

WW

All RW =All RW = pink pink(heterozygous pink)(heterozygous pink)

produces theproduces theFF11 generation generation

W

5

Many genes have more than two alleles in the population

Ex. three alleles for ABO blood type in humans IA, IB, i

Co-dominanceCo-dominance

Two allelesTwo alleles are expressed equally in are expressed equally in multiple allelesmultiple alleles in in heterozygous heterozygous individualsindividuals..

Example:Example: blood type blood type

1.1. type Atype A = I= IAAIIAA or I or IAAi i 2.2. type Btype B = I= IBBIIBB or I or IBBii 3.3. type ABtype AB = I= IAAIIB B ((equally equally

expressedexpressed)) 4.4. type Otype O = ii (= ii (recessive allelesrecessive alleles))

7

Co-dominance ProblemCo-dominance Problem

Example: homozygous male Type B (IBIB)

X heterozygous female Type A (IAi)

8

IAIB IBi

IAIB IBi

1/2 = IAIB

(A & B = equally expressed)

1/2 = IBi (B dominant over i)

IB

IA i

IB

Another Codominance ProblemAnother Codominance Problem

9

• Example:Example: male Type O (ii) x female type AB (IAIB)

IAi IBi

IAi IBi

1/2 = IAi1/2 = IBi

i

IA IB

i

Co-dominanceCo-dominance

QuestionQuestion::If a boy has a blood type O and If a boy has a blood type O and

his sister has blood type AB, his sister has blood type AB, what are the genotypes what are the genotypes

and and phenotypes of their phenotypes of their parents?parents?

boy - boy - type O (ii) type O (ii) X girl - X girl - type AB type AB (I(IAAIIBB))

10

Co-dominanceCo-dominance

Answer:Answer:

11

Sex-linked TraitsSex-linked Traits

Traits (genes) located on the Traits (genes) located on the sex chromosomessex chromosomes

Sex chromosomes are Sex chromosomes are X and YX and YXXXX genotype for females genotype for femalesXYXY genotype for males genotype for malesMany Many sex-linked traitssex-linked traits carried carried on on XX chromosome chromosome

12

Sex-linked TraitsSex-linked Traits

13

Sex ChromosomesSex Chromosomes

XX chromosome - female Xy chromosome - male

fruit flyeye colour

Example: Example: Eye colour in fruit Eye colour in fruit fliesflies

Sex-linked Trait ProblemSex-linked Trait Problem Example: Eye colour in fruit flies (red-eyed male) x (white-eyed

female) XRY x XrXr

Remember: the Y chromosome in males does not carry these traits.

RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female

14

XR

Xr Xr

Y

Sex-linked Trait Solution:Sex-linked Trait Solution:

15

XR Xr

Xr Y

XR Xr

Xr Y

50% red eyed female

50% white eyed male

XR

Xr Xr

Y

Female CarriersFemale Carriers

16

Polygenic traits - A single trait may be influenced by many genes

Quantitative traitsskin colour, height, eye colour

Fra

cti

on

of

po

pu

lati

on

Skin pigmentation

Sex-linked disorders affect mostly males

Most sex-linked human disorders are due to recessive allelesEx: hemophilia,

red-green color blindness

These traits appear mostly in males. Why?If a male receives a single X-linked recessive

allele from his mother, he will have the disorder; while a female has to receive the allele from both parents to be affected

Pedigree Chart: Inheritance Pattern for an X-linked Recessive Disease

A high incidence of hemophilia has plagued the royal families of Europe

QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

Fetal testing can spot many inherited disorders early in pregnancy

Karyotyping and biochemical tests of fetal cells can help people make reproductive decisionsFetal cells can be obtained through

amniocentesis

Figure 9.10A

Amnioticfluid

Fetus(14-20weeks)

Placenta

Amnioticfluidwithdrawn

Centrifugation

Fetalcells

Fluid

Uterus Cervix Cell culture

Severalweeks later

Karyotyping

Biochemicaltests

Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping

Figure 9.10B

Fetus(10-12weeks)

Placenta

Chorionic villi

Suction

Several hourslater

Fetal cells(from chorionic villi)

Karyotyping

Some biochemical

tests

Examination of the fetus with ultrasound is another helpful technique

Figure 9.10C, D

PGD - Preimplantation Genetic Diagnosisgenetic analysis of embryos from in

vitro fertilization (IVF)before inserting into womb