Genetics

Introduction to Genetics


Alleles


Multiple Alleles


Genotypes & Phenotypes


GENOTYPE PHENOTYPE
BB Brown-eyed person
Bb Brown-eyed person
bb Blue-eyed person

GENOTYPE PHENOTYPE
TT Tall pea plant
Tt Tall pea plant
tt Short pea plant


Pedigrees (Family Trees)

Pedigree Symbols Male without characteristic being studied
Female without characteristic being studied
Male with characteristic being studied
Female with characteristic being studied

Pedigrees Diagram


Sex Determination


Dominant-Recessive Inheritance


  B B
b Bb Bb
b Bb Bb

Possible genotypes of children = all Bb
Possible phenotypes of children = all brown-eyed children


  B b
b Bb bb
b Bb bb

Possible genotypes = 2Bb : 2bb
= 1Bb : 1bb
Possible phenotypes = 2 brown : 2 blue
= 1 brown : 1 blue

This means that about 12 of the children will be brown-eyed and the other 12 will be blue-eyed.


Test-Cross


Monohybrid Cross

  B b
B BB Bb
b Bb bb

Possible genotypes = 1 BB : 2 Bb : 1 bb
Possible phenotypes = 3 brown : 1 blue

This means that 34 of the children will be brown-eyed, and 14 will be blue-eyed.


Incomplete Dominance
(or Co-Dominance)

GENOTYPE PHENOTYPE
RR Red flowers
WW White flowers
RW Pink flowers

If a red-flowering plant produced pollen that fertilised a white-flowering plant's egg, the possible offspring would be ...

  R R
W RW RW
W RW RW

Possible genotypes = all RW
Possible phenotypes = all pink-flowering plants


Co-Dominance & Multiple Alleles
(An Example of ABO Blood Types)

BLOOD TYPE OR PHENOTYPE GENOTYPE ANTIGEN PRESENT ANTIBODY PRODUCED
A AA or AO A Anti-B
B BB or BO B Anti-A
AB AB A and B none
O OO none Anti-A and Anti-B

  A B
O AO BO
O AO BO

Possible genotypes = 1 AO : 1 BO
Possible phenotypes = 1 A : 1 B

12 the children will be A blood type, and the other 12 will be B blood type.


  A A
A AA AA
B AB AB

Possible genotypes = 1 AA : 1 AB
Possible phenotypes = 1 A : 1 AB

12 the children will have blood type A, and the other 12 will have blood type AB.


Second Possibility

  A O
A AA AO
B AB BO

Possible genotypes = 1AA : 1AO : 1AB : 1BO
Possible phenotypes = 2 A : 1 AB : 1 B

12 will have blood type A, 14 will have blood type AB, and 14 will have blood type B.


Antigens & Antibodies in ABO Blood Types


Complete Dominance or Dominant-Recessive Inheritance

Blood Type ABO Antigens Present Rhesus Antigens Present ABO Genotype Rhesus Genotype
A+ A yes Aa or AO RR or Rr
A- A no AA or AO rr
B+ B yes BB or BO RR or Rr
B- B no BB or BO rr
AB+ A and B yes AB RR or Rr
AB- A and B no AB rr
O+ none yes OO RR or Rr
O- none no OO rr


Sex-Linked Inheritance

XH - allele for normal blood clotting
Xh - allele for haemophilia

XHXH - genotype of normal female
XhXh - genotype of haemophiliac female
XHXh - genotype of carrier female (with normal blood-clotting ability, but who can pass the defective gene to her children)

XHY - genotype of normal male
XhY - genotype of haemophiliac male


  XH Y
Xh XHXh XhY
Xh XHXh XhY

Possible genotypes = 1 XHXh : 1 XhY
Possible phenotypes = 1 carrier female : 1 haemophiliac male


  XH XH
Xh XHXh XHXh
Y XHY XHY

Possible genotypes = 1 XHXh : 1XHY
Possible phenotypes = 1 carrier female : 1 normal male


Xc - allele for normal colour vision
Xc - allele for colour-blindness

Xc Xc - genotype of normal female
Xc Xc - genotype of colour-blind female
Xc Xc - genotype of carrier female (with normal colour vision, but who can pass the defective gene to her children)

Xc Y - genotype of normal male
Xc Y - genotype of colour-blind male


  Xc Y
Xc XcXc XcY
Xc XcXc XcY

Possible genotypes = 1 Xc Xc : 1 Xc Y
Possible phenotype = 1 carrier female : 1 colour-blind male


  Xc Xc
Xc XcXc XcXc
Y XcY XcY

Possible genotypes
= 1 XcXc: 1 XcXc: 1 XcY: 1 XcY
Possible phenotypes
=1 normal female: 1 carrier female: 1 normal male: 1 colour-blind male