ABO Blood Group System Inheritance

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In ABO blood group system, an individual can be blood group A, B, AB or O based on the types of antigens on the surfaces of red blood cells.

A cross between an individual with blood group A and an individual with blood group B

=> A and B are codominant whereas O is recessive
=> For blood group A: IAIA or IAIO
=> For blood group B: IBIB or IBIO
=> For blood group AB: IAIB
=> For blood group O: IOIO

Parent phenotypes:    Blood group A   X   Blood group B
Parent genotypes:                    IAIO      X      IBIO
Gametes:                               IA      IO        IB      IO
F1 genotypes:                    IAIB   IAIO     IBIO   IOIO
F1 phenotypes:                    AB       A          B        O
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Sex-Linked Inheritance - Colour Blindness Inheritance

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Sex-linked inheritance is caused by a gene defect on the X chromosome. For example, colour blindness.

Females have two X chromosomes, therefore they need to have two copies of alleles to express the trait.

Males have one X chromosome, therefore they need only one copy of allele to express the trait.

A cross between a father who is normal and a mother who is a carrier of colour blindness

=> Normal is dominant whereas colour blind is recessive
=> R - normal, r - colour blind
=> For male: XRY (normal), XrY (colour blind)
=> For female: XRXR (normal), XRXr (carrier), XrXr (colour blind)

Parent phenotypes:        Father, normal  X  Mother, carrier
Parent genotypes:                    XRY           X          XRXr
Gametes:                              XR           Y            XR          Xr
F1 genotypes:                  XRXR      XRXr       XRY       XrY
F1 phenotypes:        Daughter,  Daughter,     Son,        Son,
                                    normal       carrier          normal    colour blind
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Monohybrid Inheritance - Monohybrid Cross

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A monohybrid cross is a cross involving one trait only. For example, plant height.

A cross between a tall plant (true breed) and a dwarf plant (true breed)

=> Tall is dominant whereas dwarf is recessive
=> T - tall, t - dwarf
=> F2 ratio is 3 : 1

Parent phenotypes:      Tall    X   Dwarf
Parent genotypes:         TT     X     tt
Gametes:                    T      T        t      t
F1 genotypes:            Tt     Tt      Tt    Tt
F1 phenotypes:       Tall   Tall   Tall   Tall

F1 self-crossed

F1 phenotypes:             Tall    X    Tall
F1 genotypes:                 Tt     X     Tt
Gametes:                    T      t        T      t
F2 genotypes:           TT    Tt      Tt     tt
F2 phenotypes:      Tall   Tall   Tall  Dwarf
F2 ratio:                      3 (Tall) : 1 (Dwarf)
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Dihybrid Inheritance - Dihybrid Cross

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A dihybrid cross is a cross involving two traits. For example, seed shape and seed colour.

A cross between a plant with round, yellow seed (true breed) and a plant with wrinkled, green seed (true breed)

=> Round is dominant whereas wrinkled is recessive
=> Yellow is dominant whereas green is recessive
=> R - round, r - wrinkled, Y - yellow, y - green
=> F2 ratio is 9 : 3 : 3 : 1

Parent phenotypes:   Round, yellow   X   Wrinkled, green
Parent genotypes:                 RRYY      X        rryy
Gametes:                          RY        RY           ry         ry
F1 genotypes:                RrYy     RrYy      RrYy     RrYy
F1 phenotypes:           Round,  Round,  Round,  Round,
                                       yellow    yellow    yellow    yellow

F1 self-crossed

F1 phenotypes:           Round, yellow   X   Round, yellow
F1 genotypes:                         RrYy        X         RrYy
Gametes:                      RY   Ry   rY   ry     RY   Ry   rY   ry

F2 genotypes and phenotypes:

RRYY (Round, yellow)
RRYy  (Round, yellow)
RrYY  (Round, yellow)
RrYy  (Round, yellow)

RRYy (Round, yellow)
RRyy  (Round, green)
RrYy  (Round, yellow)
Rryy  (Round, green)

RrYY  (Round, yellow)
RrYy  (Round, yellow)
rrYY  (Wrinkled, yellow)
rrYy  (Wrinkled, yellow)

RrYy  (Round, yellow)
Rryy  (Round, green)
rrYy  (Wrinkled, yellow)
rryy  (Wrinkled, green)

F2 ratio:  9 (Round, yellow) : 3 (Round, green) : 3 (Wrinkled, yellow)  : 1 (Wrinkled, green)
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