GENETICS MAJORS ONLY PLEASE, AND DO NOT COPY THE LAST ANSWER GIVEN ON THIS SITE

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GENETICS MAJORS ONLY PLEASE, AND DO NOT COPY THE LAST ANSWER GIVEN ON THIS SITE AS IT WAS INCORRECT!!!!!

Question: A mother is heterozygous for an over-expressed plastin allele on her X chromosome, thus has longer than normal axons. She is also heterozygous for the SMN1 gene on chromosome 5. Her children’s father is normal for plastin and has normal length axons. He too is heterozygous for the SMN1 gene. Both genes influence spinal muscular atrophy type 1 (SMA1); two abnormal copies of the SMN1 gene are required to cause spinal muscular atrophy, but their effect is prevented by high expression of the plastin gene (Lewis 92).

a) Give the genotypes of the mother and the father; provide a legend for your symbols. For each gene, explain why the abnormal alleles are dominant or recessive.       (4 marks)

b) List all the possible genotypes and proportions of their offspring for the plastin gene.                                               (1 mark)

c) List all the possible genotypes and proportions of their offspring for the SMN1 gene.                                               (1 mark)

D) Showing your work, determine the probability that this couple will have a daughter with SMA.                                           (2 marks)

e) Showing your work, determine the probability that this couple will have a son with SMA.                                               (2 marks)

Explanation / Answer

A)

A mother is heterozygous for an over-expressed plastin allele on her X chromosome and her genotype is XPX.

She is also heterozygous for the SMN1 gene on chromosome 5 and her genotype is

Ss XPX

Her children’s father is normal for plastin and is heterozygous for the SMN1 gene and genotype is SsXY.

Recessive gene ‘ss’ cause muscular atrophy but in that case due to presence of XP that can suppress it. So genotype of father is SsXY.

The abnormal gene for SMA1 is recessive because two defective copies are required to cause muscular atrophy. The abnormal gene of plastin is dominant because one copy is sufficient to suprees the other SMA1 gene. So probabitity is 1/16

B)       and C)       

              SX                                 SXP                                                      s XP                                       sX

SX     SSXX                    SS XPX                            Ss XPX                                  SsXX

      (Normal female)   (Normal Plastin female) (Normal Plastin female)   (Normal female)

SY    SSXY                    SS XPY                         S s XP Y                               SsXY

     (Normal male)      ( Normal palstin male)    (Normal palstin male)         (Normal male)     

sX     SsXX                     Ss XPX                          ss XPX                                   ssXX

      (Normal female)   (Normal Plastin female) (Normal Plastin female) (SMA Normal female)  

sY       SsXY                 Ss XPY                             ss XP Y                               ssXY

      (Normal male)    (Normal palstin male)    (Normal palstin male) (SMA Normal male)   

When they meet the possible offsprings are:

Mother = Ss XPX ; Gametes are= SX, SXP , sXP,   sX.   

Father= S s X Y ; Gametes are= SX, SY , sY,   sX.

SX

SXP

sXP

sX

SX

SSXX    

SS XPX      

Ss XPX

SsXX

SY

SSXY

SS XPY

Ss XPY      

Ss XY

sX

SsXX

SsXXP

ss XPX                                  

ssXX

sY

SsXY

SsXPY

ssXPY

ssXY

Proportions of their offspring for the plastin gene is 8/16

Proportions of their offspring for the SMN1 gene is 2/16

D.

The probability that this couple will have a daughter with SMA is 1/16

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