Genetics

Example of a Punnett square. In this example i...

Example of a Punnett square. In this example in peas, the color yellow is determined by the dominant allele Y and the color green is determined by a recessive allele y. (Photo credit: Wikipedia)

Biology 42 – Human Biology (Spring 2007)

Genetics

[Students and professors, please read.]

a.  “A woman with type A blood gives birth to a baby with type O blood.”

“Can the father have type B blood?”

Yes.  He can have the dominant allele for B and the recessive allele for O, if the mother has the dominant allele for A and the recessive allele for O.  Mother and father would contribute their recessive alleles for O to their type O baby.

“Can he have type A blood?

Yes, if both mother and father have the dominant allele for A and the recessive allele for O, and both contribute their recessive alleles for O to their type O baby.

“Can he have type AB blood?”

No, even if the mother is type A with a recessive allele for O, because the father would not be able to contribute a recessive allele to his type O baby.  Two recessive alleles for O are required to produce an O phenotype.

“Can he have Type O blood?”

Yes, if the mother, again, has a dominant allele for A and a recessive allele for O.  Both parents would contribute a recessive allele for O to their type O baby.  On the other hand, if the mother has two dominant alleles for A, they could only produce a type A baby – a baby with one dominant A allele and one recessive O allele.

b. “A black dog and a white dog have puppies. All the puppies have black fur.”

“Which trait is dominant?”

The trait for black fur is dominant.  Since the white dog must have two recessive alleles for white fur, and none of the dogs turned out white, then the black dog must have two dominant alleles for black fur, and all of their puppies must have one dominant allele for black fur and one recessive allele for white fur, which explains why they are all black.

“If the puppies were to mate, what ratio of black and white offspring would you expect?”

A Punnet square helps to solve this.  Since both parents are Bw (B is dominant black, w is recessive white), they will have one BB, two Bw, and one ww.  The one BB and two Bw (for a total of 3) will have black fur, while the one ww will have white fur.  That’s a 3-to-1 phenotypic ratio.  3/4ths of the puppies will probably be black, while 25% of them will probably be white.

c.  “Intelligence is a sex-linked trait (on the X chromosome).”

“If an unintelligent male mates with a homozygous intelligent female can they produce an intelligent son?  Can they produce an intelligent daughter?”

Yes to both, and here’s why — If an unintelligent male (XiY) mates with a homozygous intelligent female (XIXI), according to the Punnet square, their possible offspring are XIXi, XIXi, XIY, XIY – therefore, all of their children, male or female, would be intelligent.

“If an unintelligent male mates with a heterozygous intelligent female what kind of intelligence can they expect in their children, both male and female?”

If an unintelligent male (XiY) mates with a heterozygous intelligent female (XIXI), according to the Punnet square, half their daughters will probably be intelligent (XIXi) and half of them unintelligent (XiXi), while half their sons will be intelligent (XIY) and half of them unintelligent (XiY).

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