Question

. Carlos and Maria feel blessed and are ecstatic to have two healthy babies. Within the first hour, the babies have been cleaned and measured and their heels pricked for blood samples. These initial blood tests are to assess for PKU, TH levels, cystic fibrosis, and several other metabolic disorders. To their surprise, their new daughter tests positive for PKU. Neither carlos nor Maria have PKU. Thankfully, their son tests negative for PKU. But as they are discussing genetic testing, Maria remembers that her father was color blind. She wonders whether that means her son will also be color blind. Carlos and Maria are so happy with their new twins that they are thinking about trying to have more children, maybe in a few years. a) What is the % chance their son will also have PKU? b) It won’t be possible to determine yet, but what are the % chances that Maria and Carlos’s son will be color blind? Explain briefly your reasoning. c) Having had one boy and a girl, what are Maria and Carlo’s chances of having another boy? Will the time of fertilization influence their chances of having a boy?

Answer 1

Phenylketonuria or PKU is an autosomal recessive trait. If the father and mother are both unaffected it means they both were carriers of PKU, which means they both had one recessive allele which is the diseased one and another normal dominant allele. In autosomal recessive traits, there is 25 % chance of an offspring to be affected and 75% chance to be normal. As they have twins with opposite sex, they are most probably dizygotic twins. So their Son will have 25% of chance of having PKU.

Color blindness is an x linked recessive trait, thus it is only seen in males as they have single x chromosome. As Maria's father was color blind and her father must have passed this x chromosome to Maria. Now Maria has one x chromose from her mother and another from her father which is the diseased one. Maria will pass one x chromosome to her son. If that chromosome is that of her father then the boy will be color blind and if the x chromosome is from her mother, the boy will be normal. Thus the boy has 50% chance of being color blind.

Having a boy or a girl depends on the Y chromosome from father. A father can pass X or Y chromosome to his progeny, so there is 50% chance of them having a Boy. Time of fertilisation will not affect the gender of the baby as it is only determined by the fathers Y chromosome.