FatherCarrier Hearing ChildNoncarrier Hearing ChildCarrier Hearing ChildCarrier Deaf ChildCarrier Hearing MotherCarrier copy with a recessive mutation Figure 5 13 Autosomal Recessive Conditions “Autosomal” conditions affect males and females equally. “Recessive” conditions also are due to changes in genes, but they run in families in a different way than dominant conditions. This is because people who have one copy of a recessive mutation do not have the condition. They are called “carriers”. If two carriers have a child together, there is a 25% (1 in 4) chance that the child will get two copies of the mutation and, therefore, have the condition. “Autosomal” genes are genes that are found on one of the first 22 pairs of chromosomes, the pairs that are the same for both males and females. In the case of “recessive” conditions, a child will have the condition only if he or she does not have a usual copy of the gene. Figure 5 shows how recessive conditions run in families. In this example, each parent is a carrier and has one usual copy (symbolized by ) and one copy with a recessive mutation (symbolized by ). Each parent will give the child either the usual copy ( ) or the copy with the recessive mutation ( ). Each has a 50% (1 in 2) chance of happening. If the child gets one usual copy ( ) from one parent and one copy with the recessive mutation ( ) from the other parent, the child will be a carrier like both parents. If the child gets the copy with the recessive mutation ( ) from both parents, and therefore doesn’t have a usual copy, the child will have the condition. When both parents are carriers, there is a 25% (1 in 4) chance that each child will get a copy with a recessive mutation from both parents and, therefore, have the condition. 14 X-Linked Recessive Conditions X-linked conditions usually affect only males. In such instances, a mutation is passed in the family through female carriers who do not have the condition. However, each son of a female carrier has a 50% chance of inheriting the mutation and, therefore, of having the condition. “X-linked” genes are genes that are found on the X chromosome. “Recessive” means that a person has the condition only if he or she does not have at least one usual copy of the gene. A female has two X chromosomes and, therefore, two copies of each X-linked gene. A woman who has one usual copy and one copy with a recessive mutation is called a carrier. A carrier does not have the condition, but can pass the copy with the recessive mutation on to her children. A male has only one copy of the X chromosome, which he has gotten from his mother. His other chromosome is a Y chromosome that he has gotten from his father. Therefore, a male has only one copy of each X-linked gene. If the male gets his mother’s X chromosome that has the recessive mutation, he will have the condition. If he gets his mother’s X chromosome that has the usual copy of the gene, he will not have the condition. Therefore, a son of a carrier mother has a 50% chance of having the condition. Because a male has only one X chromosome and, therefore, must pass that X chromosome to each of his daughters, each daughter will be a carrier. Because a male has only one Y chromosome, he will pass that Y chromosome (and not the X chromosome) to each of his sons, so none of his sons will have the condition or be carriers. Have you ever wondered if it is possible to genetically engineer “Designer Babies”? Can parents today make choices for their pre-born children regarding certain traits? Actually, there is a medical procedure available that is a kind of “first-step” in this direction. It’s called Pre-implantation Genetic Diagnosis, or PGD. This activity is designed to engage your critical thinking skills. Based on what you learn, you will be asked to make a decision as to whether or not you would choose to use PGD to design your child. SCENARIO “Kim tells me that you and Natalie are considering starting your family.” “Yes,” Drew replied. “But we have some concerns.” He and his work colleague Jeff were on their car pool commute to work. Drew and his wife Natalie had been friends with Jeff and Kim ever since both couples had moved to the city. “Like what?” asked Mike. “Well, it’s not a huge thing, but we feel a little pressure from my family to have a boy. My dad is an only boy, and I’m an only boy. Continuing the family name seems to be a big deal for some in my family,” said Drew. “Have you ever thought of using PGD?” asked Mike. “What do you mean? I’ve never hear of it,” answered Drew. Mike explained, “PGD stands for Pre-Implantation Genetic Diagnosis. It’s sort of a combination of in vitro fertilization and genetic screening, all in one. Kim and I are seriously considering it because we both have relatives with Down Syndrome, and we would like to improve our chances of having a child who doesn’t have that condition.” “How can you do that?” Drew was curious. “Well,” answered Mike, “it basically works like this: The doctor takes eggs and sperm from the prospective mother and father. An in vitro fertilization (IVF) is done, in which the eggs are fertilized outside the woman’s body, and several embryos are produced. One cell from each embryo can be removed and genetic testing done. Removing the one cell does not harm the embryo. The testing can be used for various purposes, like to determine the sex of the child or if a chromosomal mutation – like the one that causes Down Syndrome - is present. Testing can also show if the child will have certain genetic diseases – Cystic Fibrosis, for example - or even if there is an increased risk of diseases like breast cancer or early-onset Alzheimer’s Disease. The parents then have the choice of what embryos are implanted into the mother, and the ones you don’t want are discarded. “So we could find out which embryos were going to result in male babies, and have only those implanted into Natalie?” asked Drew. “I guess,” said Jeff. “PGD is used mostly to check for genetic diseases. But choosing the sex of your child is possible. In some countries it’s illegal to use PGD for that, but not in the United States.” QUESTION-PRODUCT Pretend you are Drew or Natalie and six months have passed since Drew and Jeff’s conversation in the car. In that time you have