Directional selection only occurs in response to naturally occurring events.
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Attached is the pedigree. I found the exercise on the internet.
The individuals that are missing a symbol are: II-5, II-6, II-8, III-10, III-11, III-12, III-13.
The individual II-5 would have the half black/half white square. A square because in the introductory text says that it's John's paternal grandmother (I-2) that has the disease. Half black/half white because his mother had the disease so she passed one allele that's necessarily a disease allele, and his father doesn't carry the disease or manifest it which means that from him, John's father (II-5) only received a normal allele.
The individual II-6 would have a question mark in a circle. A circle because she is John's mother once his father is the individual II-5. A question mark because we don't have information as for the manifestation of the disease in her, though we do know that she is either a carrier of the disease or inflicted by the disease because she has a daughter (John's sister) that has the disease meaning that John's sister received two alleles for the disease.
The individual II-8 would have the half black/half white circle. A circle because she is Sue's mother once her father is the individual II-7 (a square). Half black/half white because her father had the disease so he passed one allele that's necessarily a disease allele, and her mother doesn't carry the disease or manifest it which means that from her, Sue's mother (II-8) could only received a normal allele.
The individual III-10 would have a question mark in a circle. A circle because she is John's sister as said in the introductory text. A question mark because we can't affirm whether she is a carrier of one disease allele or does not carry the disease at all. We know by the introductory text that she doesn't have any signs of the disease but she could've have received a disease allele from her father or her mother if her mother is simply a carrier of one disease allele, or would definitely received a disease allele from her mother, and not from her father, if her mother has the disease.
John, the individual III-11 would have a question mark in a square. A square because is John, a male. A question mark because we can't affirm whether he is a carrier of one disease allele or does not carry the disease at all. We gather, by the introductory text, that he doesn't have signs of the disease but he could've have received a disease allele from his father or his mother if his mother is simply a carrier of one disease allele, or would definitely received a disease allele from his mother, and not from his father, if his mother has the disease.
Sue, the individual III-12 would have a question mark in a circle. A circle because is Sue, a female. A question mark because we can't affirm whether she is a carrier of one disease allele or does not carry the disease at all. By the introductory text, we gather that she doesn't have signs of the disease, but she could've have received a disease allele from her mother, once her mother is a carrier of a disease allele, turning her into a carrier as well, or could've received the normal allele from her mother. From her father she only received a normal allele.
The individual III-13 would have a question mark in a square. A square because he is Sue's brother according to the introductory text. A question mark because we can't affirm whether he is a carrier of one disease allele or does not carry the disease at all. We know, by the introductory text, that he doesn't show any signs of the disease, but he could've have received a disease allele from his mother, once his mother is a carrier of a disease allele, turning him into a carrier as well, or could've received the normal allele from his mother. From his father he only received a normal allele.
The individuals that are missing a symbol are: II-5, II-6, II-8, III-10, III-11, III-12, III-13.
The individual II-5 would have the half black/half white square. A square because in the introductory text says that it's John's paternal grandmother (I-2) that has the disease. Half black/half white because his mother had the disease so she passed one allele that's necessarily a disease allele, and his father doesn't carry the disease or manifest it which means that from him, John's father (II-5) only received a normal allele.
The individual II-6 would have a question mark in a circle. A circle because she is John's mother once his father is the individual II-5. A question mark because we don't have information as for the manifestation of the disease in her, though we do know that she is either a carrier of the disease or inflicted by the disease because she has a daughter (John's sister) that has the disease meaning that John's sister received two alleles for the disease.
The individual II-8 would have the half black/half white circle. A circle because she is Sue's mother once her father is the individual II-7 (a square). Half black/half white because her father had the disease so he passed one allele that's necessarily a disease allele, and her mother doesn't carry the disease or manifest it which means that from her, Sue's mother (II-8) could only received a normal allele.
The individual III-10 would have a question mark in a circle. A circle because she is John's sister as said in the introductory text. A question mark because we can't affirm whether she is a carrier of one disease allele or does not carry the disease at all. We know by the introductory text that she doesn't have any signs of the disease but she could've have received a disease allele from her father or her mother if her mother is simply a carrier of one disease allele, or would definitely received a disease allele from her mother, and not from her father, if her mother has the disease.
John, the individual III-11 would have a question mark in a square. A square because is John, a male. A question mark because we can't affirm whether he is a carrier of one disease allele or does not carry the disease at all. We gather, by the introductory text, that he doesn't have signs of the disease but he could've have received a disease allele from his father or his mother if his mother is simply a carrier of one disease allele, or would definitely received a disease allele from his mother, and not from his father, if his mother has the disease.
Sue, the individual III-12 would have a question mark in a circle. A circle because is Sue, a female. A question mark because we can't affirm whether she is a carrier of one disease allele or does not carry the disease at all. By the introductory text, we gather that she doesn't have signs of the disease, but she could've have received a disease allele from her mother, once her mother is a carrier of a disease allele, turning her into a carrier as well, or could've received the normal allele from her mother. From her father she only received a normal allele.
The individual III-13 would have a question mark in a square. A square because he is Sue's brother according to the introductory text. A question mark because we can't affirm whether he is a carrier of one disease allele or does not carry the disease at all. We know, by the introductory text, that he doesn't show any signs of the disease, but he could've have received a disease allele from his mother, once his mother is a carrier of a disease allele, turning him into a carrier as well, or could've received the normal allele from his mother. From his father he only received a normal allele.