Genetics — 8 Common Exam Traps

1.Trap one: dominance does NOT mean better or more common. Dominance only means the heterozygote phenotype matches one homozygote. Frequency in the population is unrelated. For example, Huntingtons disease is a dominant genetic disorder despite being rare.

陷阱一：顯性不代表比較好或比較常見。顯性只代表異型合子表現型與某個同型合子相同，與族群頻率無關。例如亨丁頓舞蹈症是顯性遺傳病，但很罕見。

2.Trap two: confusing q and q squared in Hardy-Weinberg. q squared equals frequency of homozygous recessive aa — that is the phenotype frequency. q equals frequency of the recessive allele.

陷阱二：Hardy-Weinberg 中混淆 q 和 q 平方。q 平方是隱性同型合子 aa 的頻率，即表現型頻率。q 是隱性等位基因的頻率。

3.When the problem says disease rate is one percent, that means q squared equals zero point zero one — NOT q equals zero point zero one. You then take the square root to get q equals zero point one.

題目說患病率為 1%，是 q 平方等於 0.01——不是 q 等於 0.01。然後開根號得 q 等於 0.1。

4.Trap three: Hardy-Weinberg for X-linked genes works differently by sex. Males have only one X, so color blindness frequency in males equals q — not q squared. Females have two X, so color blindness frequency in females equals q squared. Common mistake: treating male q as q squared and taking the square root.

陷阱三：X 連鎖基因的 Hardy-Weinberg 男女不同。男性只有一條 X，色盲頻率等於 q——不是 q 平方。女性有兩條 X，色盲頻率等於 q 平方。常見錯誤：把男性的 q 當 q 平方來開根號。

5.Trap four: confusing epistasis with dominance. Dominance acts within the same locus — A versus little a. Epistasis acts between different loci — A locus versus B locus, where one gene masks the expression of another. These two concepts are entirely different.

陷阱四：混淆上位與顯隱性。顯隱性作用於同一基因座內，如 A 對 a。上位作用於不同基因座之間，如 A 基因座與 B 基因座，一個基因遮蔽另一個的表現。這兩個概念完全不同。

6.Trap five: test cross ratios for linked genes. Independent assortment gives test cross offspring ratio of one to one to one to one. Linked genes give offspring where parental types far outnumber recombinant types. Common mistake: seeing a non one-to-one ratio and assuming you calculated wrong.

陷阱五：連鎖基因的試交比例。獨立分配產生 1:1:1:1 比例。連鎖基因的後代親型遠多於重組型。常見錯誤：看到不是 1:1:1:1 就以為算錯了。

7.Trap six: maximum recombination frequency RF is fifty percent. RF equals fifty percent does NOT mean two genes are on different chromosomes — they could be on the same chromosome but very far apart. RF does not equal physical distance, especially at greater distances, because of double crossovers reducing apparent recombination.

陷阱六：重組頻率 RF 最大為 50%。RF 等於 50% 不代表兩基因在不同染色體上——可能在同一染色體但距離很遠。RF 不等於物理距離，特別是距離較遠時，因為雙互換減少觀察到的重組。

8.Trap seven: pedigree analysis assumptions. Do not assume an inheritance pattern — use process of elimination across all possibilities. Note whether the problem says the disease is rare. If rare, individuals marrying into the family are usually assumed homozygous normal. The term carrier applies only to recessive inheritance.

陷阱七：族譜分析的假設。不要假設遺傳模式——用排除法檢視所有可能。注意題目是否說「該疾病罕見」。若罕見，嫁入或娶入的人通常假設為正常同型合子。「帶因者」只用於隱性遺傳。

9.Trap eight: ABO blood type paternity testing. Two type O parents cannot produce type A or type B children. Two type AB parents cannot produce type O children. But type A times type B CAN produce type O — if both parents are heterozygous I-A i and I-B i, the child can inherit two i alleles.

陷阱八：ABO 血型親子鑑定。雙親 O 型不可能生出 A 型或 B 型小孩。雙親 AB 型不可能生出 O 型小孩。但 A 型乘 B 型可以生出 O 型——如果雙親都是異型合子 I-A i 和 I-B i，小孩可繼承到兩個 i。