– Yellow (AY) in mice – Manx (M) in cats（马恩
岛无尾猫，）
– Curly wing (Cy翻翅) in Drosophila
• Dominant lethal if either heterozygotes or homozygous mutants die; rare, e.g. Huntington disease（亨亭顿舞蹈症）
– 环境改变所引起的表型改变，有时与一种基因突变 引起的表型变化很相似，这种现象叫做表型模写。
– An environmentally caused condition has symptoms and a recurrence pattern similar to those of a known inherited trait (Infection)
ABO血型的遗传与生化
ABO抗原的前体
Gal：D-半乳糖 Fuc：L-岩藻糖 Nacgal：N-乙酰基半乳糖苷
H或O抗原
A抗原
B抗原
The ABO gene is located on chromosome 9 (9q34.1)
孟买型(bombay phenotype)与H抗原
1952 年在孟 买发现的， 无B、A、H、 抗原
Establishing a Dominance Series
Conclusion: A > at > a
烟草的自交不亲和性
●栽培的普通烟草为自花授粉植物，但烟草属中有 两个野生种(Nicotiana forgationa与N. alata)表现出 自交不亲和性。
Some Alleles are Lethal in Homozygous Form
The Ay allele is a dominant coat color allele that is lethal when carried in homozygous form.
显性致死基因（dominant lethal） 只要有一个拷贝就会导致个体死亡。
Normal red blood cell Sickled red blood cell
Pleiotropy (one gene, many pheotypic effects)
二、等位基因间的相互作用
不完全显性(incomplete dominace) 共显性(codominance) 镶嵌显性(Mosaic dominace) 致死基因（lethal genes） 复等位基因(Multiple Alleles)
不完全显性（Incomplete dominance）
杂合体表型为双亲中间型
Heterozygous expresses a different phenotype from either homozygous. Usually intermediate between the two. CRCR = Red CWCW = White CRCW = Pink
Pleiotropy-the sickle cell example
HbA is a normal allele for one polypeptide chain in the hemoglobin molecule HbS is a mutant form of the allele, which produces abnormal hemoglobin polypeptides The genotype HbS/ HbS causes the disease sickle cell anemia. The abnormal hemoglobin clumps together in red blood cells.
Mirabilis jalapa（紫茉莉）
共显性 Codominance
一对等位基因的两个成员在杂合体中都表达的遗传现象 2 alleles affect the phenotype in separate, distinguishable ways Example of MN blood type in humans MM – protein M on red blood cell surface NN – protein N on red blood cell surface MN – both proteins on red blood cell surface LM LM gives M phenotype LM LN gives MN phenotype LN LN gives N phenotype LM LN X LM LN produces 1/4 LM LM, 1/2 LM LN, 1/4 LN LN
一般人都能合成H抗原
hh相当于 抑制基因
Rh血型与母子间不相容
发生：(i)多次输血之后；
(ii)经产妇之中。
症状：母亲血液中的抗体通过胎盘进入胎儿,使胎儿的红细
胞破坏,造成胎儿死亡,活下来的胎儿全身浮肿,有重症黄疸的贫 血,肝腹脾中有活动旺盛的造血巢,血液中有很多有核红血细胞， 所以一般称为新生儿溶血症（erythroblastosis fetalis）
表现度和外显率的关系
人类多指（趾）（ Brachydactyly）的遗传， 受显性基因控制 外显率为90%，外显不全； 表现度：多指的数量、大小表现程度不一致 既有表现度又有外显率的问题
多指畸形 Polydactyly
表现度：多指的大小表现不一致
Both the degree of penetrance (complete or incomplete) and expressivity (invariant or variable) are influenced by modifier genes and the environment.
人类的Huntington disease基因H:
杂合体 （Hh）的 个体在30岁 以后脑神经 发生渐进性 损坏，直到 死亡。
American folk singer and composer Woody Guthrie died from this disease at age 39.
复等位基因(Multiple Alleles)
镶嵌 显性 (Mos aic domi nanc
e)
致死基因 Lethal genes
A gene whose expression results in the death of the organism, usually during embryogenesis. -The American Heritage® Dictionary of the English Language: Fourth Edition. 2000
– 是环境变化的影响产生的表型，并不会传至下一代。 – 例1：残翅果蝇的幼虫在高温下饲养 – 例2：人类隐性遗传病短肢畸形，孕妇服用药物以
后引起的表型变化。
Penetrance and Expressivity – Modifications of Phenotypes Associated with Particular Alleles
老鼠毛色基因 Mouse coat colors
black yellow
agouti
Agouti × agouti Yellow × yellow Agouti × yellow
All agouti 刺豚鼠皮毛上的深浅环纹
2/3 yellow, 1/3 agouti ？
1/2 yellow, 1/2 agouti
More than 2 alleles per gene ABO blood type in humans (3 alleles)
A – genotypes IAIA or IAi B – genotypes IBIB or IBi AB – genotype IAIB O – genotype ii
外显率 Penetrance
For some genes, a given genotype may or may not show a given phenotype Level of penetrance＝proportion of individuals of a particular genotype that have the same phenotype.（一定基因型的个体在特定的环境 条件下形成预期表型的比例） 举例：黑腹果蝇隐性间断翅脉 90%
- gene expression and the resultant phenotype are often modified – interaction between a genotype and the internal and external environment. 表现度（Expressivity）：是指某一特定的基因 型在不同个体间的表现程度。 举例：人类成骨不全
these can be either dominant or recessive in nature（显性致死和隐性致死）
Lethal Alleles
• Recessive lethal if heters mutants die; common, e.g.
A gene's whose effect on the phenotype is profoundly negative, and causes the organism to exhibit many disadvantageous characteristics, which can eventually lead to death.---Online Biology Dictionary 使生物体不能存活的等位基因
A：IAIA, IAi B：IBIB, IBi O ：ii AB：IAIB