性別が異なる二卵性双生児を特に異性双生児という。日本では異性双生児のことを「ミックスツイン」と呼称する場合も多い。ただし、英語のmixed twinsは混血 (Multiracial) の親から生まれた双子を指し、異性双生児を指す英語はmixed sex twinsまたはopposite sex twinsである。
かつては絨毛膜や胎盤の数をもって卵性を識別していたが、これは誤りである。一卵性双生児の3分の1は2絨毛膜性双胎であり、胎盤数は二つとなる。また視認された胎盤数が一つであっても二卵性双生児の場合もある(視認された胎盤の数が一つであっても検査の結果、癒合性双胎胎盤であることも多い)。さらに、一絨毛膜性双胎であっても、二卵性双生児である可能性もわずかではあるが存在する。よって現在では出生時に二卵性双生児と言われていてもその実は一卵性双生児であったり、一卵性と判断されていても二卵性双生児であるケースが多数存在している。特に一卵性双生児が二卵性であると誤認されているケースが多い。例えばタレントの三倉茉奈・佳奈は二卵性双生児だと本人達も信じていたが、DNA検査の結果として一卵性双生児の確率が極めて高いと判断された[87]。アメリカのタレントであるオルセン姉妹も本人達は二卵性であると主張しているが、双生児研究の専門家であるNancy L. Segalはオルセン姉妹はその外見等から一卵性である可能性が高いと指摘している[88]。一卵性双生児が自身を二卵性だと信じ込みがちな理由として、(1)一卵性ならば何もかもが類似していると思い込み、相違点の存在を意識しすぎる、(2)外見や行動における個性重視の風潮から、一卵性双生児であることを重荷に感じる、という二点をSegalは挙げている。またSegalは、誤認していた卵性を正しく知った一卵性双生児が「自身らが元は一つの受精卵であり、分裂していなかった場合はどちらか一方が存在していなかった」ことを自覚して衝撃を受けた実例を紹介しつつ、双生児自身やその周囲が出生後出来るだけ速やかに正確な卵性を知ることは、相違性や類似性を正しく理解した健全な成長過程を過ごすにあたって役立つであろうと主張している(三倉茉奈・佳奈が自身らの卵性を知った際にも紹介例と同様の混乱が見られる)。
国際多胎組織協議会 (International Council of Multiple Birth Organizations)。各国に存在する全国的な多胎育児支援組織がISTS(国際双生児研究学会)に加盟した場合に自動的に登録される[132]。多胎育児支援に関する活動報告や情報交換、および広報を行う。レジストリ(研究協力者登録簿)当事者である多胎家庭と、ISTSに所属する研究者を仲介する機能も担っている。前身はISTSの一種の委員会であったCOMBO (Council of Multiple Birth Organizations)。日本における加盟組織は日本多胎支援協会 (JAMBA)。
毎年の8月第1週の週末 (first full weekend of August)。米国オハイオ州サミット郡にあるツインズバーグでは双生児・多胎児とその親の祭典として、ツインズ・デーズ・フェスティバル(The Twins Days Festival)[136]が開催される。各種の企業と研究機関が双子に関する研究調査ブースを設けている[137]。
双生児に対して迷信的な意味での好意と隔意は、その地における双生児出生率と関係があると、カリフォルニア州立大学のNacy L. Segalは以下のように指摘している。双子出生率が高いナイジェリアのIgbo Ora地域においては、双生児は好意をもって迎えられた。一方、世界的に多胎出生率が低かった日本のような地域では、迷信として古代や近世において忌避感が持たれた地域もあった。しかし現代の日本社会では双生児出生率が向上し、そのような双生児に対する隔意は一般には無くなった[150]。
19組の一卵性双生児を調査した報告によると、一方が神経症等を有しているなど表現型に違いがあった9組の双子にはその全てに後天的なDNA配列上の違い(染色体の欠損)かコピー数多型(CNV:Copy Number Variation)かの存在を確認できた。胚発生時のDNA配列が同一の一卵性双生児であっても双子の成長につれコピー数多型の差は次第に大きくなり疾病感受性等の違いをもたらすのではないかと予測され、環境要因が人間にもたらす影響を調査するための重要な双子研究の一つとなっている[168][169]。
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