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View All Every parent of twins faces the question "Are your twins identical or fraternal? Many think that being identical twins refers to how twins look and not how they form. Take a look at the biology that determines the sex combinations that are possible with identical and fraternal twins. The terms identical and fraternal don't describe what the twins look like, but actually how they form. Identical monozygotic twins are always of the same sex because they form from a single zygote fertilized egg that contains either male XY or female XX sex chromosomes. Fraternal twins can be either two girls, two boys, or one of each.For homosexuality, prevalence rates for women and men were % and Probandwise Concordances From Twin Studies of Sexual Orientation. Study. Males. Bailey, J. Michael and Richard Pillard, “A Genetic Study of Male Sexual Scott, “ A Twin Registry Study of Male and Female Sexual Orientation,” journal of Sex. kinship the extent to which two individuals have genes in common. twin design Based on “A Genetic Study of the Male Sexual Orientation,” by J. M. Bailey...the end of the story look at the video above ↑ ↑ ↑