1 Two sperm enter the same egg
Sperm 1
Egg
Sperm 2
2 The egg now has three sets of chromosomes
Father’s
first set
Mother’s
set
Father’s
second set
3 Three cells develop
Mother-father
(X-Y)
Mother-
father
(X-X)
Father-father
(X-Y)
4 The cells divide and multiply
Father-father
X-Y cells die
5 The ball of cells splits with different ratios of X-X and X-Y cells
Twin 1
Twin 2
6 Semi-identical twins develop in the womb
Twin 1
Twin 2
1 Two sperm enter the same egg
2 The egg now has three sets of chromosomes
Father’s first set
Sperm 1
Mother’s set
Sperm 2
Egg
Father’s
second set
4 The cells divide and multiply
3 Three cells develop
Mother-father (X-Y)
Mother-
father (X-X)
Father-father
X-Y cells die
Father-father (X-Y)
5 The ball of cells splits with different ratios of X-X and X-Y cells
6 Semi-identical twins develop in the womb
Twin 1
Twin 2
Twin 1
Twin 2