Suggested problems: Chapter 7: 4, 5, 6, 7, 10, 14, 16, 17, 18, 19, 21, 22
Mechanisms of sex determination
” indicates no chromosome)
This type occurs in several insects, including
In grasshoppers all gametes will have one each of 11
autosomes and either 1 or no X chromosomes (either 11
or 12 chromosomes per gamete).
Regarding reproduction in grasshoppers the males (XO)
will produce ½ X and ½ O gametes and the females (XX)
will produce all X gametes. Thus 1 x ½ = ½ of the
offspring will be XX (females) and 1 x ½ = ½ of the
offspring will be XO (males).
This provides for equal numbers of the two sexes in the
offspring, and is thus a good strategy for continuation of
Some organisms have XO as females and XX as males.
A more widespread mechanism is to have one sex be
” (XY or ZW, that is producing gametes with
sex chromosomes) and the other to be “homogametic
(XX or ZZ, that is producing gametes with only one type of sex
most higher animals, some “dioecious
Some birds, moths, butterflies, reptiles, and fish have ZZ
males and ZW females.
Regarding reproduction, consider humans (XX females
and XY males) as an example.
All eggs will contain an
X, while half of the sperm will contain an X and half a Y.
Thus 1 x ½ = ½ of the offspring will be XX (females)