10 THE ORIGIN OF PLANT AND ANIMAL CHILDREN 
4. Why are only half of the father’s transmissible qualities con¬ 
veyed to the young? 
5. How does it come that every cell of the young contains the 
same kind of chromosomes? 
Sibs. — This is a convenient term used to designate all 
the offspring, brothers and sisters, common to two parents. 
Since each sib owes its characteristics to the color bodies 
which happen to be retained in the maturation discard, the 
sibs will be much alike if in their several cases many of the 
same chromosomes are retained, and they will be very differ¬ 
ent if they carry few of the same chromosomes. Since with 
even a few chromosomes thousands of combinations are 
possible, it is quite unlikely that any two sibs (except identi¬ 
cal twins) will have exactly the same inherited qualities. A 
girl may inherit from her father a large frame and strong 
muscles, while the brother inherits from the mother a small 
stature and delicate features — no one can foretell just what 
the combination of characteristics will be. 
1. Why are sibs often alike? 
2. Why are sibs often very different ? 
3. Why are we unable to foretell which parental characteristics 
will appear in the offspring ? 
Sex Determination. — Animal breeders have always 
wished they could produce male or female animals in their 
herds at will, and parents often wish they could fix as they 
like the sex of the child to be born, but no method has been 
discovered by which the sex of the offspring can be controlled. 
We now know that the sex is determined at the time of the 
fertilization of the egg cell. One of the chromosomes of the 
sperm cell has the power to make the offspring to which it is 
transmitted a female. When the sperm cell divides into 
two spermatozoa, the chromosome with female potential¬ 
ities goes into one of the spermatozoa. If this spermatozoon 
fertilizes the egg cell, the offspring is a female. If the other 
