Chance and the Distribution. of Families. In genetic experiments with plants and other animals, the most direct inferences as to method of inheritance come from a count of the progeny. For example, if one' were to make a cross two pure lines. intercross the FJ,and from the inter-. cross obtaip approximately three-fourths of one parental type and one-fourth of the other in thl! F z , a simple Mendelian model. one locus witbJ»'P...".aJJeJes...!'lnd-c1Qmi.". This conclusion would be reinforced if the backcross of an Fr hybrid (to the pure-line parental type that had not appeared in the F 1 ) pro.: duced progeny of which approximately one-half resembled the. aforementioned; parental ty)X's and one-hctlf the Fr hybrid. In man, the number of progeny from any single family is usually so that. these conclusions would not be warranted. A mating of two .heterozygotes for a. recessive defect can produce 4:0, 2:2, I :3, and 0:4 ratios, as ,well as the expected J: I, in families offour children. Similarly, the mating A/a X a/a having two chil- dren, can produce 2:0 and 0:2 ratios as well as the expected 1:1. A bit of reflection will convince the reader that this is not surprising. Consider., . for example, the couple with genotypes A/a and a/a, respectively, having two chil- dren. Genetic theory expects them to have one A/a child and one a/a child because the A/a parent is expected to produce 1/2 A gemetes and 1/2 a gametes. ffthe A/a. parent is the male, this ratio will usually be realized among the sperm. Is there,' however, any guarantee that these"sperm will take turns, so to. speak, in fertilizing the egg'! Obviously not. The sperm involved in producing the two children could. well be A sperm in both cases or a sperm in both. If the A/a parent is the female, there is not even a guarantee that the gametic ratio will be 1/2 A and 1/2 a, since each meiosis ordinarily produces only one gamete and the various meioses are independent events: what happens in one meiosis does not influence what is to happerl the next or any subsequent one. Hence, it could happen easily that the two' children of this A/a X a/a mating are both A/a or both a/a. Clearly the prediction of genetic results is fraught with uncertainties, events over which no one has control. Such uncertainty is generally referred to as' "chance." One author has recognized the large element of chance involved byenti. t1ing his book <?n genetics The Dice of Destiny. The reader may well ask: how can . 171 , ! ! .t I f
Transcript
1. Chance and the Distribution.of Families.In genetic
experiments with plants and other animals, the most direct
inferences asto method of inheritance come from a count of the
progeny. For example, if one'were to make a cross betwee~ two pure
lines. intercross the FJ,and from the inter-. cross obtaip
approximately three-fourths ofone parental type and one-fourth of
theother in thl! F z, a simple Mendelian model. one locus
witbJ'P...".aJJeJes...!'lnd-c1Qmi.".
,-llilllc.e-~oJJiJ1..b.elI1kII.~~ This conclusion would be
reinforced if the backcross ofan Fr hybrid (to the pure-line
parental type that had not appeared in the F 1) pro.:duced progeny
of which approximately one-half resembled the.
aforementioned;parental ty)X's and one-hctlf the Fr hybrid. In man,
the number of progeny from any single family is usually so ;m~ll
that. these conclusions would not be warranted. A mating of two
.heterozygotes for a. recessive defect can produce 4:0, 2:2, I :3,
and 0:4 ratios, as ,well as the expected J: I, in families offour
children. Similarly, the mating A/a X a/a having two chil dren, can
produce 2:0 and 0:2 ratios as well as the expected 1:1. A bit
ofreflection will convince the reader that this is not surprising.
Consider., . for example, the couple with genotypes A/a and a/a,
respectively, having two chil dren. Genetic theory expects them to
have one A/a child and one a/a child because the A/a parent is
expected to produce 1/2 A gemetes and 1/2 a gametes. ffthe A/a.
parent is the male, this ratio will usually be realized among the
sperm. Is there,' however, any guarantee that these"sperm will take
turns, so to. speak, in fertilizing the egg'! Obviously not. The
sperm involved in producing the two children could. well be A sperm
in both cases or a sperm in both. If the A/a parent is the female,
, there is not even a guarantee that the gametic ratio will be 1/2
A and 1/2 a, since! each meiosis ordinarily produces only one
gamete and the various meioses are independent events: what happens
in one meiosis does not influence what is tohapperl the next or any
subsequent one. Hence, it could happen easily that the two'children
of this A/a X a/a mating are both A/a or both a/a.Clearly the
prediction of genetic results is fraught with uncertainties,
eventsover which no one has control. Such uncertainty is generally
referred to as'"chance." One author has recognized the large
element of chance involved byenti.t1ing his book