Transmission genetics is exemplified by Mendel’s rules, which describe how genetic information is transmitted from parents to offspring. Consider a monohybrid cross:
Population genetics describes how genetic information is transmitted from a population of parents to a population of offspring. Consider, for example, the following data from the Est-3 locus of Zoarces viviparus:1
Genotype of offspring | |||
Maternal genotype | |||
305 | 516 | ||
459 | 1360 | 877 | |
877 | 1541 |
This table describes, empirically, the relationship between the genotypes of mothers and the genotypes of their offspring. Using these data we can also make some inferences about the genotypes of the fathers in this population, even though we didn’t collect them.2
305 out of 821 male gametes that fertilized eggs from
877 out of 2418 male gametes that fertilized eggs from
How many of the 2,696 male gametes that fertilized eggs from
We don’t know the paternal genotypes or we wouldn’t be asking this question.
There is no way to tell which of the 1360
Regardless of what the genotype of the father is, half of the offspring of a heterozygous mother will be heterozygous.3
Heterozygous offspring of heterozygous mothers contain no
information about the frequency of
How many of the 1336 homozygous progeny of heterozygous mothers
received an
459 out of 1336 (34%)
How many of the offspring where the paternal contribution can be
identified received an
(305 + 459 + 877) out of (305 + 459 + 877 + 516 + 877 + 1541) or 1641 out of 4575 (36%)
The above calculations tell us what’s happening for this particular data set, but those of you who know me know that there has to be a little math coming so that we can describe the situation more generally.4 Here’s the notation:
Genotype | Number | Sex |
---|---|---|
female | ||
female | ||
female | ||
male | ||
male | ||
male |
Using that notation,
Since every individual in the population must have one father and one
mother, the frequency of
Question: Why do those assumptions matter?
Answer: If
This is an example of what philosophers call a zero force law. Zero force laws play a very important role in scientific theories, because we can’t begin to understand what a force does until we understand what would happen in the absence of any forces. Consider Newton’s famous dictum:
An object in motion tends to remain in motion in a straight line. An object at rest tends to remain at rest.
or (as you may remember from introductory physics)7
What we might call the “First Law of Population Genetics” is analogous to Newton’s First Law of Motion:
If all genotypes at a particular locus have the same average fecundity and the same average chance of being included in the breeding population, allele frequencies in the population will remain constant from one generation to the next.
For the rest of the semester we’ll be learning about the processes that cause allele frequencies to change and learning how to infer the properties of those processes from the changes that they induce. But you must always remember that while we can infer that some evolutionary process is happening if allele frequencies change from one generation to the next, we cannot infer the absence of an evolutionary process from a lack of allele frequency change.8
These notes are licensed under the Creative Commons Attribution License. To view a copy of this license, visit or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.