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BISC403
Genetic and Evolutionary Biology
Summary of basic points of Mendelian Genetics
Most of this information was presented in class.
My assumption is that the very large majority of students in this class are familiar with
the fundamentals of Mendel’s experiments and his proposed model, probably from both
high school and BISC207 experience. I am interested in using class time efficiently and
have decided not to start at the very beginning. However, I am also concerned about
providing appropriate guidance and not leaving people behind. Therefore, I have
prepared this summary to make as clear as I can which concepts I consider essential as
the starting point for this course. As I have stressed in lecture, working through the
problems I identified for you will be very valuable in consolidating your knowledge of
and ability to work with this information. Note that the answers to most of the oddnumbered problems are in the back of the book, just before the glossary and index.
Key terms with brief comments:
Self-fertilization
Cross-fertilization
Pure-breeding lines
(note use of "strain", "biovar", and "breed" as alternatives to "lines")
Hybrids
Dominant and recessive traits
Phenotype – obser ved char acter istic
Genotype – infor mation that br ings about the phenotype (often with str ong
influence from environmental factors)
Alleles - basic units of information transmitted by each parent to the zygote.
The alleles combine to form the genotype and the genotype determines what
the phenotype will be.
Homozygous and heterozygous – ter ms to indicate if the genotype has two identical
(homozygous) alleles or two different ones (heterozygous)
Monohybrid cross – exper iment consider ing only one featur e at a time
Dihybrid cross – exper iment consider ing two featur es together
Test cross – key pr ocedur e to deduce genotype
These terms and princples are illustrated with various figures in Chapter 2. Study these
figures along with the terms above in the appropriate place:
Fig 2.7 – Pea reproduction, Mendels’s key contribution (controlled crossing)
Fig 2.8 – Pea traits Mendel studied
Fig 2.9 – Monohybrid cross
Fig 2.11 – Punnett Square. This is essentially a bookkeeping process to follow the
transmission and combination of all alleles)
Fig 2.12 and 2.13 – Genotype-phenotype relation
Fig 2.14 and 2.18 – Test cross, monhybrid and dihybrid, respectively. Be sure to
understand these figures thoroughly.
Fig 2.15 – Dihybrid cross
Other important points:
Mendel placed great emphasis on organizing his results in terms of ratios. It is good to be
familiar with the basic genotype and phenotype ratios that are observed in monohybrid
and dihybrid crosses of traits that display complete dominance. This is the only kind of
trait that Mendel studied although we will consider alternative situations in the near
future. The following table is the simplest summary of these results showing the ratios in
the F2 generation ("F2" means "second filial generation" where "filial" means "child" or
"offspring"). Study these results until it is completely clear why the numbers are as they
are. Memorizing the numbers is not very important; being able to derive them and most
importantly being able to explain where they come from are very important
Genotype ratios
Phenotype ratios
Monohybrid Cross
1:2:1
3:1
Dihybrid Cross
1:2:1:2:4:2:1:2:1
9:3:3:1
The book has small, detailed features (called "boxes" in other books) that offer extensions
of the basic material in a more specific way than the text. Two that are important with
respect to the Mendelian results are presented in Figure A and Figure B on pages 20 and
21. These examples give modern interpretation and uses for Mendel’s ideas. Read these
and learn the material just as though it were part of the text.
Finally, the chapter ends with a very nice extension of these concepts to human
applications, especially the procedure called pedigree analysis. Read and understand the
material on pages 31-35. You will need to be familiar with analyzing pedigrees and quite
possibly in drawing them.