Human: INTRODUCTION TO GENETICS
Table of Contents
Heredity, historical perspectives | The Monk and his peas | Principle
of segregation
Dihybrid Crosses | Mutations | Genetic Terms
| Links
Heredity, Historical Perspective
of how babies were conceived and how heredity worked. Clearly they were
conceived, and clearly there was some hereditary connection between
parents and children, but the mechanisms were not readily apparent. The Greek philosophers had
a variety of ideas: Theophrastus proposed that male flowers caused female
flowers to ripen; Hippocrates speculated that "seeds" were
produced by various body parts and transmitted to offspring at the time
of conception, and Aristotle thought that male and female semen mixed
at conception. Aeschylus, in 458 BC, proposed the male as the parent,
with the female as a "nurse for the young life sown within her".
the 1700s, Dutch microscopist
a "little man" (homunculus) inside each sperm. These scientists
formed a school of thought known as the "spermists". They
contended the only contributions of the female to the next generation
were the womb in which the homunculus grew, and prenatal influences
of the womb. An opposing school of thought, the ovists, believed that
the future human was in the egg, and that sperm merely stimulated the
growth of the egg. Ovists thought women carried eggs containing boy
and girl children, and that the gender of the offspring was determined
well before conception.
was an idea that males and females formed "pangenes" in every
organ. These pangenes subsequently moved through their blood to the
genitals and then to the children. The concept originated with the ancient
Greeks and influenced biology until little over 100 years ago. The terms
"blood relative", "full-blooded", and "royal
blood" are relicts of pangenesis. Francis Galton, Charles Darwin's
cousin, experimentally tested and disproved pangenesis during the 1870s.
theories of inheritance supplanted the spermists and ovists during the
19th century. The mixture of sperm and egg resulted in progeny that
were a "blend" of two parents' characteristics. Sex cells
are known collectively as
to the blenders, when a black furred animal mates with white furred
animal, you would expect all resulting progeny would be gray (a color
intermediate between black and white). This is often not the case. Blending
theories ignore characteristics skipping a generation. Charles Darwin
had to deal with the implications of blending in his theory of
not the major principle), and suggest that science of the mid-1800s
had not yet got the correct answer. That answer came from a contemporary,
Gregor Mendel, although Darwin apparently never knew of Mendel's work.
The Monk and his peas
that would become the modern science of
discrete units, independently inherited. These eventually were termed
Gregor Mendel, the Austrian monk who figured
out the rules of hereity. The above photo is from
Mendel reasoned an organism for genetic experiments should have:
- a number of different traits that can be studied
- plant should be self-fertilizing and have a
flower structure
that limits accidental contact - offspring of self-fertilized plants should
be fully fertile.
are termed the
flower are the
opening of the pea flower. The pollen grain grows a
reaching the ovary. The ripened ovary wall becomes the fruit (in this
case the pea pod). Most flowers allow cross-pollination, which can be
difficult to deal with in genetic studies if the male parent plant is
not known. Since pea plants are self-pollinators, the genetics of the
parent can be more easily understood. Peas are also self-compatible,
allowing self-fertilized embryos to develop as readily as out-fertilized
embryos. Mendel tested all 34 varieties of peas available to him through
seed dealers. The garden peas were planted and studied for eight years.
Each character studied had two distinct forms, such as tall or short
plant height, or smooth or wrinkled seeds. Mendel's experiments used
some 28,000 pea plants.
Some of Mendel's traits as expressed in garden
peas. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
problem, use of clear-cut variables and application of mathematics (statistics)
to the problem. Gregor Using pea plants and statistical methods, Mendel
was able to demonstrate that traits were passed from each parent to
their offspring through the inheritance of genes.
Mendel's work showed:
- Each parent contributes one factor of each
trait shown in offspring. - The two members of each pair of factors segregate
from each other during gamete formation. - The blending theory of inheritance was discounted.
- Males and females contribute equally to the
traits in their offspring. - Acquired traits are not inherited.
Principle of Segregation
involving only one trait is referred to as a
smooth-seeded plants with a variety that had always produced wrinkled
seeds (60 fertilizations on 15 plants). All resulting seeds were smooth.
The following year, Mendel planted these seeds and allowed them to self-fertilize.
He recovered 7324 seeds: 5474 smooth and 1850 wrinkled. To help with
record keeping, generations were labeled and numbered. The parental
generation is denoted as the P1 generation. The offspring of the P1
generation are the F1 generation (first filial). The self-fertilizing
F1 generation produced the F2 generation (second filial).
Inheritance of two alleles, S and s, in peas.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
Punnett square explaining the behavior of the S and s alleles.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
P1: smooth X wrinkled
F1 : all smooth
F2 : 5474 smooth and 1850 wrinkled
Meiosis, a process unknown in Mendel's day, explains how the traits
are inherited.
The inheritance of the S and s alleles explained
in light of meiosis. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
rather than continuous characters which are often difficult to distinguish. When "true-breeding" tall
plants were crossed with "true-breeding" short plants, all
of the offspring were tall plants. The parents in the cross were the
P1 generation, and the offspring represented the F1 generation. The
trait referred to as tall was considered
as those which appeared in the F1 generation in crosses between true-breeding
strains. Recessives were those which "skipped" a generation, being expressed
only when the dominant trait is absent. Mendel's plants exhibited
or recessive, not "in between".
members of the F1 generation were crossed, Mendel recovered mostly tall
offspring, with some short ones also occurring. Upon statistically analyzing
the F2 generation, Mendel determined the ratio of tall to short plants
was approximately 3:1. Short plants have skipped the F1 generation,
and show up in the F2 and succeeding generations. Mendel concluded that
the traits under study were governed by discrete (separable) factors.
The factors were inherited in pairs, with each generation having a pair
of trait factors. We now refer to these trait factors as
of traits "skipping" generations.
Summary of Mendel's Results:
- The F1 offspring showed only one of the two
parental traits, and always the same trait. - Results were always the same regardless of
which parent donated the pollen (was male). - The trait not shown in the F1 reappeared in
the F2 in
about 25% of the offspring. - Traits remained unchanged when passed to offspring:
they did not blend in any offspring but behaved as separate units. - Reciprocal crosses showed each parent made
an equal contribution to the offspring.
Mendel's Conclusions:
- Evidence indicated factors could be hidden
or unexpressed, these are the - The term
- Male and female contributed equally to the
offsprings' genetic makeup: therefore the number of traits was probably
two (the simplest solution). - Upper case letters are traditionally used to
denote dominant traits, lower case letters for recessives.
gamete formation (remember, meiosis was not yet known!) to retain the
number of traits at 2. The
with each gamete receiving one or the other factor, usually not both.
Organisms carry two alleles for every trait. These traits separate during
the formation of gametes.
A hypertext version (in German or English,
annotated also available) of Mendel's 1865 paper is available by clicking
Dihybrid Crosses
phenotype ratios. The two traits, if considered to inherit independently,
fit into the principle of segregation
Instead of 4 possible genotypes from a monohybrid cross, dihybrid crosses
have as many as 16 possible genotypes.
Mendel realized the need to conduct his experiments
on more complex situations. He performed experiments tracking two seed
traits: shape and color. A cross concerning two traits is known as a
dihybrid cross.
Crosses With Two Traits
Smooth seeds (S) are dominant over wrinkled (s) seeds.
Yellow seed color (Y) is dominant over green (g).
Inheritance of two traits simultaneously, a
dihybrid cross. The above graphic is from the Genetics pages
at McGill University (http://www.mcgill.ca/nrs/dihyb2.gif).
Again, meiosis helps us understand the behavior of alleles.
The inheritance of two traits on different
chromosomes can be explained by meiosis. Image from Purves et
al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
(http://www.sinauer.com/) and WH Freeman (
used with permission.
Methods, Results, and Conclusions
Mendel started with true-breeding plants that had smooth, yellow seeds
and crossed them with true-breeding plants having green, wrinkled seeds.
All seeds in the F1 had smooth yellow seeds. The F2 plants self-fertilized,
and produced four phenotypes:
315 smooth yellow
108 smooth green
101 wrinkled yellow
32 wrinkled green
not present.The 3:1 ratio was seen separately and was in accordance
with the Principle of Segregation. The segregation of S and s alleles
must have happened independently of the segregation of Y and y alleles.
The chance of any gamete having a Y is 1/2; the chance of any one gamete
having a S is 1/2.The chance of a gamete having both Y and S is the
product of their individual chances (or 1/2 X 1/2 = 1/4). The chance
of two gametes forming any given genotype is 1/4 X 1/4 (remember, the
product of their individual chances). Thus, the Punnett Square has 16
boxes. Since there are more possible combinations to produce a smooth
yellow phenotype (SSYY, SsYy, SsYY, and SSYy), that phenotype is more
common in the F2.
the results of the second experiment, Mendel formulated the
traits assort independent of each other during gamete formation, the
results of the dihybrid cross can make sense. Since Mendel's time, scientists
have discovered chromosomes and DNA. We now interpret the Principle
of Independent Assortment as alleles of genes on different chromosomes
are inherited independently during the formation of gametes. This was
not known to Mendel.
Punnett squares deal only with probability
of a genotype showing up in the next generation. Usually if enough offspring are produced, Mendelian ratios
will also be produced.
Step 1 - definition of alleles and determination of dominance.
Step 2 - determination of alleles present in all different types of
gametes.
Step 3 - construction of the square.
Step 4 - recombination of alleles into each small square.
Step 5 - Determination of Genotype and Phenotype ratios in the next
generation.
Step 6 - Labeling of generations, for example P1, F1, etc.
easier to do. The term "true-breeding strain" is a code word
for homozygous. Dominant alleles are those that show up in the next
generation in crosses between two different "true-breeding strains".
The key to any genetics problem is the recessive phenotype (more properly
the phenotype that represents the recessive genotype). It is that organism
whose genotype can be determined by examination of the phenotype. Usually
homozygous dominant and heterozygous individuals have identical phenotypes
(although their genotypes are different). This becomes even more important
in dihybrid crosses.
Mutations
the work of Mendel, recognized that occasional abrupt, sudden changes occurred in the
patterns of inheritance in the primrose plant. These sudden changes
he termed mutations. De Vries proposed that new alleles arose by mutations. Charles Darwin,
in his Origin of Species, was unable to describe
how heritable changes were passed on to subsequent generations, or how
new adaptations arose. Mutations provided answers to problems of the appearance of
novel adaptations. The patterns of Mendelian inheritance explained the
perseverance of rare traits in organisms, all of which increased variation,
as you recall that was a major facet of Darwin's theory.
work was published in 1866 but not recognized until the early 1900s
when three scientists independently verified his principles, more than
twenty years after his death. Ignored by the scientific community during
his lifetime, Mendel's work is now a topic enjoyed by generations of
biology students (;))
Genetic Terms
Definitions of terms. While we are discussing
Mendel, we need to understand the context of his times as well as how
his work fits into the modern science of genetics.
one trait or character.
every gene, sometimes as many a three or four.
allele is expressed.
of the second allele.
allele is the same (e.g. short plants are homozygous for the recessive
allele).
under study.
Links
- History of Genetics Web Pages (University of California,
Davis) - Quantitative Genetics Resources (University of Arizona)
- MendelWeb Hey, how many folks have their own webpages
over a century after their deaths? - Classic Papers in Genetics These files are downloadable
in Adobe Acrobat format. The site has a link to get the viewer from
Adobe. - Mendelian Genetics (Bio 181 at the University of
Arizona) Lecture notes, a genetics tutorial, and some very nice graphics. - Monohybrid Problem Set (The Biology Project, U of
AZ) Tutorial on single-trait crosses. - Dihybrid Problem Set (The Biology Project, U of
AZ) Tutorial on two-trait crosses. - The Virtual Fly Lab Conduct online genetics crosses
with virtual Drosophila.
An excellent, much cited and visited site. - MIT Hypertextbook Chapter on Mendelain Genetics
- The History of Genetics (Whitman College) An outline.
- Course Outline in Genetics (McGill University) An
outline and many quality graphics and animations. - Interactive Pea Experiment (Bill Kendrick) Select
peas to breed. Nice introduction to genetics experiments. - Glossary of Genetics Terms
Text ©1992, 1994, 1997, 1999, 2000, 2001, by Nolvyhindarto.bio, all
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Email: nolvyhindarto@gmail.com
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