Bio A – Mendelian Genetics
INTRODUCTION TO MENDELIAN GENETICS
http://anthro.palomar.edu/mendel/mendel_1.htm
For thousands of years farmers and herders have been selectively breeding their plants and
animals to produce more useful hybrids It was somewhat of a hit or miss process since the actual
mechanisms governing inheritance were unknown. Knowledge of these genetic mechanisms
finally came as a result of careful laboratory breeding experiments carried out over the last
century and a half.
By the 1890's, the invention of better microscopes allowed biologists to discover the basic facts
of cell division and sexual reproduction. The focus of genetics research then shifted to
understanding what really happens in the transmission of hereditary traits from parents to
children. A number of hypotheses were suggested to explain heredity, but Gregor Mendel, a
little known Central European monk, was the only one who got it more or less right. His ideas
had been published in 1866 but largely went unrecognized until 1900, which was long after his
death. His early adult life was spent in relative obscurity doing basic genetics research and
teaching high school mathematics,
While Mendel's research was with plants, the basic underlying principles of heredity that he
discovered also apply to people and other animals because the mechanisms of heredity are
essentially the same for all complex life forms.
Through the selective cross-breeding of common pea plants (Pisum sativum) over many
generations, Mendel discovered that certain traits show up in offspring without any blending of
parent characteristics. For instance, the pea flowers are either purple or white--intermediate
colors do not appear in the offspring of a white pea plant crossed with a purple pea
plant. Mendel observed seven traits that are easily recognized and apparently only occur in one
of two forms:
1.
2.
3.
4.
flower color is purple or white
flower position is axial or terminal
stem length is long or short
seed shape is round or wrinkled
5. seed color is yellow or green
6. pod shape is inflated or constricted
7. pod color is yellow or green
This observation that these traits do not show up in offspring plants with intermediate forms
was critically important because the leading theory in biology at the time was that inherited
traits blend from generation to generation. Most of the leading scientists in the 19th century
accepted this "blending theory." Charles Darwin proposed another equally wrong theory
known as "pangenesis". This held that hereditary "particles" in our bodies are affected by
the things we do during our lifetime. These modified particles were thought to migrate via
blood to the reproductive cells and subsequently could be inherited by the next
generation. This was essentially a variation of Lamarck's incorrect idea of the "inheritance of
acquired characteristics."
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Bio A – Mendelian Genetics
Mendel picked common garden pea plants for the focus of
his research because they can be grown easily in large
numbers and their reproduction can be manipulated. Pea
plants have both male and female reproductive organs. As
a result, they can either self-pollinate themselves or crosspollinate with another plant. In his experiments, Mendel
was able to selectively cross-pollinate purebred plants with
particular traits and observe the outcome over many
generations. This was the basis for his conclusions about
the nature of genetic inheritance.
In cross-pollinating plants that either produce
yellow or green pea seeds exclusively, Mendel
found that the first offspring generation (f1)
always has yellow seeds. However, the
following generation (f2) consistently has a 3:1
ratio of yellow to green.
This 3:1 ratio occurs in later generations as well. Mendel realized that this was the key to
understanding the basic mechanisms of inheritance.
He came to three important conclusions from these experimental results:
1. Inheritance of each trait is determined by "units" or "factors" that are passed on to
descendents unchanged(these units are now called genes )
2. Individuals inherits one such unit from each parent for each trait
3. A trait may not show up in an individual but can still be passed on to the next generation.
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Bio A – Mendelian Genetics
It is important to realize that, in Mendel’s experiment, the starting parent plants were
homozygous for pea seed color. That is to say, they each had two identical forms (or alleles)
of the gene for this trait--2 yellows or 2 greens.
The plants in the f1 generation
were all heterozygous . In
other words, they each had
inherited two different alleles-one from each parent plant. It
becomes clearer when we look
at the actual genetic makeup,
or genotype of the pea plants
instead of only the phenotype , or observable physical.
Note that each of the f1 generation plants (shown above) inherited a Y allele from one parent and
a G allele from the other. When the f1 plants breed, each has an equal chance of passing on
either Y or G alleles to each offspring.
With all of the seven pea plant traits that Mendel examined, one form appeared dominant over
the other, which is to say it masked the presence of the other allele. For example, when the
genotype for pea seed color is YG (heterozygous), the phenotype is yellow. However, the
dominant yellow allele does not alter the recessive green one in any way. Both alleles can be
passed on to the next generation unchanged.
Punnet squares are utilized to show all possible combinations of alleles that an offspring would
get from his/her parents. Each parent has two alleles. For a single trait (like eye color), each
parent would have two genes (remember 2N means 2 of each gene). A gene in a punnet square
is indicated by a letter (A or a, B or b..etc). Alleles from the same gene are given the same letter.
Mendel looked at specific physical characteristics (what we see) which had only two alleles
(dominant or recessive). An allele that is dominant is given a capital letter. An allele from the
same gene that is recessive is given the same letter, but it is lower case.
So if brown is dominant over blue
Brown = B
blue = b
If a person having brown eyes is “purebred” that means that both alleles are the same (BB or bb)
in that individual. If a person having brown eyes is heterozygous, it means that the person has
one dominant allele (B) and one recessive allele (b) and his/her genotype is Bb (capital letters
always come first. This person is heterozygous for the trait, and in the case of a dominance
pattern (one allele dominant over the other), the phenotype will always be that of the dominant
allele.
In a punnet square you put one parents alleles across the top, the other parent’s alleles down the
side, and create the four possible combinations that you might get in an offspring (of that gene).
Mendel's observations from these experiments can be summarized in two principles:
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Bio A – Mendelian Genetics
1. the principle of segregation
2. the principle of independent assortment
According to the principle of segregation, for any particular trait, the pair of alleles of each
parent separate and only one allele passes from each parent on to an offspring. Which allele in a
parent's pair of alleles is inherited is a matter of chance. We now know that this segregation of
alleles occurs during the process of sex cell formation (i.e., meiosis
).
Segregation of alleles in the production of sex cells
According to the principle of independent assortment, different pairs of alleles are passed to
offspring independently of each other. The result is that new combinations of genes present in
neither parent are possible. For example, a pea plant's inheritance of the ability to produce
purple flowers instead of white ones does not make it more likely that it will also inherit the
ability to produce yellow pea seeds in contrast to green ones. Likewise, the principle of
independent assortment explains why the human inheritance of a particular eye color does not
increase or decrease the likelihood of having 6 fingers on each hand. Today, we know this is due
to the fact that the genes for independently assorted traits are located on different chromosomes.
These two principles of inheritance, along with the understanding of unit inheritance and
dominance, were the beginnings of our modern science of genetics. However, Mendel did not
realize that there are exceptions to these principles
By focusing on Mendel as the father of genetics, modern biology often forgets that his
experimental results also disproved Lamarck's theory of the inheritance of acquired
characteristics described in the Early Theories of Evolution tutorial. Mendel rarely gets credit
for this because his work remained essentially unknown until long after Lamarck's ideas were
widely rejected as being improbable.
NOTE: Some biologists refer to Mendel's "principles" as "laws".
NOTE: One of the reasons that Mendel carried out his breeding experiments with pea plants
was that he could observe inheritance patterns in up to two generations a year. Geneticists
today usually carry out their breeding experiments with species that reproduce much more
rapidly so that the amount of time and money required is significantly reduced. Fruit flies and
bacteria are commonly used for this purpose now. Fruit flies reproduce in about 2 weeks from
birth, while bacteria, such as E. coli found in our digestive systems, reproduce in only 3-5 hours.
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Bio A – Mendelian Genetics
VOCABULARY and PROBLEMS
Read through the three previous pages. Define the words (below) in your own words
WITHOUT using the internet (you may use your book). Then do the problems on pages 6 and 7.
Hybrid
Genetics
Cross-pollination
Purebred
Homozygous
Allele
Heterozygous
Genotype
Phenotype
Dominant
Recessive
Characteristic
Trait
Selective breeding
F1 generation
F2 generation
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Bio A – Mendelian Genetics
Genetics Practice problems:
ALL parts must be completed. Zero credit is given for the “answer” alone.
Part I: Complete dominance
Problem 1: In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross
of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should
have spherical seeds?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Problem 2: In dogs, wire hair (S) is dominant to smooth (s). In a cross of a female homozygous
wire-haired dog with a male smooth-haired dog, what will be the phenotype of the F1 generation
(offspring)?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
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Bio A – Mendelian Genetics
Problem 3: Woodrats are medium sized rodents with lots of interesting behaviors. You may
know of them as packrats. Let's assume that the trait of bringing home shiny objects (H) is
dominant to the trait of carrying home only dull objects (h). Suppose two heterozygous
individuals are crossed. What would be the expected genotypic ratio of the progeny (offspring)?
ALSO how many of the progeny bring home shiny objects?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
Problem 4: The common grackle is a species of blackbird that is fairly common in the United
States. Suppose that long tails (L) were dominant to short tails in these birds. A female shorttailed grackle mates with a male long-tailed grackle who had one parent with a long tail and one
parent with a short tail. What percentage of the offspring have a genotype of Ll? What
percentage of the offspring would have a long tail?
a.. Parental Genotypes (Dad’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
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Bio A – Mendelian Genetics
Part II: Incomplete dominance
Problem 1: In Japanese four o’clock flowers, color is inherited by genes that show incomplete
dominance. In such flowers, a cross between a homozygous red (R) flower and a
homozygous white (r) flower will always result in pink flowers. A cross is made between
two pink flowers. What is the probability for each phenotype?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Problem 2: In chickens, feather color is incompletely dominant. A white rooster mated with a
black hen creates chicks with grey feathers. What phenotypic AND genotypic ratios of the
offspring would you expect from the mating of a grey rooster and a black hen?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
Draw a picture of the result using colored pencils
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Bio A – Mendelian Genetics
Part III: Co-dominance
Problem 1: In shorthorn cattle, the hybrid between red and white is roan. Roan means that the
animal has BOTH red hairs AND white hairs. If a roan is bred with a white, what will the
phenotypic ratio be?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
Draw a picture of the result using colored pencils:
c. Answer:
Problem 2: Blood type in humans is determined by the presence of 2 of 3 possible alleles. The
alleles are IA, IB and iO. IA is completely dominant to iO. IB is completely dominant to iO. IA and
IB are co-dominant with each other. Suppose a female who is homozygous for Type A blood
marries a male who is heterozygous for type B blood. Their first child is type AB and their
second child is type O. Do both these children belong to the same father? Explain.
a. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer:
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Bio A – Mendelian Genetics
Part IV: Sex-linked Problems
Problem 1: The ability to see color is determined by a gene found on the X chromosome. When
this gene is defective, it causes colorblindness. A colorblind person can not distinguish between
certain colors, such as red and green. The good news is that the “normal” allele is dominant to
the colorblind allele. Suppose a Colorblind Female marries a normal male. Can they have any
male children who can see color? Can they have any female children who can see color?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
Problem 2: Hemophilia is a disease where the blood does not clot very well. The mutated gene
that causes this disease is found on the X chromosome. Again, the mutation is recessive and the
normal allele is dominant. Suppose you have a Homozygous normal female and a male with
hemophilia. What is the probability of getting a healthy child (male or female)? What fraction
of the male children will be healthy?
a.. Parental Genotypes (mom’s first) ____________________ x _____________________
b. Punnet Square:
c. Answer: (2 parts)
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Bio A – Mendelian Genetics
Part V: Test Crosses
Problem 1: A florist knows the up-coming holiday will put a large demand on red roses (R). He
wants to make sure that his Red rose plant is PURE so that he doesn’t get a lot of white flowers
(r) when he pollinates the plant.
a. What possible genotypes might his red plant be?
b. What genotype should he cross his rose with to identify its genotype?
c. Draw out BOTH possible punnett squares below:
d. By looking at the offspring of the test cross, how will he tell if his plant is pure?
Problem 2: A dog breeder knows he can sell a beagle that is PURE for a “slim neck” for more
than he can sell a hybrid. A hybrid has a chance of producing the recessive “thick neck” beagle,
which is considered undesirable. He crosses his “slim neck” beagle with a “thick neck” beagle
and 8 puppies are born. 5 are “slim neck” and 3 are “thick neck”.
a. Can he determine if his beagle is pure or not?
b. If so, how? If not, why not?
Problem 3: A farmer wants to produce all yellow corn. He knows that sometimes corn comes
out yellow, sometimes it comes out white and sometimes it comes out with both yellow and
white kernels. He has heard that test crosses can help his case. To make sure that his plant can
only produce yellow kernels, he crosses his yellow plant with a yellow/white plant. He gets a
mixture of yellow and yellow/white offspring. He decides his plant is NOT pure for the yellow
allele.
a. Did the farmer have to perform a test cross to determine the genotype of
his corn? Explain?
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Bio A – Mendelian Genetics
VI. Dihybrid Crosses
Problem 1: In mice the gene for coat color as two forms. The allele for dark coat is
dominant to the allele for albino. There are two forms for the gene controlling whiskers,
as well, straight is dominant to bent.
Cross two mice that are heterozygous for both these genes.
a. What is the genotype of each mouse?
b. What possible gametes can each mouse make (remember each gamete
contains one allele for EACH trait being considered)?
c. Draw out the Punnett square for this cross
d. What proportion of the offspring are albino?
e. What proportion would have bent whiskers?
f. What is the phenotypic ratio of ALL the offspring? (be sure to include the
traits!)
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Bio A – Mendelian Genetics
Problem 2: If a maize plant heterozygous for the alleles for pigmy and crinkly-leaf (both
recessive to normal size of plant and normal leaf) is self-pollinated and 208 seeds are
subsequently collected and germinated, how many would you expect to show:
a. crinkly leaves?
b. normal size?
c. normal leaves and normal size?
d. normal leaves and pigmy size?
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Bio A – Mendelian Genetics
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