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Unit 5: Genetics – Guided Notes
(Make sure you know the bold-face terms!)
Basic Mendelian Genetics
Before Gregor Mendel…
1) When Mendel started his work, most people believed in the blending theory of
inheritance. (Inheritance, Heredity, and Genetics meant the same thing)
2) People thought that both sexes contributed equally to their offspring.
3) People also believed that parents of contrasting traits always ended up with
offspring of intermediate traits.
For example: A short mom and a tall dad would make an average height child.
4) If the genetic information is blended like they thought, then how could we have so
much variation?
Gregor Mendel’s Experiment
1) Monk – studied genetics in the 1860’s. See page 154 in the textbook for his work.
2) Mendel worked with garden peas on his experiment. Why?
They were easy to grow, reproduced quickly (had a short generation time) and were
capable of both self-fertilization and cross fertilization. There were also many pea
plant varieties available.
3) The pea plants Mendel used were true-breeding or pure-breeding, meaning the
pea plants were like their parents and like each other when they self-pollinated. In
other words, only one trait would show up time and time again. For example: The
purple plants that self-pollinated always produced purple plants.
4) Mendel could make the plants self-pollinate and cross-pollinate. He removed the
stamens of flowers to make sure the plants were cross fertilized and to control how
they cross fertilized. He removed the stamens (male part) of the purple flowers, for
example, and transferred the pollen (sperm) from these stamens to a carpel (female
part) of a white flower, so the sperm could fertilize the eggs. He then planted the
seeds from this cross.
5) The parents were called the P generation and the plants that grew from the seeds
were the F1 generation.
6) Because the F1 generation was a mixture from parents with different traits, the F1
were considered hybrids instead of true breeding.
7) Mendel then cross-fertilized the F1 plants with each other.
8) The seeds from the F1 plants became the F2 generation.
The Results of Mendel’s First Experiment
1) Because Mendel only crossed purple flowers and white flowers in his experiment, he
called this a monohybrid cross. He was only crossing 1 trait (flower color). The parents
only differed by 1 trait.
2) When he crossed the true breeding purple flowered plant with the true breeding
white flowered plant (the P generation), all the F1 plants from this cross were purple!
(Blending had not occurred)
3) Mendel concluded that the white flower trait must have been lost.
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4) When he crossed the F1 plants (purple flowers with purple flowers), the F2 generation
resulted in 3 purple flowered plants for every one white flowered plant. In other words,
75% were purple flowered while 25% were white flowered.
5) He realized that the white flowered trait didn’t disappear in the F1 generation, it
was only hidden.
Conclusions from Mendel’s Monohybrid Experiment
 There are alternative forms of genes that determine traits. He called them factors;
we now called the alternative form of a gene an allele.
For example, the gene for flower color has alleles for purple or white.
 For each inherited trait (gene), an organism has 2 alleles (1 allele from each
parent). Remember that in meiosis, sperm and egg got half the number of
chromosomes (which contain genes). Mendel called this the Law of Segregation
because alleles segregate from each other when gametes are made in meiosis.
 When the sperm and egg unite in fertilization, each contributes a gene with an
allele that makes a pair of alleles in the offspring.
 One allele may be fully expressed whenever it is present no matter what the other
allele is. This is called the dominant allele. This is represented with a capital letter.
 One allele may be hidden or can only show up if the other allele is not dominant.
This is called the recessive allele. This is represented with a lower case letter.
 The genotype of an organism is the actual genetic makeup of an individual and is
represented with 2 letters that show dominant and/or recessive alleles.
If there are 2 dominant alleles (TT), the genotype is described as homozygous
dominant (and in problems is often referred to as “true breeding”).
If one allele is dominant and the other allele is recessive (Tt), the genotype is
described as heterozygous.
If both alleles are recessive (tt), the genotype is described as homozygous
recessive or just recessive (and again is often referred to as “true breeding”).
 The phenotype of an organism is the physical appearance an organism expressed
due to the genotype.
For Example: The genotype of a flowered plant may be PP. The phenotype
from this genotype is purple flowers.
Diagram Mendel’s Monohybrid Experiment Using Proper Symbols
Use one letter for the gene,
Dominant Allele is Upper Case,
recessive allele is lower case,
Always write the dominant one first
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Homologous Chromosomes
with Genes/Alleles:

and
The Law of Segregation states that homologs separate in Anaphase I of Meiosis I;
therefore alleles separate when gametes are made. So the chromosome on the left
(big G) will be in a separate gamete than the chromosome on the right (little g)
Punnett Squares are used to determine all the possible outcomes in offspring when
gametes unite in fertilization.
Example 1: Suppose brown eyes are dominant to blue eyes. Make a Punnett square
to predict the offspring from a cross between a male who is heterozygous for brown
eyes and a female with blue eyes. Give the genotypic and phenotypic ratio. (This
problem is a cross between an organism showing the dominant trait and an organism
that is homozygous recessive so it is called a test cross.)
Example 2: Purple flowers are dominant to white flowers. Make a Punnett square to
predict the offspring from a cross from a homozygous purple flower and a
heterozygous purple flower. Give the genotypic and phenotypic ratio.
Do the Monohybrid practice problems and this chi square for next class.
Chi Square Test and Genetics
A chi square test is often used in genetics to determine if the predicted results from a
cross deviate significantly from what actually occurred. Consider the cross we did on
page 2 of the notes. What was your phenotypic ratio of the F2 generation? __________
If you had 7324 plants in total (5474 purple and 1850 white), would those results be
significantly different from the ratio you expected?
Determine if we should accept or reject the Null Hypothesis.
Round
Wrinkled
Observed
Expected
O-E
(O-E)2
(O-E)2/ E
Sum of the last row:____________
If the critical value is 3.84, is our ratio of 3:1 accepted?
Are the results significantly different from the 3:1 ratio?
Do you accept or reject the Null Hypothesis?
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Mendel’s Dihybrid Experiment
1) Mendel also performed genetic crosses with two traits. He crossed a plant with
round and yellow seeds with another plant with wrinkled and green seeds.
2) Round is dominant to wrinkled and yellow is dominant to green.
3) Write the genotype for the true breeding parent that is round and yellow: _________
and the genotype for the true breeding parent that is wrinkled and green: ___________
4) The F1 generation were all round and yellow. Mendel then began to wonder if the
characteristics were always transmitted together and could they separate. In other
words, did round always go with yellow?
5) He then crossed the F1 generation and got the following results in the F2 generation:
9 were round and yellow, 3 were round and green, 3 were wrinkled and yellow, and 1
was wrinkled and green. This gave a phenotypic ratio of 9:3:3:1.
6) From his results, he proved that the characteristics could in fact separate and didn’t
have to stay together….you could be round and green, not just round and yellow.
7) From this, he came up with the Law of Independent Assortment which states that
each pair of alleles segregates independently of each other in meiosis.
Diagram Mendel’s Dihybrid Cross with Symbols and Punnett Square
Do the Dihybrid practice problems for next class.
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As you can see, doing a Punnett Square with dihybrid cross is very long! If you did a
trihybrid cross, it would be much worse. There would be 64 boxes in the Punnett
Square! Because of this, you will be using a mathematical way to predict the chance
of allele combinations.
Predicting Offspring Genotypes with Probability
Probability and Genetics:
Probability ranges from a scale of 0 to 1.
An event certain to happen has a probability of 1.
An event certain to not happen has a probability of 0.
All possible events for an outcome must add up to 1.
For example: If you tossed a coin, you would have ½ chance of heads and a ½
chance of tails. These are independent events.
Simple Probability Genetics Example:
What is the probability of a gamete getting a B allele if the mother has a genotype of
BB?
What is the probability of a gamete getting a b allele if the mother has a genotype of
Bb?
What if you tossed 2 coins simultaneously? This is considered a compound event. You
would have to multiply the probabilities together. This is known as the Rule of
Multiplication. You will always use the rule of multiplication when you have 2
independent events that will happen at the same time. (AND situation)
What is the probability that if you flip 2 coins at the same time, both will lands heads
up?
Rule of Multiplication Genetics Example:
If you cross 2 dogs that have the genotypes Bb, what is the probability that the F1
generation dog will have the bb genotype?
First, find the probability that the sperm will have a b.
Second, find the probability that the egg will have a b.
Now, we want these independent events to happen at the same time so multiply the
probabilities together to get the answer.
Check your answer by doing a Punnett Square:
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You may also have to use the Rule of Addition in some genetics problems.
The Rule of Addition states that the chance of an event that can occur in 2 or more
independent ways is the sum of their individual chances. So, instead of multiplying,
you add probabilities together if it can happen in 2 different ways. (OR situation)
Rule of Addition Genetics Example:
If you cross 2 dogs that have the genotypes Bb, what is the probability that the F1
generation dog will have the Bb genotype?
Other Practice Problems Using Probability:
1) In Mendel’s Dihybrid Cross, what is the probability of getting the following offspring?
Parent 1: RrYy
Parent 2: RrYy
Offspring 1: RRYy
Do Offspring 2 and 3 for next class
Offspring 2: RrYy
Offspring 3: rrYY
2) What is the probability that the following 2 parents will produce offspring that are
homozygous dominant (AABBCC) for all 3 traits?
Parent 1: AABbCC
Parent 2: AaBbCc
What about an offspring which is recessive (aabbcc) for all 3 traits?
3) Do for next class: What is the probability that the following 2 parents will produce
offspring that are heterozygous (AaBbCcDd) for all four traits?
Parent 1: AaBbCCDd Parent 2: AaBbCcDD
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Nonmendelian Genetics
Everything we have been talking about so far has assumed complete dominance and
basic genetic conditions. Most gene reactions, however, do not work out that way.
1. Dominance may have degrees.
Incomplete dominance: when neither allele is completely dominant over
the others. Example: Red flowers and white flowers produce pink flowers.
Codominance: when both alleles are fully expressed, they both create an
effective product. For example – blood groups and blood types.
2. Multiple Alleles: more than two alleles exist for a particular gene, although
each individual still only gets two. For example – blood groups again.
3. Epistasis: when the product of one gene may modify the phenotypic expression of
another. Basically, genes interact to produce a phenotype. For example – hair
and skin color is a two-step process: step 1 is to make the pigment and step 2 is
to add the color. If there is no step 1, there cannot be a step 2. So the
genotype for trait 2 doesn’t matter because trait 1 masks it.
Albinism is a breakdown of this process and an example of Epistasis.
4. Continuous Variation and Polygenic Theory of Inheritance: some traits have
multiple genes that act together to determine phenotype, for example height
and weight. Since there are many genes that determine these factors and
these genes segregate independently of one another, we see a gradation of
the degree of difference when many individuals are examined. This continuous
variation often sees a bell curve distribution, as in a bell-shaped graph.
5. Pleiotropy: when an allele has more than one effect on a phenotype.
Examples include: white cats with blue/green eyes, yellow mice, dwarfism
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6. Sex Linkage:
Human females are XX and males are XY. The Y chromosome carries very few
traits, most are on the X. Therefore, if the X chromosome has a mutation on it,
then it will be passed easier to a male and will usually show up in males before
females. It is not impossible for females to have sex-linked traits, just rare.
Remember, since fathers pass the Y chromosome down to their sons, the X
chromosome with the sex-linked trait must come from the mother.
Examples of sex-linked traits: Color blindness, Hemophilia, Muscular Dystrophy,
Adrenoleukodystrophy (ALD – the disease from “Lorenzo’s Oil”),
and white eyes (albinism) in fruit flies
Do the sex-linked practice problems for next class.
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Human Diseases that Follow Mendelian Patterns of Inheritance
Autosomal Recessive Disorders: With recessive disorders, there is an increased
possibility of passing on recessive traits if close relatives mate. This is called inbreeding
and is seen a lot in zoo animals and some dogs because of unethical dog breeding in
puppy mills.
Cystic Fibrosis – sodium/potassium pumps malfunction, resulting in high extracellular
chloride concentration which causes mucus to be thicker and stickier than usual. It
builds up in the lungs, pancreas, and digestive tract and other glands. Because
one gene causes multiple effects, it is an example of pleiotropy. It affects 1 out of
2500 people of European descent. If untreated, children die before the age of 5
but can live into their 30s if treated. The current best treatment is a lung transplant.
Sickle Cell Anemia – red blood cells become sickle-shaped instead of
round/concave. When oxygen is low, the sickled cells clump together and clog
blood vessels. Symptoms include weakness, pain, organ damage, paralysis.
Heterozygote carriers may have some symptoms, which is evidence of incomplete
dominance. They also have an advantage in some areas because they are
resistant to malaria. It affects 1 out of 400 African Americans and people of
Mediterranean descent.
Phenylketonuria (PKU) – the liver cannot break down the amino acid phenylalanine
so it accumulates in the blood. Symptoms include a mousy odor, broad shoulders,
missing skin pigment, difficulty walking, and mental retardation. It is easily
manageable if individuals with PKU do not eat phenylalinine. There are warning
labels on food for them.
Albinism – lack of pigment in the skin and hair. An example of epistasis. Occurs in 1
out of 22,000 individuals.
Tay Sachs – the lysosomes cannot break down lipids so they accumulate in the
brain cells. Babies are normal at birth, become blind by age 1, and are usually
dead by age 5. Affects 1 in 300,000 but is much more common in the Jewish
population.
Autosomal Dominant Disorders: Dominant disorders originally appear by mutation. If
they are immediately lethal, the mutated allele is never passed on. If they are not,
then they will appear in offspring in a homozygous or heterozygous situation.
Achondroplasia – the allele codes for shortened bone growth. This results in a form
of dwarfism where the torso is normal sized but the arms and legs are smaller.
Normal sized individuals are homozygous recessive. Dwarfs are heterozygous.
Homozygous dominant individuals do not make enough bone tissue at all and die
as an embryo. This is known as the dominant lethal allele. This also means it is not
passed down as often as it could be. It occurs in 1 out of 25,000 people.
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Huntington’s Disease – a degenerative disease of the brain and nervous system. It
is irreversible and fatal. Because of this it is very rare, only affecting 1 out of 100,000
people. It is still passed on because the symptoms do not appear until the
individual is around 40 years old, and by then most of them have reproduced. Any
child born to an affected parent has a 50% chance of getting it. Because it is so
deadly and hard to identify early, it was one of the first genes tracked during the
Human Genome Project and we know it is on the tip of chromosome 4 so we can
test for it.
Polydactly – extra fingers and toes. Affects 1 in 1,000 on average, but 1 in 200 in
people of African descent. Most extra fingers are on the pinky side, but some can
be on the thumb side or even in the middle.
Sex-Linked Recessive Disorders
These alleles are inherited on the X chromosome. Since males only need 1 of these
alleles to show the trait, they are much more common in males than in females.
Red-Green Color Blindness – the cones in the eyes are unable to distinguish
between red and green pigments. A normal person sees 150 colors, color blind
individuals see 25 or less.
Hemophilia – a blood plasma disorder where clotting factors are missing or nonfunctioning, causing excessive bleeding.
Duchenne Muscular Distrophy – an allele for a muscle protein is missing or nonfunctioning, causing a progressive weakening and loss of muscle. Occurs in 1 out
of 3,500 males, and individuals rarely live past their 20’s.
Multifactorial Disorders – remember that many disorders may be polygenic or they
may have a multifactorial basis. There is a genetic and environmental component.
Examples are: heart disease, cancer, alcoholism, weight, height.
When studying human disorders, you can’t perform breeding experiments. You need
to analyze the results of matings that have already occurred. To do that we use:
Pedigree Analysis
Autosomal Recessive
Autosomal Dominant
Sex-Linked Recessive
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Chromosomes and Genetics
When Mendel first published his work, scientists did not know about chromosomes.
Later, they started noticing parallels between how chromosomes behaved and the
behavior of Mendel’s inheritance factors. This led to the development of The
Chromosomal Theory of Inheritance: Genes occupy specific positions or loci on the
chromosome and it is the chromosomes that undergo segregation and independent
assortment during meiosis. This accounts for inheritance patterns.
Genes and Chromosomes
 There are many more genes in a cell than chromosomes.
 Each chromosome could actually have thousands of genes.
 If genes are located very closely on a chromosome, they tend to travel together
when the chromosome segregates during meiosis.
 Genes that are very close on the same chromosome are often called linked genes.
 In the drawing, genes 2 and 3 are more linked because they are closer together. 1
and 4 are less linked because they are farther apart.
 Remember from Mendel’s dihybrid cross experiment that he always got a ratio of
9:3:3:1. Based on this, he determined the Law of Independent Assortment that
stated that traits are inherited independently of each other.
 Scientists performing dihybrid crosses later on found that the results sometimes
deviated from that. They realized that some traits did seem to always stick
together.
 These results deviated because these traits were found on genes that were located
very near each other on the same chromosome.
 Scientists also realized that genes on a chromosome may still not end up traveling
together when they segregate during meiosis because of crossing over.
 Genes that have crossed over are often called recombinant genes.
 If two genes located on the same chromosome are farther apart, they have a
greater chance of being separated from each other during crossing over.
 Thomas Morgan studied this and determined that you could look at the
percentage of the offspring and determine the positions of genes on
chromosomes.
 He called it the recombination frequency – the percentage of recombinant
offspring there are for the linked traits.
 Recombinant offspring do not look like either parent; they are a combination of
traits from both parents (for the linked traits).
 Morgan determined that the more recombinant offspring there are, the more
crossing over has occurred, and the further apart the genes are on the
chromosome.
 Using his data, he mapped out the genes with their relative positions on
chromosomes.
 SO – if there is a genetics problem and the ratios do not come out to the 9:3:3:1
expected (or whatever the expected ratio is), linkage and crossing over is involved.
(We will do examples on the back of this page)