Download Chapter 8 – Fundamentals of Genetics

Fundamentals of Genetics
A trait is any characteristic that is passed from the parents to the offspring. From early days, people have
been breeding animals for desired traits. About 2300 years ago, Greek philosopher Aristotle developed an
explanation for how we inherit traits. He said that we had pangenes in our blood, and they contained a memory
of each structure in our body. From his idea comes the term “bloodline” we use today. In the 1600’s, scientists
realized that both parents must contribute to the offspring. But no one understood how it happened.
In the 1860’s, Austrian monk Gregor Mendel became known as the father of modern genetics through a
series of experiments he did with garden peas. The scientific study of heredity is called genetics.
Mendel’s Experiments
Mendel experimented with hybrid pea plants. A hybrid is an organism that receives different genetic
information for a trait from each parent. He used the garden peas for a few different reasons: 1) they selffertilize in nature, 2) each of the 7 traits he studied appeared in only two forms (i.e. round vs. wrinkled), 3) they
were easy to obtain and grew quickly. Mendel began with pure-bred plants. These are plants that when
allowed to self-fertilize, produced only offspring with the parental trait. Here is a pictorial diagram of what he
did and the results he obtained:
Parental Generation
Pure-bred tall plant
x
Pure-bred short plant
First Offspring Generation (F1)
Hybrids (all tall plants) – this one self fertilizes for the next generation
Tall plant
Second Offspring Generation (F2)
Tall plant
Tall plant
Short plant
Mendel noticed a couple of things: 1) all plants produced were either short or tall – no medium-sized plants;
2) there was a ratio of 3:1 in the F2 generation; 3) the entire F1 generation resembled the parents. From these
observations, he hypothesized that there were 2 kinds of traits: dominant traits, or traits that showed in
hybrids, and recessive traits, or traits that did not show in a hybrid. Above, tallness was dominant and
shortness was recessive. He received the same results each time he did the experiment.
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From all of Mendel’s pea plant studies, he hypothesized that each trait is controlled by something called a
“gene” in the organism. Many genes can express two or more forms, and that one trait is dominant over
another. He knew that both parents must contribute to an organism’s inheritance. He hypothesized that the two
factors must separate when gametes are formed (because reducing from 2N to N). During fertilization, each of
the parent’s 1 trait join together to make a pair in the offspring. He also reasoned that factors (genes) must
remain separate and distinct in the offspring. Different versions of a gene for the same trait are called alleles.
For example, short and tall are two alleles for plant height.
Modern Genetics
All of these basic rules of genetics are now called “Mendel’s Laws”. Remember: genes and chromosomes had
not yet been discovered. Here are his laws:
The Law of Segregation
Factors (genes) separate when gametes form. (We now know this to be true because of what happens in
meiosis I and II.)
The Law of Dominance
When the two alleles in a gene pair are different, one allele can control the trait and the other can be hidden.
A recessive allele can be expressed only when the organism has NO copy of the corresponding dominant
allele.
The Law of Independent Assortment
Gene pairs separate segregate into gametes randomly and independently of each other. Example: seed color
and seed shape are inherited separately from one another, because when you breed the parents, you will find
all possible combinations of seed shape and color.
Language of Genetics
Alleles – scientists often use letter for different alleles. For example, short vs. tall in pea plants would be
represented as T vs. t. The upper case T represents a dominant allele, and the lower case t represents a recessive
allele.
Genotype vs. Phenotype – the genotype of an organism is its’ actual genetic makeup (i.e. what the GENES
look like, not the organism). The phenotype of an organism is what is actually observed in the organism.
Example: In pea plants, TT is the genotype of a certain pea plant. However, the phenotype is tall. Tt is the
genotype in another pea plant, but the phenotype is still tall. And last, tt is the genotype in another pea plant,
but the phenotype is short.
Homozygous vs. Heterozygous – An organism in which the two alleles for a particular trait are identical (such
as in TT or tt) are homozygous. If their two alleles are different (such as in Tt), the organism is said to be
heterozygous.
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Problem: In rabbits, B is an allele for black coat and b is an allele for brown coat. Write the genotypes
for a rabbit that is homozygous for black coat and another rabbit that is heterozygous for black coat.
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Probability, Genetics and the Punnett Square
Geneticists use probability to predict the phenotypes and genotypes of offspring in breeding experiments. This
can predict the number and type of offspring BEFORE an actual cross is performed. A Punnett square is a grid
for organizing genetic information and the probabilities. Remember: this shows probabilities, not actual
results. Here are the steps for doing Punnett squares:
1. Determine the alleles in the gametes of the parents.
2. Place the alleles of the gametes of one parent along the top of the grid and those of the other
parent along the left side of the grid.
3. Combine the alleles inside the boxes of the Punnett square.
4. Determine the genotypes and phenotypes of the offspring inside the boxes.
There are three types of crosses we will study in this class: monohybrid crosses, dihybrid crosses and test
crosses.
 Monohybrid Crosses:
Let’s study the trait of pod color and ignore all other pea plant traits. G is the allele
for green color, and g is the allele for yellow color. We are going to begin with a homozygous plant for
green pods (GG) and cross it with a homozygous plant for yellow pods (gg). See square below:
Notice that ALL of the offspring have the genes Gg. Now let’s cross the two hybrid Gg plants (selffertilize them):
Notice that you had 1 GG, 2 Gg, and 1 gg. This is in a 1:2:1 ratio. This is the genotypic ratio. However,
the phenotypes of the above offspring are 3:1, that is, 3 green plants to 1 yellow plant. So the phenotypic
ratio is 3:1.
 Dihybrid Crosses: This is a cross where you study the inheritance of 2 traits at one time.
Let’s do a cross
with peas again. This time we will use color and shape. Green color is G and yellow color is g. Round
seed shape is R and wrinkled seed shape is r. To do this kind of cross, you first have to figure out all of the
possible gamete combinations of each of the parents. Let’s say that the genotype of the male is GGRR. The
genotype of the female is ggrr. Then the possible combinations would be GR for the male and gr for the
female. Here is the cross below of the P generation:
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OK. Now let’s cross the hybrids of the F1 generation to produce an F2 generation. The possible combination of
gametes for each hybrid is: GY, Gy, gY, gy. Place these combinations along the square, and cross again:
Notice that in this dihybrid cross, you have a phenotypic ratio of 9:3:3:1.
 Test Cross:
In a test cross, an unknown genotype is crossed with a homozygous recessive individual. If the
unknown genotype is heterozygous, ½ of the offspring should show the recessive phenotype. If the
unknown genotype is homozygous dominant, the all of the offspring will show the dominant phenotype.
Other Patterns of Heredity
There are a few other types of inheritance that are exceptions to Mendel’s rule. They are:
1. Incomplete Dominance – a heterozygote shows a phenotype that is intermediate between the two
homozygous phenotypes. Neither allele expresses itself fully. Example: you cross a red flower with a white
flower and end up with a pink flower (1/2 way between both). Since there are no dominant or recessive alleles
in this type of inheritance, the alleles are expressed differently. They are shown with superscripts as follows:
FrFr = red
FrFw = pink
FwFw = white
2. Codominance – both alleles express themselves fully. Example: blood types. If a person receives the type
A allele from one parent and type B allele from the other parent, they will have type AB blood.
3. Polygenic Traits – traits controlled by two or more gene pairs. Example: the color of human eyes. Eye
color is a matter of pigment, tone, amount, and distribution of pigment. Also human skin color is the same.
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