Introduction to Genetics-Teacher Notes
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Mendelian Genetics
a. Gregor Mendel
i. The ‘father’ of modern-day genetics
ii. Mendel's pea plant experiments conducted between 1856 and 1863 established
many of the rules of heredity, now referred to as the laws of Mendelian
inheritance
iii. Mendel worked with seven characteristics of pea plants: plant height, pod shape
and color, seed shape and color, and flower position and colo
b. Why pea plants?
i. Advantages of pea plants for genetic study:
ii. There are many varieties with distinct heritable features. (flower color, seed
color, etc)
iii. Each pea plant has sperm-producing organs (stamens) and egg-producing
organs (carpels)
iv. Cross-pollination can be achieved by dusting one plant with pollen from another
c. Mendel chose to track only those characteristics that varied in an “either-or” manner.
i. Two varieties of a trait, such as purple or white flowers
d. He also used varieties that were “true-breeding” (plants that produce offspring of the
same variety when they self-pollinate)
Mendel’s Experiments
a. In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a
process called hybridization.
b. The true-breeding parents are the P generation.
c. The hybrid offspring of the P generation are called the F1 generation.
d. When F1 individuals self-pollinate, the F2 generation is produced.
Ratios in heredity!
a. When Mendel crossed contrasting, true-breeding white and purple flowered pea plants,
all of the F1 hybrids were purple
b. When Mendel crossed the F1 hybrids, about 75% of the generation had purple flowers,
25% had white flowers.
i. WHY?!
c. Mendel called the purple flower color a dominant trait and white flower color a
recessive trait.
d. Mendel observed the same pattern of inheritance in six other pea plant characters, each
represented by two traits
e. These traits were all controlled by individual genes, which are segments of DNA within
different chromosomes.
Mendel’s First Concept is that alternative versions of genes account for variations in
inherited characteristics.
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a. For example, the gene for flower color in pea plants exists in two versions, one for
purple flowers and the other for white flowers
b. These alternative versions of a gene are now called alleles
c. Each gene resides at a specific locus on a specific chromosome
i. A locus is a LOCATION
Mendel’s second concept is that for each characteristic, an organism inherits two alleles:
one from each parent.
a. Mendel made this deduction without knowing about the role of chromosomes
b. The two alleles can be identical, such as with a true-breeding pea plant.
c. The two alleles can also differ, such as in a hybrid.
Mendel’s third concept is that if the two alleles at a locus differ, the dominant allele will be
expressed, the recessive allele will be hidden.
Mendel’s fourth concept states that an egg or a sperm gets only one of the two alleles that
are present in the somatic cells of an organism.
a. This is now called independent assortment.
Genetics Continued
I.
The testcross
a. How can we tell the genotype of an individual that is exhibiting the DOMINANT
phenotype, or physical characteristic?
i. The answer is to conduct a testcross
1. Breed the mystery individual with an individual that is homozygous
recessive
a. If any offspring display the recessive trait, then the unknown
parent had to be heterozygous!
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The Law of Segregation
a. Each individual will randomly pass along 1 of 2 alleles for a trait
i. For example: If a pea plant is Pp for color, there’s a 50% chance a gamete will
get the big P and 50% chance it will get the little p. It will not get both!
Monohybrid crosses
a. Crossing for one particular trait
b. Use a 4 box punnett square!
i. Bb x Bb
Dihybrid cross
a. Crossing for two particular traits
i. AaBb x AaBb
ii. Resulting Punnett square is 16 boxes
iii. You must account for each combination of alleles from each parent
1. Parent 1 from i: AB, Ab, aB, ab
2. Parent 2 from i: AB, Ab, aB, ab
Law of independent assortment
a. Each pair of alleles segregates independently of other pairs
i. I.e. The distribution of one allele does not depend on the distribution of others
Probability
a. Mendelian genetics is based on probability
i. Just like tossing a coin ( ½ heads (50%) and ½ tails (50%) )
ii. Multiplication Rule can be used
1. Odds of two events occurring in a row ( ½ x ½ = ¼ )
Other patterns of inheritance
a. Many heritable characters are not determined by only one gene with two alleles
b. However, the basic principles of segregation and independent assortment apply even to
more complex patterns of inheritance
3 levels of dominance in genetics
a. Complete dominance
i. occurs when phenotypes of the heterozygote and dominant homozygote are
identical
1. Example: PP and Pp both produce purple flowers in pea plants.
b. Codominance
i. both alleles are dominant and have different effects.
ii. If you have a flower that is red (R) and one that is white (W) and cross them, you
get a red-white flower!
iii. Also, human blood types are examples of codominance
1. A is AA or Ai
2. B is BB or Bi
3. AB is a combination of both dominant alleles
4. O is ii (recessive!)----but most common blood type in the world!
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6. This is also an example of multiple alleles because there are more than
two variations on the trait….A-B-AB and O all are alleles for blood type
c. incomplete dominance
i. neither allele is dominant, and the hybrid phenotype is a mixture of the two
parents.
1. If you use the Red (R) and White (W) flower example, this time the
flowers, when combined, would show as PINK
a. Neither trait establishes dominance, so neither is shown
Pleitropy
a. Most genes have multiple phenotypic effects, a property called pleiotropy
b. For example, pleiotropic alleles are responsible for the multiple symptoms of certain
hereditary diseases, such as cystic fibrosis and sickle-cell disease
Epistasis
a. In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second
locus
b. For example, in mice and many other mammals, coat color depends on two genes
c. One gene determines the pigment color
d. The other gene determines whether the pigment will actually be deposited in the hair
Recessively inherited disorders
a. Many genetic disorders are inherited in a recessive manner
b. Recessively inherited disorders show up only in individuals homozygous for the allele
c. Carriers are heterozygous individuals who carry the recessive allele but are
phenotypically normal
Polygenic Inheritance
a. Quantitative characteristics are those that have an entire spectrum of variation.
i. Examples: Skin color, height
b. This is caused by polygenic inheritance, when two or more genes affect the same trait.