Download Chapter 25 Mendelian Genetics Chapter 24: Sexual Reproduction in

Chapter 25
Mendelian Genetics
Chapter 24: Sexual Reproduction in Plants
Plants have both male and female reproductive structures. Some plant species actually have male and female
individuals but most are hermaphrodites.
Fertilization in plants (pollination) can occur naturally or artificially in one of 2 ways:
1) cross pollination in which the pollen from one plant travels to the female structure and fertilizes the egg of another
plant
or
2) self pollination in which the pollen from an individual plant fertilizes the egg of the same plant. Self pollination can
be prevented by removing the male structures of a plant before they mature. This allows a researcher to control which
plants mate.
Flower Structure http://www.urbanext.uiuc.edu/gpe/case4/c4facts1a.html
Section 25-1-> Mendel’s Principle of Heredity
Genetics: the branch of biology that studies the ways in which hereditary material is passed on from parents to offspring
Gregor Mendel
Gregor Mendel was an Austrian monk who studied pea plants from 1857-1865. Pea plants are such a good
species for genetics studies because:
1.
They are easy to grow.
2.
They mature quickly.
3.
Pollination can easily be controlled.
4.
They produce a large number of offspring from one mating.
5.
They have sharp contrasting traits which are easily distinguished.
1) seed shape - round or wrinkled
2) seed color - green or yellow
3) seed coat color - grayish brown or white
4) pod color - green or yellow
5) pod shape - inflated or wrinkled
6) stem length - long or short
7) flower position - lateral or terminal
During his studies, Mendel crossed various pea plants and collected and planted the seeds from each mating. He
grouped and counted the various types of offspring and analyzed the results. Throughout this process he kept careful
records. He died in 1884 without being recognized for his work. In 1900, his work and results were duplicated and
then the study of “Mendelian Genetics” began.
Mendel`s Observations and Experiments
■
Mendel observed that after repeated generations of self pollination, some pea plants showed a certain trait. For
example, plants were always tall or short, round seeded or wrinkle seeded, etc. These plants were said to be
pure for these traits. He always started his experiments with plants that were pure for specific traits. He used
the following notation to keep track of his mating generations:
■
P generation - the “parent” generation which the experiments always started with
F1 generation- the “first filial” generation which is the first generation of offspring from a mating experiment
F2 generation - the “second filial” generation which is the second generation of offspring in an experiment
First Experiment
1.
Mendel wondered what would result if a pure tall plant was cross pollinated with pure short plant. Prediction?
The results were that all the F1 generation were tall.
2.
Mendel now wondered what happened to the short trait so he allowed the F1 generation (the tall plants) to self
pollinate. The resulting offspring (F2) were about 3/4 tall and 1/4 short. The short trait reappeared in the F2
generation.
1st Experiment http://science.nhmccd.edu/biol/monohybr/monhybr.html
Definitions
alleles - different copies of the same gene which control a certain trait. For example, there are two alleles which control
plant height in pea plants -> tall and short
hybrid/heterozygous - an individual which possesses two different alleles for a trait
pure/homozygous - an individual which possesses two similar alleles for a trait
From mathematical analysis of these results, Mendel proposed two theories to explain why this happened.
These theories are now known as laws.
Definitions http://science.nhmccd.edu/biol/monohybr/terms.html
Law of Dominance - when an individual is hybrid for a pair of different alleles, only the dominant allele is expressed.
For example, Mendel`s F1 generation were hybrids but the tall allele was expressed. It is the dominant trait and
is denoted using a capital letter. The trait which is not expressed is said to be recessive and is a small letter. Short is
the recessive trait. Thus, for plant height in pea plants:
“T” is tall and “t” is short.
Law of Segregation - The alleles of a gene occurs in pairs which are separated from each other during gamete formation
(meiosis) and recombine during fertilization.
The allele for the tall trait was separated from the allele for the short trait during meiosis. Remember that gametes have
half the genetic information of somatic cells. The correct number of alleles for each trait are reestablished at
fertilization.
Definitions
genotype - the genetic makeup of an individual organism. It is simply the pair of alleles for a certain trait. For example,
Mendel`s pea plants could be:
pure: TT or tt
hybrid: Tt
genotypic ratio - the ratio of the various genotypes in a generation of offspring
- ratios are usually written in the following order
“pure dominant: hybrid: pure recessive”
phenotype - the physical trait observed in an individual as a result of its genotype
TT
-> tall
Tt
-> tall
tt
-> short
phenotypic ratio - the ratio of the various phenotypes in a generation of offspring
- ratios are usually written “dominant: recessive”
Punnett Squares
Mendel`s Second Experiment
Mendel wondered what ratios would result from following more than just one trait at a time. In his first
experiment he observed only the trait of plant height. In his second experiment he observed pea shape and pea color at
the same time.
Once again he began with parents which were pure for both plants. He crossed a pure round, yellow pea plant
with a pure wrinkled, green pea plant. In the F1 generation, all the offspring were yellow and round. Thus, yellow and
round are the dominant traits.
He allowed the F1 plants to self pollinate and recorded the resulting offspring. The offspring had the following
ratio:
yellow & round
yellow & wrinkled
green & round
green & wrinkled
-> 9/16
-> 3/16
-> 3/16
-> 1/16
When you examine these numbers with the traits separate, you see that you still have the 3:1 dominant to
recessive ratio. For example, 12 yellow: 4 green or 12 round: 4 wrinkled. because of this, Mendel developed a third
law for genetics, the Law of Independent Assortment. This law states that different traits are inherited independently
from one another. Pea shape is independent of pea color.
2nd Experiment http://science.nhmccd.edu/biol/dihybrid/dihybrid.html
Independent Assortment Animation http://www.csuchico.edu/~jbell/Biol207/animations/assortment.html
Test Cross http://science.nhmccd.edu/biol/monohybr/test.html
Exceptions to Mendel’s Laws
1) Incomplete Dominance: traits in which both alleles contribute equally (blended) to the phenotype of hybrids
e.g. The Japanese four o’clock plant can be homozygous dominant (WW) which gives a white phenotype. It can also
be pure (RR) for a red flower. However, if these two plants were crossed, neither trait will dominate. The result is a
pink flower (R.W.) in which the traits are blended. Note that all traits are exhibited in capital letters.
2) Codominance: traits in which two dominant alleles are expressed at the same time. There is no blending of traits.
e.g. Shorthorn cattle can be pure red (CRCR) or pure white (CWCW). If two of these cattle are bred, a hybrid (CRCW)
results which has both completely red and completely white hairs. This type of coat is called roan. Note that all traits are
exhibited with capital letters and superscripts.
3) Multiple Alleles: traits which are controlled by more than two alleles. Human blood type (A, B, AB, O) is controlled
by three different types of alleles. The notation for multiple alleles is similar to that for codominance. Superscripts
must be used.
Phenotype
Corresponding Allele
A
IA
B
IB
O
i
Blood Type
Possible Genotypes
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
Blood Type Calculator http://www.biology.arizona.edu/human_bio/problem_sets/blood_types/btcalcA_popup.html