Mendelian Genetics

1
Mendelian Genetics
2
About Gregor Mendel
Gregor Mendel was an Austrian monk who studied
science and math in the mid-19th century
(1850s) -- Mendel was also a priest and a high
school teacher at the monastery One of the main
jobs of Gregor Mendel was to tend the garden at
the monastery, and one of Mendels favorite
plants to grow was the garden pea -- Mendel
cultivated several varieties of garden
pea Through his work over many years with pea
plants, Mendel stumbled upon the secret of
inheritence -- thus, Mendel is considered the
father of modern genetics
3
About Pea Plants
Pea plants are special because they can
self-pollinate -- in other words, the pollen
(male sex cells formed in plant meiosis) and the
ovary (female sex cells formed in plant meiosis)
from the same plant join together to form plant
offspring -- therefore, all of the offspring of
the plant have the same DNA and therefore the
same genes and traits as the parent -- pea
plants that self-pollinate are called true
breeding Pea plants can also pollinate ovaries of
other pea plants through a process called cross
pollination -- pea plants that cross-pollinate
are called cross breeding hybrids (or hybrids)
4
What Mendel Did With Peas
Mendel looked at 7 specific traits of his pea
plants. Each trait came in two possible
varieties -- Seed Shape (either round or
wrinkled) -- Seed Color (either yellow or
green) -- Seed Coat Color (either gray or
white) -- Pod Shape (either smooth or
constricted) -- Pod Color (either yellow or
green) -- Flower Position (either axial or
terminal) -- Plant height (either tall or short)
5
What Mendel Did (contd)
Mendel took his true-breeding pea plants and did
what is called selective cross-breeding -- in
other words, he intentionally pollinated the
flowers of a plant with one variety of a trait
with the pollen of a plant with the other variety
for a trait -- to ensure that the pea plants
didnt self-pollinate, he removed the stamens
(pollen producers) from the plant receiving the
pollen What was Mendels hypothesis? Mendel
believe that the offspring would show traits that
were a combination of the traits of the parents.
6
Mendels Results
The results of Mendels experiment were quite
surprising! Each of the offspring (F1 generation)
had only one variety of the trait shown in the
parent generation (P generation) For example,
when looking at seed shape, in a cross between a
round seed parent plant and a smooth seed parent
plant, all of the F1 plants had round
seeds! Mendel called the variety of the trait
that appeared in the F1 generation dominant
7
The Results of Mendels Experiment
Parent Generation
F1 Generation Gray Seed Coat Only All axial
flowers
Seed Coat Color White Gray
All Yellow Seeds
All Round Seeds
All Smooth Seeds
All Green Pods
All Tall Plants
8
Mendels Second Cross
If the results of Mendels first experiment were
surprising, the results of his second experiment
were even more surprising. . . For his 2nd
experiment, Mendel let the plants of the F1
generation self-pollinate to create an F2
generation Mendels Hypothesis Mendel assumed
that all of the plants of the F2 generation would
have the exact same traits as the F1 parent The
Result Most of the F2 plants had the same
variety of the trait as the parent, however,
about ¼ of the F2 plants showed the other variety
of the trait, that was missing in the F1
generation, but present in the parent generation
9
Tall vs. Short Plants An Example of Mendels
Crosses
P (parent) generation Tall x Short
F1 (1st filial) generation All tall plants
F2 (2nd filial) generation ¾ tall ¼ short
10
In Terms of Modern Genetics. . .What Really
Happened?
In modern genetics, we call all the varieties of
a trait alleles Mendel happened to test plants
with two different alleles -- for example, for
the trait of plant height, there were two
alleles, T, for tall plants, and t, for short
plants we give both alleles for the same
trait the same letter. The dominant allele is
capitalized and the recessive allele is lower
case What you need to remember is that cells
that express traits are diploid in chromosome
number. . .and that each chromosome in a pair
contains genes for the same trait.
11
Why is Diploid Important?
Even though both homologous chromosomes contain
genes for the same trait, each chromosome may
have a different allele for the trait Lets look
at the cross of tall and short plants. . . In the
parent generation, each plant was true breeding,
meaning both copies of the chromosome coding for
plant height had the same allele for the trait of
plant height -- the tall plant had two tall
alleles, T and T -- the short plant had two
short alleles, t and t When the plants were cross
pollinated, the offspring got one chromosome, and
therefore one allele from each parent
12
The F1 Generation of Pea Plants
Since all of the F1 generation got one allele
from each parent, each plant in the F1 generation
got one tall allele, T, and one short allele,
t -- all the plants were Tt, however, since T
(tall) is dominant to t (short), all the plants
appeared tall When the F1 plants were allowed to
self-pollinate, however, the cross could have
gone in several different ways -- the pollen
either gave a T allele or a t allele -- the
ovary either gave a T allele or a t allele
Remember that through meiosis, the sex cell
contains only one of the two chromosomes, which
one it contains is completely random
13
The F2 Generation
The F2 generation could have several possible
combinations of alleles from the Tt x Tt cross. .
. If the pollen gave If the ovary gave The F2
plant would be T T TT (tall) T t Tt
(tall) t T tT (tall) t t tt
(short) Therefore, ¾ of the offspring should be
tall and ¼ of the offspring should be short.
Exactly what Mendel figured out!
14
Genetics Vocabulary
Trait a characteristic expressed by an
organism -- in other words, your height is a
trait Allele the variety of gene that you have
that codes for a trait -- pea plants can either
be tall or short, tall and short are alleles for
the trait of height Phenotype the actual trait
that is expressed, what we see -- we either see
the pea plant to be tall or short. Tall or short
is the phenotype of the pea plant Genotype the
combination of alleles that an organism has for a
trait -- for example, pea plants can either be
TT, Tt, or tt
15
More Genetics Vocabulary
Dominant the allele that is expressed when both
alleles are present -- in peas, tall is dominant
to short Recessive the allele that is not
expressed when both alleles are present -- in
peas, short is recessive Homozygous when an
organism has two of the same alleles for a
trait -- if a pea plant is TT, it is homozygous
dominant -- if a pea plant is tt, it is
homozygous recessive Heterozygous when an
organism has two different alleles for the same
trait -- all Tt pea plants are heterozygous
16
Predicting the Offspring
Now that we know how alleles are passed onto the
next generation, we can predict what future
generations will look like For example, when a
pea plant with yellow seeds is crossed with a pea
plant with green seeds, what will the offspring
look like? -- as long as we know the genotypes
of the parents, and which trait is dominant and
which is recessive, we can figure out the
genotypes and phenotypes of offspring. . . -- we
use a tool called a Punnett Square to figure out
the traits of the offspring
17
The Punnett Square
A Punnett Square is a box made of four squares
(at least simple ones have four squares). The
alleles of one parent are written across the top
and the alleles of the other parent are written
across the side, so that one allele sits across
each row and above each column. The combination
of alleles that result are the possible results
for the offspring of that cross
Female Parents alleles
Male Parents alleles
18
Punnett Square Practice Problems
In pea plants, the allele for tall plants is
dominant to the allele for short plants. If a
homozygous dominant parent is crossed with a
heterozygous pea plant, what would be the
genotypic and phenotypic ratios of the offspring?
Genotypic Ratio Phenotypic Ratio
19
Punnett Square Practice Problems
In pea plants, the allele for yellow seeds is
dominant to the allele for green seeds. If a
heterozygous yellow pea plant is crossed with a
green pea plant, what are the genotypic and
phenotypic ratios of the offspring?
Genotypic Ratio Phenotypic Ratio