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11.1 The Work of Gregor Mendel
Genetics = the study of heredity (passing down of
characteristics from parent to offspring)
Gregor Mendel = “the father of genetics”
Born in 1822 – Austrian monk
Worked with pea plants that were self-pollinating and
true-breeding (the offspring always looked like the
parent)
Mendel’s Pea Plants
Mendel cross pollinated his true-breeding plants
Peas
The original pair of plants is called the __ (parental
generation)
The offspring are called the __ (first filial
generation)
________= offspring of crosses between parents with
different characteristics
P
F1
Hybrids
Important Genetic Terms
Trait = a specific characteristic (pea color, hair color)
Gene = the factors that are passed from parent to
offspring
Allele = the different forms of a gene
Height in Peas
Mendel’s Conclusions
An individual’s characteristics are determined by
factors (genes) that are passed from one parental
generation to the next
Principle of dominance = some alleles are
dominant and some are recessive
Dominant = need one allele (form of the gene) for the
trait to be expressed
Recessive = need two alleles for the trait to be
expressed
Segregation of alleles
Gametes = reproductive cells (sperm, egg, pollen, ovule)
During the formation of gametes, the alleles for the trait separate from each other
Each gamete gets 1 allele (copy of the gene)
When fertilization occurs – the plant gets one allele from each parent (2 total)
11.2 Applying Mendel’s Principles
Mendelian genetics is based on probability = the
likelihood that an event would occur
Genetics and probability
Dominant alleles are written in upper case T = tall
Recessive alleles are written in lower case t = short
In this example:
There is a 50% chance that the plant the offspring will get a “T” allele
There is a 50% chance the plant will get a “t” allele
Even more genetic terminology
Genotype = the genetic
makeup of an organism
Homozygous = organisms that
have two identical alleles for a
gene (BB or bb)
Heterozygous = organisms that
have two different alleles for a
gene (Bb)
Phenotype = the physical
appearance of an organism
Oh no! You need to think!!!!
For each example, write the genotype and
phenotype.
1) The Rr flower
Genotype ____________
Phenotype____________
2) The rr flower
Genotype ____________
Phenotype ____________
Rr
Purple
rr
white
Punnett Squares
Punnett squares = a diagram that uses probability
to predict the possible genotype and phenotype
combination in crosses
T = tall
t = small
(choose a letter from the
dominant allele)
Monohybrid cross
In peas, yellow seeds are dominant to green. Complete the following cross Yy x yy
1) Make a key – yellow = ____
green = ____
2) Parental genotypes – if not given
yy x Yy
3) Set up the Punnett square
Y y
y
y
Yy yy
Yy yy4) Figure out the phenotypic and
genotypic ratio
Phenotypic ratio - _______________________
Genotypic ratio - _______________________
1 yellow : 1 green
1 Yy : 1 yy
Monohybrid cross
In peas, yellow seeds are dominant to green. Complete the following cross Yy x Yy
1) Make a key – yellow = ____
green = ____
2) Parental genotypes – if not given
Yy x Yy
3) Set up the Punnett square
Y y
Y
y
YY Yy
Yy yy4) Figure out the phenotypic and
genotypic ratio
Phenotypic ratio - _______________________
Genotypic ratio - _______________________
3 yellow : 1 green
1 YY : 2Yy : 1 yy
Gene
Trait
Allele
Gamete
Dominant allele Recessive allele
Phenotype
Genotype
HomozygousHeterozygous
Recall: Monohybrid cross
In peas, yellow seeds are dominant to green. Complete the following cross Yy x Yy
1) Make a key – yellow = ____
green = ____
2) Parental genotypes – if not given
Yy x Yy
3) Set up the Punnett square
Y y
Y
y
YY Yy
Yy yy4) Figure out the phenotypic and
genotypic ratio
Phenotypic ratio - _______________________
Genotypic ratio - _______________________
3 yellow : 1 green
1 YY : 2Yy : 1 yy
Dihybrid cross
When there are 2 traits it is a dihybrid cross.
Genes for different traits can segregate
independently during the formation of gametes
Dihybrid cross
EXAMPLE PROBLEM
Cross two plants that are heterozygous for height
and pod color. Tall is dominant to short and green
pods are dominant to yellow
Step 1 – Make a key and determine the parents
Tall = Green =
Short = Yellow =
Step 2 – Write the genotypes of the parents
T
t
G
g
TtGg x TtGg
Dihybrid cross
Step 3 – Determine the
possible allele
combinations for the
gametes
Step 4 – Set up the
16-square Punnett
square
Dihybrid cross example
Step 5 – Complete the Punnett square
Step 6 – Determine the phenotypic ratio
9 tall green: 3 tall yellow: 3 short green: 1 short yellow
11.3 Exceptions to Mendel’s rules
Incomplete Dominance
Codominance
Multiple Alleles
Polygenic Traits
Incomplete Dominance
Incomplete dominance = one allele is not completely dominant over another
Phenotype is a combination of the two alleles
EXAMPLE: Four o’clock flowers
R = Red
W = White
What are the genotypes of the following?
Red _____ White ______ Pink______
What are the phenotypes of the following?
RR _________ RW________ WW_______
RR
Red WhitePink
RWWW
Cross a white flower with a red flower
Codominance
Codominance = both alleles are seen in phenotype
The phenotype shows each allele NOT a combination
Example – Some varieties of chickens
W = White
B = Black
WW = White BB = Black BW = Black AND White
What is the phenotypic ratio when you cross two BW
chickens?????
1 Black : 2 Black and White : 1 White
Multiple Alleles
Multiple alleles = there are more than 2 alleles for
a trait
Example – rabbit's fur color, human blood types
Disorders caused by individual genes –
codominant and multiple alleles
ABO Blood Types
A (IA) and B (IB) are codominant
O (i) is recessive
Polygenic Traits
Polygenic trait = traits produced by more than one
gene
Examples – human skin color and height
Genes and the Environment
Genes provide a plan for development, but
environment also plays a role in phenotype
11.4 Meiosis
Meiosis = the process in which the number of
chromosomes per cell is cut in half
Occurs through separation of homologous chromosomes
(matching chromosomes from a female and male parent)
Creates gametes (sex cells – sperm, eggs, pollen, etc.)
Diploid= a cell that contains both sets of
homologous chromosomes (2N)
Haploid = a cell that contains
a single set of chromosomes (N)
Meiosis meiosis
Meiosis has two divisions (before meiosis 1 the cell is in interphase and replicates the chromosomes)
Meiosis I
Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
Meiosis 2
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Meiosis 1
Prophase1
Each chromosome matches with its homologous
chromosome (forms a tetrad)
Crossing over occurs.
(chromatids crossover and exchange ends)
Meiosis 1 cont.
Metaphase 1
Homologous chromosomes line up
in the center of the cell
Anaphase 1
Homologous chromosomes are
pulled toward opposite ends of
the cell by spindle fibers
Telophase 1
Nuclear membrane forms around
each nucleus
Cytokinesis follows
The end of Meiosis 1
At the end of meiosis 1 there are two daughter cells
Each has 1 set of chromosomes (haploid)
Chromosomes do not replicate before Meiosis II
Meiosis II
Prophase II
Chromosomes become visible
Metaphase II
Chromosomes line up at the
center of the cell
Anaphase II
Chromatids separate
Telophase II
The nuclear membrane reforms
The result of Meiosis
The result of meiosis is 4 haploid (N) daughter cells
In our example each cells has 2 chromosomes
(1/2 of the starting number)
ORIGINAL CELL 4 DAUGHTER CELLS
(4 chromosomes) (2 chromosomes each)
Comparing Mitosis and Meiosis
End of
Meiosis I
End of
Meiosis II
Mitosis ends with 2 genetically
identical diploid daughter cells
Meiosis ends with 4 genetically
different haploid cells
Gene linkage
Alleles of different genes tend to be inherited together when those genes are located on the same chromosome (linked)
Chromosomes assort independently
Gene maps = location of genes on a chromosome
Crossovers between genes that are close are rare
More crossing occurs with genes that are farther apart
Researchers looked at data
to determine location of genes
Gene linkage cont.
Life Cycle of the Fruit Fly
Fruit flies can live up to eight weeks under optimal conditions.
There are four distinct
stages include:
1. Egg
2. Larva
3. Pupa
4. Adult
Why use Fruit Flies for genetic study
There are 4 advantages for using fruit
flies as model organisms for genetic
testing.
1. Flies have a relatively short lifespan
2. Easy to anesthetize
(put to sleep for close examination)
3. Can be taken care of and
manipulated easily
4. Males and females are easy to tell
apart from one another
“P” Parental Generation
A female Drosophila can store
and use the sperm from a
single insemination for the
major portion of her adult life
when she is reproductively
active.
Its important to have virgin
females for the experiment
because…
Anesthetizing the Flies
In order to count the flies from one
generation to the next we will
anesthetize flies with “Fly nap”
(putting the flies to sleep for a short
period of time)
Chromosomes and Genetic Inheritance
Fruit flies have 4 chromosomes
Many genes are inherited
together because of their close
proximity to one another on
their respective chromosomes.
Sexing Flies
Male fruit flies have a much more rounded abdomen
than females
The posterior part of male flies is darker than females
Females have more sternties (hairs under their abdomen)
Fruit Fly Generations
P = parental
generation
F1 = first filial
generation
F2 = second filial
generation
Homeotic Genes and Body Patternsadapted from http://learn.genetics.utah.edu/content/basics/hoxgenes/
Every organism has a unique body pattern. Although
specialized body structures, such as arms and legs, may be
similar in makeup (both are made of muscle and bone),
their shapes and details are different. While an embryo
grows, arms and legs develop differently due to the actions
of homeotic genes, which specify how structures develop
in different segments of the body.
1. What are homeotic genes and when do these
genes get activated in an organism?
How did scientists discover genes that
determine body pattern?
Scientists discovered homeotic genes by
studying strange transformations in fruit flies,
including flies that had feet in place of mouth
parts, extra pairs of wings, or two pairs of
balance organs (called halteres) instead of
wings. Some even had legs growing out of
their heads in place of antennae!
Scientists called these modifications
"homeotic transformations," because one
body part seemed to have been replaced by
another. Researchers, including a group
headed by Ed Lewis at Caltech, discovered
that many of these transformations were
caused by defects in single genes, which they
termed homeotic, or Hox, genes.
(a) Fruit fly with legs as antenna
How did scientists discover genes that
determine body pattern?
This work demonstrated that
antennal cells carry all of the
information necessary to
become leg cells. This is a
general principle: every cell
in an organism carries, within
its DNA, all of the
information necessary to
build the entire organism.
Top: (Left) Normal fruitfly; (Right) Fruitfly with
mutation in antennapedia gene Bottom: (Left)
Normal fruitfly; (Right) Fruitfly with a homeotic
mutation that gives it two thoraxes. Bottom
images courtesy of the Archives, California
Institute of Technology.
Shared characteristics
Fruit flies begin life as worm-like creatures made
up of repeating units, or segments. Early in
development, Hox genes are switched on in
different segments. Patterns of Hox gene activity
give each segment an identity, telling it where it is
in the body and what structures it should grow.
For instance, genes that are active in the head
direct the growth of mouth parts and antennae,
while genes that are active in the thorax direct the
growth of legs and wings.
Changes to Hox gene expression change a
segment’s identity. For example the first segment
of the thorax normally grows legs, the second
grows legs and wings, and the third grows legs
and halteres. When the Hox gene activity in the
third segment is made the same as that in the
second, both segments grow legs and wings (see
Shared characteristics
While studying the DNA sequences of homeotic
genes in fruit flies, researchers found that they all
shared a similar stretch of about 180 bases; they
named this stretch the homeobox. The homeobox
is just a portion of each gene. If the words below
were homeotic genes, the capital letters would
represent the homeobox: togeTHEr - THEoretical -
gaTHEring – boTHEr
Researchers used DNA-sequence similarity to find
genes with homeoboxes in other species, including
other insects, worms, and even mammals.
Together, these genes make up the Hox gene
family (Hox is short for homeobox).
Interestingly, Hox genes are arranged in clusters.
Typically, their order on the chromosome is the
same as the order in which they appear along the
body. In other words, the genes on the left control
2. Where did scientists discover homeotic genes and what
kind of observations did they make?
3. How many genes were involved in these mutations?
4. What did the work of researches like Ed Lewis at Caltech
that led to the discovery of Hox genes reveal about all cells
in an organism?
5. What body part helps flies balance themselves in flight?
Do you think an extra pair of wings would be a helpful
mutation for the fly? Explain.
How did scientists discover genes that
determine body pattern?
Shared characteristics
6. What do the segments in fruit fly larvae correspond to in
their adult form?
7. What is the homoebox and how is it related to Hox
genes?
8. Hox genes are found in clusters along the chromosome.
What did this arrangement reveal about Hox gene
placement on the chromosome and body patterns?
Name ____________________ Period ____ Date _____________
Probability and Genetics
Procedure:1. In pea plants, yellow seeds (A) are dominant over green seeds (a). Using a Punnett square, determine the probable
color of the seeds produced by pea plants whose parents are both heterozygous for the seed-color trait.2. Record the expected genotypic and phenotypic ratios in the appropriate places in the data table.3. Using a penny to represent each of the parents, with heads = (A) and tails = (a), place the 2 pennies into the cup.
With your hand over the top, shake the cup and empty the coins onto the table. On a piece of scratch paper, tally the results for 10 tosses and record them on the data table.
4. Now toss the coins 100 times and record your results on the data table and on the chart on the board.5. From class results, record the results for 1000 tosses on the data table.6. Determine the total number of seeds with the yellow phenotype for each series of tosses. Record on data table. 7. Using the data, calculate the genotypic and phenotypic ratios for each series of tosses. This is done by dividing
each number in the ratio by the ratio's smallest number and rounding off to the nearest tenth's place.Example: Suppose for 100 tosses you have 23 AA, 51 Aa and 26 aa. The smallest number is 23, so you divide each number by 23 and round to the nearest tenth. (23/23 = 1, 51/23 = 2.2, 26/23 = 1.1) This gives you a ratio of 1:2.2:1.1
Phenotype Yellow Green Total number of yellow seeds
(AA + Aa)
Genotypic Ratio Phenotypic Ratio
Genotype AA Aa aa Expected Experimental Expected Experime
ntal
10 tosses
100 tosses
1000 tosses
Genotype Phenotype Genetic Name
AA yellow Homozygous dominant____
___________
______________________________
