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GENETICS and HEREDITY
* Gregor Mendel and his pea plants experiments (1857-1865)
Genetics
wonderful review of
l genetics--right now
focus on the
questions What is
eredity? and What is
a trait?
learn some of th
history behind
Gregor Mendel
To download this
straight to your
computer...
...CLICK HERE!
genetics: the branch of biology that studies the transmission of hereditary
information from parents to offspring
WHY MENDEL? WHY PEAS?
Tim and Moby
discuss heredity
Mendel was a monk who was educated--he knew MATH! He also had a lot of free
time.
As for the peas? Well, they were:
* easy to grow
* easy to pollinate (either self or cross)
* easy to study their TRAITS (characteristics) such as: height, seed pod shape,
seed color, seed pod color, seed texture,flower position, and seed shape
Mendel kept very accurate records for hundreds of individual plants. He used
statistical analysis (MATH) to study the traits of different generations. Through his
findings, he proposed that the traits were passed on by some kind of hereditary
factors (now know to us as DNA).
Here's basically what he did:
One trait he studied was plant height. Mendel pollinated all tall plants for many
generations to get a pure population of tall plants. He also pollinated all short plants
to get a pure population of small plants. See picture below...
X means to cross
(as in cross-pollinate)
tall plants X tall plants as well as short plants X short plants
He then cross-pollinated a member of the tall pure population with a member of the
short pure population...
P1 (pure parent generation)
P1 TALL PLANT X P1 SHORT PLANT
ALL TALL PLANTS
F1 (first filial [family] generation)
Where did the short trait go? Mendel then crossed two tall members of the F1
generation.
F1 TALL PLANT X F1 TALL PLANT
THIS IS CALLED A MONOHYBRID CROSS (hybrids known for one trait)
F2 (second filial generation)
For every 3 TALL PLANTS there was 1 SHORT PLANT
So, the short trait did not disappear. It was present in the F1 generation, but IT
WAS NOT EXPRESSED (this means it did not show up!) Somehow, it was
expressed (visible) in the F2 generation.
I. Mendel's Principal of Dominance
* because the tall trait showed up more than short, Mendel call this trait DOMINANT
* the short trait, because it seemed 'weaker' than the tall trait, was called
RECESSIVE
After this discovery...well, um...NOBODY CARED! Later on, when scientists studies
meiosis in Drosophila (genus name for fruit flies), they linked together Mendel's
factors with the chromosomes in gametes.
* Mendel's factors are now called GENES
* genes are segments of chromosomes (DNA) that code for a characteristic
* these characteristics can be:
a) physical traits (can be seen like eye color, hair color, height, etc.)
b) chemicals produced in the body (like for hormones or enzymes)
genes are lined up on chromosomes in a certain order, like beads on a string
* homologous (similar) chromosomes have the same order of
genes...HOWEVER..
* these chromosomes might have the same forms of a gene
ex. trait=eye color can be blue, black, brown, hazel, green, etc.
* these different forms of a gene are called alleles
* in simple patterns of inheritance, there are 2 different forms of a gene (alleles),
where one is dominant and one is recessive
* DOMINANT IS SHOWN BY CAPITAL LETTERS
* recessive is shown by lowercase letters
ex. in Mendel's peas T = tall t = short
* diploid (having 2 sets of chromosomes) organisms have 2 copies of genes, one on
each chromosome they have
* the 2 copies can be the same--HOMOZYGOUS, like TT (homozygous dominant)
or tt (homozygous recessive)
*the 2 copies can be different--HETEROZYGOUS Tt (heterozygous)
Notice that what the organism looks like and the kind of genes it has are two
separate ways to classify an organism.
phenotype: what the appearance of an organism is (how it looks) ph = physical
genotype: what the genetic makeup of an organism is (what genes it has) gen =
genes
genotype can be:
homozygous dominant----------> TT
heterozygous (a hybrid--a mix)---> Tt
homozygous recessive----------> tt
recessive phenotypes can only be expressed (show up) when there are 2 copies of
a recessive gene present at the same time...if a dominant gene was there, then that
would be expressed, hiding the recessive gene!
II. Mendel's Principle of Segregation and
Recombination
Genetics
* when gametes are formed during meiosis (DIPLOID to
MONOPLOID) the homologous chromosomes (which contain the
many different genes) separate randomly
* this is called SEGREGATION
* if the organism has a homozygous (the same) genotype for a
given trait (TT or tt), then all gametes will also have that trait
TT <----diploid----> tt
meiosis
T or T <----monoploid----> t or t
a wonderful review of all genetics--right now focus
on the questions What is heredity? and What is a
trait?
To download this straight to your computer...
...CLICK HERE!
* if the organism has a heterozygous (not the same) genotype for a
given trait (Tt), then half of the gametes will have one trait, and the
other half will have the other trait
Tt <----diploid
meiosis
T or t <----monoploid
* when the monoploid gametes fuse during fertilization, the diploid
number of chromosomes is restored
* this can result in a new combination of genes
* this is called RECOMBINATION
Let's look back at Mendel's first few pea plant crosses.
P1 (pure parent generation) cross was: TT X tt
Crossing terminology.
F1 (first filial generation) cross was Tt X Tt
F2 (second filial generation) offspring had genotypes of TT, Tt, tT,
and tt
How to determine ratios from the crosses..
Now, compare the different phenotypes (physical appearance) and
genotypes (genetic makeup).
For the F2 generation what is the PHENOTYPIC RATIO?
3 tall : 1 short
For the F2 generation, what is the GENOTYPIC RATIO?
1 homozygous dominant : 2 heterozygous : 1 homozygous
recessive
Tim and Moby Review Punnett squares.
PHENOTYPE DOES NOT ALWAYS EQUAL GENOTYPE!
* to show segregation (separation of genes/chromosomes) and
recombination (during fertilization), we use Punnett Squares
Practice using Punnett squares.
Punnett Square Method
Follow my steps to carry out a Punnett square
problem...(unfortunately the steps are shown in detail
in class. These notes will make more sense if you
weren't absent. If you were, set up a time after school
to learn it!)
Problem: A homozygous tall pea plant is crossed
with a short pea plant. (Remember, tall is dominant
over short for pea plants!) What would the expected
genotypes (the genotypic ratio) and phenotypes (the
phenotypic ratio) of their offspring?
a wonderful review of all genetics--right now focus on the
questions What is heredity? and What is a trait?
STEP 1: set up the key that will show how each allele
(form of a gene) will be shown
T = tall
t = short
To download this straight to your computer...
...CLICK HERE!
STEP 2: set up the parents that will be crossed; use
the information from the word problem itself
TT x tt
a tall homozygous parent a short parent
remember, to be short, it has to have both short alleles!
STEP 3: do the Punnett square; that is, put one parent
on the top and the other parent on the left of the
square...then from a blank box, take the allele on the
top and the allele from the left and fill them in
STEP 4: show the results (ratios) of the cross
GENOTYPIC RATIO:
PHENOTYPIC RATIO:
TRY THESE PUNNETT SQUARE PROBLEMS
1) Use a Punnett square to show the offspring of a
cross between two pea plants that are heterozygous
for height (Tt). Give the phenotype and genotypes of
the offspring. (That is, what is the phenotypic and
genotypic ratios!)
2) Use a Punnett square to show the offspring of a
cross between a pea plant that is homozygous tall and
one that is heterozygous. Give the phenotypes and
genotypes of the offspring.
3) Use a Punnett square to show the offspring of a
cross between a pea plant that is heterozygous tall and
one that is homozygous recessive for height. Give the
phenotypes and genotypes of the offspring.
A) Incomplete Dominance:
* when one allele (form of a gene) is partially dominant over the other
* it is a blending of the alleles
ex. Four O'clock Flowers (key below)
R = red
W = white (can also use R')
RW or WR= pink can also be RR' or R'R)
a
More practice problems
4) Use a Punnett square to show the offspring of a cross between a red Four
O'clock flower and one that is heterozygous. Give the phenotypes and genotypes of
the offspring.
5) Use a Punnett square to show the offspring of a cross between a homozygous
white Four O'clock flower and one that is heterozygous pink. Give the phenotypes
and genotypes of the offspring.
B) Codominance:
* this time, both the alleles are expressed, but they are not blended together
* in humans, this is like hair texture and blood groups
ex. cattle coat color (key below)
CR = red
CW = white
CWCR or CRCW = roan (where both red and white are expressed
More practice problems
6) Use a Punnett square to show the offspring of a cross between a homozygous
white fur cattle and one that is roan. Give the phenotypes and genotypes of the
offspring.
C) Multiple Alleles:
* when there are more than 2 alleles for a gene
* in human blood types there 3 possible alleles for the ABO blood typing system
IA = A antigen
IB = B antigen
i = no antigen
a wonderful review of all genetics-right now focus on the questions
What is heredity? and What is a
trait?
To download this straight to your
computer...
...CLICK HERE!
" both IA and IB are dominant over i but,
" both IA and IB are codominant with each other (that means, they are both
expressed equally)
More practice problems
7) Four newborn babies in the delivery room of the hospital at the same time were
mixed up by the nurse who attached the wristbands. The blood types of the four
babies were known to be AB, O, A, and B. How did the doctors find out which baby
belonged to which set of parents? Carry out all possible crosses to determine which
baby belongs to which set of parents. (HINT: You may not have to use all the
Punnett squares!)
Parents #1 had blood types O and AB
Blood type of baby: _______
Parents #2 had blood types AB and B
Blood type of baby: _______
Parents #3 had blood types O and O
Blood type of baby: _______
Parents #4 had blood types O and A
Blood type of baby: _______
8) If a heterozygous A person and a homozygous B person have children, can they
have an A type baby?
D) Sex-Linked Genes:
* most diploid organisms have 2 different types of chromosomes
1) autosomes- contains genes on regular 'body chromosomes'
2) sex chromosomes- contains genes for sex differences (male-female)
* humans have 22 pairs of autosomes and 1 pair of sex-chromosomes
XX = female XY = male
* women (XX) always pass an X chromosome; men can pass an X or a Y (a 50%-50% chance)
* this is determined during the random selection process during the production of gametes
(meiosis)
*besides the sex characteristics, there are also other traits on these chromosomes
ex. hemophilia
color blindness
* Thomas Hunt Morgan (the fruit fly guy) noticed that some traits are inherited more frequently in
males than females
* these were found to be recessive alleles found on the X chromosome
* because the Y chromosome is actually smaller than the X chromosome, men only have one cop
of certain genes...
* so...if the X chromosome has a recessive allele, that will be expressed because that is all that is
there! (key below)
X = normal X chromosome
X* = carrier X chromosome
Y = normal Y chromosome
More practice problems
9) Use a Punnett square to show the offspring of a cross between a woman with normal vision and
a color blind male. Give the phenotypes and genotypes of the offspring including the sex of the
children. Can any of them be color-blind?
10) Use a Punnett square to show the offspring of a cross between woman who carries the
hemophilia gene and a man who is a hemophiliac. Give the phenotypes and genotypes of the
offspring. Can any of their daughters be a hemophiliac?