Name: __________________________
Per. __________
Unit 8- Meiosis & Mendelian Genetics
Although the resemblance between generations of organisms had been noted for thousands of
years, it wasn’t until the 1800s that scientific studies were carried out to develop an explanation for
this. Today we know that we resemble our parents because of heredity, which is the set of
characteristics we receive from our parents. The study of heredity is known as genetics.
I.
SEXUAL REPRODUCTION & MEIOSIS (270-276)
 In sexual reproduction, an egg and sperm cell fuse together to create a fertilized egg or
zygote.
 Egg and sperm cells are known as gametes.
 Gametes are created through a special process of cell division called meiosis which halves
the chromosome number. Meiosis only occurs in the ovaries of females and the testes of
males
A. Chromosome Number
1. Somatic Cells: (body cells)
 Human Somatic cells contain 46 chromosomes and
are diploid or 2n because they have a double set of
chromosomes. 23 from mom and 23 from dad.
 These “matching” chromosomes are known as
homologous pairs
 Homologous pairs- same pair of chromosomesone from each parent, with the same type of
characteristics or genes.
2.
Gametes: (sex cells)
 Egg & Sperm- contain 23 chromosomes and are haploid or n.
 These contain ½ the total number of chromosomes or single set of
chromosomes.
 Fertilization – when gametes fuse together- egg + sperm = zygote (n + n= 2n)
B. Formation of Gametes
 Formed in Meiosis
 Occurs in ovaries of females and testes of males
 In Meiosis, DNA is replicated once in S of interphase, but the cell goes through 2 divisions,
resulting in 4 daughter cells with ½ the original chromosome number.
Genetics is the Study of
_________________
What type of cells
undergoes Meiosis?
What type of cells
undergoes Mitosis?
What is a gamete? Give
examples.
What is a somatic cell?
____________________
Give an example:
____________________
If 2n= 800, than
n=________
What are homologues
pairs of chromosomes?
Why would you want a
gamete to only have ½
the # of chromosomes?
Meiosis occurs in 2 stages:
1. Meiosis 1-





Before meiosis I - DNA is replicated in
S of interphase. End up with
chromosomes, each consisting of 2
sister chromatids.
Prophase I- pairing of homologous
chromosomes & Crossing Overproduces exchange of genetic info (so siblings look different from each other)
Metaphase I- Sister chromatids line up in the middle of the cell in homologous pairs
Anaphase I & Telophase I- the homologous pairs separate, but the sister
chromatids are still connected at the centromere
Two cells are formed with 23 pairs of Sister Chromatids = 23 chromosomes, but
there are no longer any homologous pairs so the cells are haploid or n.
How many times does a
cell divide during
meiosis?
2. Meiosis II The two cells Do NOT go through interphase (DNA does NOT replicate again)!!!
 Prophase II- Chromosomes condense (again) & spendles form
 Metaphase II- Chromosomes align in the middle
 Anaphase II- Sister chromatids are pulled apart
 Telophase II- Four nuclei form around chromosomes
 4 new cells are formed, each with ½ the original number of chromosomes as the
original.
C. Crossing Over:
 Occurs during prophase I when homologous
pairs of chromosomes come together.
 Part of one sister chromatid breaks off and
exchanges with the corresponding portion of
a sister chromatid of the homologous
chromosome.
 Very common and Increases the genetic
variability in offspring.
Draw, Color, Label & Describe the Phases of Meiosis: (pg. 273)
What is the end result of
Meiosis?
What would happen if
the sister chromatids of
one pair did not
separate during
anaphase II?
What is crossing over?
What is a benefit to
crossing over?
II.


III.
Meiosis in Men and Women:
During Spermatogenesis (the production of
sperm), Men will produce 4 sperm cells for
each somatic cell.
During Oogenesis (the production of ovum or
egg cell), the woman will produce 1 egg and 3
polar bodies (non-viable cells that get
“flushed” out) for each somatic cell
Comparison of Mitosis & Meiosis (pg. 275)
Mitosis
Meiosis
Definition:
Definition:
Cells where process occurs:
Cells where process occurs:
Number of DNA Replications:
Number of DNA Replications:
Number of Cell divisions:
Number of Cell divisions:
Number of Resulting Daughter cells:
Number of Resulting Daughter cells:
Chromosome number in Original Cell:
Chromosome number in Original Cell:
Chromosome number in Daughter cell:
Chromosome number in Daughter cell:
Purpose for cell division:
Purpose for cell division:
Illustration: Label each cell as n or 2n and describe each
step using pg. 275 in book:
Illustration: Label each cell as n or 2n and describe each
step using pg. 275 in book:
IV. History of Genetics (pg. 277-285)
A. Gregor Mendel “Father of Genetics”
 Did experiments on pea plants
 Studied 7 traits: plant height, seed color,
follower color ect..
 Used true-breeding (purebred, prueline)
which means the characteristics always
show.
 Generation of true-breeding plants is known
as P generation.
 Trait- an inherited characteristic
 Cross-pollinated – pollen from one plant
fertilizes an egg from another.
 Self-pollinate- pollen can fertilize egg from
same plant.
 Mendel did experiments by controlling cross-pollination and preventing selfpollination.
B. Mendel’s Results:
1. P generation:- Cross a purebred with one trait with a purebred with another trait.
Example: Tall plant x short plant
2. F1 generation- offspring produced from P x P. In F1, one trait disappears
Example: Tall plant x short plant = all tall plants (Tt)
3. F2 generation- Offspring produced from F1 x F1. In F2 the trait that disappeared in F1
comes back in ¼ of the offspring; the other ¾ shows the trait seen in F1
C. Mendel’s Principles1. Law of Dominance For each gene, an organism inherits two alleles, one from each parent. Alleles are
different forms or possibilities of a gene.
 The dominant trait is the trait shown and is represented by a capital letter. T=
tall
 The recessive trait is the trait that is hidden and is represented by a lower case
letter, t = short.
 Homozygous= same TT = tall or tt= short
 Homozygous Dominant- TT = Tall or RR= Round or BB= Brown
 Homozygous Recessive- tt= short or rr= wrinkled or bb = blond
 Heterozygous= (hybrid) different Tt the plant will be tall, but they carry the
recessive trait
2. Genotype and Phenotype Phenotype- the physical description of the trait. Ex: tall, short, brown, blond
 Genotype- The genetic make-up of the organism’s alleles. Ex: TT, Tt, tt
Why is Mendel known as
the Father of Genetics?
What does P generation
mean? ________________
Explain what true-breeding
means:
If you cross a true-breeding
plant with yellow flowers
with a true-breeding plant
that also has yellow flowers,
what are you going to get?
If you cross a golden
retriever with a golden
retriever what are you going
to get?
P=
F1 =
F2 =
Pure Tall= TT
Pure short = tt
Cross a TT x tt
What do you get?
Cross the offspring from the
problem above:
Genotype: ____________
(Homo Dom : Hetero: Homo rec)
Phenotype: ___________
(Dom : Rec)
3. Mendel’s law of segregation In meiosis the two alleles for a trait segregate (separate). Each egg or sperm
receives a copy of one of the two alleles. There is a 50% chance that a copy of
that allele will end up in the gamete.
 During fertilization the two alleles (one from mom and one from dad) will unite.
4. Mendel’s Law of Independent assortment Random distribution of alleles occur during gamete formation
 Genes on separate chromosomes sort independently during meiosis.
B=Black, b= grey
Cross a Homozygous
black rabbit with a
heterozygous rabbit.
Genotype: _________
Phenotype:_________
If mom has the alleles
Tt and dad has the
alleles tt what alleles
can the offspring get?
Explain the law of
dominance in your own
words:
Explain the law of
Segregation in your
own words:
Explain Independent
assortment in your own
words:
G= green, g= yellow
Write the alleles for:
Homozygous DomHomozygous recHeterozygous-
V.
Analyzing Inheritance
A. Probability
 Due to the law of segregation if you know the genotype of the parents, you can
predict the outcome of the offspring.
 Probability can be written 3 ways: The probability of a coin coming up heads:
1. Fraction: ½
2. Ratio: 1:2
3. Percent: 50%
B. Punnett Squares
 Used to predict the possible outcomes of meiosis and fertilization.
 Predict the probability of certain traits appearing in offspring
VI.
Punnett Practice:
A. Monohybrid Cross- (Cross one trait at a time)
1. Construct a Punnett square to determine the probability of white flowers if a
heterozygous purple (Pp) flower is crossed with a homozygous white (pp)
flower.
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
2.
Construct a Punnett square to determine the probability of short pea plants if
a homozygous tall (TT)plant is crossed with a heterozygous tall (Tt) plant.
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
3. In Flowers Red is dominant over white. One flower is heterozygous red and it is
crossed with a homozygous white plant.
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
4. In certain species of pine trees, short needles are dominant (S) to long needles
(s). What is the probability of the offspring having long needles if you cross a
heterozygous with a long needle plant?
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
In purple people eaters
the color Purple is
dominant over red.
P= purple
P = red
Cross a Hetero with a
red purple people eater:
Give the probability of:
Geno: _____________
Pheno: _____________
What terminology would
be described if I said
“The dog was black”?
What terminology would
be descried if I said “The
dog was BB”?
B. Dihybrid Cross- (Cross two traits at a time)
1. If a pea plant with genotype RRYy (round, yellow peas) is crossed with a pea plant
with genotype rrYy (wrinkled, yellow peas), what would the results be?
Key: R = round, r = wrinkled; Y = yellow, y = green
Cross: ________________________________________
How many Round and Yellow _____
How many Round and green _____
How many wrinkled and Yellow ______
How many are wrinkled and green ______
2. Cross a GgRr x GgRr
Key: G = gray body
R = red eyes
g = black body
r = black eyes
Cross: ___________________________________
How many Gray and Red eyes __________
How many Gray and black eyes _________
How many black and Red eyes _________
How many black and black eyes ________
3. In Purple People Eaters Having horns is dominant to no horns and being purple is
dominant to being red. If you cross a heterozygous for horned red monster with a
no horned heterozygous purple monster what would you get?
Key: __________________________________________________________________
Cross: ___________________________________
How many Horned Purple monsters ______
How many Horned red monsters ________
How many no horned purple monsters _________
How many no horned red monsters ________
VII.
A Closer Look at Heredity
A. Incomplete Dominance
 Neither allele has “complete” dominance over the other - heterozygous
phenotype is a blend of the two colors
 For example, in snapdragons if you cross a Red snapdragon (R) with a White
snapdragon (R’) you will get a Pink snapdragon (RR’)
1. Cross a red snapdragon with a pink snapdragon
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
2. In some breeds of rabbits if you cross a black bunny with a white bunny you can get a grey
bunny. What will you get if you cross two grey bunnies together?
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
B. Co-dominance
 Both alleles of a gene are dominant and both are seen
 For example in some breeds of chickens if you cross a black chicken (B) with a
white chicken (W) you will get a checkered (BW) chicken
1. Cross a checkered chicken with a white chicken
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
2. In some breeds of cows and horses if you cross a red animal (R) with a white (W) animal you
can get a Roan (RW) animal. What will you get if you cross two roan animals?
Key: ____________________________
Cross: _________________________
Genotype ratio: = ___________________
Phenotype ratio:= ___________________
C. Polygenic Traits- “Many genes”
 Many traits are controlled by more than one gene.
 Examples include: hair color, eye color and skin color
D. Multiple Alleles
 Many genes have options for more than two alleles, although an individual can
only express two alleles, one from mom and one from dad.
 Example: blood type. There are 3 possible alleles for this: A, B & O (AB is codominant so someone can be blood type AB)