1. Answer the following questions regarding meiosis.
a. What is its purpose? To create haploid gametes for sexual reproduction. The
purpose of this is to INCREASE GENETIC VARIATION.
b. Fill in the following chart demonstrating the differences between mitosis and
meiosis in humans.
1
1
2
46
1
2
4
23
Centromeres
Homologous
Chromosomes
Identical
Different
Grow Repair
Replace
Make Gametes
c. What type of cells does meiosis make? haploid gametes (Why?)
d. Describe the major occurrences in the stages of meiosis (Picture is just there to
help answer the questions).


i. Prophase I (explanation must include crossing over, tetrad, and synapse)
(Like mitosis) - Nucleus break down, spindle fibers form, chromatin goes to
chromosomes
(Different from mitosis) - Homologous chromosomes synapse forming a tetrad.
Crossing over between homologous chromosomes occurs here.



ii. Metaphase I (explanation must include homologous chromosomes and the
law of independent assortment)
Homologous chromosomes line up at the metaphase plate (middle) of the cell. The
law of independent assortment means that each homologous pair lines up
independently of the others. (For example, they could line up as follows: 12 34 or
12 43)
iii. Anaphase I (explanation must include the law of segregation)
Homologous chromosomes separate. (Anaphase, apart, and away). The law of
segregation just says that alleles separate from one another. This is true because
they are on the homologous chromosomes that are separated.
iv. Telophase I
Nuclei reform – You now have 2 haploid cells.
v. PMAT II is basically just like mitosis. The only difference is that the 2
cells are now haploid (23 chromosomes) and will divide into a total of 4
haploid cells containing 23 chromosomes still (because we have separated
the sister chromatids). There is nothing to answer for this question.
e) What happens in metaphase of meiosis that explains the laws of segregation and independent
assortment?
Law of independent assortment – The homologous chromosomes line up independently of
one another.
Law of segregation – The homologous chromosomes are separated after they line up in
metaphase.
f. If an intestinal cell in a grasshopper contains 24 chromosomes, a grasshopper sperm cell contains
how many chromosomes? 12 chromosomes (haploid = half)
g. If a frog gamete has 10 chromosomes, how many chromosomes are in its somatic cells?
20 (diploid = couble)
h. Why is it important that EVERY SEXUALLY REPRODUCING ORGANISM (humans, plants,
everything) be capable of alternating between a haploid and diploid stage in their life cycles. (For
humans, we are diploid and our reproductive cells are haploid).
So that we can get ½ DNA from mom and ½ from dad to make 1 whole baby
2. Explain the difference between the following & provide an example of each:
a. genotype & phenotype
Genotype = genes (remember a gene is enough DNA to code for 1 protein) – represented by
letters (Ex. TT, Tt, or tt)
Phenotype = physical trait that is coded for by the genotype (Ex. Tall or short)
b. homozygous & heterozygous
homozygous = same allele for a gene (Ex. TT or tt)
heterozygous = different allele for a gene (Tt)
c. recessive & dominant
Recessive means that the DNA sequence for that allele does not code for a functioning
protein
Dominant means that the DNA sequence for that allele does code for a functioning protein.
This is why dominant alleles “mask” recessive alleles.
Note: Dominant does not mean better or more common.
d. complete dominance, incomplete dominance, and codominance
Complete dominance = Simple dominant and recessive traits like described in part c.
Incomplete dominance = Neither allele dominates the other. Heterozygous phenotype is a
blend of the 2 homozygotes. (Black + white = grey)
Codominance = both alleles dominate. Heterozygous phenotype shows both homozygous
phenotypes. (black + white = black & white)
e. true breed & hybrid
True breed = homozygous
Hybrid = Heterozygous
f. autosome & sex chromosome
Autosomes = Chromosomes 1-22. Boys and girls inherit these traits at equal rates.
Sex chromosome = 23rd chromosome pair. Boys and girl inherit these traits at different
rates. Boys show these traits more while girls are carriers.
g. allele, gene, and homologous chromosome
Allele = Alternative form of a gene. You get 1 allele for each gene from each parent (so 2
total alleles for each gene). Ex. T = tall and t = short
Gene = enough DNA to code for 1 protein. Each gene is made of 2 alleles (1 from mom and
1 from dad).
Homologous chromosome = chromosomes that are that same size, same shape, and carry
the same genes. These are the chromosomes that occur in pairs (we have 23 pairs of
homologous chromosomes). There will be 1 allele for each gene on each homologous
chromosome.
h. chromosome vs. sister chromatid
chromosome = 1 strand of DNA. Each chromosome contains hundreds of genes
sister chromatid = 2 identical copies of a chromosome that are held together by the
centromere. This copy is made during S phase of interphase so that each new cell gets a
complete set of DNA.
i. law of segregation vs. law of independent assortment
Law of segregation = Homologous chromosomes (and the alleles that are on them) separate
from each other during anaphase I. This is how meiosis takes a diploid cell and makes
haploid gametes.
Law of independent assortment = Homologous chromosomes (and the genes that are one
them) line up indepently of each other during metaphase; therefore, traits that are on
different chromosomes (i.e. eye color and hair color) are inherited independently of each
other.
j. polygenic inheritance vs. multiple alleles
Polygenic inheritance (Poly=many and genic=genes) – Traits that are coded for by multiple
genes. This results in a CONTINUUM of possible phenotypes (i.e. hair color, eye color, skin
color, height). To remember this type of inheritance, think poly = many so this results in
many phenotypes (hundreds of possibilities).
Multiple alleles = There are more than 2 alleles for a single gene at a single locus. (Ex.
Blood type has IA, IB, and i alleles)
k. sex-linked vs. linked
Sex-linked = genes that are on the X chromosome. Guys get these traits more because they
only have 1 X chromosome so they only have to get it from mom
Linked = these traits are inherited together because they are on the same chromosome
(they DO NOT follow the law of independent assortment). In other words, if you get one of
these traits then you’ll get the other because they are “linked” together. – Ex. red hair and
freckles.
l. haploid vs. diploid
haploid = only 1 member of homologous chromosome pairs. Represented by n (which
means number of homologous pairs). Gametes are the only cells that are haploid.
diploid = both members of homologous chromosome pairs. Represented by 2n (because you
have 2 of every homologous chromosome). All somatic cells are diploid.
m. monohybrid vs. dihybrid
Monohybrid = cross that involves only 1 trait (monohybrid means 1 difference). You only
use 1 Punnett square when you work these out.
Dihybrid = cross that involves 2 traits (dihybrid means 2 differences). You use 2 Punnett
squares when you work these out.
3. Answer the following questions about Mendel’s P, F1, and F2 generations.
a. What does each stand for?
a. P – Parent
b. F1 – Filial 1 (first son)
c. F2 – Filial 2 (second son)
b. What was crossed in each generation?
a. P - TT x tt
b. F1 – Tt x Tt
c. F2 – nothing
c. What was the genotype/phenotype ratio of the offspring in each generation (there
is space below each if you need to do the Punnett squares)?
a. P
The offspring of the P generation is the F1 generation.
G.R. = 0:4:0
P.R. = 4:0
b. F1
The offspring of the F1 generation is the F2 generation.
G.R. = 1:2:1
P.R. = 3:1
c. F2
no offspring
4. What type of cross showing complete dominance would in a 3:1 phenotypic ratio? Show
the Punnett square.
Tt x Tt
5. If a trait for free earlobes is autosomal recessive, and a female has free earlobes, what is
her genotype? What is her phenotype?
genotype = aa
phenotype = free
6. Can a male be a carrier of an autosomal recessive disorder? Why?
Yes, both males and females can be (Aa).
7. Is it possible for a male or female to be a carrier of an autosomal dominant disorder?
Why?
No. Aa would result in the dominant phenotype. You can’t be a carrier if you actually have
the trait.
8. A cross was done between 2 flowers exhibiting incomplete dominance and all of the
offspring were pink.
a. Draw a Punnett square showing this crossing.
R
r Rr
r Rr
b.
c.
d.
e.
R
Rr
Rr
What are the genotypes of the parents? RR x rr
What are the phenotypes of the parents? Red x white
What is the genotypic ratio? 0:4:0
What is the phenotypic ratio? 0:4:0
9. If you were to cross two roan (red and white) cattle, what would be the chances of the
offspring having a red phenotype (assuming the gene exhibits codominance)? Show the
Punnett square.
R
R RR
r Rr
r
Rr
rr
¼ = Red (RR)
10. Answer the following questions about the dihybrid cross of DdEe X DdEe.
a. Perform the Punnett squares.
D
D DD
d Dd
d
Dd
dd
E
E EE
e Ee
e
Ee
ee
b. How many of the offspring would be dominant for both traits? 9/16 (3/4 for “D”
trait dominant” X ¾ “E” trait dominant)
c. How many of the offspring would be ddee? 1/16 (1/4 dd X ¼ ee)
d. What is the phenotypic ratio? 9:3:3:1 (DD:Dr:rD:rr)
11. What fraction of the offspring would have the genotype aabbcc if we were to cross
AaBbCc x AaBbCc? Show the Punnett squares.
1/64 (1/4 aa X ¼ bb X ¼ cc)
A
a
A AA
Aa
a Aa
aa
B
B BB
b Bb
C
C CC
c Cc
b
Bb
bb
c
Cc
cc
12. If a female that is colorblind (a sex-linked recessive trait) and is crossed with a normal
male, what genotypes and phenotypes would be possible in the offspring?
Xb
Xb
XB XBXb
Y XbY
XBXb
XbY
Phenotypes = normal vision female and colorblind male
Genotypes = XBXb and XbY
13. Why are sex-linked traits expressed more in males? Why can't males be carriers of a sex
linked trait?
Guys only have 1 X chromosome so they cannot be carriers. They either have the trait or
they do not. Girls have 2 X’s so they can be carriers.
20. Explain why polygenic traits (like hair color, eye color, height, etc.) result in a range of
hundreds of possible phenotypes rather than a few possible phenotypes like the other types of
inheritance we have discussed.
Poly = many genes. There are many genes making many proteins so you have countless
possibilities for the phenotypes. RESULTS IN A CONTINUING RANGE OF
PHENOTYPES (LIMITLESS PHENOTYPES)
21.What is an example of linked traits, and why are these inherited together?
They are inherited together because they are on the same chromosome. Ex. red hair and
freckles.