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BIO 240 FINAL EXAM NOTES WITH 2026
ACTUAL EXAM TIPS-CHROMOSOMES,
MEIOSIS AND GENETIC DISORDERS
Double-stranded chromosomes wrapped around 8
histones=nucleosomes Supercoiling-when it bunches up;
condensing
Solenoid-
When is DNA replication during cell cycle?
- S phase
2 sets of chromosomes= 2n, 4 chromosomes 2n=4 n=2
Nondisjunction- failure of either homologous chromosomes or sister chromatids
to separate during anaphase
Aneuploidy- gain or loss of one or more chromosomes
Monosomy- loss of a single chromosome in a diploid organism
i.e 45 chromosomes in humans
Trisomy- gain of a single chromosome in a diploid organism
i.e 47 chromosomes in humans
Meiosis- 1. Homologous chromosomes separate
2. Sister chromatids separate
A diploid cell undergoes meiosis 1 and
cytokinesis Before starting meiosis 2,
this cell is haploid
Meiosis 1 Nondisjunction - all abnormal
,2
How many chromosomes should each gamete have (from image above)? 2 because
started
with 4 chromosomes
Meiosis 2 Nondisjunction - 2 normal, 2 abnormal (use image above
,3
Abnormal zygote with extra chromosome = 2n+1
Nondisjunction in meiosis: gametes with an abnormal chromosome number
Genetic abnormalities that ranges from mild to fatal
In most cases, a human embryo with an abnormal chromosome number results in a
miscarriage
- Mistakes in meiosis is the leading cause of miscarriage
Why is it so detrimental to have an abnormal chromosome number?
- Monosomy unmasks recessive lethal
- Trisomy= more viable but often lethal because of dosage sensitivity
- Haploinsufficiency- one copy is not sufficient for organisms
to survive Trisomy 21= Down syndrome (12-14 characteristics)
- Individuals express 6-8
Trisomy 13= Patau Syndrome + Trisomy 18 (Edwards syndrome)
- Survive to term
- Early lethality + manifest severe
Why is Trisomy 21 more common?
- Smaller so fewer genes and chromosome 21 can handle it
As mother’s age increases, the probability of Down Syndrome increases
- Why? – halted in prophase 1 then during ovulation, it continues the cycle
o Likely to have nondisjunction because some cells are halted for a very
long time
Turner Syndrome is the only monosomy that is viable in humans
- XO (female)
Klinefelter Syndrome- XXY, XXYY, XXXY (all males)
- Sterile, enlarged breasts (phenotypic diff.), can be treated with the
testosterone Turner Syndrome- XO
- Sterile, short, web of skin in between neck and shoulders, can treat with
estrogen
1/14/2020
How many sets of chromosomes does a diploid cell contain?
- 2 sets
, 4
*Table 8.1*
BIO 240 FINAL EXAM NOTES WITH 2026
ACTUAL EXAM TIPS-CHROMOSOMES,
MEIOSIS AND GENETIC DISORDERS
Double-stranded chromosomes wrapped around 8
histones=nucleosomes Supercoiling-when it bunches up;
condensing
Solenoid-
When is DNA replication during cell cycle?
- S phase
2 sets of chromosomes= 2n, 4 chromosomes 2n=4 n=2
Nondisjunction- failure of either homologous chromosomes or sister chromatids
to separate during anaphase
Aneuploidy- gain or loss of one or more chromosomes
Monosomy- loss of a single chromosome in a diploid organism
i.e 45 chromosomes in humans
Trisomy- gain of a single chromosome in a diploid organism
i.e 47 chromosomes in humans
Meiosis- 1. Homologous chromosomes separate
2. Sister chromatids separate
A diploid cell undergoes meiosis 1 and
cytokinesis Before starting meiosis 2,
this cell is haploid
Meiosis 1 Nondisjunction - all abnormal
,2
How many chromosomes should each gamete have (from image above)? 2 because
started
with 4 chromosomes
Meiosis 2 Nondisjunction - 2 normal, 2 abnormal (use image above
,3
Abnormal zygote with extra chromosome = 2n+1
Nondisjunction in meiosis: gametes with an abnormal chromosome number
Genetic abnormalities that ranges from mild to fatal
In most cases, a human embryo with an abnormal chromosome number results in a
miscarriage
- Mistakes in meiosis is the leading cause of miscarriage
Why is it so detrimental to have an abnormal chromosome number?
- Monosomy unmasks recessive lethal
- Trisomy= more viable but often lethal because of dosage sensitivity
- Haploinsufficiency- one copy is not sufficient for organisms
to survive Trisomy 21= Down syndrome (12-14 characteristics)
- Individuals express 6-8
Trisomy 13= Patau Syndrome + Trisomy 18 (Edwards syndrome)
- Survive to term
- Early lethality + manifest severe
Why is Trisomy 21 more common?
- Smaller so fewer genes and chromosome 21 can handle it
As mother’s age increases, the probability of Down Syndrome increases
- Why? – halted in prophase 1 then during ovulation, it continues the cycle
o Likely to have nondisjunction because some cells are halted for a very
long time
Turner Syndrome is the only monosomy that is viable in humans
- XO (female)
Klinefelter Syndrome- XXY, XXYY, XXXY (all males)
- Sterile, enlarged breasts (phenotypic diff.), can be treated with the
testosterone Turner Syndrome- XO
- Sterile, short, web of skin in between neck and shoulders, can treat with
estrogen
1/14/2020
How many sets of chromosomes does a diploid cell contain?
- 2 sets
, 4
*Table 8.1*
