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Honors Biology
Genetics Practice Exam
1.
KEY
You grow red flowers. While walking in the field one day, you are surprised to find a single
plant with blue flowers. You decide to cross this new blue flower with one of your red plants.
All 50 offspring have blue flowers. You then mate two of these second generation blues
together. From the 53 offspring, 39 have blue flowers and 14 have red flowers.
- What is the most reasonable explanation for the appearance of the blue flower?
Because it is only one flower out of thousands, it must have arose through mutation.
-
Describe the inheritance of the blue flower. Show me the Punnet Squares for the F1 and F2
generations.
Since all of the F1 flowers were blue, it must be dominant.
F1 cross: FF (Blue) x ff (red)
F2 cross: Ff x Ff
Ff
Ff
FF
Ff
Ff
Ff
Ff
ff
2.
You are a molecular biologist. While performing an electrophoretic mobility shift (EMS)
assay, you spill a large amount of 32P, a radioactive isotope of phosphorus, on your left hand.
Radiation is a potent cause of mutations. Should you be worried that exposing your hand to
radiation will cause birth defects to any future children you have?
You need not worry at all. Mutations to DNA in your hand cannot be passed on to your
offspring. Only mutations in your germ cells (in your ovaries or testes) can be passed
onto your offspring.
3.
When non-disjunction occurs, one of Mendel’s laws is violated. What is non-disjunction,
and what law is violated?
Mendel’s Law of Segregation says that only one of the two homologous chromosomes
from each parent is passed onto the child. In non-disjunction, the two homologous
chromosomes stick together in meiosis I, or the two identical alleles stick together in
Meiosis II. As a result, an offspring may receive two alleles from a parent instead of the
normal one.
4.
B encodes for brown eyes, b for blue. C encodes for curly hair, c for straight. F encodes for
freckles, while f is clear skin.
A brown-eyed, curly haired man marries a brown-eyed, curly haired woman. They have a
brown-eyed, straight haired daughter. What are the genotypes of the three?
Man: BbCc or BBCc
Woman: BbCc or BBCc
Kid: Bbcc or BBcc
A CcFf woman mates with a double recessive man. Give the phenotypic ratio of all the
possible offspring.
Offspring: [Recall that phenotype is the physical appearance!]
CcFf x ccff
¼ Freckled with curly hair
¼ Freckled with straight hair
¼ Non-freckled with curly hair
¼ Non-freckled with straight hair
5.
Why is crossing-over so important?
Crossing over allows for new combinations for genes that are linked together. It further
increases the different combination of genes, which increases the variety of organisms
produced.
6.
You mate a red flower with a white flower. All the baby flowers are pink. What is this an
example of?
This is incomplete dominance: three distinct phenotypes, one of which is a blend of the
other two.
You mate a cow with black spots with a cow with brown spots. All the calves have both brown
spots and black spots. What is this an example of?
This is codominance: both phenotypes are expressed equally in the offspring.
You mate a blue-eyed female with a brown-eyed male. All of the males are blue-eyed, but the
females are brown. What is this an example of?
This is sex linkage: there is a separation by gender.
7. What is the difference between:
- a linked and unlinked gene
Linked genes are on the same chromosome.
chromosomes.
Unlinked genes are on separate
- a haploid cell and a diploid cell
Haploid cells have only one allele for each trait. They are used as reproductive cells by
advanced life forms. Diploid cells have two alleles for each trait. They serve as somatic,
or body, cells.
- mitosis and meiosis
Mitosis occurs in all somatic (body) cells, and makes a perfect copy of the cells for
growth and repair. Meiosis creates haploid cells for reproduction. It occurs only in the
gonads.
- polygenic and monogenic (or simple) inheritance
Monogenic (one gene) inheritance follows simple Mendelian patterns: two phenotypes,
and one dominant and one recessive allele. Traits controlled by polygenic inheritance do
not have two distinct phenotypes: rather, they have a broad range. Human height is
polygenic, where there is a broad range from tall to short and many heights in between.
8. There are many human diseases that have a genetic basis. Cystic fibrosis is caused by a three
base-pair deletion in the DNA, erasing the 508th amino acid in a membrane protein. Patients
with Turner’s syndrome have only one X chromosome. Down syndrome patients have a trisomy
at chromosome 21.
Which condition could you use a karyotype to screen for?
Karyotypes can only detect problems with chromosomes, not with gene mutations. As a
result, you would be able to see Turner’s and Down syndromes, but not see the genetic
mutation that caused cystic fibrosis.
What are possible causes of the three diseases?
Cystic fibrosis is caused by a deletion mutation.
Turner’s and Down syndromes are caused by non-disjunction in meiosis.
9. What is meant by “heterozygote advantage?”
Heterozygote advantage is a rare situation that occurs with some diseases caused by
recessive genes (for example, sickle-cell anemia and malaria; cystic fibrosis and cholera).
People with both copies of the recessive allele (homozygous recessive) have the disease
and often die. However, heterozygotes (with only one copy of the disease) do not have
the disease, but the slight change in the body caused by the one copy of the allele
protects the person from a serious infection. Those that are homozygous dominant for
the alleles are free from disease, but highly susceptible to death from another infection.