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Genetics
Keystone Review
Terms You Need To Know
Gene- sequence of DNA that codes for a
protein and thus determines a trait
Trait- a characteristic that an organism can
pass on to its offspring through its genes.
Terms You Need To Know
Alleles- alternative forms of a gene, represented
by capital and lowercase letters
(You only have 2 alleles for each trait, one from your Mom
and one from your Dad)
• Dominant- allele that will overpower the
alternative allele (recessive). An individual will
show the dominant trait as long as one of the
alleles is the dominant allele.
• Recessive- allele that is overshadowed by the
dominant allele. The recessive trait only shows
if no dominant allele is present.
Terms You Need To Know
Genotypes
– The actual genes you have for a trait
TT- (homozygous dominant)
Tt- (heterozygous)
tt- (homozygous recessive)
– The letters represent actual genes inherited
(one from each parent)
Terms You Need To Know
Phenotypes
– Actual appearance (think PHoto- PHeno)
– The way the trait shows itself
– If T is the allele for tallness and t is the allele
for a dwarf plant:
TT- tall plant
Tt- tall plant
tt- dwarf plant
An example
There is a gene that
determines the trait of having
detached earlobes or attached
earlobes.
There is an allele for detached and an allele for
attached. The allele for detached is dominant. The
allele for attached is recessive.
If your genotype is homozygous dominant (EE) your phenotype
is detached earlobes.
If your genotype is heterozygous (Ee) your phenotype is
detached earlobes.
If your genotype is homozygous recessive (ee) your phenotype
is attached earlobes.
Solving Genetics
Word Problems
• On the keystone test you will be expected
to answer multiple choice questions and
written response questions about
inheritance patterns.
• Punnett squares can be used to show the
possible outcomes for a trait according
to the traits of the parents. They also
show the probability of the outcome
Ee x Ee
• Squares represent possible
offspring
• Each offspring gets one gene
from “dad” (from the top) and one
gene from “Mom” (from the side)
• What % of the offspring will have
detached earlobes?
• What % will have attached
earlobes?
• What % will be heterozygous for
the trait?
E
e
E
EE
Ee
e
Ee
ee
Mendelian Genetics
• Medelian Genetics describe situations in
which the basic rules are followed as seen
thus far.
• The first part of your worksheet are regular
dominance (or Mendelian Genetics)
problems
• There are other inheritance patterns
however, that go beyond this.
Other Inheritance Patterns
Going Beyond Simple Dominance:
1.
2.
3.
4.
5.
Incomplete Dominance
Codominance
Multiple Alleles
X- linked (or sex linked) Traits
Polygenic Traits
Incomplete Dominance
• In some cases, neither allele
truly dominates over the other.
No allele is really recessive.
• The heterozygous genotype
shows a MIX of the two traits.
• Examples- Four O’Clocks,
Snapdragons
– R- gene for red flowers, W- gene
for white flowers:
– RR- red, WW- white, RW- pink
Codominance
• In some cases, both alleles are dominant.
No allele is really recessive.
• The heterozygous genotype shows BOTH
of the two traits.
• Example- Chicken feathers
– B- gene for black feathers, W- gene for white
feathers:
– BB- black, WW- white,
BW- “erminette” Black and White!
Multiple Alleles
• Many genes have more than just two
alleles for a trait
• Example: Alleles for rabbit fur
– C-full color, dominates over Cch, Ch, c
– Cch – chinchilla, dominates over Ch and c
– Ch- himalayan, dominates over c
– c- albino, recessive to all
Practice Cross
C chCh x cc
• What are the
phenotypes of the
parents?
• Fill in the Punnett
Square.
• What is the probability
that an offspring will be
albino?
• What is the probability
that an offspring will be
himalayan?
Human Blood Types
Multiple alleles and Codominance!
• Multiple alleles- IA, IB, i
• Codominance- A and B are codominant
but both dominate over O
Sex-linked Traits
• Often called X- linked traits
• Trait can be dominant or recessive
• Probability of inheritance is altered
because the trait is on the X chromosome
• Females- XX Males- XY
Examples of
X-linked traits
• Red-green
colorblindness
• Hemophilia
• Duchenne
Muscular
Dystrophy
Polygenic Traits
• Many traits result from the interaction of
several genes.
• Polygenic traits can produce a large range
of phenotypes
• Examples: human skin color (at least 4
genes), human eye color, human height
Gradation of Human Skin Color
Meiosis
The alleles that Mendel was using represent
traits which are actually coded in genes,
just small sections along the DNA. The
DNA, remember, is coiled and condensed
into chromosomes. NOW:
How do parents actually pass on traits to
children?
• DIPLOID
– Cells with a double set of
chromosomes
– “2n”
– Humans – 23 sets/pairs,
2n=46
• HAPLOID
– Single set
– “n”
– Humans – one chromo from
each set, n=23
meiosis
• Similar to mitosis BUT…
– 2 phases of chromosome separation
– G1  S  G2  Meiosis I  Meiosis II
– Produces 4 gametes with half the normal
chromosome number
– 1 diploid somatic (normal cell)  4 haploid cells
– Distributes chromosomes “randomly” into
gametes
– One chromosome from each homologous pair
Tetrads line up
at equator
Crossing over
Homologous chromosomes
separate, NOT chromatids
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two
The chromosomes line up
The sister chromatids
haploid (N) daughter cells, in a similar way to the
separate and move toward
each with half the number metaphase stage of mitosis. opposite ends of the cell.
of chromosomes as the
original.
Telophase II
Meiosis II results in
four haploid (N)
daughter cells.
Spermatogenesis vs. Oogenesis
In summary:
Russel p. 74
Keystone-like Questions:
In a flowering plant species, red flower color is
dominant over white flower color. What is the
genotype of any red-flowering plant resulting from
this species?
A. red and white alleles present on one chromosome
B. red and white alleles present on two chromosomes
C. a red allele present on both homologous chromosomes
D. a red allele present on at least one of two homologous
chromosomes
Mitosis and meiosis are processes by which
animal and plant cells divide. Which statement
best describes a difference between mitosis and
meiosis?
A. Meiosis is a multi-step process.
B. Mitosis occurs only in eukaryotic cells.
C. Meiosis is used in the repair of an organism.
D. Mitosis produces genetically identical daughter cells.
Blood type is inherited through multiple
alleles, including IA, IB, and i. A child has
type A blood. If the father has type AB
blood, what are all the possible phenotypes
of the mother?
A. phenotypes O or A
B. phenotypes A or AB
C. phenotypes A, B, AB
D. phenotypes O, A, B, AB
Sample Written Response:
Patau syndrome can be a lethal genetic disorder in
mammals, resulting from chromosomes failing to separate
during meiosis.
• Part A: Identify the step during the process of meiosis
when chromosomes would most likely fail to separate.
• Part B: Describe how chromosome separation in meiosis
is different from chromosome separation in mitosis.
• Part C: Compare the effects of a disorder caused by
chromosomes failing to separate during meiosis, such as
Patau syndrome, to the effects of chromosomes failing to
separate during mitosis.