PART 7: Heredity
1. Genes: a segment of a chromosome that contains DNA which codes for a
visual trait.
 Trait: expressed characteristic.
 Locus: the location of a gene on a chromosome.
 Alleles: opposite pairs of genes – one from mom and one from dad;
assigned letters in pairs; may be homozygous or heterozygous (depends on
the parents and on chance), and dominant or recessive (depends on type).
o Dominant: a trait that only needs dominant one of its alleles present to
be expressed. Represented by an uppercase letter (ex., tall plant = T).
o Recessive: a trait that needs both recessive alleles present to be
expressed. It is represented by a lowercase letter (ex., short plant = t).
o Homozygous: when both alleles of a trait are the same - both recessive
(tt) or both dominant (TT). In this case, a recessive trait would be
expressed if both alleles were lowercase (ex., pp). Also, if both were
uppercase, a dominant trait could be expressed. If both alleles are
dominant and the trait of that locus is only expressed recessively, then
it won’t be expressed or passed on to offspring. Vice versa, also won’t
be expressed or handed down to offspring.
o Heterozygous: when the two alleles are opposite, with one dominant
and one recessive. If the trait of the locus is recessive, it will not be
expressed but CAN be passed to offspring, and if it’s dominant will be
expressed in both the organism and the offspring.
o Genotype: the genetic makeup of organisms; the alleles an organism
possesses.
o Phenotype: the expression of a trait of an organism; the physical
appearance as a result of the genotype.
o Generations: when crossing genotypes, we can calculate the probability
of the genotypes of different generations of offspring, starting with a
specific parent generation
 P1 Generation: the parent generation
 F1 Generation: the filial, or offspring, generation
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 F2 Generation: the generation that is the filial, or offspring, of
the F1 generation.
2. Punnett square: a diagram that is used to predict an outcome of a particular
cross or breeding experiment. There are three steps to make a Punnett square:
I. Figure out the genotype of the organisms being crossed. (ex. Two flowers
are crossed; one is tall, yellow and heterozygous, while the other is short,
white and homozygous. Yellow is dominant over white. -> The only possible
phenotypes are TtYy for the first and ttyy for the second.)
II. FOIL the individual genotypes for all possible combinations. FOIL is a series
of multiplications of the Firsts, Outsides, Insides, then Lasts.
T t Y y = TY, Ty, tY, ty
III.
t t y y = ty, ty, ty, ty
Make a table with the possibilities for each genotype each side. Match the
letters to see the new possible genotypes. From the genotypes we can figure out
the phenotypes, as extended as an example to the right of the Punnett Square.
TY
Ty
tY
ty
ty
TtYy
Ttyy
ttYy
ttyy
ty
TtYy
Ttyy
ttYy
ttyy
ty
TtYy
Ttyy
ttYy
ttyy
9 tall and green (TT or Tt + GG or Gg)
3 tall and yellow (TT or Tt + gg)
3 short and green (tt + GG or Gg)
1 short and yellow (tt + gg)
ty
TtYy
Ttyy
ttYy
ttyy
Phenotypes:
Tall, Yellow
Tall, White
Short, Yellow
Short, White
9:3:3:1 ratio will
ALWAYS be the result
of a dihybrid cross.
3. Mendelian Genetics: laws of genetics based on the lifelong studies of Gregor
Mendel in the 19th century. He developed three laws that are still used today:
the law of dominance, the law of segregation, and the law of independent
assortments.
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o Law of Dominance: a law stating that a cross of parents that are pure
[homozygous] for opposite traits will only have one of those traits
represented in the next generation. The offspring will be hybrid for the
trait (heterozygous). One trait masks the effect of another trait. This law is
only applicable for single traits, as in a monohybrid cross.
 Monohybrid cross: the crossing of two genotypes (homozygous or
heterozygous) when only one trait is being studied.
 Example: T = tall, t = short, tall is dominant, short is recessive.
P1
TT (Homozygous Tall)
F1
T
T
t
Tt
Tt
t
Tt
Tt
tt (Homozygous Short)
All are heterozygous
and tall.
o Law of Segregation: law stating that alleles can segregate and recombine
in two generations. Characteristics hidden by dominant alleles in the F1
generation (see example from the Law of Dominance) can come back in the
F2 generation. It is because of the outcome of the F2 generation that
Mendel realized that there must be dominant alleles overshadowing
recessive ones.
F1
Tt (Heterozygous Tall)
F2
T
t
T
TT
Tt
t
Tt
tt
Tt (Heterozygous Tall)
Homozygous Tall (1)
Heterozygous Tall (2)
Homozygous Short (1)
Ratio of Short to Tall = 1:3
o Law of Independent Assortment: law stating that traits can segregate and
recombine independently of other traits in just one generation. Because
they can assort themselves independently, we FOIL the genotypes to
include all possible pairings. This can be observed when two parents (of a
known genotype) are crossed as we follow two characteristics – this is
called a dihybrid cross. An example is in Vocab #2.
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4. Test cross: a monohybrid cross with the purpose of learning whether or not an
organism’s genotype is homozygous or heterozygous. We do this by crossing the
organism in question with another expressing a recessive trait. If half of the
offspring show the recessive trait, the organism in question is heterozygous. If
none show the recessive trait, then it is homozygous, as demonstrated in the
Punnett squares below.
Example Test Cross
*We have two plants. One is short (recessive trait), and the other is tall (dominant
trait). The first must be homozygous for it to show its recessive trait. All we know
about the genotype of the second is that there must be at least one dominant
allele because it shows the dominant trait – it could have one OR two dominant
alleles.
Must be tt
P1
Could be  Tt OR TT
*Now we cross the plants (in real life – this cannot be answered without being
given the results of the cross). We will either get Results 1 or Results 2, which will
determine the original genotype of the P1 generation.
Short (Recessive) = tt
Tall (one in Question) = Tt or TT
Heterozygous
Tt x tt
T
t
t
Tt
tt
Homozygous
TT x tt
t
Tt
tt
½ Tall
T
T
t
Tt
Tt
t
Tt
Tt
All Tall.
½ Short
*So if plants had
that result, the
genotype in question
was heterozygous.
*So if plants had
that result, the
genotype in question
was homozygous.
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5. Autosomes: chromosomes that are not related to gender. Humans have 22 of
them.
6. Sex chromosomes: chromosomes that determine gender. The two types are X
and Y. If a person has both, he is male. If a person has two X’s, she is female.
Below are some mutations affecting the sex chromosomes.
- XXX: Triple X syndrome is a form of chromosomal variation characterized by the
presence of an extra X chromosome in each cell of a human female.
- XYY: syndrome is an aneuploidy (abnormal number) of the sex chromosomes in
which a human male receives an extra Y chromosome
- XXY: Klinefelter's syndrome: syndrome in males that is characterized by small
testes and long legs and enlarged breasts and reduced sperm production and
mental retardation
- X: Women with Turner syndrome typically have one X chromosome instead of
the usual two sex chromosomes. Turner syndrome is the only full monosomy that
is seen in humans—all other cases of full monosomy are lethal and the individual
will not survive development.
7. Carrier: any allele that is not expressed for whatever reason (i.e., being
recessive, sex linked in women) can be handed down to offspring. These alleles
are called carries, the person with them being a carrier. This is why children with
perfectly healthy family histories can have genetic diseases from earlier
generations.
Example:
*dd = Disease
Dd = Carriers DD = Normal
Nan
Dd
Pop
DD
Nan
Dd
Mom
Dd
Pop
Dd
Dad
Dd
Sick child
dd
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8. Extended genetics: Mendel’s theories only apply to certain characteristics,
such as the ones he studied. Science has advanced to explain more complicated
types of inheritance.
 Codominance (multiple alleles): when more than one allele can be
expressed at the same time without blending at equal expression. The best
example is blood types such as AB (IAIB). In blood, i (type O) is recessive and
is overshadowed by both dominant types, IA and IB. But because type A and
B are equal, they are expressed simultaneously when together.
Ex.,
Genotype
Phenotype
A
Ii
Type A
B
Ii
Type B
AB
II
Type AB
ii
Type O
AA
II
Type A
BB
II
Type B
etc.
 Incomplete dominance: the blending of traits when neither trait
overshadows the other. almost the same thing as Codominance, the only
difference is how they’re expressed. Work them out the same way. It still
has two different dominant alleles expressed at the same time, but the
difference is that the phenotype (what you can see different) looks more
like its blending in incomplete dominance, rather than being totally
separated in Codominance. Example:
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 Polygenetic inheritance: when a trait is affected by multiple genes;
multiple genotypes create one phenotype.
 Epistasis: when a gene at one locus can affect the expression of a gene at
another locus (ex., mouse fur color is controlled by two loci, one with
alleles B (black) or b (brown), and the other with C (dominant no affect) or c
(recessive for albino). If the latter is cc, it will overshadow the other locus
(whether it’s BB, Bb, or bb) and the mouse will be albino.
 Pleiotropy: when one gene (just two alleles) shows multiple phenotypes.
An example of this would be sickle-cell disease because it has so many
symptoms from one pair of alleles.
 Linked genes: when two genes for different characteristics tend to stay
grouped together, unable to break apart even in crossing over. These traits
tend to be passed on together (ex., flower color and pollen shape are
generally together). The probability rule of a dihybrid cross does not apply
to these genes.
 Recombination mapping: we can figure out a sequence of linked genes,
how far they are from one another, and how frequently they recombine
using recombination mapping. Given the frequency between multiple
genes, we can find distance and order of the genes.
Example:
Gene A and C recombination frequency = 24%
Gene A and B recombination frequency = 15%
Gene B and C recombination frequency = 9%
A
(15 units)
B
(9 units)
C
15%
9%
_____________________________________
24%
(24 units total)
We can also find a missing frequency:
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Example:
Gene A and C recombination frequency = 21%
Gene A and B recombination frequency =
Gene B and C recombination frequency = 16%
It is simply the difference: 21 – 16 = 5%
A
(16 units)
B (5 units) C
16%
5%
_____________________________________
21%
(21 units total)
*NOTE: Letters will rarely be in alphabetical order. You may be given percentages and
asked for the order or given the order and asked for the most likely percentage.
Example 1:
Gene D – B = 50%
Gene D – A = 30%
Gene C – B = 5%
50 – 30 – 5 = 15
30%
15%
5%
_________________________________
D
A
C
B
50%
Order is DACB.
Example 2: If the order of a sequence is CADB and from A – B is 30%, what is most likely
the frequency of C-B?
a) 10%
b) 5%
c) 20%
d) 40% (The only one greater distance than 30%)
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 Sex-linked genes: genes that are carried on chromosome 23 (sex
chromosomes). Some traits carried here are haemophilia (inability to clot
blood) and color blindness (inability to identify colors). Most sex-linked
traits are carried on the X chromosome (unless otherwise stated). Sex
linked traits are similar to recessive traits in females – one defective X will
be masked by her other X. In other words, a female must be homologous
to a sex-linked defect in order for it to express. However, in males, just one
defective X chromosome will always be expressed because there no other X
chromosome to mask it.
o Barr Body: an inactive X-chromosome that is dark and condensed in
females. It is inactivated during embryonic development and
remains inactive in all cells of an adult woman EXCEPT in sex cells.
Although it is inactive, it is still replicated in cellular reproduction.
Example: Woman with color blind father has children with man of perfect
vision (X is defect). What are the chances of having a colorblind child?
*Xl = colorblind allele
Woman: XlX
Her father is XlY
(colorblind male).
She got an X from her
mother, and because
she is female she must
of gotten her father’s
Xl, carrying the
colorblindness.
P1 Generation
Xl
Woman
X
Man: XY
Non-affected
male.
Man
X
XlX
Y
(Girl that carries
colorblindness)
(Colorblind boy)
XlY
XX
XY
(Normal girl)
(Normal boy)
¼ chance of having a colorblind child.
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