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S E X and G E N E S
Recitation 10
JD Price
Multiplicity…
Asexual reproduction
•Genetic information is copied verbatim so parent
and offspring has same code
•Offspring and parent are clones
•Adaptation requires modification of the code by
organism
Sexual reproduction
•Genetic information is halved and combined with
another cell with halved information
•Unique combination of genetic information to
offspring
•Adaptation results when certain combinations are
favorable
Humans have 46 chromosomes. Shown are 1-22, plus X & Y
Women: double 1-22 and X
Men: double 1-22 and single X and Y
Each double (homologous pair) contains
similar, but not identical information
Each chromosome consists of DNA coiled
around and wrapped by proteins. Each
side of the chromosome contains the same
sequence (it’s a copy).
Let’s just follow genes
1-4. The parent cell
contains diploid pairs.
The blue chromosome
in each pair contains
same trait controls but
different coding.
In Meiosis, the process
of making sex cells,
pairs are split and then
each chromosome is
split, rendering two
pair of genetically
identical sex cells
(haploid).
In sexual reproduction, one of the
haploid cells combines with another
haploid cell. In many cases, the
second cell is from another individual.
Two homologous pairs are brought
together (their information is then
duplicated).
In reality, sometimes portions of the
chromosome pairs are swapped – this
is called “cross-over,” and complicates
the picture of heredity even further.
This is just one generation, with the
next generation, the chance of
deviation from the grandparent is even
greater.
Chance
Because of the variety of combinations, the traits of
individual offspring are a functions of statistical probability,
or the chance of outcome.
When you flip a coin there is about an even chance of it
coming up either heads or tails. In other words the chance
of the coin being heads is roughly one in two.
Of course if you flip a coin only twice, you may get tails both times.
Statistical probability predicts likely outcome, which will manifest itself over a
large sample. If you flip a coin 1000 times, you are most likely to get 500
heads and 500 tails.
So when you calculate the likely outcome of genetic combinations, you may
find that 1 out of every 10 children is likely to have type B blood. This
doesn’t mean that if a set of parents has 10 kids, 1 will definitely type B
blood, but only that it is likely to be the case.
Humankind has been playing with the genetic code
of organisms for thousands of years.
Animal husbandry: the mating of domesticated
animals to produce offspring with desirable
characteristics
Gregor Mendel
(1822-1884)
Agriculture: the merging of organisms to produce
crops with desirable characteristics.
Mendel is credited with making statistical sense of offspring.
By
pollinating pea plants with distinct traits with pollens from plants with
different traits, Mendel was able to sort out the likelihood of a trait to
appear in subsequent generations.
Mendel’s work took advantage of
several attributes of pea plants.
•Ease of pollination
•Rapid germination and growth
•Distinctive characteristics over
generations.
He found that many traits were
either on or off; purple vs. white
flowers, tall vs. short, round seed vs.
rumpled.
Mendel found traits expressed in
parents may not be expressed in the
first generation but may be carried
over into subsequent generations.
Parents of different attributes (like
white or purple flowers) could be
mated to find which traits were
passed along in offspring.
What Mendel couldn’t know at the time is that genetic information
is stored as chemical sequences in deoxyribonucleic acid (DNA), and
that there are two sets of the same DNA region for one trait aspect.
However, the statistics remain valid.
Alleles are parts of the genetic code (DNA) that defines a trait.
There may be more than one allele for a given trait in the cell.
(example T for tall, t for short)
Genotype refers to all of the alleles present for a trait
(example Tt)
Phenotype refers to the traits expressed
(example: a Tt genotype produced a tall phenotype)
Alleles were found by Mendel to be
dominant or recessive
If a tall plant with genotype TT
were mated to a short one of
genotype tt, the offspring would
have mixed genes, but all would
be tall because that allele is
dominant
Mating these offspring together
results three tall plants out of four,
one of genotype TT, two of Tt,
and one short tt
Short (tt) alleles
donated to sperm
Lets examine the cross in detail. Mendel mated a tall plant (TT) with a short
plant (tt). One can make an array of the allele combinations. This makes it easy
to see the combinations that result. (The matrix below is known as a Punnett
square)
Tall (TT) alleles donated
to egg
F1
T
T
t
Tt
Tt
t
Tt
Tt
Offspring
Four out of every
four offspring will
have the Tt
genotype. Tt is a
tall pea plant
because the T allele
is dominant.
If these offspring are mated (don’t worry,
incest is an accepted practice among
plants), the array shows us their offspring
are a mixture.
Tall and short (Tt) alleles
donated to sperm
Tall and short (Tt) alleles donated to egg
F2 T
t
T
t
T
TT
Tt
TT
Tt
t
Tt
tt
Tt
tt
T
TT
Tt
TT
Tt
t
Tt
tt
Tt
tt
Four out of every
sixteen offspring are
short (tt). The
remaining twelve are
tall, but only four are
the genotype TT.
Flys need love too…
Meet Drosophila melanogaster, or more
commonly known as the fruit fly. These
arthropods are favorite subjects for introductory
genetics labs because they breed quickly and
have easily distinguishable traits. We will take
some time to look at their reproductive
characteristics. Unfortunately, we lack the time
to breed them ourselves, which means you won’t
have the dubious joy of watching flies twitch in
their sleep at 100x magnification.
Two traits we can follow: the development of red vs. white eyes and the stumpy
(vestigial) vs. full (normal) wings.
One further complication: eye color is carried on the gender chromosomes X and
Y. The X chromosome can code for either color; the Y chromosome always
codes for the recessive white eyes in these critters.
Shorthand for alleles
XR = red eye
Xr = white eye
Y = male chromosome (white eye)
N = full (normal) wings
n = stumpy (vestigial) wings
Females have XX, males have XY (like humans)
Types
Wild = Red eyes and normal wings
Mutant = white eyes and stumpy wings
Wild Type
Drosophila melanogaster
F
XRXR = red eye
NN = normal wings
XRY = red eye
NN = normal wings
M
Mutant
F
M
XrXr = white eye
nn = vestigial wings
XrY = white eye
nn = vestigial wings
Cross A
Wild Type Female x Mutant Male
XR XR NN x Xr Y nn
What are the probable traits for the first and
second generation offspring?
F1
Xrn
Yn
XRN
XRN
Key
XR = red eye Xr = white eye
N = normal wings n = vestigial wings
Cross A
Wild Type Female x Mutant Male
XR XR NN x Xr Y nn
F1
XRN
XRN
Xrn
XRXrNn XRXrNn
Yn
XRYNn
XRYNn
1/2 Females: red eyed, normal wings
1/2 Males: red eyed, normal winged
F2
XRN
XRn
YN
Yn
XRN
XRn
XrN
Xrn
F2
XRN
XRn
XRN
XRXRNN XRXRNn XRXrNN XRXrNn
XRn
XRXRNn XRXRnn XRXrNn XRXrnn
YN
XRYNN XRYNn
XrYNN
XrYNn
Yn
XRYNn
XrYNn
XrYnn
XRYnn
XrN
Xrn
6/16 Females: red eyed, normal wings
3/16 Males: red eyed, normal wings
2/16 Females: red eyed, vestigial wings
1/16 Males: red eyed, vestigial wings
3/16 Males: white eyed, normal wings
1/16 Males: white eyed, vestigial wings
Of course the flies and the peas (and the birds and
the bees) are interesting in their own right, but they
are really useful examples because these apply to all
organisms that reproduce sexually (including a large
bipedal heterotroph like yourself)
Let’s look at a simple human characteristic: blood type
There are four types A, B, AB, and O
These are phenotypical expressions of the genotypes for blood
AA = A
AO = A
AB = AB
OO = O
BB = B
BO = B
This tells us both A and B are dominant alleles (codominant), and O is
recessive. Note that if A and B are present on the two different
chromosomes, both are expressed as blood type AB.