Transcript Genetics

GENETICS
Why do we
show the
traits we do?
MENDELIAN GENETICS
Father of genetics, but mostly a monk!
Published research on inheritance in pea
plants
Inheritance = heredity
Pea plants could be easily cross-pollinated
by hand
Used data to make inferences about how
traits were inherited
Features of Pea Plants
A. Vocabulary
1. Characters – physical features that are
inherited
• Flower color in peas is a character
2. Trait – one of several possible forms of a
character
•
The flower colors of purple or white are traits
3. Hybrid – offspring of a cross between
parents with different traits
•
If a purple flowered peas crosses with a white
flowered pea, its offspring would be a hybrid
B. Pea Plant Characters – Mendel observed
7 characters
1. Height of plant’s stem (tall or short)
2. Position of flower on stem (mid-stem or end
of stem)
3. Pod Color (green or yellow)
4. Pod Appearance (smooth or bumpy)
5. Seed Texture (round or wrinkled)
6. Seed Color (green or yellow)
7. Flower Color (purple or white)
C. Pollination – Mendel manually took
pollen from an anther (male) &
transferred it to the stigma (female).
1. Self-Pollination: a flower will pollinate
itself or another flower on the same plant
2. Cross Pollination: a flower will pollinate a
flower on a different plant
**Mendel uses both forms of pollination in his
experiments.
III. Mendel’s Experiment
A. Mendel first used true-breeding plants.
1. True-Breeding – a plant that is pure for a
trait; when self-pollinated, its offspring will
always have that same trait
•
A true-breeding purple flowered plant will always
produce purple flowered plants!
2. Mendel formed 14 strains that were truebreeding.
3. These true-breeding plants were labeled the
P1 generation since they were the original
parents.
B. Two P1 generation plants of contrasting
traits were cross-pollinated. Their
offspring were labeled the F1 generation.
C. An F1 generation plant was allowed to
self-pollinate. Its offspring were labeled
the F2 generation.
D. An Example of Mendel’s Results
P1 = True-Breeding Purple X True-Breeding White
F1 = 100% were Purple Flowers
F2 = 75% Purple flowers & 25% White flowers
* = 3 Purple : 1 White Ratio
**Mendel crossed all 14 traits and always received
a 3:1 ratio in the F2 generation
INHERITANCE OF TRAITS
Original cross between Parent (P) generation
Offspring = F1 generation
Offspring of F1 generation = F2 generation
Cross of yellow and green seeds
F1 – all yellow seeds
F2 – 3:1 ratio of yellow to green…how?
Must be two forms of the seed trait in plants
These different forms are called alleles
DOMINANCE (VOCABULARY)
Must be two forms of the seed trait in plants
These different forms are called alleles
Allele – an alternative form of a single gene
passed on from generation to generation
Dominant allele (Y) – appeared in F1 generation
Recessive allele (y) – masked in F1 generation
DOMINANCE (VOCABULARY)
Phenotype – green or yellow (color)
Genotype – allele pairs (YY, Yy, or yy)
Homozygous – two of the same allele
YY or yy
Heterozygous (hybrids) – two different alleles
Yy
LAW OF SEGREGATION
Two alleles join during fertilization and
separate during meiosis
What do I mean by fertilization?
PUNNETT SQUARES
TYPES OF DOMINANCE
Incomplete dominance – heterozygous
phenotype is an intermediate between
the two homozygous phenotypes
Snapdragons – red, pink, white flowers
Codominance – both alleles are
expressed in the heterozygous condition
Both are expressed!
Now let’s practice!
Incomplete
Dominance
Codominance
LAW OF INDEPENDENT ASSORTMENT
Genes on separate chromosomes sort
independently during meiosis
Color and Round v. Wrinkled
Example: four allele combinations each equally
as likely to occur (YR, Yr, yR, yr) in Heterozygous
dihybrid
 Math in Biology – FOIL!
Results in a new phenotypic ratio
Probability – the more offspring, the closer we
get to these “magic” ratios
DIHYBRID CROSSES
How can we look at more than one trait
at once?
Round (R - dominant) v. Wrinkled (r)
seeds
How can we add this to our monohybrid?
Yellow/Round x Green/Wrinkled
YYRR x yyrr
F1 is YyRr but what about F2?
GENE LINKAGE
Genes that are located close to each
other on the same chromosome travel
together during meiosis
Exception to Mendel’s law (not independent)
Studied in fruit flies
Crossing over
Can separate once close genes; bring far
apart genes closer
More likely to occur in genes farther apart
POLYPLOIDY
Most species have diploid
cells, but some have
polyploid cells (more than
2n)
Triploid – 3n
In humans it is lethal but…
In plants they increase vigor
and size!
As many as 8n – sugar cane
SICKLE CELL AND MALARIA
 Caused by just one DNA base change
 Disease that affects a blood cell’s ability to
transport oxygen
 Block circulation in small blood vessels
 Heterozygous – normal and sickle cells
 Sickle cells are also malaria resistant!
DIHYBRID CROSSES – SLEEPING
AROUND!
 With a partner, pick two of the traits from the list below
and run two dihybrid crosses for your “offspring.”
 Assume complete dominance
 If dominant, flip a coin
 Heads = homozygous dominant, Tails = heterozygous
 Traits
 Widow’s Peak (W) v. Continuous hairline (w)
 Freckles (F) v. no freckles (f)
 Attached earlobes (A) v. unattached (a)
 PTC paper taste (P) v. no taste (p)
 Can roll tongue (T) v. can’t (t)
 Dimples (D) v. not (d)
 Hand clasping left (L) v. not (l)
 Repeat two more times with different partners!