Transcript Ch. 11 Introduction to Genetics

Ch. 11
Introduction to
Genetics
11.1, Work of Gregor Mendel
Experiments of Gregor Mendel

Individual’s looks determined by
traits passed to offspring from parents
 heredity: delivery of characteristics from
parent to offspring
 genetics: scientific study of heredity
11.1, work of Mendel
Mendel worked w/ ordinary garden peas
 peas = “model system”
 Model system because they
have few traits and they
grow fast!
 Mendel did experiments in
short time that would take
years to do.
w/ humans

11.1, work of Mendel
Role of Fertilization
 male part of each flower makes pollen,
which contains sperm (male reproductive
cells)
 female part of each flower makes eggs
(female reproductive cells)
 pea flowers normally “self-pollinating”
(sperm fertilize eggs from w/in same
flower)
11.1, work of Mendel
fertilization: in sexual reproduction, egg &
sperm join to produce new cell
 in peas, new cell develops into tiny embryo
(the peas in the pod).
 Because peas self-pollinate, they inherit all
its characteristics from its single parent
plant

11.1, work of Mendel
Mendel had peas that were “true-breeding”
(produced offspring w/ identical traits to
themselves)
 trait: specific characteristic of individual
(seed color, plant height, etc.) that may vary
from 1 individual to another
 Mendel decided to “cross-pollinate” his
stocks (transfer pollen to cause 1 plant to
reproduce w/ another plant)

11.1, work of Mendel
cross-pollination let Mendel breed plants w/
traits different from their parents & study
results
 hybrids: offspring of crosses between parents
w/ different traits
Genes & Alleles

original organisms studied = P (parental) generation
offspring of P = F1 generation
11.1, work of Mendel
For all 7 traits Mendel studied, every
offspring looked identical to the trait of 1
of their parents (no “blended” traits)
Other parent’s trait “disappeared”
11.1, work of Mendel

Mendel’s 1st conclusion: an individual’s
characteristics are determined by
factors passed from 1 generation to next
 factors = genes
 alleles: genes for different forms of a
given trait
In the picture, the
Alleles are “R” and “r”
F1 recieves 1 allele from
each parent.
11.1, work of Mendel

Mendel’s 2nd conclusion: principle of
dominance
 some alleles are dominant & others are
recessive
dominant allele= organism will show that form of
trait (represented with : CAPITAL LETTER)
 recessive allele= organism will exhibit that form
only if no dominant allele is present (represented
with: lowercase letter)

11.1, work of Mendel
Segregation

Each parent has 2 alleles, the genes
segregate from each other, so each sex cell
only carries one allele.
 WHERE DID THE
RECESSIVE GO?!?
Solution: Mendel let his
F1 hybrids self-pollinate,
making F2 hybrids
11.1, work of Mendel
Results of crossing F1 Generation
 recessive alleles reappeared in F2 THEY
CAME BACK! (1/4 had recessive trait)
 Dominant allele hid
recessive allele in F1
 Reappearance of
recessive trait indicated
that, at some point,
short allele separated
from tall allele.
11.1, work of Mendel
Formation of Gametes
 Assume each F1 plant has inherited 1 tall
allele from its tall parent & 1 short allele
from its short parent.

each F1 plant in Mendel’s cross produced 2 kinds
of gametes — ½ w/ tall allele (T) & ½ w/ short
allele (t)
This could go two ways….
1. If each 2 gametes with “t” allele paired to
produce F2 plant, that plant was short (tt)
2. If either of the two gametes in F2 plant was
T, that plant was tall (TT or Tt).
http://www.youtube.com/watch
?v=GTiOETaZg4w
11.2, applying Mendel’s
principles
Probability & Punnett squares
 Punnett squares use mathematical
probability to help predict genotypes
& phenotypes in genetic crosses
probability: likelihood that particular
event will occur
past outcomes DO NOT affect
future ones
11.2, applying
To find probability of multiple events,
multiply probabilities of each
example: probability of flipping
head is 1/2, so probability of 3 heads
in a row is: 1/2 × 1/2 × 1/2 = 1/8
How the alleles segregate during gamete
formation is just as random as coin flip
11.2, applying
How alleles
segregate during
gamete formation is
just as random as
coin flip
ex: F1 parent w Tt
genotype has 1/2 chance of t
in gamete, so chance of F2
w/ tt is 1/2 × 1/2 = 1/4, &
Mendel had 1/4 short
11.2, applying
homozygous: w/ 2 identical alleles
for trait (ex.: TT or tt)
Homozygous dominant: TT (2 dominant)
Homozygous recessive: tt (2 recessive)
heterozygous: w/ 2 different alleles
for trait (ex.: Tt)
11.2, applying
Genotype & Phenotype
genotype: genetic makeup for trait
phenotype: physical trait (what you
see)
genotype determines phenotype,
but organisms with the same
phenotype may have a different
genotype
ex.: TT or Tt both look tall
11.2, applying
Punnett square for 1-factor cross
1. write genotypes of parents &
possible alleles from each
2. write one parent’s alleles across top
& other down side
3. fill boxes
4. count ratios, from most
to least dom.
Genotype ratio = 1:2:1
Phenotype ratio = 3:1
11.2, applying
principle of independent assortment:
genes for different traits can segregate
independently during formation of
gametes
monohybrid cross: genes for 1 trait
dihybrid cross: 2 different traits
trihybrid cross: 3 traits
11.2, applying
Mendel used true-breeding parent
plants to create F1 that were
heterozygous for 2
traits (dihybrid)
crossing F1
produces 4 combo
phenotypes
(G & Y, G & y,
g & Y, or g & y)
11.2, applying, cont
since seed color & pod color didn’t
affect each other,
Mendel concluded
that 1 trait had no
effect on another
during gamete
formation
(independent
assortment)
 beyond
dominant & recessive alleles

some alleles are neither
dominant nor recessive ( called
incomplete dominance or
codominance)
Many genes exist in several different forms
(have multiple alleles)

Many traits are produced by the
interaction of several genes (polygenic traits)

 beyond
dominant & recessive
 incomplete dominance: 1 allele is not
completely dominant over another;
phenotype is intermediate (ex: red &
white alleles making pink flowers)
 codominance: both
alleles show up full
strength (A & B blood)
 multiple alleles: more
than 2 possible genes
(A, B, or O blood)
http://www.youtube.c
om/watch?v=G_9_CF02qI
IA
IA
IAIA
(Type A)
IB
IAIB
(Type AB)
i
IAi
(Type A)
Ibi
(Type B)
 genes
& environment

environmental conditions can
affect gene expression & influence
genetic traits

temperature, nutrition, diseases during
development. Remember Jurassic Park??
11.4, meiosis
 chromosome #

diploid cells of most adult
organisms have 2 sets of inherited
chromosomes w/ 2 complete sets of
genes
 chromosomes: strands of DNA &
protein inside cell nucleus
 genes located in specific positions
on chromosomes
11.4, meiosis, cont
 chromosome #, cont
 diploid cell: any cell w/
2 sets of chromosomes,
1 from each parent
 homologous chromosomes: same
type; have genes for same traits
 haploid cell: cell w/ 1 set of
chromosomes, generally gametes
11.4, meiosis, cont
 phases of meiosis
 2 divisions, each w/ stages similar to
mitosis

meiosis I (a.k.a. reduction division)
separates homologous chromosomes
into 2 haploid cells, still w/ 2 chromatids
for each chromosome

meiosis II (a.k.a. mitotic division)
splits sister chromatids in each cell from
meiosis I, forming 4 haploid gametes
11.4, meiosis, cont
 phases, cont
 meiosis I

prophase I
 like mitosis, chromosomes, spindle
fibers, centrioles form,
nuclear envelope
breaks down
 homologous
chromosomes pair up, forming tetrads
(4 similar chromatids)
11.4, meiosis, cont
 phases, cont
 meiosis I, cont

prophase I, cont
 crossing-over: homologous
chromosomes “swap” genes
 chromatids overlap
sections
 crossed sections cut &
spliced, exchanging genes
 produces new combinations of alleles
11.4, meiosis, cont
 phases, cont
 meiosis I, cont
metaphase I: tetrads line up
across cell’s center w/ spindle
attached to chromosomes
 anaphase I: spindle fibers
pull each homologous
chromosome pair toward
opposite ends of cell
 telophase I & cytokinesis: nuclear
membranes form & cytoplasm splits

11.4, meiosis, cont
 phases, cont
 meiosis II




prophase II: chromosomes &
spindles form
metaphase II: chromosomes line
up & spindles attach to chromatids
anaphase II: chromatids separate
telophase II & cytokinesis: nuclear
membranes form & cytoplasm splits
11.4, meiosis, cont
 phases, cont
 gametes to zygotes

haploid cells produced by meiosis II are
gametes
 in males, these gametes are called
sperm (plant sperm enclosed in pollen)
 in female animals, usu. only 1 cell from
meiosis becomes egg (other 3 form
small cells called polar bodies)
 in female plants, might make 4 eggs or 1
egg & 3 polar bodies
11.4, meiosis, cont
 phases, cont
 gametes to zygotes, cont
fertilization: fusion of male & female
gametes
 zygote: cell formed by fertilization; forms
new organism w/ new combination of
genes diff. from parents
 gene linkage & gene maps

alleles of diff. genes tend to be
inherited together when those genes are
on same chromosome

11.4, meiosis, cont
 phases, cont
 gene linkage & maps, cont


Thomas Hunt Morgan studied genetic traits
& chromosomes in fruit flies, w/ 2
conclusions:
 each chromosome is actually a group of
linked genes
 chromosomes assort independently, not
individual genes
gene map: Alfred Sturtevant figured out that
crossing-over was more frequent when
genes were closer together on chromosome