Download genetics 2014

Genetic Vocabulary
• True – breeding – over many generations of self
polinating , produce only the same traits as the parent
plant. (eg: purple flower plants only produce purple
flower plants)
• Hybridization – the mating (or crossing) of two true
breeding varieties (eg: purple x white)
Homozygous - pair of identical alleles for a gene
Heterozyous - two different alleles for a gene
Phenotype –organism’s trait (physical appearance)
Genotype – genetic makeup (expressed with alleles)
Genetics
• The study of heredity.
• Gregor Mendel (1860’s) discovered the
fundamental principles of genetics by breeding
garden peas.
Mendel’s Experiment
• P generation – true
breeding parents
• F1 generation – hybrid
offspring of P
generation
• F2 generation –
offspring of hybrids
Mendel’s Model
• Gene – Character – a heritable feature that varies
among individuals (eg: flower color)
1. Alternative versions of genes called
alleles
– (genes – factors passed from parent to offspring)
• Account for variations
• Ex: white flower allele or purple
flower allele
2.
Inherit two alleles for each trait
3.
Principle of Dominance
If two alleles differ, one determines
appearance, dominant allele;
the other has no noticeable affect,
recessive allele
4.
Law of Segregation – the two alleles for a
heritable character segregate during gamete
formation
Homologous Chromosomes
eye color locus
B = brown eyes
eye color locus
b = blue eyes
This person would
have brown eyes (Bb)
Paternal Maternal
Meiosis - eye color
B
sperm
B
B
Bb
haploid (n)
b
diploid (2n)
b
b
meiosis I
meiosis II
Punnett square and
Probability
• A Punnett square is used to show the
possible combinations of gametes or
probability of genotypes
• Monohybrid Cross
• tracks the inheritance of a single trait.
Breed the P generation
• purple (PP) vs. white (pp) flowers
P
p
p
P
purple (PP) vs. white (pp) flowers
P
P
p
Pp
Pp
produces the
F1 generation
p
Pp
Pp
All Pp purple
(heterozygous)
Breed the F1 generation
• purple (Pp) vs. Purple (Pp) flower
plants
P
P
p
p
Purple (Pp) vs. purple (Pp) flower
plants
P
P
p
PP
Pp
p
Pp
pp
produces the
F2 generation
1/4 (25%) = PP
1/2 (50%) = Pp
1/4 (25%) = pp
1:2:1 genotype
3:1 phenotype
Monohybrid Cross
• Example:
Cross between two heterozygotes
for brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
b
B
Bb x Bb
b
female gametes
male
gametes
Monohybrid Cross
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
1/4 = BB - brown eyed
1/2 = Bb - brown eyed
1/4 = bb - blue eyed
1:2:1 genotype
3:1 phenotype
Test Cross
• Perform a test cross to
determine the genotype of
an organism with a
dominant phenotype
• Cross the organism with
the unknown genotype with
an organism that is
homozygous recessive
Dihybrid Cross
• A breeding experiment that tracks the inheritance
of two traits.
• Mendel’s “principle of independent
assortment”
a. each pair of alleles segregates independently
during gamete formation (metaphase I)
b. formula: 2n (n = # of heterozygotes)
Independent Assortment
Dihybrid Cross
• Example:
R
r
Y
y
= round
= wrinkled
= yellow
= green
cross between round and yellow
heterozygous pea seeds.
RrYy x RrYy
RY Ry rY ry x RY Ry rY ry
possible gametes produced
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
RY
RY RRYY
Ry RRYy
Ry
RRYy
RRyy
rY
RrYY
RrYy
ry
RrYy
Round/Yellow:
9
Round/green:
3
Rryy
wrinkled/Yellow: 3
rY RrYY
RrYy
rrYY
rrYy
wrinkled/green:
ry
Rryy
rrYy
rryy
9:3:3:1 phenotypic ratio
RrYy
1
Incomplete Dominance
• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
• Example: snapdragons (flower)
• red (RR) x white (WW)
R
RR = red flower
WW= white flower
W
W
R
Incomplete Dominance
R
R
W RW
RW
W
RW
RW
produces the
F1 generation
All RW = pink
(heterozygous pink)
Codominance
• Two alleles are expressed (multiple alleles)
in heterozygous individuals.
• Example: blood
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
I A IB
ii
Codominance
• Example:
homozygous male B (IBIB)
x
heterozygous female A (IAi)
IB
IB
IA
IA I B
IA IB
i
IB i
IB i
1/2 = IAIB
1/2 = IBi
Codominance
• Example: male O (ii) x female AB (IAIB)
IA
IB
i
IA i
IB i
i
IA i
IB i
1/2 = IAi
1/2 = IBi
Codominance
• Question:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents.
• boy - type O (ii) X girl - type AB (IAIB)
Codominance
• Answer:
IA
IB
i
i
IA IB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Example:
fruit flies
(red-eyed male) X (white-eyed female)
Sex-linked Traits
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
Sex-linked Traits
• Example:
fruit flies
(red-eyed male) X (white-eyed female)
• Remember: the Y chromosome in males
does not carry traits.
RR = red eyed
Rr = red eyed
rr = white eyed
Xy = male
XX = female
XR
Xr
Xr
y
Sex-linked Traits
XR
Xr
XR Xr
y
Xr y
1/2 red eyed and female
1/2 white eyed and male
Xr
XR Xr
Xr y
Population Genetics
• The study of genetic changes in populations.
• The science of microevolutionary changes in
populations.
• Hardy-Weinberg equilibrium:
the principle that shuffling of genes that occurs
during sexual reproduction, by itself, cannot
change the overall genetic makeup of a population.
• Hardy-Wienberg equation:
1 = p2 + 2pq + q2
Important
• Need to remember the following:
p2 = homozygous dominant
2pq = heterozygous
q2 = homozygous recessive
Question:
• Iguanas with webbed feet (recessive trait) make
up 4% of the population. What in the population
is heterozygous and homozygous dominant.
Answer:
1. q2 = 4% or .04
q2 = .04
2. then use 1 = p + q
1 = p + .2
q = .2
1 - .2 = p
.8 = p
3. for heterozygous use 2pq
2(.8)(.2) = .32 or 32%
4. For homozygous dominant use p2
.82 = .64 or 64%
Hardy-Wienberg equation
1 = p2 + 2pq + q2
•
•
•
•
64% = p2
32% = 2pq
04% = q2
100%
= homozygous dominant
= heterozygous
= homozygous recessive