Download Genetics - POTOSI SCHOOL DISTRICT

Genetics
Heredity and Genetics
• Heredity is the passing of traits
from parents to offspring
• Traits are controlled by genes,
so GENETICS is the study of
how traits are inherited through
the action of alleles
Gregor Mendel – “Father of Genetics”
• Austrian monk born in
1822 who is responsible
for the laws governing
the inheritance of traits
• Between 1856 and 1863,
Mendel cultivated and
tested over 28,000 pea
plants
• Mendel performed crosspollination by
transferring pollen from
one plant to selected ova
of other plants, thereby
controlling which plants
mixed
Mendel’s Experiments
• Mendel produced pure strains by allowing
plants to self-pollinate, I.e., pollen (male
gamete) from one plant fertilizes ova (female
gamete) of same plant
Mendel’s Peas….
s
S
Y
y
I
i
G
g
Mendel’s Peas….
P
p
A
a
T
t
Mendel’s Results….
(F2 from crossing to heterozygous F1s)
Genetics Terminology
• Traits – any characteristic that can be passed
from parents to offspring
• Heredity – the passing of traits from parents to
offspring
• Alleles – one form (dominant or recessive) of a
gene
• Sex cells have ONE form of a gene on their
chromosomes
• Body cells have TWO forms or alleles for a
single gene (you get one from mom and the
other from dad)
• Dominant – is always expressed; masks a
recessive trait
• Recessive – can only be expressed if there are
no dominant alleles present
Genetics Terminology
• Dominant alleles are represented by a capital letter
• Recessive alleles are represented by a lower case letter
Example: B = Brown eye color (dominant)
b = Blue eye color (recessive)
• “Purebred” species have two of the same alleles; this is
also called HOMOZYGOUS, e.g., BB (homozygous
dominant) or bb (homozygous recessive)
• Species with two different alleles are called
HETEROZYGOUS, e.g., Bb
• Genotype: the alleles present in the organism, i.e., BB,
Bb, or bb
• Phenotype: the expression of the genes; what is
observed
Genotype and
Phenotype in Flower
• All genes occur in pairs, so TWO alleles affect a
trait.
• Possible combinations if:
R = Red flower
r = Yellow flower
• Genotypes
• Phenotypes
RR
Red
Rr
Red
rr
Yellow
BrainPop - Heredity
Genetic Crosses
• Monohybrid Cross: cross involving ONE trait,
e.g., eye color
• Dihybrid Cross: cross involving TWO traits, e.g.,
eye color and hair color
• Offspring’s genotype and phenotype is determined
using a Punnett square
B
b
B
BB
Bb
b
Bb
bb
Punnett Square, cont.
Generations in Crosses
• Parental Generation (P1) = the parental generation
in a breeding experiment
• First Filial Generation (F1) = the first generation
of offspring in a breeding experiment
• Second Filial Generation (F2) = the second
generation of offspring in a breeding experiment
TT
tt
Tt
Tt
TT
Tt
Tt
tt
P1 Monohybrid Cross
• Trait: Seed Color
• Alleles: Y – Yellow
y – Green
• Cross: Yellow seeds X Green seeds
YY
X
yy
Crossing two true-breeding (pure) plants
y Yy
y
Yy
Y
Yy
Yy
Y
Genotype: Yy
Phenotype: Yellow
Genotypic Ratio: 100% Yy
Phenotypic Ratio: 100% Yellow
F1 Monohybrid Cross
• Trait: Seed Color
• Alleles: Y – Yellow
y – Green
• Cross: Yellow seeds X Green seeds
Yy
X
Yy
Crossing to heterozygotes (hybrids)
Y YY
y
Yy
Y
Yy
yy
y
Genotype: YY, Yy, yy
Phenotype: Yellow and Green
Genotypic Ratio: 25% YY, 50%
Yy, 25% yy (1:2:1)
Phenotypic Ratio: 75% Yellow,
25% Green (3:1)
Dihybrid Crosses
• A breeding experiment that tracks the inheritance
of two traits
• Mendel’s “Law of Independent Assortment”
• Each pair of alleles segregates independently
during gamete formation
• Formula: 2n (n = # of heterozygotes)
Example:
1. RrYy: 2n = 22 = 4 possible gametes
RY
Ry
rY
ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD
ABCd
AbCD
AbCd
aBCD
aBCd
abCD
abCD
Dihybrid Crosses
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
copyright cmassengale
18
Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
copyright cmassengale
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
19
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
copyright cmassengale
20
Test Cross
•Test crosses involve breeding the individual in
question with another individual that expresses a
recessive version of the same trait. If all
offspring display the dominant phenotype, the
individual in question is homozygous dominant; if
the offspring display both dominant and recessive
phenotypes, then the individual is heterozygous
y Yy
y
Yy
Y
Y
Yy
Yy
Offspring all yellow!
Y
y
y
y
Yy
yy
Yy
yy
½ Offspring yellow; ½ Offspring green!
Test Cross
• A mating between an individual of unknown
genotype and a homozygous recessive
individual.
• Example: bbC__ x bbcc
•
•
•
•
•
•
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
copyright cmassengale
22
Test Cross
Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
copyright cmassengale
bC
b___
c
bbCc
bbcc
23
Incomplete Dominance
and
Codominance
copyright cmassengale
24
Incomplete Dominance
• F1 hybrids have an appearance
somewhat in between the phenotypes
of the two parental varieties.
• Example: snapdragons (flower)
red (RR) x white (rr)
r
RR = red flower
rr = white flower
r
R
R
copyright cmassengale
25
Incomplete Dominance
r
r
R Rr
Rr
R Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
copyright cmassengale
26
Incomplete Dominance
copyright cmassengale
27
Codominance
• Two alleles are expressed (multiple
alleles) in heterozygous individuals.
• Example: blood type
•
•
•
•
1.
2.
3.
4.
type
type
type
type
A
B
AB
O
=
=
=
=
IAIA or IAi
IBIB or IBi
I AI B
ii
copyright cmassengale
28
Codominance Problem
• Example:
homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
copyright cmassengale
1/2 = IAIB
1/2 = IBi
29
Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
copyright cmassengale
1/2 = IAi
1/2 = IBi
30
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)
copyright cmassengale
31
Codominance
• Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
copyright cmassengale
32
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Sex chromosomes are X and Y
• XX genotype for females
• XY genotype for males
• Many sex-linked traits carried on
X chromosome
copyright cmassengale
33
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
copyright cmassengale
34
Sex-linked Trait Problem
• Example: Eye color in fruit flies
•
(red-eyed male) x (white-eyed
female)
Xr
Xr
XRY
x
XrXr
• Remember: the Y chromosome
in
XR
males does not carry traits.
• RR = red eyed
Y
• Rr = red eyed
• rr = white eyed
copyright cmassengale
35
Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
copyright cmassengale
36
Female Carriers
copyright cmassengale
37