Download Mendelian Genetics

Mendelian Genetics
Bio 181 – Week 1
Goals
• Describe simple genetic
dominance, incomplete
dominance, and lethal
inheritance.
• Describe possible genotypes for
some of your personal traits
inherited as dominant and
recessive genes.
• Explain the importance of
Mendel’s Law of Segregation
and Law of Independent
Assortment
• Distinguish between an
organism’s phenotype and
genotype.
Today
• 17.1 – Determine genotypic and phenotypic ratios for
albinism
• 17.2 – Determine color ratios for corn plants
• Incomplete dominance
• 17.3 – Determine blood type for ABO system
• 17.4 – Determine Rh
• Sex-linked inheritance
• 17.5 – Analyze a pedigree of inheritance of cystic fibrosis
• 17.6 – Analyze a pedigree of inheritance of Huntington’s
disease
• Work in groups of 2-4, complete handout and turn in
before leaving.
Gregor Mendel
• “Experiments in Plant
Hybridization” – published in
1866, noticed in 1900.
• Replaced “blending theory”
of inheritance with
“particulate theory”
Gregor Mendel – Experimental Design
• Used the garden pea, Pisum
sativum as a model organism
– Easily recognizable traits: e.g.,
flower color, seed shape
– Could control which individuals
were mating (removed male
reproductive organ before
pollen formed)
• Obtained “pure lines” or “truebreeding lines” from breeders
in which a trait was always
seen
1
Removed stamens
from purple flower
2 Transferred sperm-
bearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
3 Pollinated carpel
Stamens
Carpel (male)
(female)
matured into pod
4 Planted seeds
from pod
5 Examined
First
generation
offspring
(F1)
offspring:
all purple
flowers
Mendel’s Experiments with a Single Trait
• Blending inheritance
hypothesis: claimed that
traits observed in a mother
and father blend together to
form the traits observed in
their offspring. As a result,
traits are an intermediate
between the mother’s and
father’s.
Parental
X
?X?
? ?
? X ? F1
? ? F2
Mendel’s Experiments with a Single Trait
• Mendel let the F1 generation
mature and self-pollinate.
• Monohybrid cross –
between two parents that
each carry two different
genetic determinants for the
same trait (each F1
individual had a smooth and
wrinkly parent)
• Outcome: 5474 round, 1850
wrinkly! (~3:1 ratio)
X
Parental
?X?
? X ? F1
F2
Mendel’s Experiments with a Single Trait
• Outcome: 5474 round, 1850
wrinkly! (~3:1 ratio)
• Mendel coined these terms:
– Recessive – the wrinkled-seed
shape seemed to recede or
become hidden.
– Dominant – the round seeds
appeared to dominate.
X
Parental
?X?
? ?
? X ? F1
? ? F2
Mendel’s Experiments with a Single Trait
• Repeated this experiment
with other traits, had
identical results, with a
ratio of 3:1 in F2.
• Mendel formulated the
Particulate Inheritance
Hypothesis – Hereditary
determinants maintain
their integrity from
generation to generation.
Instead of blending
together, they act as
discrete entities or
particles.
X
Parental
?X?
? ?
? X ? F1
? ? F2
Particulate Theory
• Inherited characters are determined by
particular factors (now called genes).
• These factors occur in pairs (i.e., genes occur
on maternal and paternal homologous
chromosomes).
• When gametes form, these genes segregate so
that only one of the homologous pair is
contained in a particular gamete.
Mendel’s Laws
• Mendel’s First Law, Law of Segregation – Members
of each gene pair must separate into different
gamete cells during the formation of eggs and
sperm. Each gamete has an equal chance of
possessing either member of a pair of homologous
Parental Genotype SS
chromosomes.
x
ss
Parental Gametes
F1 Genotype
S
s
Ss
Punnett Squares
•
•
•
•
•
Monohybrid cross
A = Yellow
a = Green
Genotype ratio: 1:2:1
Phenotype ratio: 3:1
Example (17.1)
• Say we have a yellow pea plant that had a green pea
plant as the mother. We cross the yellow pea plant to
a green pea plant.
• A = Yellow
• a = Green
1 x?
• What is the genotype
of pea #2?
• Of pea #3?
• What possible gametes can
pea #2 produce?
2 x3
17.2 - Determine color and height
ratios for corn plants
• Examine ears of corn with
different color grains
• Record observations and
determine the probable
genotypes of the parents of
each cross.
• Each kernel is a seed/individual.
• Cross involving two traits
Experiments with Two Traits
• Do different genes segregate
together or independently?
• The Dihybrid Cross
YYRR
– Crossing two individuals heterozygous
for two traits.
• Possibilities:
– Independent assortment: seed shape
and color will separate from each
other and be transmitted
independently.
– Allele for seed shape and seed color
present always as seen in parent
plants, meaning genes are transmitted
together.
Parental
yyrr
F1
x
YyRr
YyRr
Mendel’s Laws
• Mendel’s Second Law, Law of Independent
Assortment – Genes on nonhomologous or
different chromosomes will be distributed
randomly into gametes.
• Anaphase of Meiosis I
– Chromosomes segregate randomly
Codominance - Blood Type
• Phenotypes of both
alleles expressed at
the same time.
• Example: AB blood
phenotype, IAIB
genotype
• Today: Test blood for
phenotype,
determine genotype
and possible
paternity
Sex Linkage and Chromosome Theory
• Sex-linked traits are located on
the sex chromosomes (X or Y)
fruit fly
eye color
XX - female
XY - male
Polygenic Traits
• Trait influenced by more than one gene
• Examples: Blood type (e.g., O-); fur color in
many species
Pleiotrophy
• One gene influences more than one trait
• Example: Sickle-cell anemia causes sickleshaped red blood cells but also resistance to
malaria
Environmental Influence on Gene
Expression
• Phenotypes of most genes strongly influenced
by the environment
• PKU (Phenylketonuria)
• Winter coats
Mendelian Genetics in Real Life
• Most traits are controlled by many genes, so
Punnett squares may oversimplify
• Commonly used in selective breeding on
simple traits
• May be used in genetics counselling
Today
•
•
•
•
•
•
•
•
17.1 – Determine genotypic and phenotypic ratios for albinism
17.2 – Determine color ratios for corn plants – use from blue bin
Incomplete dominance
17.3 – Determine blood type for ABO system
17.4 – Determine Rh
Sex-linked inheritance - info only in real lab manual
17.5 – Analyze a pedigree of inheritance of cystic fibrosis
17.6 – Analyze a pedigree of inheritance of Huntington’s disease
• Answer questions on handout and turn in before leaving
• Work in groups of 2-4, complete handout and turn in before
leaving.
• Study for Quiz next week!