Transcript Mendelian Genetics - Tri-County Technical College

Mendelian Genetics
Bio 100
Tri-County Tec. College
Pendleton, S. C.
Contributions of Mendel
• Inherited characteristics are carried
as discrete units.
• These discrete units are parceled
out in different ways with each
generation.
Mendel and Garden Peas
• Mendel chose garden peas to work with.
• Good Choice
– abundantly available at his monastery
– easily cultivated
– varieties had clearly defined characteristics
which bred true (“wild-type” trait)
– pollen of the pea is contained within each
flower--prevent accidental crosspollination
Garden Pea Flower
Genetic Terminology
• Gene
– a unit of heredity; a segment or locus on a
chromosome influencing a single trait
• Allele
– alternate forms of the same gene
– A plant has a gene for flower color with an
allele for red flowers and an allele for white
flowers.
Terminology, ctd.
• Phenotype
– the visible characteristics of an organism; for
example: tallness or shortness
• Genotype
– the genetic makeup of an organism that you
cant see; for example TT for tallness or tt for
shortness
Terminology, ctd.
• Homozygous
– describing the genotypic condition
present when members of a gene pair
determining a single trait on homologous
chromosomes are the same. TT for
tallness is homozygous. tt for shortness
is also homozygous Both are alleles of
the gene for height.
Terminology, ctd.
• Heterozygous
– Term that describes the genotypic condition
present when members of a pair of alleles are
different on homologous chromosomes. For
example, Tt.
• Dominant
– describes a trait (allele) that is expressed
whenever it is present
Terminology, ctd.
• Recessive
– describes a trait (allele) that is expressed
whenever the dominant allele is not a part of
the genotype.
• Please note***
– Dominant alleles are usually noted with a
capital letter; while recessive alleles are noted
in small caps.
Terminology, ctd.
• P1 (parental generation)
– the generation that you start with in a genetic
study
• F1 (first filial generation)
– offspring of the P1 parents
• F2 (second filial generation)
– offspring of the crossing of members of the F1
generation
Law of Segregation of Alleles
• It says-– a pair of alleles is segregated or separated
during the formation of gametes during
reduction division (meiosis)
• For example:
– “Aa” alleles will segregate with the A allele
going into one gamete and the a going into
another gamete.
Traits Mendel Studied
Probability vs. possibility
• Weather forecast states probability of rain
tomorrow is 30%….WRONG!!!
• Probability is defined as the number of
times an event occurs divided by total
number of possible outcomes
• Probability of rain on any given day is
50%…it will either rain or it will not rain
• Time for an Estesism
Explanation
P1 Generation:
F1 Generation:
PP x pp
Pp
(all purple)
Explanation
Cross-pollination of members of the F1
Pp x Pp
F2
PP Pp Pp pp
Purple
White
Gametes
• The genotype of the P1 is “PP” and “pp”.
– the “PP” parent will produce gametes
containing only the “P” allele
– the “pp” parent will produce gametes
containing only the “p” allele
• The genotype of the F1 parents is “Pp”.
– 50% of the gametes will contain “P”
– 50% of the gametes will contain “p”.
Making the cross
• The cross between P1
is no problem
– “P” combines with “p”
to produce “Pp”
• To do the cross
between F1 members
– Punnett square
• Each space is 25%
chance.
P p
P PP Pp
p Pp pp
Dated but still useful concept
• Field of corn and we can see its all tall and
we know tall is dominant trait
• Do we know the genotype?
• In old days, testcross would be used to
determine genotype
• TESTCROSS is mating of an unknown
genotype with individual homozygous
recessive for the trait
• Do the Punnett square
The Dihybrid Cross
• Some characteristics are inherited together
in pairs.
• Mendel worked with seed color and seed
surface texture.
• Phenotypes
– yellow and green seeds; smooth and wrinkled
– yellow dominates green; smooth dominates
wrinkled
P1
pure breeding X purebreeding
smooth-yellow
F1
wrinkled-green
all smooth-yellow
smooth-yellow x smooth-yellow
F2
315 smooth-yellow
101 wrinkled-yellow
108 smooth-green
32 wrinkled-green
9:3:3:1 ratio
P1 Gametes
• SSYY bred with ssyy
• SSYY can produce only one kind of
gamete: SY
• ssyy can produce only one kind of gamete:
sy
• When these are combined you get the
genotype for the F1 generation: SsYy
Determining F2 Gametes
• To determine the gametes for the F2 we
have to segregate each pair of alleles
independently of each other
S
s
Y
y
Y
y
SY
Sy
sY
sy
Making The Cross
• Use a 16 chambered Punnet Square
SY
Sy
sY
sy
SY
SSYY SSYy SsYY SsYy
Sy
SSYy SSyy SsYy Ssyy
sY
SsYY SsYy ssYY ssYy
sy
SsYy Ssyy
ssYy ssyy
Summarizing The Results
• Since smooth dominates wrinkled and
yellow dominates green
–
–
–
–
S_Y_ : smooth yellow -- 9 of those
S_ yy: smooth green -- 3 of those
ssY_ : wrinkled yellow -- 3 of those
ssyy : wrinkled green -- 1 of those
Blood Types
• Due to an antigen (protein) on the RBC
• Due to multiple alleles
• Three alleles for blood type
– A-allele
– B-allele
– O-allele
• Only two of these alleles per person
Codominance of Blood Types
• The A and B alleles dominate the O allele
• When they are together, the A and B alleles
show incomplete dominance (codominance)
• All the possible genotypes for blood types
–
–
–
–
type A genotypes: AA and AO
type B genotypes: BB and BO
type AB genotype: AB
type O genotype: OO
Incomplete or Lack of
Dominance
• Really does look like “blending”
• Offspring is intermediate between the two
parents
• Red flower crossed with white flower
giving all pink offspring
• White hen and black rooster=all gray chicks
• Red bull and white cow = roan calf
Polygenic Inheritance
• Prefix “poly” means many
• Polygenic inheritance is controlled by the
interaction of many genes
• Human skin color
– AA BB CC DD -- very dark skin
– Aa Bb Cc Dd -- medium
– aa bb cc dd -- very light
• height is also polygenic controlled
Pleiotropy
• Opposite to polygenic inheritance
• In pleiotropy, one gene results in the
expression on many different phenotypes
• Best example is sickle cell disease
• Study 9.21 on page 157
Sex Determination
• A human female always produces an ovum
with an X-chromosome
• 50% of sperm contain a X-chromosome
• 50% of sperm contain a Y-chromosome
• X-sperm + X-ovum = XX (female)
• Y-sperm + X-ovum = XY (male)
Punnet Square for Sex
Determination
X
X
X
XX
XX
Y
XY
XY
Sex Linkage
• The X-chromosome carries alleles for
various traits.
• The Y-chromosome does not carry alleles
for the traits we will discuss.
• Some traits are sex-linked because their
alleles are linked to the X-chromosome
• We will look at colorblindness and
hemophilia.
Colorblindness
•
•
•
•
•
•
XCY -- normal male
XcY -- colorblind male
XCXC-- normal female (homozygous)
XCXc-- normal female (heterozygous)
XcXc -- colorblind female (homozygous)
Colorblind females are relatively rare.
Recessive disorders
• Majority of disorders affecting humankind
are recessive in nature
• Albinism
• Cystic fibrosis
• Galactosemia
• Phenylketonuria (PKU) (care for diet soda?)
• Sickle-cell disease
• Tay-Sachs disease
Dominant disorders
• Achondroplasia
• Alzheimer’s disease (this one is still kind of
if’y)
• Huntington’s disease
• Hypercholesterolemia
• ???markers for alcoholism, homosexuality,
some mental disorders…????
Pedigree Analysis
• Pedigree is family tree describing the occurrence
of heritable characteristics in parents and offspring
across many generations
• Where would one find most of the pedigrees in
this part of the country?
• Useful in genetic counseling
• Like it or not…”Roots” are important, especially
as one gets older and more aware of his/her
mortality
Can’t choose your parents, yet!!
• A carrier is an individual who is
heterozygous for a recessive inherited
disorder and who therefore DOES NOT
show symptoms of that disorder
• Carrier does have probability of passing the
recessive gene on to offspring
• Queen Elizabeth and the royal houses of
Europe
Fetal testing procedures
•
•
•
•
•
Ultrasound
Amniocentesis
Chorionic villus sampling (CVS)
Fetoscopy
All except ultrasound carry some risk for
both mother and fetus
• Big time difference in obtaining results
• What choice would (should) one make???