Download Mendelian Genetics Gregor Mendel Generations Law of

Mendelian Genetics
Gary Bates PhD
Gregor Mendel
• Austrian monk who formulated
fundamental laws of heredity in early
1860s.
– Studied science and mathematics at
University of Vienna.
– Conducted breeding experiments with the
garden pea Pisum sativum.
– Formulated the the particulate theory of
inheritance
•
•
•
•
•
•
•
•
Monohybrid
Dihybrid
Incomplete
Polygenic
Sex and x linked
Linkage maps
Changes in chromosome number
Changes in chromosome structure
Generations
– Performed reciprocal crosses
• Parental generation = P
• First generation offspring = F1
• Second generation offspring = F2
• Genes reshuffled
Law of Segregation
Mendel’s Monohybrid Cross
• Each individual has two factors for each trait.
• The factors segregate during gamete
formation.
• Each gamete contains only one factor from
each pair of factors.
• Fertilization gives each new individual two
factors for each trait.
1
Mendel’s Monohybrid Cross
Modern Genetics View
• Each trait in a pea plant is controlled by
two alleles (alternate forms of a gene).
• Dominant allele (capital letter) masks the
expression of the recessive allele (lowercase).
• Alleles occur on a homologous pair of
chromosomes at a particular gene locus.
– Homozygous = identical alleles
– Heterozygous = different alleles
Genotype Versus Phenotype
• Genotype
– Refers to the alleles an individual receives at
fertilization.
• Phenotype
– Refers to the physical appearance of the
individual.
Testcross
Testcross
• A monohybrid testcross is used to
determine if an individual with the
dominant phenotype is homozygous
dominant or heterozygous for a particular
trait.
Two-Trait Inheritance
• Mendel performed cross using truebreeding plants differing in two traits.
– Dihybrid Cross
• Observed phenotypes among F2 plants.
– Formulated law of independent assortment
2
Law of Independent Assortment
Dihybrid Cross
• Each pair of factors segregates
independently of the other pairs.
• All possible combinations of factors can
occur in the gametes.
Dihybrid Cross
Two-Trait Testcross
• A two-trait testcross is used to determine if
an individual is homozygous dominant or
heterozygous for either of the two traits.
Two-Trait Testcross
Human Genetic Disorders
• Autosome - Any chromosome other than a
sex chromosome.
– When a genetic disorder is autosomal dominant,
an individual with AA or Aa has the disorder.
– When a genetic disorder is autosomal recessive,
only aa individuals have the disorder.
• Carriers - Individuals unaffected by a disorder but can
have an affected child.
3
Autosomal Recessive Pedigree
Autosomal Recessive Disorders
• Tay-Sachs Disease
– Progressive deterioration of psychomotor
functions.
• Cystic Fibrosis
– Mucus in bronchial tubes and pancreatic
ducts is particularly thick and viscous.
• Phenylketonuria
– Lack enzyme for normal metabolism of
phenylalanine.
Autosomal Dominant Pedigree
Autosomal Dominant Disorders
• Neurofibromatosis
– Tan or dark spots develop on skin and
darken. Small, benign tumors may arise from
fibrous nerve coverings.
• Huntington Disease
– Neurological disorder leading to progressive
degeneration of brain cells, in turn causing
severe muscle spasms and personality
disorders.
Incomplete Dominance
Incomplete Dominance
• Exhibited when the heterozygote has an
intermediate phenotype between that of
either homozygote.
4
Multiple Allelic Traits
Inheritance of Blood Type
• When a trait is controlled by multiple
alleles, the gene exists in several allelic
forms.
– ABO blood types
Phenotype
A
B
AB
O
Genotype
IAIA,IAi
IBIB,IBi
IAIB
ii
Polygenic Inheritance
Polygenic Inheritance
• Occurs when a trait is governed by two or
more sets of alleles.
• Occurs when a trait is governed by two or
more sets of alleles.
– Each dominant allele has a quantitative effect
on the phenotype, and these effects are
additive.
– Each dominant allele has a quantitative effect
on the phenotype, and these effects are
additive.
• Result in continuous variation of phenotypes.
• Result in continuous variation of phenotypes.
Terminology
Environment and Phenotype
• Pleiotropy - A gene that affects more than
one characteristic of an individual.
• Sickle-cell (incomplete dominance)
• Codominance - More than one allele is
fully expressed.
• Himalayan Rabbits - Enzyme coding for
black fur is active only at low
temperatures.
– Black fur only occurs on extremities.
– ABO blood type (multiple allelic traits)
• Epistasis - A gene at one locus interferes
with the expression of a gene at a different
locus.
– Human skin color (polygenic inheritance)
5
Chromosomal Inheritance
• All but one pair of chromosomes in males
and females are the same.
– Autosomes - Nonsex chromosomes
• The different pair, sex chromosomes,
determines the sex of an individual.
– X-linked (sex-linked) is the term used for
genes carried on the X chromosome.
Male (YX)
X-Linked Alleles
• X-linked alleles have a different pattern of
inheritance than alleles on autosomes
because the Y chromosome is blank for
these alleles.
– Inheritance of a Y chromosome cannot offset
the inheritance of an X-linked recessive allele.
Human X-Linked Disorders
• Color Blindness
Female (XX)
– In humans, color vision receptors in the retina
are three different classes of cone cells.
• Only one type of pigment is present in each class
of cone cell.
– The allele for blue-sensitive is autosomal, but the redand green-sensitive proteins are on the X chromosome.
- Only males can determine Sex
Human X-Linked Disorders
Hemophilia Pedigree
• Muscular Dystrophy
– Absence of protein dystrophin allows calcium
to leak into muscle cells.
• Hemophilia
– Hemophilia A due to lack of clotting factor IX
and hemophilia B due to lack of clotting factor
VIII.
6
Gene Linkage
• The existence of several alleles on the
same chromosome.
– Alleles on the same chromosome form a
linkage group because they tend to be
inherited together.
• Can be used to build a linkage map.
Crossing-Over
Constructing a Chromosome
Map
• A linkage map can also be called a
chromosome map because it tells the
order of gene loci on chromosomes.
– If crossing-over occurs between two linked
alleles of interest, a dihybrid produces four
types of gametes instead of two.
Linkage Data
• The percentage of recombinant phenotypes
can be used to map chromosomes because
there is a direct relationship between the
frequency of crossing-over and the distance
between alleles.
– Assumed 1% crossing-over equals 1 map unit.
– A Map unit is also call a centa-Morgan (cM)
Linkage Data
• Black body and vestigial Wings are 18.5 cM
apart
• Black body and Purple Eyes are 6cM apart
• Purple eyes and Vestigial wings are 12.5cM
•
What is the arrangement of the genes ?
Linkage Data
• Green Teeth and Blue Hair is 25cM apart
• Blue Hair and Big eyes are 12cM apart
• Big eyes and Green teeth are 13cM apart
- Draw a linkage map -
7
1. First Look for the genes with the farthest linkage- (Draw)
25cM
GT
BH
Changes in Chromosome
Number
• Polyploidy
2. Draw individual maps for the other genes. Keep
the genes in the same order.
BigE
12cM
GT
BH
13cM
– Occurs when eukaryotes have more than 2n
number of chromosomes.
BigE
3. Combine the Maps in a way that makes sense
25cM
GT
GT
13cM
BigE
BigE
BH
12cM
• Named according to number of sets of
chromosomes.
• Some estimate 47% of flowering plants are
polyploids.-Cotton, Wheat
BH
Changes in Chromosome
Number
• Monosomy and Trisomy
Down Syndrome
• Down syndrome (Trisomy 21) is caused by
three copies of chromosome 21.
– Monosomy (2n - 1) occurs when an individual
has only one of a particular type of
chromosome.
– Trisomy (2n + 1) occurs when an individual
has three of a particular type of chromosome.
Changes in Sex Chromosome
Number
Changes in Chromosome
Structure
• An abnormal sex chromosome number is
the result of inheriting too many or too few
X or Y chromosomes.
– Nondisjunction during oogenesis or
spermatogenesis.
8
Changes in Chromosome
Structure
Extra d e
c and d
Swap positions
9