Transcript Mendel`s Laws of heredity

Chapter 10.1
MENDEL’S LAWS OF
HEREDITY
Who is Gregor Mendel?
 A monk in an Austrian
monastery
 Carried out the first
important studies of
heredity
 First to succeed in
predicting how traits are
passed from parents to
offspring
 Bred garden pea plants to
study inheritance
Why Mendel Chose Pea Plants
 They are genetically
simple
 Mendel could study one
trait at a time
 They reproduce sexually
 He could control breeding
 Mendel chose which plants
to cross & studied one trait
at a time
 They grow quickly
Breeding
Plants
 Reproductive Parts:
 Male = Stamen
 Female = Pistil
 Pollination = pollen from the anther is transferred
to stigma
 Pollination can be controlled
Traits observed
•Flower color
•Stem length
•Seed color
•Seed shape
•Flower Position
•Pod color
•Pod shape
Monohybrid Crosses
 Mendel carefully chose purebred (true-breeding) pea
plants.
 Monohybrid crosses look at one trait at a time
 Example: flower color
Crossing Pea Plants:
1. Mendel crossed purpleflowered plants with whiteflowered plants
2. Mendel planted the seeds,
then allowed the F1 plants to
self-fertilize
3. The resulting offspring F2
showed a 3:1 ratio of purple
flowers
Terminology
P = parental generation
F1 = first Filial generation
F2 = second Filial generation
Rule of Unit Factors
 Organisms have 2 factors that control each
trait
 one from each parent
 Alleles = different forms of a gene
 Example: height may be tall or short; peas
may be yellow or green, round or wrinkled.
Rule of Dominance
 Dominant = the trait that is observed
whenever it is present
 Shown as CAPTAL letters
 In pea plants, Tall is dominant written as T
 Recessive = the trait that is hidden if a
dominant trait is present
 Shown as lower-case letters
 In pea plants, Short is recessive  written as t
Mendel’s 2 laws
1. Law of Segregation
 Each parent has 2 alleles
that separate (segregate)
during meiosis
 Gametes form random
pairs during fertilization
2. Law of Independent Assortment
 Genes for different traits are inherited
independently of one another
Genotypes & Phenotypes
 Phenotype = the way an organism
looks and behaves
 Usually a description
 Example: Plant height = tall or short
 Genotype = the genetic
combination for an organism
 Usually the combination of alleles
 Example: TT or Tt both produce tall
plants, tt produces a short plant
 You can’t always identify the
genotype from the phenotype
Genotypes
 Homozygous = two like alleles (TT or tt)
 True-breeding plants are generally
homozygous
 Heterozygous = two different alleles (Tt)
Crosses
 Monohybrid Cross = a cross involving one trait
 Dihybrid cross = a cross involving two different
traits
 Parents: True-breeding Round, Yellow peas &
Wrinkled, Green peas
 F1 generation = all Round, Yellow peas
 F2 generation – F1 generation self-pollinated to
produce a mixture of offspring
 Yellow, round; Yellow, wrinkled; Green, round; Green,
wrinkled
 The F2 phenotype ratio was 9:3:3:1
Punnett Squares
 Created by Reginald
Punnett in 1905
 Used as a tool to predict all
of the possible outcomes of
a genetic cross
Monohybrid Cross
 Punnett Square is 2 x 2
 Steps:
 set up the square
 do the cross
 determine offspring genotypes
 determine offspring phenotypes
DRAW THIS
Genotype Ratios
 Genotype looks at all of the
possible genetic combinations
 To determine ratios, look at each
pair inside of the square:
TT = homozygous dominant (tall)
2. Tt = heterozygous
3. tt = homozygous recessive (short)
1.
 Genotypes:
 1 TT, 2 Tt, 1 tt
 Genotype Ratio = 1:2:1
Phenotype Ratios
 Phenotype looks at the trait
that will be expressed
 Doesn’t account for “hidden”
recessive alleles
 Any offspring combination that
has the dominant allele will show
the dominant trait.
 Phenotypes:
 3 Tall, 1 short
 Ratio = 3:1
Dihybrid Cross
 Punnett Square is 4 x 4
 Steps:
1.
2.
3.
4.
5.
Figure out the gametes
set up the square
do the cross
determine offspring
genotypes
determine offspring
phenotypes
Probability
 The chance of getting a certain outcome
 Example: A coin has two sides – heads & tails
 The probability of getting heads is ½ or 1:2
 Probability in Punnett
Squares: Tt x Tt
 The probability of getting a
tall plant is ¾ or 75%
Another Type of Cell Division to Produce Gametes
MEIOSIS
What is Meiosis?
 A process
 Occurs in sex cells
 Only occurs in
eukaryotes.
 Plants
 Animals
 Reduces the
amount of
chromosomes by
half
 Makes gametes
 Reproduction Cells
 Occurs in the
gonads
Number of Chromosomes in a
Cell
 Haploid: contains one set of chromosomes
 N = 23
 Gamete cells
 Diploid: contains two sets of chromosomes
 One from each parent
 2n = 2(23) = 46
 Humans (except for gametes)
 Some plants and animals
Number of Chromosomes in a
Cell
 Homologous
 Pair of
chromosomes
 One member
obtained from the
mother
 The other is
obtained from the
father
Phases of Meiosis
 Two successive nuclear
divisions
 Meiosis I (Reduction of genes)
 Meiosis II (Division)
 Produces 4 haploid cells
Meiosis I

Prophase I: two events occur.
Homologues chromosomes pair up.
2. Crossing-over may occur at this point.
 Chromatids break and may be reattached to a
different homologous chromosome.
1.
Prophase I
Metaphase I
 Tetrads line-up along
the equator of the
spindle
 Spindle fibers attach to
the centromere
Anaphase I
 Tetrads separate
 Independent
assortment of
homologous
chromosomes.
 Drawn to opposite
poles by the spindle
fibers
 Centromere in
anaphase I remain
intact
Telophase I
 One set of
(replicated)
chromosomes is in
each "cell.
Meiosis II (Similar to
Mitosis)
 Prophase II
 Nuclear envelopes (if
they formed during
telophase I) dissolve
 Spindle fibers reform
Metaphase II
 spindles moving
chromosomes into
equatorial area and
attaching to the
opposite sides of the
Centromere
Anaphase II
 Centromere split and
the former
chromatids (now
chromosomes) are
segregated into
opposite sides of the
cell.
Telophase II
 identical to Telophase
of mitosis
Cytokinesis
 Cytokinesis
separates the cells
Nondisjunction,
 An abnormality that
occurs by the failure of
replicated
chromosomes to
segregate during
Anaphase II.
 Trisomy
 Extra chromosome
 Monosomy
 Missing a chromosome
 Triploidy
 Extra set of
chromosomes