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Chapter 10.1
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
•Seed color
•Flower Position
•Seed shape
•Pod shape•Pod color
•Stem length
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 purple-
flowered plants with white-flowered 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
F2 = second Filial generation
F1 = first Filial generation
P = parental 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:1. TT = homozygous dominant
(tall)2. Tt = heterozygous3. tt = homozygous recessive
(short) 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. Figure out the gametes
2. set up the square3. do the cross4. determine offspring
genotypes5. 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
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.1. Homologues chromosomes pair up.2. Crossing-over may occur at this point. Chromatids break and may be
reattached to a different homologous chromosome.
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