Chapter 10.1. Who is Gregor Mendel? A monk in an Austrian monastery Carried out the first...

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