CHAPTER 11 INTRODUCTION TO

GENETICS11-1 The Work of Gregor Mendel

genetics – the scientific study of heredity

A. Gregor Mendel’s Peas

- Austrian monk who worked with pure strains of pea plants called true-breeding plants

- True-breeding plants would self-pollinate and produce offspring identical to themselves (one parent)

Ex. tall plants would produce only tall plants

- pea plants can also cross-pollinate meaning male sex cells in pollen from one plant can fertilize the eggs of a flower from another plant (two parents)

- Mendel selected certain characteristics and performed experiments by crossing the seeds of those selected plants

B. Genes and Dominance

- Mendel studied 7 specific pea plant traits

- A trait is a specific characteristic such as seed color or plant height

- Each of the 7 traits studied, had different forms such as green seed color and yellow seed color, tall or short height, etc. These different forms are called alleles

- Mendel crossed each trait and studied the offspring

P = parent generation

F1 = first offspring generation

Hybrids – the offspring of crosses between parents with different traits

- Mendel’s F1 hybrids showed characteristics of only one parent and it appeared that the second parental trait disappeared

Traits Mendel Tested

Two conclusions were drawn from Mendel’s experiments:

1. Biological inheritance is determined by factors that are passed from one generation to another. These factors are called genes.

2. Principle of dominance

- some alleles are dominant and some are recessive

- the dominant allele or form of the trait always shows

- the recessive allele is hidden and only is visible when the dominant form is absent

C. Segregation

- to determine if the recessive alleles disappeared or were simply hidden, Mendel crossed all F1 hybrid generations by self-pollination to produce an F2 generation

F1 x F1 = F2 (second filial generation)

1. The F1 cross

- traits controlled by the recessive allele reappeared

- ¼ (25%) of F2 plants showed the trait was controlled by the recessive allele

2. Explaining the F1 cross

- reappearance of the trait controlled by the recessive allele indicated that the allele had at some point been separated from the dominant allele (segregation)

- Mendel suggested that segregation of different alleles occurs during the formation of gametes (sex cells)

- Each gamete carries only one copy of each gene

- Each F1 plant produces two types of gametes; those with the dominant and those with the recessive allele

- The alleles get paired up again during fertilization when the gametes fuse in the F2 generation leaving the F2 generation with new combinations of alleles

11-2 Probability and Punnett Squares

A. Genetics and Probability

Probability

- the likelihood that a particular event will occur

- past outcomes do not affect future ones

- the way in which alleles segregate is completely random, therefore, we can use the principles of probability to predict the outcomes of genetic crosses

B.Punnett Square

- a diagram used to determine the gene combinations that might result from a genetic cross (See Fig 11-7)

- organisms with two identical alleles for a particular trait (true-breeding) are homozygous (TT)

- organisms that have two different alleles for a particular trait (hybrid) are heterogygous (Tt)

- the phenotype represents the physical characteristics (what you see) (tall or short)

- having the same phenotype does not mean they are genetically the same (genotype)

Ex. TT (homozygous) tall

Tt (heterozygous) tall

- therefore, plants with the same phenotype may have a different genotype

C. Probability and Segregation

See. Fig 11-7

¼ of F2 plants are TT (tall)

2/4 (1/2) of F2 plants are Tt (tall)

¼ of F2 plants are tt (short)

3 : 1 ratio

3 tall : 1 short

- Mendel’s prediction of 3 tall to 1 short proved to be correct for each of the 7 crosses and segregation occurred according to his model

D. Probabilities Predict Averages

- probabilities predict the outcomes of a large number of events but not of an individual event

- this is also true of genetics; the larger the number of individuals, the closer the resulting offspring number will get to the expected values

11-3 Exploring Mendelian Genetics

Does the segregation of one pair of alleles affect the segregation of another?

A. Independent Assortment

- Mendel performed an experiment called the two-factor cross

1.The two-factor cross: F1

See Fig 11-9 pg. 270

RRYY (yellow,round) x rryy(wrinkled, green)

- F1 generation was RrYy (round,yellow) showing the alleles for yellow and round are dominant over the alleles for wrinkled and green

See. Fig 11-10 pg. 271

- The F1 cross did not show whether or not the genes assorted or segregated independently

2. The two-factor cross: F2

- Mendel crossed the F1 generation which were all heterozygous for both seed shape and seed color: RrYy x RrYy = F2

- the F2 generation offspring showed combinations of alleles and phenotypes not seen in either parent meaning that the alleles for seed color segregated independently from those of seed shape

- this is a principle known as independent assortment and the genes for seed shape and seed color do not influence each other’s inheritance.

***** the principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes

B. Summary of Mendel’s Principles

1. The inheritance of biological characteristics is determined by individual traits known as genes. Organisms that reproduce sexually pass genes from parent to offspring.

2. Where two or more forms of a gene exist, some forms of genes may be dominant and some may be recessive

3. In sexually reproducing organisms, each adult has two forms of a gene (one from each parent) and those genes are segregated from each other when the gametes are formed

4. Alleles for different genes usually segregate independently from one another

C. Beyond Dominant and Recessive Alleles

- there are exceptions to the principles discovered by Mendel

- some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes

1. Incomplete dominance

- cases in which one allele is not completely dominant over another Fig 11-11 pg. 272

- the heterozygous phenotype is in between the two homozygous phenotypes (blending)

Incomplete Dominance

2. Codominance

- both alleles contribute to the phenotype of the organism

- seen in some varieties of chicken and cattle

3. Multiple Alleles

- some genes have more than two alleles

- seen in rabbit coats and blood types

- the combination of alleles display a pattern of dominance that can result in different phenotypes

4. Polygenic Traits

- traits controlled by two or more genes exhibiting a wide range of phenotypes

Ex. skin color: controlled by as many as 4 different genes

D. Analyzing Mendel’s Principles

- the fruit fly, Drosophila melanogaster, is used frequently to study genetics because it can breed a new generation of offspring every 14 days with as many as 100 offspring.

- Mendel’s principles also apply to humans and have been used to study the inheritance of human traits and calculate probabilities of their appearance in the next generation

Ex. Albinism (no melanin)

11-4 MeiosisThe process by which gametes (sex cells) are formed and end up with only one set of genes

All other cells in the body have two sets of genes, one from each parent

A. Chromosome number

- cells that contain two complete sets of homologous chromosomes and two complete sets of genes are said to be diploid

- this agrees with Mendel’s principle that cells of adult organisms contain two copies of each gene

- gametes of sexually reproducing organisms contain only one set of chromosomes and one set of genes

- gametes are said to be haploid

B. Phases of Meiosis

- Meiosis is a process of reduction division by which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell

- Meiosis involves two phases, Meiosis I and Meiosis II

- At the end of Meiosis II, the diploid cell that entered Meiosis has become 4 haploid cells

1. Meiosis I

- replication has already taken place

- cells begin to divide in a way similar to mitosis

Prophase I

- each chromosome pairs with its corresponding homologous chromosome to form a tetrad (contains 4 chromosomes)

- as homologous chromosomes pair up and form a tetrad, they may exchange portions of their chromatids in a process called crossing-over

- crossing-over results in the exchange of alleles between homologous chromosomes and produces new combinations of alleles

Crossing Over

Metaphase I

- spindle fibers attach to the chromosomes

Anaphase I

- Spindle fibers pull the homologous chromosomes to the opposite ends of the cell

Telophase I and Cytokinesis

- the nuclear membrane forms, the cell separates into two new cells each with 46 chromosomes

Meiosis II:

- The second meiotic division without replication

Prophase II

- the chromatids pair up

Metaphase II

- The chromosomes line up in the center of each cell attached to spindle fibers

Anaphase II

- the paired chromatids separate and move to opposite ends of the cell

Telophase II and Cytokinesis

- each cell divides into two daughter cells (4 cells) each with one half the number (haploid) of chromosomes

C. Gamete Formation (sex cells)

Sperm – haploid gametes produced by the male

Egg – haploid female gamete

- divisions at the end of Meiosis I and Meiosis II are uneven with most of the cytoplasm going to the one egg cell. The other three are called polar bodies and usually do not participate in reproduction.

D. Comparing Mitosis and Meiosis

Mitosis

- results in 2 genetically identical diploid cells

Meiosis

- results in 4 genetically different haploid cells

11-5 Linkage and Gene Maps - A gene map shows the relative location of genes on a chromosome

- The frequency of crossing over between genes is used to produce a map of distances between genes

- The farther apart the genes, the more likely they are to be separated during crossing-over in Meiosis, therefore, the frequency of crossing-over is equal to the distance between two genes