Chapter 11 Introduction to Genetics

Chapter 11 Introduction to Genetics

Section 11-1 The Work of Gregor Mendel

Be Able To:

• Describe the steps involved in Mendel’s experiments on garden peas.

• Distinguish between dominant and recessive traits.

• State two laws of heredity that were developed from Mendel’s work.

• Explain the difference between an allele and a gene.

• Describe how Mendel’s results can be explained by the scientific knowledge of genes and chromosomes.

Key Terms: ^{Pg 308}

The Experiments of Gregor Mendel

• Genetics is the field of study devoted to understanding how characteristics are transmitted from parents to offspring.

• Gregor Mendel founded

genetics with his detailed study of heredity.

• Mendel chose the common garden pea Pisum sativum.

• Mendel protected his pea plants from self-pollination and

manually transfered the pollen thus cross-pollinating his

garden pea plants.

Gregor Mendel (1822-1884)

Mendel’s Experiments

• The pure parent plants or P₁ generation always produce offspring with a specific trait.

• Mendel’s experiment began with cross pollinating the P₁

generation.

• The offspring called the filial or F₁ generation were allowed to self-pollinate.

• The offspring of the F₁

generation were called the

second filial generation or F₂ generation..

Mendel’s Results

Segregation

• Mendel reasoned that there were a pair of factors

controlling each trait, either dominant or recessive

factors.

• Mendel’s law of segregation states that a pair of factors is separated during the

formation of gametes.

• Mendel’s law of independent assortment states that factors for different characteristics are distributed to gametes

independently.

Chapter 11 Introduction to Genetics

Section 2 Applying Mendel’s Principles

Be Able To:

• Explain how probability is used to predict the results of genetic crosses.

• Use a Punnett square to predict the results of monohybrid and dihybrid crosses.

• Explain how a testcross is used to show the genotype of an individual whose phenotype is dominant.

• Differentiate a monohybrid cross and from a dihybrid cross.

Key Terms: codominance, complete dominance, dihybrid cross, genotype, genotypic ratio, heterozygous, homozygous, incomplete

dominance, monohybrid cross, phenotype, phenotypic ratio, probability, punnett square, and testcross.

Genotype and Phenotype

• The genetic makeup of an individual is its genotype.

• The appearance of an

organism as a result of its genotype is called its

phenotype.

• If an organism is homozygous for a

characteristic they have a pair of alike allleles.

• Heterozygous organisms for a characteristic have two different alleles.

Are Addy and her baby heterozygous for fur color or homozygous?

Probability

• The likelihood that a

specific event will occur is probability.

• Probabilities can be

expressed as fractions or percentages.

• The results predicted by probability are more likely occur when there are many trials.

Predicting Results of Monohybrid Crosses

• A monohybrid cross is a mating cross between

individuals that involve one pair of contrasting traits.

• A punnett square is a model used to establish the

probabilities of the results of a genetic cross.

• The combinations of alleles in the four boxes of a punnett square indicate the possible genotypes that can result from the cross.

• What is the genotypic ratio of a cross between a female pug and a male pug that are both heterozygous for fur color?

What is the phenotypic ratio?

: :

Predicting Results of Monohybrid Crosses

• A testcross is the crossing of an

individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

• An inheritance relationship in which one allele is completely dominant over the other is complete

dominance.

• An inheritance relationship that

occurs when both alleles influence the phenotype is incomplete

dominance.

• An inheritance relationship that

occurs when both alleles for a gene are expressed in heterozygous

offspring is codominance.

B B

B b

b b

b b

Predicting Results of Dihybrid Crosses

• A dihybrid cross is a cross between

individuals that

involves two pairs of contrasting traits.

• All possible allele combinations in the parents for the two traits are crossed.

Chapter 7 Extending Mendelian Genetics Section 7-2 Complex Patterns of Inheritance

Be Able To:

• Provide and discuss some examples of non-Mendelian genetics.

• Explain the effect of environmental factors on phenotype.

Key Terms: ^{Pg 319}

Section 7.2 Complex Patterns of Inheritance

• A polygenic trait is a trait that is controlled by two or more genes.

• Skin color is caused by the effect of three to six genes.

• Each gene result in a certain amount of pigment called melanin.

• Multiple allele traits are controlled by three or more alleles of the same gene that code for a single trait, ie human A, B, AB, O bloody

type.

• An inheritance relationship that occurs when both alleles influence the

phenotype is incomplete dominance.

• An inheritance relationship that occurs when both alleles for a gene are

expressed in heterozygous offspring is codominance

Chapter 11 Introduction to Genetics

Section 4 Meiosis

Be Able To

• Explain how many genes are found in most adult organisms.

• Summarize the events of Meiosis I and II.

• Explain how two alleles from different genes can be inherited together.

Key Terms

Chromosome Number

• Sex chromosmes are

chromosomes that determine the sex of an organism.

• Autosomes are chromosomes that are not involved in determining the sex of the organism.

• Homologous chromosomes are one of a pair of similar

chromosomes.

• A karyotype is a photomicrograph of the chromosomes of a dividing cell.

• A diploid cell (2n) contains both

chromosomes of a homologous pair, e.g, skin cell or a muscle cell.

• A haploid cell (n) contains only one chromosome of a homologous pair, e.g. sperm or egg cell.

Is this individual male or female and did these

chromosomes come from a diploid or

haploid cell?

Phases of Meiosis

• Meiosis is a process of

nuclear division that reduces the number of chromosomes in new cells by half the

number in the original cell.

• Human gametes, such as egg cells and sperm cells, are haploid (n) due to

meiosis.

• The fusion of these two cells results in a diploid cell (2n) called the zygote.

Original diploid

cell

S phase

Meiosis I

Meiosis II

Meiosis I

• Synapsis is the pairing up of

homologous chromosomes during prophase I.

• Crossing-over is a process in which portions of chromatids may break off and attach to adjacent chromatids on the homologous chromosome.

• Crossing-over permits the exchange of genetic material between maternal and paternal chromosomes resulting in

genetic recombination.

• During metaphase I, tetrads line up randomly along the mid-line of the cell in a process called independent

assortment.

• Independent assortment results in the random separation of maternal and paternal chromosomes and more genetic variation

Meiosis I

Each new cell is haploid, but contains 2 copies of a

chromosome.

Meiosis II

• Meiosis II occurs in each cell formed by meiosis I.

• The stages of meiosis II occur in the same manner as in mitosis.

• Cytokinesis II results in four new haploid cells.

• In humans, meiosis occurs only in the testes and ovaries.

• Through meiosis in males, spermatogenesis creates 4 haploid cells called spermatids

which will later develop into mature sperm cells.

• Through meiosis in females, oogenesis creates one mature egg cell.

• During cytokinesis I and II the cytoplasm is not distributed equally resulting in one mature egg cell.

• The other 3 cells created by meiosis

degenerate and are called polar bodies.

Meiosis

Gene Linkage

• Do genes on same chromosomes assort independently of each other during gamete formation?

• Thomas Hunt Morgan’s fly lab discovered linked genes.

• His fly lab students determined that they could group all of their fruit

flies genes into four linkage groups.

• For some organisms, alleles of

different genes tend to be inherited together when those genes are

located on the same chromosomes.

• The farther away two genes are on the chromosome the more likely they will be inherited together.