Why is Genetics interesting?

Dominant BB

Recessive bb

Recessive Epistasis ee (B or

b)

Introduction to Genetics

- Chapter 11

What is Inheritance?

• Every living thing has a set of characteristics inherited from its parent or parents.

• Genetics is the scientific study of heredity.

11-1 The Work of Gregor Mendel

• He was an Austrian monk

• He worked with different true-breeding (pure bred) pea plants

• Pea plants were a good choice because:• They were self-pollinating• Seed in one season• Many different true-breeding types

• Mendel worked with 7 different traits:

• Seed Shape & Color• Seed Coat Color• Pod Shape & Color• Flower Position• Plant Height

Genes and Dominance

• What is a trait?• A characteristic

like eye color

• Example:• Tall vs.. Short –

Height • Round vs.

Wrinkled – Pea Shape

• Mendel called the original plants the P1 (parent) generation

• Offspring = F1 generation

• If you cross two parents with different traits, the offspring are called hybrids.

This is what Mendel saw.

The offspring had the characteristics of only one of its parents

Mendel’s F1 Cross

• Mendel concluded:1. Inheritance is determined by factors that are

passed from one generation to the next

2. Chemical factors that determine traits are called genes

3. Different forms of the same gene are called alleles

Example: Gene for height

Alleles: tall vs. short

4. The Principle of Dominance : Some alleles

are dominant and some alleles are recessive.

• Dominant Traits are always expressed

• Recessive Traits are can only be expressed when the dominant allele is not present

• Mendel wondered if the recessive alleles had dissapeared or were they still present in the F1 plants

• He decided to allow all seven kinds of F1 hybrids to produce F2 offspring.

Segregation• Mendel looked at the results of his

F1 and F2 crosses:

P 1 tall plants x short plants

F 1 tall plants

( F1 become the parents for the next generation )

P2 tall plants x tall plants

F2 tall plants , short plants

This is What Mendel Saw

• Because the trait, short, reappeared, Mendel reasoned that the alleles for tallness and shortness had separated from each other when gametes (sex cells) form.

T = Tall

t = short F1 : the F1 plant produces

2 kinds of sex cells

T t

T t

11-2 Probability and Punnett Squares

• The chances that a particular event will happen is called probability.

• The principles of probability can be used to predict the outcomes of genetic crosses.

Punnett Squares

• Punnett Squares can be used to determine the genetic combinations that might result from a genetic cross.

• The letters in a Punnett Square represent alleles:• Capital Letters = Dominant Alleles (G)• Lowercase Letters = Recessive Alleles (g)

Punnett Square Terms

• Homozygous = True Breeding (Pure)

• Heterozygous = Hybrid (Mixed)

• Phenotype = Physical Characteristics(Tall or short)

• Genotype = Genetic Makeup (T or t)

Predicting Averages• Probabilities can predict the average

outcome of genetic crosses.

• The larger the number of offspring resulting from a cross, the closer the results will be to the expected values.

Offspring

Ratios:

P - 75% Tall, 25% short

G – 1:2:1

11-3 Independent Assortment

• What happens if there is more than one gene?

• Does inheriting a certain gene for seed color affect the inheritance of another trait like plant height?

2 Factor Crosses

• Mendel performed experiment to follow two different genes as they passed from one generation to the next.

• These experiments are known as two factor (Dihybrid) crosses.

• Mendel crossed plants that were true-breeding for two different traits.• Round, yellow peas• Genotype RRYY

• Wrinkled green peas• Genotype rryy

Which traits are dominant and which traits are recessive?

ry ry ry ry

RY

RY

RY

RY

RRYY x rryy

Why are there so many boxes?

•Each parent can produce 4 different kinds of sex cells (gametes)

•Each gamete has an equal chance of combining with each of the other parents 4 types of gametes.

F2: Dihybrid Cross

• Each of the offspring in the example are hybrids for BOTH traits - Dihybrids

• Mendel crossed these offspring to produce another generation of plants (F2)

• If the genotype of each parent is RrYy, What kinds of gametes will each parent produce?

Gametes

• Parent 1: RrYy• _____ _____ _____ _____

• Parent 2: RrYy• ______ ______ _____ _____

____

____

____

____

____ ____ ____ ____

RrYy x RrYy

Results

• The results of the F2 cross showed that the alleles for the two different traits segregated independently into the gametes.

• The offspring from this cross showed a 9:3:3:1 ratio of the different phenotypes.

Summary of Mendel’s Principles

• Inherited traits are determined by genes. Genes are passed from parents to offspring

• Some forms of the gene may be dominant and others may be recessive

• The genes segregate during meiosis so only one copy of a gene goes into the gamete

• Alleles for different genes usually segregate independently of one another.

Exceptions to Mendel’s Principles

• Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes.

Incomplete Dominance.

• One allele is not completely dominant over another.

• The heterozygous phenotype is somewhere in between the two homozygous phenotypes.

•R = red flowers•r = white flowers•Rr = pink flowers.

________

____

____

Codominance

• Both alleles contribute to the phenotype of the organism

• Example: Horses, allele for red hair is codominant with allele for white hair. Animals with both alleles have both red and white hairs. The color is called roan.

Blue Roan Red Roan

Multiple Alleles

• A particular trait has more than just two alleles.

• You only inherit two of those alleles at a time.

• Examples: coat color in rabbits, hair color in humans, and human blood types.

Hair Color

Human Blood Types

Polygenic Traits

• Traits produced by the interaction of many genes.

• Examples: human skin color, height, cystic fibrosis

Applying Mendel’s Principles

• Albinism in humans is caused by a recessive trait.

• If two people with normal skin color have a child with albinism, what are the odds that a second child will also have albinism?

A = normal, a = albino

•Chances for a normal child?•___________•Changes for an albino child?•___________

________

____

____

11-4 Meiosis

Introduction

• Multicellular organisms use mitosis to replace cells that are lost due to injury or damage or to grow.

• These cells (somatic cells) are identical to the parent cells because all of the DNA is first copied and then two copies of the DNA separate when the daughter cells form.

• The daughter cells are identical to the parent cells

Meiosis is Different• Multicellular organisms reproduce

sexually.

• In order to keep the number of chromosomes the same from generation to generation, the sex cells have to reduce the number of chromosomes to one half of the number that you find in a somatic cell (body cell)

• Meiosis is the process that reduces the number of chromosomes to 1/2.

Meiosis I

• The chromosomes have replicated during S phase of the cell cycle.

• During Prophase I, the chromosomes become visible and the chromosomes pair off--that is chromosomes that carry the same information called homologs, and form structures called tetrads.

• Something important happens during this process--the homologous chromosomes can twist around each other and some times they break off. When they re-attach, they may attach to the other chromosome.

• This event is called crossing over and is an important process in genetics.

After prophase I, the tetrads of chromosomes line up at the equator of the cell for metaphase I

In anaphase I, the pairs of chromosomes separate from each other. Each daughter cell receives one copy of each type of chromosome.

Metaphase I Anaphase I

• In telophase I, two daughter cells form. Each of these cells have 1/2 of the number of chromosomes that we started with.

Meiosis II

• In this second cell division, the two daughter cells from meiosis I undergo one more cell division

• The steps in this cells division are very much the same as mitosis except the DNA is NOT copied again

• Prophase II: chromosomes become visible

• Metaphase II: chromosomes line up at the equator

Anaphase II: centromeres split, and the copies of the DNA are pulled apart

• Telophase II: 4 daughter cells form.

Products of Meiosis

• 4 daughter cells

• Cells are not genetically identical to the parent cell

• Crossing over may have shuffled the genes from the maternal and paternal chromosomes.

11-5 Linkage and Gene Maps

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