Ch. 11: Introduction to Genetics

“When in doubt, Punnett!”

Chapter 11 Outline• 11-1: The Work of Gregor Mendel

– Gregor Mendel’s Peas– Genes and Dominance– Segregation

• 11-2: Probability and Punnett Squares– Genetics and Probability– Punnett Squares– Probability and Segregation– Probabilities Predict Averages

Chapter 11 Outline• 11-3: Exploring Mendelian Genetics

– Independent Assortment– A summary of Mendel’s Principles– Beyond Dominant and Recessive Alleles– Applying Mendel’s Principles– Genetics and the Environment

• 11-4: Meiosis– Chromosome Number– Phases of Meiosis– Gamete Formation– Comparing Mitosis and Meiosis

• 11-5 Linkage and Gene maps– Gene Linkage– Gene Maps

11-1 The Work of Gregor Mendel

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

Genetics: The scientific study of Hereditiy

Gregor Mendel’s Peas• Gregor Mendel was an Austrian Monk,

and he is called the “Father of Genetics” because of his research at a monastery

• He carried out his work on pea plants. The Male parts of plants produce pollen, which contain sperm. Female flower parts produce egg cells. When male and female reproductive cells join (fertilization), a new cell is produced.

Gregor Mendel’s Peas• Pea Plants are normally self-pollinating

(fertilization happens on the same flower). This means they have only one parent and show all the characteristics of that parent

• Mendel worked with true-breeding peas– True Breeding: If allowed to self-pollinate,

they would produce offspring identical to themselves.

– Ex: Tall Peas plants only produce Tall plants

Gregor Mendel’s Peas• Mendel wanted to produce new cells

(seeds) by joining male and female reproductive structures from two different pea plants.

• To do this, he had to prevent self-pollination. (So, he removed the Male parts)

• He then put some pollen from one plant onto another pea plant.– Cross-pollination: produces seed that

have two different plants as parents.

Genes and Dominance• Mendel studied seven different traits in

pea plants.– Traits: a specific characteristic that varies

from one individual to another• Ex: Seed Color or Plant Height

• Each original pair of Pea plants were called the P (parent) generation and their offspring were called the F1 (first filial) generation.– Hybrids: The offspring of crosses between

parents with different traits

Genes and Dominance• The Results of Mendel’s Crosses:

– The F1 generation did not look like a blend of the two parents. The offspring had the trait of only one of the parents.

• Conclusions:1. Biological inheritance is determined by

factors that are passed from one generation to the next. (Today we call those factors genes• Genes exist in different forms. One gene

controls height (tall or short pea plants). The different forms of genes are called alleles.

Genes and Dominance

2. The Principle of dominance: Some alleles are dominant and others are recessive

• Dominant alleles will always shown• Recessive alleles will only be seen

when there are no dominant alleles present• EX: Tall is dominant over short in peas

Segregation• After this work, Mendel had another

question: Had the recessive alleles disappeared in the F1 plants?– To answer this, he allowed F1 plants to self-

pollinate and produce the next generation: F2

• The results of the F1 cross:– The Traits controlled by recessive alleles

reappeared in about ¼ of the F2 plants.

Segregation• Explaining these results:

– Mendel assumed that a dominant allele had masked the corresponding recessive allele. The reappearance of the recessive alleles indicated that at some point the allele for shortness (for example) had separated from the allele for tallness.

– Segregation: the separation of alleles

Segregation• How did this Segregation happen?

– Alleles segregated from each other during the formation of sex cells, or gametes.

– For example, in a cross of tall x short pea plant, the F1 plants have one allele for tallness and one for shortness. But when the F1 plants to produce gametes (sex cells that will fuse with other gametes to produce embryos), the two alleles segregate from each other so that each gamete only carries a single allele (either the tall or short in our example).

Genetic and Probability• Probability: the likelihood that an event

will occur• Mendel realized he could apply the

principles of probability to explain the results of genetic cross.

• Example of probability: Coin Flipping– Each time you flip a coin, you have a 1 in 2

chance of getting heads, and a 1 in 2 chance of getting tails. This probability is ½ or 50%.

Genetics and Probability

• What is the probability of getting 3 heads in a row?

½ x ½ x ½ = 1/8 • The principles of probability can be

used to predict the outcomes of genetic crosses.

Punnett Squares

• Punnett square: a diagram used to show gene combinations that can result from a cross.

• The letters in the Punnett square represent alleles.

Punnett Square

Punnett squares• Homozygous: organisms that have two

identical alleles for a trait.• Heterozygous: organisms that have two

different alleles for a trait• Phenotype: physical characteristics (what you

see)• Genotype: the actual genetic makeup

– Example: the phenotype of a tall plant is “tall” and the genotype is TT or Tt

• The data from Mendel’s experiment fit the model of Punnett squares.– When he crossed two heterozygotes (F1 generation) he

got a 3:1 ratio.

Exploring Mendelian Genetics

• After showing that alleles segregate, Mendel wondered if the segregation of one pair of alleles affects another pair of alleles

• Independent Assortment– To figure this out, Mendel performed

an experiment to follow two different colors as they pass from generation to generation

Independent Assortment

• Two-Factor crosses:– Mendel crossed true breeding round,

yellow seed plant (RRYY) with wrinkled, green pea plants (rryy). All of the F1 offspring were round and yellow

• Punnett square for this cross: dihybrid cross

RrYy x RrYy

Independent Assortment• The results show an F1 generation of all

RrYy. Then, the F1 self-pollinated and produced the following results: 9 round yellow: 3 round green: 3 wrinkled yellow: 1 wrinkled green

• This clearly meant that the alleles for seed shape and seed color segregated separately of each other

Independent Assortment

• Independent assortment: the independent segregation of genes during the formation of gametes.

• This helps account for genetic variation

Beyond Dominant and Recessive Alleles

• There are several exceptions to Mendel’s Principles. Not all genes show dominant or recessive patterns. Many genes have more than two alleles.

• Incomplete Dominance– One allele is not dominant over the other.

The heterozygous phenotype is a blend of the two parent phenotypes.

– Ex. Four o’clock flowers: RR (red) x WW (white) produces an F1 generation of all RW (pink)

Incomplete Dominance

Codominance

• Both alleles contribute to the phenotype.– Ex. Chickens with black feathers

crossed with chickens with white feathers produces black and white speckled feathers.

Multiple Alleles

• More than two alleles for a gene• Ex. Coat color in rabbits, blood

type for humans.

Polygenic Traits

• Traits controlled by two or more genes. Usually show a wide range of phenotypes because of this– Ex. Human Skin Color

Applying Mendel’s Principles

• Mendel’s principles apply not only to plants but to animals (including humans!)

• Thomas Hunt Morgan tested genetic principles on fruit flies, called Drosophila. After him, many other scientists followed in the research of genetics.

Genetics and the Environment

• The environment also influences phenotype. It is not determined by genes alone.– Ex. Plant genes affect their height and

color, but climate and soil conditions also affect the same things

Meiosis• Mendel’s principles of genetics require two

things:1. Each organism must inherit a single copy of every

gene from both its parents.2. When an organism produces its own gametes,

those two sets of genes must be separated from each other, so that each gametes only have one set of genes.

• Thus, there must be some process that separates the two sets of genes. Mendel didn’t know it, but there IS a process that does this, and it’s called Meiosis

Chromosome Number• To illustrate this process we are

going to look at the chromosome of Drosophila. A body cell in an adult fruit fly has 8 chromosomes (4 from mom and 4 from dad)– Homologous chromosomes: a pair

of chromosomes that contain similar information (1 from mom and 1 from dad)

Chromosome Number• So the Drosophila have 4 homologous pairs of

chromosomes.• Diploid: a cell that has both sets of

chromosomes (the one from each parent)– Symbol for a diploid cell is 2N

• For Drosophila 2N=8. Diploid cells contain two complete sets of chromosomes.

• The Gametes (sex cells) of organisms only contain one set of chromosomes so they can fuse with other gametes to make a complete set. Cells with only one set of chromosomes are called haploid (N). For Drosophila, N=4.

Phases of Meiosis• Overview

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

– Two divisions of Meiosis:• Meiosis I• Meiosis II

Meiosis• Meiosis I

– Before the start of this phase, the DNA is replicated. The cells divide in a process similar to mitosis. EXCEPT in metaphase, homologous pairs line up at the equator.

– Tetrad: 2 homologous chromosomes paired together (made up of 4 chromatids b/c each chromosome has replicated)

Meiosis• When a tetrad forms at the center of a cell,

some chromatids exchange small portions of their DNA. This is called crossing-over.– This produces new combinations of alleles and leads

to genetic variation.• The result: Homologous pairs separate and

make two cells each with only one set of genes now (but each chromosome still has two sister chromatids)

• Another division is necessary to separate the chromatids.

Meiosis II• There is no replication before this

division. Again this process is just like mitosis. The chromosomes line up at the equator and sister chromatids separate.

• The result: A total of four HAPLOID daughter called gametes.

• The purpose: To make cells with only one set of genes so it can fuse with another gamete to make a zygote.

Meiosis I

Section 11-4

Figure 11-15 Meiosis

Meiosis I

Section 11-4

Figure 11-15 Meiosis

Meiosis I

Meiosis I

Section 11-4

Figure 11-15 Meiosis

Meiosis I

Section 11-4

Figure 11-15 Meiosis

Meiosis I

Section 11-4

Figure 11-15 Meiosis

Meiosis I

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells.

Section 11-4

Figure 11-17 Meiosis II

Gamete Formation

• In male animals, the haploid gametes are called sperm. In females, they are called eggs. In females, of the four gametes produced by one division only one is involved in reproduction. The others are called polar bodies.

Comparing Mitosis and Meiosis

• Mitosis results in the production of two genetically identical diploid cells.– Body uses these cells in growth and

replacement of other cells• Meiosis produces four genetically

different haploid cells.– Body uses these in reproduction

11-5 Linkage and Gene Maps

• Since it is chromosomes that assort independently, individual genes do not!

• Genes that travel on the same chromosome are “linked genes”, because they will be inherited together (unless crossing over happens in that area)

Linkage and Gene Maps

• The farther apart two genes are on a chromosome, the more likely it is that they will become unlinked by crossing over.– Gene Map: a diagram showing the

relative locations of genes on a chromosome

Gene MapExact location on chromosomes Chromosome 2