Chapter 11 Introduction to Genetics

11.1 The Works of Gregor Mendel

The Experiments of Gregor Mendel Genetics was founded by an Austrian monk named Gregor Mendel. He was in charge of the monastery garden, where he was able to do the work that changed biology forever. Mendel carried out his work with ordinary garden peas. A single pea plant can produce hundreds of offspring. Today we call peas a “model system.” http://www.youtube.com/watch?v=MWkFxWXHTnw

The Role of Fertilization During sexual reproduction, male and female reproductive cells join in a process known as fertilization to produce a new cell. In peas, this new cell develops into a tiny embryo encased within a seed.

The Role of Fertilization Mendel’s garden had several kinds of pea plants that would produce offspring with identical traits to themselves. In other words, the traits of each of the following generations would be the same. A trait is a specific characteristic of an individual, such as seed color or plant height, and may vary from one individual to another. Turn to your neighbor and list three (3) traits you can see in them? For example, hair color, height, etc. __________________ , _________________ , __________________

The Role of Fertilization Cross-pollination allowed Mendel to breed plants with traits different from those of their parents and then study the results.

The Role of Fertilization Mendel studied seven different traits of pea plants, each of which had two contrasting characteristics. Look at these pictures. What differences can you notice? ____________________ , ______________________ , ____________________

The Role of Fertilization Mendel crossed plants with each of the seven contrasting characteristics and then studied their offspring. The offspring of the crosses between parents with different traits are called hybrids. When doing genetic crosses, we call the original pair of plants the P, or parental, generation. Their offspring are called the F1, or “first filial,” generation.

Genes and Alleles For each trait studied in Mendel’s experiments, all the offspring had the characteristics of only one of their parents, as shown in the table. In each cross, the nature of the other parent, with regard to each trait, seemed to have disappeared.

Genes and Alleles From these results, Mendel drew two conclusions. 1st – An individual’s characteristics are determined by genes – factors that are passed from one parental generation to the next. Example: gene for height The different forms of a gene are called alleles. Example: tall vs. short 2nd – is called the principle of dominance. This principle states that some alleles are dominant and others are recessive. http://www.youtube.com/watch?v=eOvMNOMRRm8

Dominant and Recessive Traits Looking at the tall vs. short example: Tall is dominant and short is recessive T = tall and t = short Always use one letter. Capital is dominant Lower case is recessive Dominant traits – effect shown even when there’s only one copy of an allele Recessive traits – only shown when dominant allele not present

Genotype and Phenotype In Mendel’s experiments, the allele for tall plants was dominant and the allele for short plants was recessive. For a tall plant, the gene for height can be: TT or Tt For a short plant, the gene for height can only be: tt Genotype: the genetic makeup; the combination of letters (TT, Tt, tt) the for that trait Phenotype: the physical description of the traits (Tall, short) the for that trait http://www.youtube.com/watch?v=fQrmiG-MaBw

A Closer Look at The F1 Cross The traits for short plants reappeared in the F2. Roughly ¼ of the F2 plants showed the trait controlled by the recessive allele. Conclusion: At some point, the allele for shortness had separated from the allele for tallness.

The Formation of Gametes How did this separation, or segregation, of alleles occur? F1 plants must have segregated from each other during the formation of the sex cells, or gametes.

The Formation of Gametes In the example, each F1 plant in Mendel’s cross produced two kinds of gametes—those with the allele for tallness (T) and those with the allele for shortness (t).

11.2 Applying Mendel’s Principles

What is Probability? Probability is the likelihood that a particular event will occur. Formula to know: Number of Actual Outcomes Number of Possible Outcomes

Using Probability to Make Predictions Using statistics to predict what traits will show up in the offspring Mendel studied 30,000 pea plants over a period of 8 years Increased his chances of seeing a repeatable pattern Valid scientific conclusions need to be based on results that can be duplicated

Example: A coin toss Number of Actual Outcomes Number of Possible Outcomes What is the probability that a coin flip will give heads up? Try writing your answer 3 different ways below: _________ or _________ or __________ ratio percentage fraction What is the probability (fraction) of flipping 3 heads in a row? * You could have used tails instead and gotten the same results. Heads Tails

Results of the coin toss experiment If you flip a coin once, the probability of flipping heads up once is: 1/2 The probability of flipping three heads in a row is: 1/2 × 1/2 × 1/2 = 1/8 Results: Each coin flip is an independent event, with a one chance in two probability of landing heads up. Past outcomes do not affect future ones. * So, just because you’ve flipped 3 heads in a row does not mean that you’re more likely to have a coin land tails up on the next flip.

What we can conclude so far…….. The larger the number of offspring, the closer the results will be to the predicted values. So: If an F2 generation contains just three or four offspring, it may not match Mendel’s ratios. Also: When an F2 generation contains hundreds or thousands of individuals, the ratios usually come very close to matching Mendel’s predictions.

Homozygous and Heterozygous Organisms that have two identical alleles for a particular gene—TT or tt in this example—are said to be homozygous. Organisms that have two different alleles for the same gene—such as Tt—are heterozygous.

Using Punnett Squares One of the best ways to predict the outcome of a genetic cross is by drawing a simple diagram known as a Punnett square. Punnett squares allow you to predict the genotype and phenotype combinations in genetic crosses using mathematical probability. T t T t 1/4 1/4 1/4 1/4

How To Make a Punnett Square for a One-Factor Cross (4 BOXES) 1st – Write the genotypes of the two organisms that will serve as parents in a cross. In this example we will cross a male and female osprey that are heterozygous for large beaks. They each have genotypes of Bb. Genotype Parent #1: Bb Parent #2: Bb

How To Make a Punnett Square 2nd – Determine what alleles would be found in all of the possible gametes that each parent could produce. Parent 1 would be Bb therefore they would give a B or a b Parent 2 would be Bb therefore they would give a B or a b

How To Make a Punnett Square 3rd – Draw a table with enough spaces for each pair of gametes from each parent. 4th – Enter the genotypes of the gametes produced by both parents on the top and left sides of the table. B b  Genotype for Parent #2 B b Genotype forParent #1  * You could have put Parent #1 on the side and Parent #2 on the top. It is your choice.

How To Make a Punnett Square 5th – Fill in the table by combining the gametes’ genotypes. Start by filling in all the alleles for each column (top to bottom). Then fill in all the alleles for each row (left to right). B b BB Bb bb B b *** You must always wtrite the capital letter first. For example, you would write Bb instead of bB.

How To Make a Punnett Square Calculate the percentage of each. In this example, the probability is that three fourths (3/4) of the chicks will have large beaks, but only one in two (1/2) will be heterozygous (Bb). B b BB Bb bb The 4 genotypes that result are possible genotypes for the offspring. It does not mean there will be 4 offspring, each one showing one of the four genotypes shown. B b

Monohybrid and Dihybrid Crosses dihybrid cross: two different genes monohybrid crosses: single-gene crosses http://www.youtube.com/watch?v=MrRNBDNkHb4

Mendel’s Experiment

The Two-Factor Cross: F1 Mendel crossed true-breeding plants that produced only round yellow peas with plants that produced wrinkled green peas. The round yellow peas had the genotype RRYY, which is homozygous dominant. The wrinkled green peas had the genotype rryy, which is homozygous recessive.

The Two-Factor Cross: F1 (16 BOXES) All of the F1 offspring produced round yellow peas. These results showed that the alleles for yellow and round peas are dominant over the alleles for green and wrinkled peas. The Punnett square shows that the genotype of each F1 offspring was RrYy, heterozygous for both seed shape and seed color.

Independent Assortment The alleles for seed shape segregated independently of those for seed color.   Genes that segregate independently—such as the genes for seed shape and seed color in pea plants—do not influence each other’s inheritance. Mendel’s experimental results were very close to the 9:3:3:1 ratio that the Punnett square shown predicts. The principle of independent assortment : states that genes for different traits can segregate independently during gamete formation.

Other Patterns of Inheritance 11.3 Other Patterns of Inheritance

Incomplete Dominance Cases in which one allele is not completely dominant over another Results in a “BLENDING” Example: Red + white = pink

Codominance Cases in which the phenotypes produced by both alleles are clearly expressed are called codominance. For example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as “erminette,” speckled with black and white feathers.

Multiple Alleles A single gene can have many possible alleles. A gene with more than two alleles is said to have multiple alleles. Example: Blood Type

Polygenic Inheritance When a group of gene pairs act together to produce a trait Example: Eye color Skin color Hair color

11.4 Meiosis

Chromosomes Located in the nucleus 2 types of chromosomes 1.) AUTOSOMES – non-sex chromosomes In humans: pair# 1 -22 2.) SEX CHROMOSOMES In humans: pair #23 XX: female XY: male        KARYOTYPE- photographic map of a species chromosomes   

Karyotype

Homologous Chromosomes Humans inherit one set of chromosomes from their moms and one from their dads. Homologous Chromosomes: are the same size and contain the same information Example Homologous Pair # 1 (large pair) Homologous Pair # 2 (small pair) The light green chromosome came from one parent The dark green chromosome came from the other parent. The light yellow chromosome came from one parent The dark yellow chromosome came from the other parent.

Homologous Chromosomes in Humans Since all of our cells contain homologous pairs (2 copies), our cells are called diploid.  The diploid number (total number) of chromosomes is represented by the symbol 2N.   In humans, the 2N = ______.

Haploid Cells Our gametes (egg and sperm) are different from all of the other cells in our bodies. This is because there are no homologous chromosomes in these cells. Gametes are called haploid because they contain only half the chromosomes as our other cells. If all of the cells in our bodies contain 46 chromosomes, how many chromosomes do gametes have? _______

Comparing Meiosis and Mitosis forms 2 diploid daughter cells that are identical to the diploid parent. Meiosis forms 4 haploid daughter cells that are very different from the diploid parent.