Section 11-1: The Work of Gregor Mendel Chapter 11 – Introduction to Genetics Section 11-1: The Work of Gregor Mendel

Genetics = the scientific study of heredity (how characteristics are inherited from parents) Gregor Mendel was important to understanding biological inheritance He was an Austrian monk and worked in the monastery gardens – this is where he did a lot of the work that helped him understand inheritance

Mendel carried out his work with ordinary garden peas Each plant contained a part that produced pollen (the male reproductive cells) Each plant also contained a female part that produced eggs cells

During sexual reproduction the male and female cells would join The fertilization produced a new cell that would be encased in a seed

Pea plants are normally self-pollinating (one parent) True-breeding = producing offspring that are identical to the parent

True breeding plants were the basis for Mendel’s work  he knew what traits to expect Mendel wanted to cross breed plants (give them 2 parents) He would use a brush to dust pollen from one plant to another, giving it 2 different parents and 2 different traits

Mendel studied 7 different pea plant traits Trait = a specific characteristic that varies from individual to individual Each of the 7 traits studied had 2 contrasting characters; ie. green seed color and yellow seed color

Each original pair of plants was the P (parental) generation The offspring are called F1 (first filial) – filius and filia are Latin for “son” and “daughter” The offspring of crosses between parents with different traits = hybrids Traits did not blend in hybrids, instead all of the offspring had the character of one parent and the other seemed to disappear

Mendel drew 2 conclusions from this set of experiments 1. Biological inheritance is determined by factors (genes) that are passed from one generation to the next Each trait was controlled by 1 gene with 2 contrasting forms The different forms of the gene = alleles

2. The principle of dominance, which states that some alleles are dominant and others are recessive An organism with the dominant trait will always show that trait An organism will only show the recessive trait if they DO NOT have the dominant trait

Common single gene traits in humans Dominant Recessive # in class Dominant Cleft Chin Absent Present Dimples Earlobes Dangling Attached Freckles Hairline Widow’s peak Straight Handedness Right Handed Left Handed Hitchhiker’s Thumb Toe Length 2nd toe longer than 1st 1st toe longer than 2nd Tongue Rolling Ability to roll Inability to roll

Mendel still wondered about the trait controlled by the recessive allele that seemed to disappear in F1 – was it completely gone or still present? To answer this question, he allowed the F1 generation to self-pollinate to produce an F2 generation

In observing the F2 generation, the recessive allele(trait) reappeared So why did the trait disappear in one generation and reappear in another?

The reason the trait reappeared has to do with segregation (separation) Mendel suggested that the alleles for 2 contrasting characteristics segregated from each other during gamete formation Gamete = sex cells

We represent the dominate allele with a capital letter, tall = T We represent the recessive allele with a lower case letter, short = t

Ex. Let’s assume that each F1 plant inherited a tall trait from one parent and a short trait from the other T t Tt Tt T t t

Probability and Punnett Squares Section 11-2: Probability and Punnett Squares

When Mendel preformed crosses, he labeled/counted the offspring and noticed patterns He realized that the principles of probability could explain the results of genetic crosses Probability = the likelihood that a particular event will occur

Ex. In flipping a coin, there are 2 outcomes; heads up or tails up The probability of either outcome is equal; (½ or 50%) Each coin flip is independent, it has a probability of ½ Ex. If I flip heads, what is the probability I will flip it again?

An important thing to remember is that past outcomes DO NOT affect future ones The way in which alleles segregate is completely random, just like flipping a coin

The gene combinations that might result from a genetic cross can be represented in a Punnett square The types of gametes produced by each F1 parent are shown along the top and left sides of the square The possible gene combos for the F2 offspring appear in the 4 boxes that make up the square

Organisms that have 2 identical alleles (TT and tt) are homozygous (true-breeding) Organisms that have 2 different alleles for a trait (Tt) are heterozygous (hybrids)

However, they do not have the same genotype = genetic makeup All of the tall plants have the same phenotype = physical characteristic However, they do not have the same genotype = genetic makeup What is the phenotype of chicken A? What about chicken B?

Draw the Punnett square in your notes! Punnett squares can compare the genetic variations in a cross Ex. ¼ of the F2 plants have 2 alleles for tallness (TT) 2/4, or ½ of the F2 plants have one of each allele (Tt) Draw the Punnett square in your notes! And ¼ of the F2 plants have 2 alleles for shortness (tt)

Phenotype ratios compare the physical traits Genotypic ratios compare the genes (letters) Probabilities are better for predicting the outcome of a large number of events, rather than a particular event What is the phenotypic ratio? What is the genotypic ratio?

Exploring Mendelian Genetics Section 11-3: Exploring Mendelian Genetics

Mendel wondered if one set of alleles would have an effect on another or if they were independent? To answer the question, Mendel performed a cross to follow 2 different genes (2-factor cross)

First, Mendel produced plants that only produced round, yellow peas (genotype RRYY) and wrinkled, green peas (genotype rryy) In the cross he found that all of the F1 offspring produced round, yellow peas – this just showed that yellow and round are dominant over green and wrinkled This Punnett square shows the cross and that all offspring had a genotype of RrYy

Next, he crossed 2 F1 generation – now he could see if the genes segregated independently Mendel found some new phenotypes; round, green peas and wrinkled, yellow peas This meant that the alleles for shape & color segregated independently – they did not influence each other

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

There are some exceptions to Mendel’s principles: Incomplete dominance happens when one allele is not completely dominant over another In incomplete dominance, the heterozygous phenotype is a blend of the two homozygous phenotypes Ex. In crossing a red flower (RR) with a white flower (WW), the F1 generation will consist of pink flowers (RW)

Codominance happens when both alleles contribute to the phenotype (similar to incomplete dominance) In codominance, the heterozygous phenotype will show both of the homozygous phenotypes instead of blending them Ex. In some chickens, the allele for black feathers is codominant with the allele for white feathers  heterozygous chickens have feathers that are speckled with both black and white

Multiple alleles – many genes have more than 2 alleles Individuals don’t end up with more than 2 alleles (they can only get one from each parent), it just means that there are more than 2 possibilities Ex. Rabbit’s coat color: the coat color of a rabbit is controlled by a gene that has at least 4 different alleles

Polygenic traits – traits that are controlled by 2 or more genes Ex. There is a wide range in skin color in humans because more than 4 genes help control this trait Ex. Human eye color is controlled by at least 3 different genes so there is a wide variety of eye colors

Mendel’s principles don’t only apply to plants, but they apply to animals/humans as well Another important organism used to study genetics is the common fruit fly Fruit flies have many traits to study, they produce many offspring, and have short lifespans

Section 11-4: Meiosis

Genes are located on chromosomes in the nucleus of the cell Mendel’s principles of genetics required two things; 1. Each organism must inherit 1 copy of every gene from each of its “parents” 2. When the organism forms gametes, that the two sets of genes must be separated so that each gamete only contains one set of genes

The two sets of chromosomes an organism has are homologous = each chromosome from the male has a corresponding chromosome from the female

Diploid = a cell that contains both sets of homologous chromosomes Diploid cells contain 2 complete sets of chromosomes  this satisfies requirement number 1

Gametes only contain a single set of chromosomes (one set of genes) Haploid = cells that contain only one set of chromosomes  this satisfies requirement number 2

So how do we get haploid cells from diploid cells So how do we get haploid cells from diploid cells?  that’s where meiosis comes in! Meiosis = a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell Meiosis usually involves 2 sets of divisions; meiosis I and meiosis II

By the end of meiosis II, the diploid cells that we started with produces 4 haploid cells Meiosis works in a cycle very much like mitosis, the cells undergo interphase and then go through prophase, metaphase, anaphase, and telophase

Meiosis I Interphase I - Before meiosis I begins, each chromosome is replicated Prophase I – Each chromosome pairs up with its homologous chromosome forming a tetrad  there are 4 chromatids in a tetrad

While forming tetrads, the chromatids swap parts (crossing-over) Crossing-over helps to produce new combos of alleles Why is this important?

Metaphase I – Spindle fibers attach to each chromosome Anaphase I – The fibers pull the homologous chromosomes towards centriole Telophase I/ Cytokinesis – Nuclear membranes form – now 2 separate cells

Unlike mitosis, the two resulting cells DO NOT have a complete set of chromosomes since the homologous chromosomes were split up The two cells have different sets of chromosomes and alleles

Meiosis II There is no interphase 11! Prophase II – Each cell has half the chromosomes as the original cell Metaphase II – Chromosomes line up at the center of the cell

Anaphase II – The sister chromatids separate and move toward opposite ends of the cell Telophase II/ Cytokinesis – Each of the 4 daughter cells receives 2 chromatids – they are each a haploid cell

Gamete formation in males and females is a little different In males, the gametes are called sperm During the process of meiosis, 4 sperm are produced from 1 diploid cell

In females, the gametes are called eggs The cell divisions at the end of meiosis I and meiosis II are uneven so that 1 cell gets most of the cytoplasm  this one cell becomes the egg The other 3 cells are known as polar bodies and usually do not participate in reproduction

Comparing mitosis and meiosis (very different end results) Mitosis results in the production of 2 genetically identical diploid cells Mitosis allows an organism’s body to grow and heal Meiosis produces 4 genetically different haploid cells Meiosis is how sexually reproducing organisms produce gametes