Introduction to Genetics Chapter 11 Introduction to Genetics

11- 1 The Work of Gregor Mendel Every living thing – plant or animal, microbe or human being – has a set of characteristics inherited from its parents Since the beginning of recorded history, people have wanted to understand how that inheritance is passed from generation to generation

Genetics The scientific study of heredity

Gregor Mendel Austrian Monk Born 1822 in Czech Republic Worked at monastery and taught high school Tended the monastery garden Grew peas and became interested in the traits that were expressed in different generations of peas

True breeding If allowed to self pollinate they would produce offspring identical to themselves He was also able to cross breed peas for different traits

Genes and Dominance Mendel studied seven different pea plant traits Each trait he studied had a contrasting form

Pea Plant Traits

Genes and Dominance The offspring of crosses between parents with different traits are called Hybrids When Mendel crossed plants with different traits he expected them to blend, but that’s not what happened at all. All of the offspring had the character of only one of the parents

Mendel drew two conclusions 1. Inheritance is determined by factors that are passed from generation to generation – today we call these factors genes

Alleles Different forms of a gene

Mendel’s 2nd conclusion 2. The Principal of Dominance Some alleles are dominant and some are recessive

dominant Covers up the recessive form Ex.) T = tall

recessive Gets covered up in the presence of a dominant allele Ex.) t = short

Segregation Mendel wanted to answer another question Q: Had the recessive alleles disappeared? Or where they still present in the F1 plants? To answer this he allowed the F1 plants to produce an F2 generation by self pollination

P1 Parental F1 F2 Tall Short All Tall 3 tall : 1 short 75% tall

The F1 Cross The recessive traits reappeared! Roughly 1/4 of the F2 plants showed a recessive trait

Explanation of the F1 Cross The reappearance indicated that at some point the allele for shortness had been separated from the allele for tallness Mendel suggested that the alleles for tallness and shortness in the F1 plants were segregated from each other during the formation of sex cells or gametes When each F1 plant flowers, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. Therefore, each F1 plant produces two types of gametes – those with the allele for tallness and those with the allele for shortness

11-2 Probability and Punnett Squares Mendel kept obtaining similar results, he soon realized that the principals of probability could be used to explain the results of genetic crosses

Probability The likelihood that a particular event will occur The way in which alleles segregate is random like a coin flip

Punnett Square Vocab If you do not know the following vocabulary words you will fail miserably

Punnett Square Diagram used to determine genetic crosses

Homozygous Organisms that have 2 identicle alleles for a trait Ex.) TT , tt

Heterozygous Have two different alleles for a trait Ex.) Tt

Phenotype Physical characteristics – (words) Ex.) tall

Genotype Genetic make-up - (letters) Ex.) Tt, TT, tt

11-3 Exploring Mendelian Genetics

Mendel wondered if alleles segregate during the formation of gametes independently Does the segregation of one pair of alleles affect the segregation of another pair of alleles? For example, does the gene that determines whether round or wrinkled in shape have anything to do with the gene for color? Must a round seed also be yellow?

All heterozygous 9:3:3:1 Ratio

Independent Assortment Genes that segregate independently do not influence each others inheritance

A Summary of Mendel’s Principles The inheritance of biological characteristics is determined by individual units known as _______________. In organisms that reproduce sexually, _______________ are passed from parents to offspring Genes Genes

A Summary of Mendel’s Principles In cases in which 2 or more forms of a gene are present, some forms of the gene may be _______________________ or ___________________________ In most sexually reproducing organisms, each adult has two copies of each gene – one from each parent. These genes are segregated from each other when gametes are formed The alleles for different genes usually segregate independently of one another dominant recessive

Incomplete Dominance When one allele is not dominant over another Four o’clock flowers The heterozygous phenotype is somewhat in-between the two homozygous phenotypes

Codominance When both alleles contribute to the phenotype of an organism Ex.) Speckled Chickens

Multiple Alleles When more than two possible alleles exist in a population Ex.) blood type IA IB i Dominant Recessive

Human Blood Types Phenotype Genotype A B AB O IAIA or IAi IBIB or IBi IAIB ii

Polygenic Traits Traits controlled by two or more genes Ex.) eye color, skin color

Genetics and the Environment The characteristics of any organism, is not only determined by the genes it inherits Characteristics are determined by interactions between genes and the environment Ex.) genes may affect a plants height but the same characteristic is influenced by climate, soil conditions and availability of water

Do Now Human hair is inherited by incomplete dominance. Human hair may be curly (CC) or straight (cc). The heterozygous genotype (Cc) produces wavy hair. Show a cross between two parents with wavy hair

Do Now A man is suing his wife on grounds of infidelity. The man claims that the child is blood type O and therefore must be fathered by someone else. Can he use this evidence in court if he and his wife both have heterozygous B genotypes? Show the cross of the two parents

11 – 4 Meiosis

Objectives What happens during the events of meiosis? What is the difference between mitosis and meiosis?

Meiosis Gregor Mendel did not know where the genes he had discovered were located in the cell Genes are located on ______________________ in the cell ______________ chromosomes nucleus

Mendel’s principles of genetics require at least 2 things Each organism must inherit… a single copy of every gene from each of its parents When an organism produces its own gametes… these two sets of genes must be separated from each other so that each gamete contains just one set of genes

Chromosome Number Ex.) fruit fly 8 chromosomes 4 from mom, 4 from dad Ex.) Humans 46 chromosomes 23 from mom, 23 from dad

Homologous Chromosomes that each have a corresponding chromosome from the opposite sex parent

Diploid A cell that contains both sets of homologous chromosomes (2N) Body cells

Haploid A cell that contains only a single set of chromosomes (1N) Sex cells (gametes)

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 Makes sex cells

Meiosis usually involves 2 divisions Meiosis I Meiosis II

Meiosis I prior to meiosis I, each chromosome is replicated The cells then begin to divide similar to mitosis

Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a structure called a _____________________ - has 4 chromatids Tetrad

Crossing over When chromosomes exchange portions of their chromatids and results in the exchange of alleles

Crossing over Leads to new combinations of alleles The homologous chromosomes separate, and 2 new cells are formed Although each cell now has 4 chromatids something is different. Because each pair of homologous chromosomes was separated, neither of the daughter cells has two complete sets of chromosomes that it would have in a diploid cell The two sets have been shuffled

Meiosis II The two cells produced by meiosis I now enter a second meiotic division Unlike the 1st division, no chromosomes are replicated Each cell’s chromosomes has 2 chromatids

Metaphase II 2 chromosomes line up in the center of each cell

Anaphase II The paired chromatids separate

Telophase II Forms 4 daughter cells each with 2 chromatids These 4 daughter cells are now haploid (N) – just 2 chromosomes each

Gamete Formation In male animals, the haploid gametes produced by meiosis are called sperm In some plants they are called pollen

Spermatogenesis

Gamete Formation In females, generally only one of the cells produced by meiosis is involved in reproduction This female gamete is called an egg The other 3 cells that do not receive as much cytoplasm as the egg are called polar bodies

oogenisis

Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells

Comparing Mitosis and Meiosis 46 46 46 23 23 23 46 23

11-5 Linkage and Gene Maps

Gene Linkage When genes are located on the same chromosome they are inherited together (Linkage) It’s the chromosomes that assort independently not individual genes

When genes are formed on the same chromosome, this does not mean that they are linked forever Crossing over during meiosis sometimes separates genes that had been on the same chromosome onto homologous chromosomes Cross over events occasionally separate and exchange linked genes and produce new combinations of alleles

Q: Why is this good? A: Generates genetic diversity

Gene Maps 1911 Alfred Sturtevant hypothesized that the further apart genes were, the more likely they were to be separated by a crossover in meiosis the rate at which linked genes were separated and recombined could then be used to produce a “map” of distances between genes

Gene map Shows the location of each gene