Introduction to Genetics Chapter 11

What traits do you see in this room? TraitVarieties# with trait

Crash Course: Biology Heredity UEA&index=9&list=PL3EED4C1D684D3ADF UEA&index=9&list=PL3EED4C1D684D3ADF

Mendel Wio Wio

Mendel’s Peas Why did Mendel choose peas? Copyright Pearson Prentice Hall

Gregor Mendel’s Peas True-breeding –if allowed to self-pollinate  offspring identical to parents Cross-pollination – Cross one with another

Genes and Dominance A trait - Mendel studied seven pea plant traits, each with two contrasting characters. He crossed plants with each of the seven contrasting characters and studied their offspring. Copyright Pearson Prentice Hall

P (parental) generation - F 1, or “first filial,” generation - Hybrids -

Looking closer at Mendel ’ s work P 100% F 1 generation (hybrids) 100% purple-flower peas X true-breeding purple-flower peas true-breeding white-flower peas

Genes and Dominance Copyright Pearson Prentice Hall Mendel’s F 1 Crosses on Pea Plants

Genes and Dominance Mendel’s Seven F 1 Crosses on Pea Plants Mendel’s F 1 Crosses on Pea Plants

Mendel's first conclusion individuals characteristics are determined by factors passed from one generation to the next. – Today these factors are known as - – Each trait is controlled by one gene – Traits come in alternative versions These different forms are called - These are located at loci on the chromosome Copyright Pearson Prentice Hall purple-flower allele & white-flower allele are two DNA variations at flower-color locus

For each trait or gene, an organism inherits 2 alleles, 1 from each parent – diploid organism like having 2 editions of encyclopedia – Encyclopedia Britannica – Encyclopedia Americana

Genes and Dominance Mendel’s 2 nd Conclusion was The principle of dominance which states that some alleles are dominant and others are recessive

Genes and Dominance What is the difference between what we observe for a dominant trait and a recessive trait?

F 2 generation 3:1 75% purple-flower peas 25% white-flower peas ???? Did the recessive alleles disappear? P X true-breeding purple-flower peas true-breeding white-flower peas PPpp 100% F 1 generation (hybrids) 100% purple-flower peas PpPpPpPpPpPpPpPp phenotype genotype self-pollinate

Segregation Results: –The traits from the recessive alleles reappeared in the in F 2 generation –but only one fourth of the offspring had the recessive trait Why did the recessive trait disappear in the F 1 generation and then reappear in the F 2 generation? Copyright Pearson Prentice Hall

Mendel’s Conclusion Because the recessive allele was seen in some of the F 2 generation, it must have somehow separated from the dominant allele. Mendel suggested that the recessive and dominant alleles must - separate or segregate from each other during gamete (egg and sperm) formation

What is Segregation? Alleles separate during gamete formation. Copyright Pearson Prentice Hall

Segregation Mendel’s 3 rd conclusion was the Law of Segregation –pairs of alleles separate during gamete formation and randomly unite at fertilization. Copyright Pearson Prentice Hall

Ted Ed pea-plants-helped-us-understand-genetics- hortensia-jimenez-diaz pea-plants-helped-us-understand-genetics- hortensia-jimenez-diaz

Genetics and Probability Probability – the likelihood that a particular event will occur The principles of probability can be used to predict the outcomes of genetic crosses. If you flip a coin what is the probability –For heads? –For Tails? What is the probability of getting heads 3 times in a row? Copyright Pearson Prentice Hall

How does this relate to Heredity? The way alleles segregate during gamete formation is like flipping a coin – 50:50 or 1/2 – If the parent has a dominant and recessive allele Each gamete has – A ½ chance of getting the dominant allele – A ½ chance of getting the recessive allele Two gametes combine to make the fertilized egg. What is the probability of both gametes carrying the recessive allele?

Probabilities Predict Averages Probabilities predict the average outcome of a large number of events. Probability cannot predict the precise outcome of an individual event. Copyright Pearson Prentice Hall

Punnett Squares Punnett squares – Used to help predict the genotype and phenotype combinations in genetic crosses Examples: Copyright Pearson Prentice Hall

Terms to Know Homozygous – Heterozygous –

Terms to Know Genotype – Phenotype –

Making crosses You can represent alleles as letters – flower color alleles  P or p – true-breeding purple-flower peas  PP – true-breeding white-flower peas  pp PP x pp PpPp F1F1 P X purplewhite all purple

Punnett squares Pp x Pp Pp male / sperm P p female / eggs PP 75% 25% 3:1 25% 50% 25% 1:2:1 % genotype % phenotype PPPpPp PpPppp PpPp PpPp F 1 generation (hybrids)

Phenotype vs. genotype 2 organisms can have the same phenotype but have different genotypes homozygous dominant PPpurplePpPp heterozygous purple How do you determine the genotype of an individual with with a dominant phenotype?

Test cross Breed the dominant phenotype that has an unknown genotype with a homozygous recessive to determine the identity of the unknown allele pp is it PP or Pp? x

PPpp How does a Test cross work? pp P P pp P p PpPppp xx PpPp PpPpPpPp PpPp 100% purple PpPp pp PpPp 50% purple:50% white or 1:1 pp

More of Mendel’s Questions Does the segregation of one pair of alleles affect the segregation of another pair of alleles? Example in True Breeding Pea Seeds: X Must a round seed always be yellow? YYRRyyrr

Mendel’s Experiment Mendel followed 2 genes as they passed from one generation to another. – This is known as a two-factor or dihybrid cross

Dihybrid Cross P generation = R R Y Y x r r y y What gametes can form for each parent? What will be the result for the F 1 generation?

F 1 Self-Pollination F 1 Generation Y y R r x Y y R r What gametes can form from each parent? What is the result of the self-pollination of the F 1 generation?

Self-Pollination Punnett Square Y y R r x Y y R r

YyRr YRYryR yr YR Yr yR yr YYRR x YYRrYyRRYyRr YYRrYYrrYyRrYyrr YyRRYyRryyRRyyRr YyRrYyrryyRryyrr 9/16 yellow round 3/16 green round 3/16 yellow wrinkled 1/16 green wrinkled

Mendel’s 4 th Conclusion The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. –Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. Copyright Pearson Prentice Hall

Summary of Mendel’s Principles 1.Inheritance is determined by genes passed from parents to offspring. (Inheritance) 2.Forms of the gene (alleles) may be dominant or recessive. (Principle of Dominance) 3.Each individual has 2 copies of each gene, one from each parent. These are segregated during gamete formation. (Principle of Segregation) 4.Alleles from different genes usually separate independently from one another. (Principle of Independent Assortment)

Exceptions to Mendel’s Ideas Some alleles are neither dominant or recessive. Many traits are controlled by multiple alleles or multiple genes.

Exception 1 Incomplete Dominance occurs when one allele is not completely dominant over the other. – The heterozygous phenotype is somewhere in between the two homozygous phenotypes. – Example: Four O’Clocks

A cross between red (RR) and white (WW) four o’clock plants produces pink- colored flowers (RW). Copyright Pearson Prentice Hall WW RR

Exception 2 Codominance occurs when both alleles contribute to the phenotype. Example: Erminette Chickens – P = Black Feathers x White Feathers – F1 = Heterozygous Chickens with black and white feathers

Exceptions Multiple Alleles occur when a gene has more than two alleles. Individuals don’t usually have this But, there can be more than two possible alleles found in a population. Example: Rabbit Fur Color Determined by a single gene that has 4 different alleles

Exception 4 Polygenic Traits are produced by the interactions of several genes. Example: Skin Color is Controlled by more than 4 different genes.

Meiosis The process of making gametes. Each organism must inherit a single copy of every gene from each of its “parents.” Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set. Copyright Pearson Prentice Hall

Chromosome Number All organisms have different numbers of chromosomes. –A body cell in an adult fruit fly has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent. –Pairs of chromosomes are said to be homologous chromosomes Copyright Pearson Prentice Hall

These sets of chromosomes are homologous. In fruit flies: Each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent. Copyright Pearson Prentice Hall

A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is represented by the symbol 2N. For fruit flies, the diploid number is 8, which can be written as 2N=8. For humans the diploid number is 46, so 2N = 46 Copyright Pearson Prentice Hall

The gametes contain only a single set of chromosomes. Gametes are called haploid. Haploid cells are represented by the symbol N. For fruit flies the haploid number is N = 4 For humans the haploid number is N = 23 Copyright Pearson Prentice Hall

Meiosis Meiosis is the process of making gametes by reducing the chromosomes from 2N to N Otherwise the gametes would have 2x too many chromosomes. hill.com/sites/ /student_view0/cha pter3/animation__stages_of_meiosis.html hill.com/sites/ /student_view0/cha pter3/animation__stages_of_meiosis.html

Phases of Meiosis Meiosis involves two divisions, –meiosis I –meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells. Copyright Pearson Prentice Hall

Phases of Meiosis Meiosis I Copyright Pearson Prentice Hall Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis Interphase I Meiosis I

Phases of Meiosis Cells undergo a round of DNA replication, forming duplicate chromosomes. Copyright Pearson Prentice Hall Interphase I

Phases of Meiosis Synapsis: Homologous chromosomes form tetrads. There are 4 chromatids in a tetrad. Copyright Pearson Prentice Hall MEIOSIS I I Prophase I

In Prophase I During synapsis – chromosomes exchange portions of their chromatids This process is called crossing over. Crossing-over produces new combinations of alleles. Copyright Pearson Prentice Hall

Metaphase I Spindle fibers attach to the chromosomes. Tetrads line up on the metaphase plate The side of the plate that the homologous chromosomes go to is random. Copyright Pearson Prentice Hall MEIOSIS I Metaphase I

Phases of Meiosis The fibers pull the homologous chromosomes toward opposite ends of the cell. Copyright Pearson Prentice Hall MEIOSIS I Anaphase I

Phases of Meiosis Nuclear membranes form around each cluster of chromosomes The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the parent cell. Copyright Pearson Prentice Hall MEIOSIS I Telophase I and Cytokinesis

Meiosis II Copyright Pearson Prentice Hall Telophase II and Cytokinesis Prophase II Metaphase II Anaphase II Telophase I and Cytokinesis I Meiosis II

Phases of Meiosis Meiosis II Unlike meiosis I, neither cell goes through chromosome replication as in Interphase I Each of the cell’s chromosomes is made of 2 sister chromatids. Copyright Pearson Prentice Hall

Phases of Meiosis In Prophase II the sister chromatids become visible. The chromosomes do not form tetrads. Copyright Pearson Prentice Hall MEIOSIS II Prophase II

Phases of Meiosis The sister chromatids line up in the center of cell. Copyright Pearson Prentice Hall MEIOSIS II Metaphase II

Phases of Meiosis The sister chromatids separate and move toward opposite ends of the cell. Copyright Pearson Prentice Hall MEIOSIS II Anaphase II

Phases of Meiosis Meiosis II results in four haploid (N) daughter cells. Copyright Pearson Prentice Hall MEIOSIS II Telophase II and Cytokinesis

Gamete Formation In male animals, meiosis results in four equal- sized gametes called sperm. Copyright Pearson Prentice Hall

Gamete Formation In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.

Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells. Copyright Pearson Prentice Hall

Comparing Mitosis and Meiosis Mitosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis. Copyright Pearson Prentice Hall

Comparing Mitosis and Meiosis Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes. Copyright Pearson Prentice Hall