Genetics Review Chapter 5 Mendel s Investigations Gregor

Genetics Review Chapter 5

Mendel’s Investigations Gregor Mendel was the first to closely examine principles of heredity. Mendel chose peas to study inheritance because they possess several contrasting traits without intermediates.

Mendel’s Investigations Mendel was not aware of the existence of chromosomes or genes. It is easier to get the big picture of heredity by combining Mendel’s results with what we know about chromosomes.

Meiosis is the special type of cell division that produces eggs and sperm. In meiosis, a diploid cell with two sets of homologous chromosomes will divide so that the daughter cells are haploid and have one set of chromosomes.

Meiosis Chromosomes have replicated during interphase just as in mitosis. Meiosis actually consists of two separate divisions. Meiosis I – serves to separate the two versions of the chromosome (homologues). Meiosis II – serves to separate the two replicas of each version (sister chromatids).

Meiosis Because there is only one replication of DNA but two cell divisions, each of the four daughter cells is haploid – has only one set of chromosomes.

Fertilization – reestablishes the diploid chromosome number. Union of egg and sperm produces a zygote (single cell). Contains chromosomes of egg and sperm – 2 sets of chromosomes (diploid).

Meiosis I Prophase I – Chromosomes become visible. The 2 versions of each chromosome pair up and exchange segments. This is called crossing over. Late in prophase, the nuclear envelope disappears.

Meiosis I Metaphase I – Spindle apparatus forms. Chromosomes line up in the middle. Which chromosome faces which pole is random. This is called independent assortment.

Meiosis I Anaphase I – The spindle is complete. Homologues are pulled apart and move toward opposite poles. Sister chromatids NOT separated yet. Each pole has half as many chromosomes (one set rather than two) as the original cell. Telophase I – the chromosomes gather at the two poles and wait for the onset of meiosis II.

Meiosis II After a brief interphase in which NO DNA synthesis occurs, meiosis II begins. Meiosis II is just like mitosis except that the sister chromatids are no longer identical due to crossing over.

Meiosis II Prophase II – nuclear envelopes break down as a new spindle forms. Metaphase II – chromosomes line up in the middle of the cell and spindle fibers bind to both sides of the centromeres.

Meiosis II Anaphase II – spindle fibers contract splitting the centromeres and moving the sister chromatids to opposite poles. Telophase II – The nuclear envelope reforms around the four sets of daughter chromosomes.

Meiosis II The resulting 4 daughter cells are haploid. No 2 cells are alike due to crossing over. In animals, these cells develop directly into gametes (eggs & sperm). In plants, fungi & many protists they divide mitotically to produce greater numbers of gametes.

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Sex Determination Sex chromosomes vs. autosomes Autosomes – chromosomes present in both sexes, do not influence sex. In humans, females have 2 X chromosomes, while males have and X and a Y.

Sex Determination Some species have XX females and X males. Others have ZZ males and ZW females. In others, sex is determined environmentally.

Mendel’s Laws Mendel’s experiments with garden peas resulted in his two laws of inheritance. Law of segregation Law of independent assortment

Mendel’s Peas The peas can self-fertilize or outcross. Mendel could control who the parents were. Mendel always started with true-breeding parents. E. g. self-fertilized white flowered parents always produced white flowered offspring.

Mendel’s Peas He could cross true breeding white with true breeding purple – this is the parental generation. Resulting in all purple offspring – this is the F 1 generation.

Mendel’s Peas Allowing the hybrid F 1 generation to self pollinate gives the F 2 generation with 3 purple: 1 white offspring. He kept careful quantitative records that allowed him to find patterns.

Mendel’s Law of Segregation Mendel’s explanation of the 3: 1 ratio of purple (dominant) to white (recessive) flowers resulted in the Law of Segregation.

Mendel’s Law of Segregation Alternative versions of genes account for variations in inherited characters. Two versions of the flower color gene are purple & white. We now call these versions alleles.

Mendel’s Law of Segregation For each character, an organism inherits two alleles, one from each parent. Mendel deduced this without knowledge of chromosomes! If there are two different alleles present only one of them – the dominant allele – determines the appearance.

Mendel’s Law of Segregation – the two alleles for a heritable character separate during gamete formation and end up in different gametes. Each egg or sperm will contain either one of the two alleles, but not both!

Genetic Terms Homozygous – both alleles are the same. PP homozygous dominant – purple flowers. pp homozygous recessive – white flowers. Heterozygous – two different alleles. Pp heterozygous, shows dominant, purple color.

Genetic Terms Genotype – the alleles that are actually present. PP, Pp, pp Phenotype – the physical appearance of the organism. Purple or white flowers.

Genetic Terms Monohybrid cross – crossing two individuals that are heterozygous for one particular trait. Pp X Pp Dihybrid cross – crossing two individuals that are both heterozygous for two separate traits. Yy. Tt X Yy. Tt

The Testcross Given a purple flowered pea plant with unknown parents, we will cross it to a homozygous recessive (white) individual to determine its genotype.

The Law of Independent Assortment Following two traits at once: Yellow (Y) vs. green (y) Tall (T) vs. short (t) Cross true-breeding yellow, tall (YYTT) with true-breeding green, short (yytt) to get F 1 individuals that are dihybrids (het for both traits – Yy. Tt).

The Law of Independent Assortment Each pair of alleles separates independently of other pairs during gamete formation. At least as long as the pairs of alleles are on separate chromosomes.

Complexities Mendel was fortunate to have chosen a simple system for study. In reality, there a number of complicating factors.

The Spectrum of Dominance The traits that Mendel examined showed complete dominance. The heterozygotes looked just like homozygous dominant individuals.

The Spectrum of Dominance Codominance occurs when both alleles affect the phenotype in separate, distinguishable ways. Both phenotypes are expressed. Not an intermediate. AB blood types

The Spectrum of Dominance In incomplete dominance, the phenotype of a heterozygote appears to be intermediate to, or distinct from, the homozygous dominant and homozygous recessive conditions.

Multiple Alleles Most genes actually have more than two different alleles. Human Blood Type – 3 different alleles. IA, I B, i IAIA, I Ai Type A blood IBIB, I Bi Type B blood IAIB Type AB blood ii Type O blood

Pleiotropy is a property where a gene has more than one effect on the phenotype of an organism. The gene that causes sickle cell disease also conveys some resistance to malaria.

Epistasis (from the Greek word for stopping) – one gene can alter the phenotypic expression of another gene.

Polygenetic Inheritance Polygenetic inheritance - Some traits have more than one gene contributing to a phenotype – like skin color in humans. Alleles have a cumulative effect.

Environmental Effects Some traits can be affected by the environment. Exposure to sunlight affects skin color in humans. Nutrition affects height in humans. Soil acidity affects color in hydrangea flowers.

Mendel & Modern View of Heredity Mendel’s fundamental principles of heredity can be expanded to understand the more complex issues. These principles can be applied to any living organism.

The Chromosomal Theory of Inheritance Genes have specific positions (loci) on chromosomes. Chromosomes undergo independent assortment and segregation.

Sex-Linked Genes located on the sex chromosomes (X or Y – usually X) are called sex-linked genes. Fathers pass on a sex-linked genes only to daughters (sons only receive the Y).

Sex-Linked Genes Color-blindness is a sex-linked trait in humans. Much more common in males. Males only need to inherit one recessive allele Females need to inherit two – one from each parent.

Experimental Evidence T. H. Morgan’s experiments provided the first evidence of the association between a specific gene & chromosome.

Experimental Evidence White eyes in fruit flies are recessive to red eyes. White eyes found only in males in F 2 Eye color gene located on X chromosome.

Autosomal Linkage Linked genes are located close together on the same chromosome. They are usually inherited together. They do not follow the rule of independent assortment!

Linked Gene Experiment Two recessive mutant traits: Black rather than grey bodies Vestigial rather than normal size wings Parental phenotypes: Grey, normal wings Black, vestigial wings

Linked Gene Experiment Testcross produced mostly the parental phenotypes.

Linked Gene Experiment Some non-parental phenotypes were produced. Genetic recombination – Crossing over explains how this happens.

Linkage Mapping A genetic map of the sequence of genes on a chromosome can be made using the frequency of recombination data for a number of traits. Crossing over more likely to separate genes that are further apart.

Alterations in Chromosome Number Errors in meiosis or mitosis may lead to one extra or one less chromosome. This is called aneuploidy. Trisomic – 3 copies of a chromosome Monosomic – only 1 copy If the organism survives, it usually has symptoms relating to the increase or decrease in proteins coded for by the extra (or missing) chromosome.

Alterations in Chromosome Number Euploidy – An addition or deletion of a whole set of chromosomes. Polyploidy is the condition where there are more than 2 complete sets of chromosomes. Triploid – 3 sets Tetraploid – 4 sets Common in plants Having an entire extra set is not as detrimental as having one extra chromosome.

Alterations of Chromosome Structure

Genes Gene gene product phenotypic expression Gene products = proteins Beadle & Tatum’s experiments using bread mold led to the idea that one gene produces one enzyme. Today’s version: a nucleic acid sequence (usually DNA) that encodes a functional polypeptide or RNA sequence.

Nucleic Acids DNA and RNA both built of nucleotides containing Sugar (deoxyribose or ribose) Nitrogenous base (ATCG or AUCG) Phosphate group

Nitrogenous Bases Nitrogenous bases can be double ringed purines or single ringed pyrimidines.

Nitrogenous Bases A purine will always pair with a pyrimidine.

DNA The phosphate group and sugar make up the backbone of the DNA molecule.

DNA The DNA backbone: Phosphate groups and pentose sugars. The 5' end of each strand has a free phosphate group attached to the 5' carbon of the pentose sugar. The 3' end has a free hydroxyl group attached to the 3' carbon of the pentose sugar.

DNA consists of two complementary chains connected by hydrogen bonds. A=T C=G

DNA synthesis occurs in the 5' to 3' direction in both strands. The DNA strands are antiparallel 5' end of one is associated with the 3' end of the other. The DNA ladder is twisted into a double helix Ten base pairs occur per turn.

RNA exists as a single polynucleotide chain. Ribose Uracil

DNA Replication DNA must replicate itself prior to cell division. Enzymes are responsible for each step of replication, including proofreading. The helix unwinds, separates, and each half acts as a template for the formation of a new complementary strand. Reaction catalyzed by DNA polymerase.

Gene Expression Gene expression – the use of information in DNA to direct the production of particular proteins. Transcription – first stage of gene expression. A messenger RNA (m. RNA) is synthesized from a gene within DNA. Translation – second stage – m. RNA is used to direct production of a protein.

DNA Coding DNA codes for the sequence of amino acids in a protein. A codon is three base-pairs long and is a segment of m. RNA that codes for an amino acid.

Transcription Messenger RNA (m. RNA) transcribes the DNA and transports it out of the nucleus.

Transcription Before leaving the nucleus, segments of m. RNA called introns are removed and the exons are spliced together. Exons contain the information coding for the protein that will be synthesized.

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Translation occurs on ribosomes outside the nucleus. m. RNA attaches to a ribosome and protein synthesis begins.

Translation Transfer RNA (t. RNA) collects free amino acids from the cytoplasm and delivers them to the polysome (m. RNA-ribosome complex) where they are assembled into a polypeptide. t. RNA has a triplet – the anticodon – that is complementary to the codon of m. RNA.

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Regulating Gene Expression Cells control the expression of genes by saying when they are transcribed, not how fast. Some regulatory proteins block the binding of the polymerase, and others facilitate it.

Storage and Transfer of Genetic Information Regulation of Gene Expression in Eukaryotes As tissues differentiate, they use only a part of the genetic instruction present every cell. In a particular cell or tissue most genes are inactive at any given moment.

Gene Mutations Gene mutations result in an alteration of the sequence of bases in the DNA. Harmful Neutral Beneficial

The Importance of Genetic Change Evolution begins with alterations in the genetic message. Mutation creates new alleles Gene transfer alters gene location Recombination shuffles these changes Chromosomal rearrangement alters the organization of entire chromosomes.

The Importance of Genetic Changes that result in the organism leaving more offspring are often preserved. Other changes result in fewer offspring – these changes are usually lost. Genetic changes can only be inherited if they occur in germ-line tissue!

The Importance of Genetic Change Genetic change through mutation and recombination provides the raw material for evolution.