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GENETICS VOCAB CARD 1 CHROMOSOMES Zygote- a single fertilised cell Gene- Units of inheritance usually occurring at specific locations, or loci, on a chromosome. (A section of DNA) These units are responsible for hereditary characteristics in plants and animals. Chromatin-DNA in its normal, functional state, condenses to form chromosomes Chromatid-is one of the two threads of condensed chromatin which forms one chromosome. The 2 chromatids are connected by a centromere. They exist only during prophase and metaphase Haploid-a cell contains only one set of chromosomes (n) Diploid-a cell that contains a double set of chromosomes, one from each parent(2n) Chromosome-each species will have a specific number of chromosomes (e.g humans 46) A human has 22 pairs of autosomes and one pair of sex chromosomes. A karyotype shows all the chromosomes DNA DNA- deoxyribose nucleic acid, a double stranded helix (spiral) The rungs of the ladder are the nitrogenous base pairs, (A, T, C, G) which are held together by a backbone of deoxyribose sugar (a 5 carbon sugar)and phosphate groups.Adenine, Thymine, Cytosine, Guanine. A and T always pair together C and G always pair together DNA 2 2 complementary strands of DNA form the helix; It is made up of-sugar-phosphate backbone -nitrogenous bases stacked in the center -antiparallel 5’ end 3’ end -twists to right ALLELE Allele-an alternative form of a gene. Because they are different, their action may result in different expressions of a trait. • Example: one allele may cause blonde hair, while another causes black • Letters are used to represent alleles • Capital letters = dominant genes • Lowercase letters = recessive genes RSA 08 DNA REPLICATION (Semi-conservative replication) DNA replication occurs prior to cell division, during interphase. It occurs in a series of steps, each controlled by enzymes, with energy supplied from ATP. -parent strands separate and serve as a template for a new strand -enzymes break the hydrogen bonds between the bases, unzipping the rungs of ladder. -copied in a 5’ 3’ direction the leading strand is copied in a straight forward manner, and the lagging strand is built in a series of steps by Okazaki fragments. -two new pieces of DNA result, one from the parent strand and a newly synthesized one. Each containing half the original DNA (hence semi (half) conservative (saved)) MITOSIS Mitosis is the division of cells for growth and repair, somatic cells. It results in 2 identical cells being formed, which are diploid (2n). Each daughter cell has the same kind and number of chromosomes as the original parent cell. Remember Interphase-in the beginning, the resting stage. Chromosomes cannot be seen theyduplicate and double in number Prophase- 2 p’s in prophase means stage 2. Chromosomes become visible Each chromosome consists of two identical chromatids. Metaphase- Chromosomes line up at the equator (centre of the cell) Spindles are formed to attach to the centromere of each chromosome Anaphase- think the godfather ‘ana then they begin to separate. Sister chromatids separate as individual chromosomes They move apart towards the opposite poles Telophase-ending in 2 new cells. Chromosomes gradually disappear. Nuclear membrane is form around each set of chromosomes Cytokinesis- is the division of the cytoplasm and formation of a membrane to produce complete new cells. LABEL THE PHASES OF MITOSIS 1 2 1 3 in te 4 te 5m lo rp pOF LAW et p SEGREGATION: ha rh ap se o ha as pe se h as RSA 08 e 1= Anaphase 2=Interphase 3=Telophase 4=metaphase 5=prophase Each pair of chromatids is segregated, or separated, from one another at the centromere during the formation of gametes. This occurs at anaphase. They separate independently of each other during meiosis and so can form different combinations. Leading to genetic variation. MEIOSIS Prophase P2 Metaphase M2 Anaphase A2 Telophase T2 SIGNIFICANCE OF MEIOSIS PRINCIPLE OF INDEPENDENT ASSORTMENT Producing cells with chromosome number half of the parent cell, forming the gametes. Gametes are cells having one chromosome from each homologous pair haploid (n) Two nuclear divisions l Meiosis I and meiosis II Four haploid cells are produced Meiosis occurs in § Plants: anthers and ovules § Mammals: testes and ovaries 1. Leads to halving of chromosome number (haploid), so to ensure that the diploid number of chromosomes can be restored after fertilization 2. Produce genetic variation at -crossing-over between homologous chromosomes during prophase I results in recombination. -independent assortment of chromosomes during metaphase I Gregor Mendel's principle of genetic inheritance stating that different pairs of genes are passed to offspring independently so that new combinations of genes, present in neither parent, are possible. In other words, the distribution of one pair of alleles does not influence the distribution of another pair--the genes controlling different traits are inherited independently of one another. The homologous chromosomes line up in random order along the equator during metaphase, therefore the genes are inherited independently. For humans with 23 pairs of chromosomes there are over 8 million possible combinations of the chromosomes in the gametes that are produced. CROSSING OVER AND RECOMBINATION The point of crossing over is called a CHIASMA RSA 08 Crossing over can only occur when homologous chromosomes come together side by side during the early stages of meiosis. They have to be homologous chromosomes as the chromatids can only cross over with corresponding gene sequences. When they become entangled they form a chiasma. They can now pass on to the second meiotic division to form 4 genetically different gametes. Two of the four chromatids have recombined genes-are recombinant, different to that of the maternal or paternal genes. and two are non-recombinant. If chromatids were not involved in crossing over, some of the gametes will have ‘original’ allele sequences. These are non-recombinant alleles. COMPARE AND CONTRAST MITOSIS AND MEIOSIS MUTATIONS It is the process of mutation that creates the new alleles which are the source of genetic variation. But it is the three processes of independent assortment, segregation, and recombination that shuffle the alleles creating new combinations which produce the great variation seen within members of a species. TYPES OF MUTATIONS Mitosis Meiosis Produces body cells(Somatic cells) Produces sex cells(Gametes) Daughter cells diploid(2N) Daughter cells haploid(N) Two daughter cells produced Four daughter cells produced In metaphase chromosomes line up singly In metaphase I chromosomes line up as homologous pairs(synapsis) The two double chromosomes are called a tetrad when they are lined up side-by-side. Crossing over occurs during the formation of the tetrad One nuclear division Two nuclear divisions Produces cells for growth and repair Produces cells for sexual reproduction Daughter cells have two sets of chromosomes(pairs) Daughter cells have only one member of each pair of chromosomes Daughter cells are genetically identical to the parent cell Daughter cells have one-half of the genes from the parent cell Insures that all daughter cells are genetically identical Generates genetic diversity through crossing over and random separation of homologous pairs of chromosomes A mutation is the change in genetic material of an organism. Mutations are spontaneous and random, and typically are rare. However, their rate may be increased by exposure to environmental factors such as ionising radiation and mutagens. (Mutation inducing chemicals such as benzene and formaldehyde) While most mutations are harmful or neutral, sometimes the altered function will be beneficial to the individual. If the mutations occur in the gametes, then the mutations will enter the gene pool of the population and become subject to natural selection. BLOCK OR CHROMOSOMAL MUTATIONS -duplication, inversion, translocation, deletion Mutation involving whole genes. This occurs when parts of chromosomes are deleted, repeated, or translocated to other chromosomes. They may remove alleles from the gene pool (through deletion) or produce different combinations of alleles (through translocation). POINT MUTATIONS These are changes in the bases in DNA, so occur within a gene. Bases maybe deleted, inserted or substituted during DNA replication. This results in a change in the base sequence of DNA that codes for a gene, so producing a new allele. RSA 08 GENETICS VOCAB CARD 2 (INHERITANCE) MONOHYBRID CROSSES What proportion of the offspring (out of 4) will be: Recessive allele-The general term for an allele that is masked in the phenotype by the presence of another allele. Dominant allele-The general term for an allele that masks the presence of another allele in the phenotype. Genotype-The genetic makeup of an individual for a trait or for all of his/her inherited traits-not the observable or detectable characteristics. Phenotype- The observable or detectable characteristics of an individual organism; the detectable expression of a genotype. Heterozygous genotype- A genotype consisting of two different alleles of a gene for a particular trait. Homozygous genotype-A genotype consisting of two identical alleles of a gene for a particular trait. True/Pure breeding-An organism, which when bred with genetically alike organisms. All the offspring are the same as the parents. They have a homozygous genotype. Sexed link genes- Genes that are located on the sex chromosome, usually the X Punnett squares - can be used to work out the probability of a characteristic being passed on to the next generation. A cross between a single characteristic governed by two alleles is called a monohybrid cross. Short Toe______ Long Toe ______ *These are your predicted ratios. TEST CROSS or BACKCROSS An individual that displays a dominant trait can either be homozygous dominant or heterozygous. To discover which we must do a test cross. Mating involving an unknown genotype with the homozygous recessive eg hh to determine the genotype of the unknown by the offspring produced. Question Corn seed colour is determined by a single gene. Purple seed (P) is dominant over yellow seed (p). Max buys some purple corn seeds from the local seed merchant. Explain what he could do to determine whether the corn seed was homozygous or heterozygous. To be sure of the genotype of the individual being tested, it must be: -crossed with a large number of recessive individuals. Or -bred a number of times; this is to ensure that the masked recessive allele will show in the offspring if present. If no recessive features occur after multiple matings, the tested individual can safely be assumed to be homozygous and so can be used for breeding. PEDIGREE CHARTS RSA 08 - Pedigree Test - Test in which you look at the offspring of parents to determine the genotypes of the parents. - The squares are for males and the circles are for females. -If the symbol has been darkened it says the person carries the trait. DIHYBRID CROSSES Is the inheritance of 2 genes controlling 2 different features. If two heterozygous dihybrids with complete dominance are crossed (TtYy x TtYy). The 9:3:3:1 ratio is always the probability of getting each phenotype. RSA 08