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KEY CONCEPTS 1. Definition 2. Revision from gr 11: • Homologous chromosomes • Paternal/maternal chromosomes • Diploid/haploid • Somatic cell/body cell 1 Genetics • Genetics is the study of heredity • It deals with the similarities and differences between parents and their offspring • In Genetics we look at how characteristics are passed from one generation to the next. • Genes carry the hereditary information on chromosomes. WHAT IS GENETICS 4 JESSICA ALBA GETS MARRIED Chromosomes, DNA & Genes KEY CONCEPTS 1. Locus(i) 2. Alleles 3. Genome 4. Dominance & Recessiveness 5. Karyotype 6. Genotype 7. Phenotype 8. Homozygous 9. Heterozygous 2 Genetics • Let us look at the human characteristic of having a free or attached earlobe • Click on the attached lobe YES This ear is attached This lobe is FREE 3 Genetics Attached lobe Free lobe 4 Attached lobe e e •Genes occur in pairs •If we represent the gene for attached ear lobe as little “e” •Then this persons gene pair will be “ee” 5 Genetic Terms e e PHENOTYPE The characteristic expressed by the gene e.g. attached ear lobe ALLELE genes are partners of each other:“e” is an allele of “e” GENOTYPE The genes that code for a characteristic e.g. ee 6 Looking at the genes on Chromosomes Attached lobe e e The genes occur in pairs on chromosomes Free lobe E E The gene for free lobe is “E” 7 Attached lobe e e Free lobe E E The genes occur at special positions on chromosomes called loci. One version of the gene is inherited from the father , the other from the mother If the two genes are the same e.g. ee or EE they are homozygous. 8 E e There can be a mixture of attached and free ear lobe genes (Ee)- heterozygous. One gene, in this case (E) is dominant over unattached lobe (e) which is called recessive In this case the E gene completely supresses the expression of e gene so this persons will have free ear lobes 9 Summary Three possible genotypes… PHENOTYPE Free lobe Attached lobe GENOTYPE e e Homozygous recessive E E E e Heterozygous dominan Homozygous dominant Alleles Alleles Bb BL**DY IMPORTANT DEFINITIONS 1. The two genes on homologous chromosomes that code for the same characteristic are found on identical locations on the pair of chromosomes, called loci (singular: locus) pg 5 2. Alleles are alternate forms of a gene located on the same locus of homologous chromosomes. 3. It seems that sometimes, one gene dominates the other of the pair. We say that the one gene is dominant, while the one that is dominated, is called the recessive gene (pg5). 4. A genotype Bb is called HETEROZYGOUS (or hybrid). Here the paired genes (ALLELS) for a particular trait (characteristic) are different (pg 6). 5. A genotype BB or bb is HOMOZYGOUS. Here the paired genes (ALLELES) for a particular trait (characteristic) ore identical (7)` BL**DY IMPORTANT DEFINITIONS 1. The characteristics that we can see in an individual, for example, brown eyes, is known as the PHENOTYPE (pg 6). 2. The letters Bb indicate to us the GENOTYPE for eye colour, that is, its genetic makeup (pg 6). 1 Looking at our example of earlobes. What possible offspring can be produced if: The male parent has The female parent has ATTACHED lobes FREE lobes (ee) (EE) 2 Firstly we need to look at the formation of sperm cells (male gametes in the testes) to see the different types of sperm cell that can be produced from this ee parent. 3 Looking at one cell in the testes dividing by meiosis e e e e Cell with double stranded chromosome pair with egenes 4 Cell divides by meiosis e e e e 5 e e e e End of the first MEIOTIC division Chromosomes have separated 6 e e e e Second MEIOTIC division 7 End of the second MEIOTIC division – FOUR sperm cells are produced, each with a e-gene e e e e 8 E ee EE e e e e e In the same way, the female will produce egg cells in the ovary. As she is EE she will only In this only one type of sperm cell produce onecase typethere of eggis cell that can be produced – all have e-gene with the E-gene 9 e E ee EE E e The sperm and egg cells fuse to form a new child All the offspring from these parents will have a Ee genotype – They will ALL have Free ear lobes Punnett Square GAMETES E E e Ee Ee e Ee Ee 1 Now what if both parents are heterozygous (Ee) What are the possible offspring? The male parent has The female parent has ATTACHED lobes ATTACHED lobes (Ee) (Ee) 2 Looking at one cell in the testes dividing by meiosis E E e e Cell with double stranded chromosome pair with E and e genes 3 Cell divides by meiosis E E e e 4 E E e e End of the first MEIOTIC division Chromosomes have separated 5 E E e e Second MEIOTIC division 6 End of the second MEIOTIC division: FOUR sperm cells are produced, TWO with an Egene and TWO with e-gene E E e e 7 E E e e Two types of sperm cells can be produced- ONE with a e-gene and one with a E-gene 8 Looking at one cell in the ovary dividing by meiosis E E e e 9 E E e e 10 E E e e 11 E E e e 12 E E e e 13 E E e e Two types of egg cells can be produced- ONE with a e-gene and one with a E-gene 14 E e 15 E E e e What possible offspring can be produced when these sperm and egg cells fuse? 16 e E E e E E E e If a E-sperm If a e-sperm fuses with a E- fuses with a Eegg, the child egg, the child will be EE -Free will be Ee -Free Lobe Lobe 17 e E E e E E e E In the same way … E e e e To simplify this we use a Punnett square to show possible offspring male gametes E female gametes 18 E e EE Ee e Ee ee Possible offspring 3 out of 4 free lobes 1 out of 4 attached lobe If for example marries P1 (first parental generation) Bb x (Brown-eyed male) Meiosis Meiosis B b Male gametes (sperm) ♀ GAMETES b b Female gametes (egg cells) Male gametes ♂ (symbol for male) We use what is called a PUNNET SQUARE Female gametes bb (blue-eyed female) B b b Bb bb b Bb bb Punnett Square GAMETES B b b Bb bb b Bb bb Male gametes ♂ (symbol for male) We use what is called a PUNNET SQUARE GAMETES B b b Bb bb b Bb bb • Phenotype: Half are Brown-eyed, half are blue-eyed (1:1) • Genotype: half are heterozygous brown (Bb) and half are homozygous blue (bb): 1Bb:1Bb Why can two brown-eyed parents have a blue-eyed child? P1 Bb x Bb P1 Meiosis B Fertilisation F1 or GAMETES b B or B b ratio of gametes b B BB Bb b Bb bb 1 homozygous brown (BB) 2 heterozygous brown (Bb) 1 homozygous blue (bb) (F1 = first filial generation, in other words, the possible types of eye-colour of children the parents may have) The PHENOTYPIC RATIO: 3 brown-eyed child:1 blue-eyed child The GENOTYPIC RATIO: 1 homozygous brown (BB): 2 heterozygous brown (Bb): 1 homozygous blue (bb) 1BB:2Bb:1bb or 25%BB:50%Bb:25%bb e.g., Tall Plants x Short Plants Let T = gene for tallness Let t = gene for shortness (note: you must use the same letter for a characteristic) P1 TT x T T tt t P1 (crossed 2 homozygous plants) t (Gametes) Fertilisation F1 (Punnet square) GAMETES t t T Tt Tt T Tt Tt All the offspring of F1 will be Tt (heterozygous tall) Thus, Mendel said that when two characteristics meet in an individual, one dominates over the other, called the recessive (LAW OF DOMINANCE AND RECESSIVENESS). Mendel took the offspring from F1 (Tt) and crossed them P2 Tt x Tt P2 (2nd Parental generation) Meiosis T t T t gametes Fertilisation F2 GAMETES T t T TT Tt t Tt tt 1 homozygous tall (TT) 2 heterozygous Tall (Tt) 1 homozygous short (tt) Mendel’s law: INDEPENDENT ASSORTMENT • Independent assortment occurs during meiosis I, specifically metaphase I of meiosis, to produce a gamete with a mixture of the organism's maternal and paternal chromosomes. Along with chromosomal crossover, this process aids in increasing genetic diversity by producing novel genetic combinations. 3 MEIOSIS – Prophase I Crossing Over As This happens homologous when partner pairs line up, chromosomes crossing over swop pieces of occurs chromatid Chromatids This mixesof Pieces from partner genetic chromosome material chromosomes are andswopped brings cross over variety 4 Crossing Over brings Variation Instead of Four different types of chromatids Two 5 MEIOSIS – Metaphase I - The Homologous chromosome pairs can line chromosomes up in different line up combinations – IN this PAIRS atbrings the equator variety 6 How many possible combinations are there ? With 2 chromosome pairs (2) there are 4 possible combinations 22 = 4 This is called independent assortment 7 How many possible combinations are there ? What possible combinations are there with 23 pairs? 223 = ? 8 388 608 Remember this is without crossing over and just in a sperm or egg cell!! Mendel’s law: INDEPENDENT ASSORTMENT • In independent assortment the homologous • chromosomes separate randomly during Anaphase I of Meiosis I. Chromosomes that end up in a newly-formed gamete are randomly sorted from all possible combinations of maternal and paternal chromosomes. Because gametes end up with a random mix instead of a pre-defined "set" from either parent, gametes are therefore considered assorted independently. As such, the gamete can end up with any combination of paternal or maternal chromosomes. INCOMPLETE DOMINANCE • In Gauteng, we often see cosmos flowers on the side of roads at the end of summer. We see red, white and purple flowers. Why? 1 Incomplete Dominance Incomplete Dominance occurs when the offspring show a combination of recessive and dominant characteristics Pure Red flowers crossed with White flowers produce all pink flowers in the F1 generation. INCOMPLETE DOMINANCE • Let R • Let W = gene for red snapdragons = gene for white snapdragons GENOTYPE PHENOTYPE CRCR Red Flowers CRCW Pink flowers CWCW White flowers INCOMPLETE DOMINANCE P1 CRCR x CWCW P1 (crossed 2 homozygous plants) CR CR Fetilisation F1 (Punnet square) GAMETES CW CW CW CW CR CR CRCW CRCW CRCW CRCW (Gametes) INCOMPLETE DOMINANCE If CRCW is crossed with CRCW the result will be F2 GAMETES CR CW CR CRCR CRCW CW CRCW CWCW • Phenotype: 1 red: 2 pink: 1 white • The genes are unaltered by this phenomenon CO-DOMINANCE PARENTS Phenotype: Red Genotype: IRIR x x White IWIW Key: R – Red coat W – White coat MEIOSIS GAMETES IR IR IW IW FERTIISATION GAMETES IW IW IR IR IR IW I RI W IRIW IRIW Looking at co-dominance when pure bred (homozygous) red and white cattle are bred. 2 P1 Genotype Red Bull IRIR White cow IW IW Gametes CR CW F1 All IRIW (Roan) Offspring are produced GAMETES W W GENOTYPE: PHENOTYPE: R R RW RW RW RW ALL HETEROZYGOUS RW ALL ROAN CO-DOMINANCE P2 (F1) Phenotype: Roan IR I W x MEIOSIS GAMETES IR IW FERTIISATION GAMETES Roan IR I W IR I W IR IW IR IR IR IR IW IW IR IW IW IW • F2Genotype: 1 IR IR: 2 IR IW: 1 IW IW • F2 Phenotype: 1 RED: 2 ROAN: 1 WHITE GAMETES R W Genotype: Phenotype: R W RR RW RW WW 1 RR: 2 RW: 1 WW 1 RED: 2 ROAN: 1 WHITE WHAT ARE THE CHANCES OF HAVING A BOY OR GIRL ON THIS BASIS ALONE? _______% 50% MALE PARENT FEMALE XY BODY CELL XX MEIOSIS GAMETES X Y X XX XY X FERTILISATION ZYGOTE [OFFSPRING/PROGENY] XX XY SEX-LINKED INHERITANCE Some Characteristics, like the gene for colour vision are found attached to the X chromosome. This means that the gene for that characteristic is linked to the sex o the individual. X X X Y Do you notice that the male’s X chromosome does not have corresponding loci on the Y chromosome because it is shorter. Thus even a recessive gene on the X chr. will be expressed. In humans, the gene for colour vision is sex-linked. The gene is linked to the X chromosome. The gene for normal colour vision (B) is dominant over the gene for colour blindness (b). B b X X XBXb b X Y XbY If the female parent (XX) has normal vision (Bb) and the male (XY) is colour blind (b – only on the X)… 8 SEX-LINKED INHERITANCE X X X Y If the female parent (XX) has normal vision (BB) and the male (XY) is colour blind (b –only on the X)… How do we link the colour blind genes to the sex chromosomes ? 9 SEX-LINKED INHERITANCE B B X b X X Y Normal Female Colour Blind male BB on the X chromosomes b on the X chromosome only 10 Normal Female B Colour Blind Male b B XX b XY X B Colour blind male possible sperm cells B X Y X Normal female possible egg cells Normal Female x Colour Blind Male 11 Normal Female B Colour Blind Male b B XX b XY Y X B X B b B X X Normal Female but carries the (b) colour blind gene Normal Female x Colour Blind Male X Y Normal Male 12 What are the possible offspring that would result from a carrier female and normal male ? Normal male gametes Carrier female gametes XB Y 1 2 XBXB XB 4 XBXb XbY Possible GENOTYPE 1 Normal female 2 Normal male XBY 3 Xb PHENOTYPE 3 Carrier female 4 Colour blind male What are the possible offspring that would result from a carrier female and normal male? P1 XBXb x XB Y meiosis gametes XB Xb XB Y FERTIISATION GAMETES F2: XB XB XB X B Xb XB X b Y XB Y XbY Genotype: _____________________________________________ 1 XBXB: 1XBXb: 1 XBY: 1 XbY NORMAL: 1 FEMALE NORMAL CARRIER: Phenotype: 1 FEMALE _____________________________________________ 1 MALE NORMAL: 1 MALE COLOURBLIND _____________________________________________ BLOOD GROUPS • CO-DOMINANCE • MULTIPLE ALLELES ABO blood groups in humans Important because ABO blood groups affect blood transfusions o Genes cause expression of sugar groups on surface of red blood cell membrane; these carbohydrates act as antigens in immune reactions o 3 alleles: A, B, O o A & B dominant over O o A & B co-dominant to each other o Type O produces no sugar antigens o 6 genotypes and 4 phenotypes Phenotype Genotypes A AA (IAIA) or AO (IAi) B BB (IBIB) or BO (Ibi) AB AB (IAIB) O OO (ii) Blood Groups e.g., Cross a homozygous group A man with a heterozygous group B women to find the F1 PHENOTYPE A (I A I A) x B (I Bi) GENOTYPE GAMETES F1 GAMETES IA IA IB IAIB IAIB i IA i IA i 50% = IA i 50% = IAIB GENOTYPE: _______________________________________________________ 50% = A 50% = AB PHENOTYPE:_______________________________________________________ Blood Groups Cross a homozygous group A man with a heterozygous group B women to find the F1 PHENOTYPE A x GENOTYPE AA B BO GAMETES F1 GAMETES A B AB O AO GENOTYPE: A AB AO 2 AB:2AO PHENOTYPE: 50% GROUP A: 50% AB BLOOD GROUPS BLOOD TRANSFUSION Before a person (recipient) receives blood, the blood of the donor has to be first tested to ensure that it is not infected and is of the right type. The table below shows the safe donor for recipients of the various blood types: BLOOD TYPE OF RECIPIENT ANTIBODIES A B AB O B A NONE A&B DONOR A, O B, O A, B, AB, O O Blood Groups BLOOD TYPE OF PARENTS AB x AB AB x A AB x B AB x O A x A A x B A x O B x B B x O O x O BLOOD TYPE OF CHILDREN A, AB, B A, AB, B A, AB, B A, B A, O AB, A, B, O A, O B, O B, O O AB X AB = A, AB, B GAMETES A A (I ) B B (I ) A (IA) AA (IAIA) AB (IAIB) B (IB) AB (IAIB) BB (IBIB) BLOOD TRANSFUSION recipient donor A A or O B B or O AB (UNIVERSAL RECIPIENT) A, B, AB, or O O (UNIVERSAL DONOR) O The pedigree diagram below shows the blood groups of individuals of a family. The blood groups are indicated inside the circle or square. The blood groups of individuals W and X are not indicated. Blood group O W Blood Group A X Write down al the possible genotypes of individuals: [a] W [b] X Blood group B (a)W = AB (IAIB) (b) AO (IAi) Blood group O Key: Male Female X = AO (IAi) OO (ii) (8) Haemophilia is a blood clotting disorder. Explain why mainly males suffer from this disorder. (4) • It is a sex-linked disease caused by a recessive • • • • allele carried on the X chromosome Males need only one recessive allele to have the disease because they have XY combination, while females have to have both recessive alleles to have haemophilia because they have an XX combination any (4) DIHYBRID CROSSES Let R = gene for round seeds Let Y = gene for yellow seeds r = gene for wrinkled seeds y = gene for green seeds If a Round, yellow seed is crossed with a wrinkled green seed: Genotype of parents (P1) RRYY x rryy meiosis Gametes RY Genotype of offspring (F1) Phenotype of offspring: RY ry ry RrYy All Round, Yellow seeds Genotype of parents (P2) RrYy x RrYy meiosis Gametes RY Ry rY ry RY Ry rY Genotypes of the offspring (F2) GAMETES RY Ry rY ry RY Ry rY ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy RrYy rrYY rrYy Phenotype of offspring (F2): Rryy Round Yellow: rrYy 9 ……… ……… 3 Round Green: ............ Wrinkled Yellow: 3 ……..... Wrinkled Green: 1 ………. rryy ry Polygenic inheritance • Some phenotypes determined by additive effects of 2 or more genes on a single character – phenotypes on a continuum – human traits • skin color • height • weight • eye color • intelligence • behaviors Polygenic inheritance ALBINISM The woman must be Aa (because parents were aa x AA or aa x Aa) Albino man normal woman X aa all a ½ Aa sperms x Aa Normal children ½ A eggs ½ ½ aa Albino children a Albinism albino Africans Johnny & Edgar Winter DOWN’S SYNDROME DOWN’S SYNDROME • Individuals with Down syndrome tend to have a lower-than-average cognitive ability, often ranging from mild to moderate disabilities. • A small number have severe to profound mental disability. The average IQ of children with Down syndrome is around 50, compared to normal children with an IQ of 100. • • • • • • • • abnormally small chin poor muscle tone a flat nasal bridge protruding tongue (due to small oral cavity, short neck, Mental retardation in the mild (IQ 50–70) to moderate (IQ 35–50) range. They also may have a broad head and a very round face. Language skills show a difference between understanding speech and expressing speech, and commonly individuals with Down syndrome Nature vs. nurture • Phenotype is controlled by both environment & genes Human skin color is influenced by both genetics & environmental conditions Coat color in arctic fox influenced by heat sensitive alleles Color of Hydrangea flowers is influenced by soil pH Non Inherited variations ENVIRONMENTAL VARIATIONS • Birth Defects also called congenital • disorders due to factors affecting foetal development, such as radiation, heat, chemicals (booze, smoking), infectious agents or maternal disease (e.g., measles) Teratogen: “monster” “born” Inherited Variations • Mutations A mutation occurswhen the order of nucleotides in the D.N.A. is changed. X-rays, excessive exposure to the sun’s heat, exposure to harmful chemicals, radiation form nuclear bomb explosions are some of the causes of mutated genes. The offspring will inherit the mutated gene Hybridisation Genetically modified (GM) foods GM foods • Genetically modified organisms have had specific changes introduced into their DNA by genetic engineering, • unlike similar food organisms which have been modified from their wild ancestors through selective breeding (plant breeding and animal breeding) or mutation breeding. • GM foods were first put on the market in the early 1990s. GM FOODS – the positives • GM foods have been modifies to: increase the crop yield; • make crops resistant to herbicides (so that weeds can be eliminated); resistance to insects which may eat the crop; • production of specific nutrients (like vitamins); • produce drought-resistance crops; • improve the taste of certain foods; GM FOODS – the negatives • Critics have objected to GM foods on several • grounds, including perceived safety issues (may cause diseases), ecological concerns (genes from GM foods may mix with nonGM foods and cause unfvourable changes in crops) and economic concerns raised by the fact that these organisms are subject to intellectual property law (premium on price for these seeds). Human Genome • The human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs. Twenty-two of these are autosomal chromosome pairs, while the remaining pair is sexdetermining. The haploid human genome occupies a total of just over 3 billion DNA base pairs. The Human Genome Project (HGP) produced a reference sequence of the euchromatic human genome, which is used worldwide in biomedical sciences. Human Genome • The haploid human genome contains ca. 23,000 protein-coding genes, far fewer than had been expected before its sequencing. In fact, only about 1.5% of the genome codes for proteins, while the rest consists of non-coding RNA genes, regulatory sequences, introns, and (controversially named) "junk" DNA. STEM CELLS • Stem cells are cells found in all multi cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiate into a diverse range of specialized cell types. • The two broad types of mammalian stem • • • cells are: embryonic stem cells that are isolated from the inner cell mass of blastocysts, and adult stem cells that are found in adult tissues. In a developing embryo, stem cells can differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells, but also maintain the normal turnover of regenerative organs, such as blood, skin, or intestinal tissues. AB X A (AA OR AO) = A, AB, B GAMETES A B A AA AB A (O) AA (AO) AB (OB) AB X B (BB OR BO) = A, AB, B GAMETES A B B AB BB B (O) AB (AO) AB (OB) AB X O = A,B GAMETES A B O AO BO O AO BO AA (AO) X AA (AO) = A,O GAMETES A A (O) A AA AA (AO) A (O) AA (AO) AA (OO) AA (AO) X BB (BO) = A, B, AB, O GAMETES A A (O) B AB AA (BO) B (O) AB (AO) AB (OO) AA (AO) X OO = A,O GAMETES A A (O) O AO AO (OO) O AO AA (OO) BB (BO) X BB(BO) = B, O GAMETES B B (O) B BB BB (BO) B (O) BB (BO) BB (OO) BB (BO) X OO = B,O GAMETES B B (O) O BO BO O BO BO (OO)