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MENDELIAN GENETICS 1. Monohybrid Crosses Show the inheritance of a trait 2.

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Presentation on theme: "MENDELIAN GENETICS 1. Monohybrid Crosses Show the inheritance of a trait 2."— Presentation transcript:


2 Monohybrid Crosses Show the inheritance of a trait 2

3  Allele  Genotype  Phenotype  Homozygous  Heterozygous 3

4 BB BB bb bb Bb BbBb Bb B = b = BB = Bb = bb = 4

5  The brown eye colour allele is dominant over the blue allele  Question-  A brown eyed homozygous male and a blue-eyed female have a child. Draw a diagram to show the colour of their child’s eyes.  The child later marries a blue-eyed woman. What chance would they have of having a blue-eyed child? Dominance 5

6  A pure-breeding purple rabbit was crossed with a pink rabbit. All the offspring were purple.  Draw a genetic diagram to show why this was. Steps to answering genetics questions  Decide on suitable letters  Work out which characteristic is dominant – the offspring phenotype will tell you this  Write the genotypes of the parents  Draw a Punnett square  Always put the dominant feature 1 st  Write down the phenotypes of each genotype and state the numbers of each type 6

7 Cystic fibrosis  F = normal CTFR allele  f = faulty CTFR allele  CTFR gene codes for a protein channel allowing exit of chloride ions from cells  Mutated CTFR protein are not placed in the plasma membrane.  This causes a build up of thick mucus in the lungs and other organs. 7

8  Draw a genetic diagram to determine the chance of a heterzygous man and homozygous dominant woman having a child with cystic fibrosis  Explain what is wrong with each of these statements  A couple are both carriers for CF will have 4 children, 1 with CF and 3 without’  If a couple’s first child has CF, their second child will not. 8

9 Co-dominance Occurs when both alleles are expressed in the phenotype  Example:  A cat has 2 alleles for coat colour black (B) or white (W).  A cat with 2 black alleles has black fur, a cat with 2 white has…..  A heterozygote produce both white & black pigments resulting in grey fur. 9

10 Co-dominance example: FBFBFBFB FWFWFWFW Gene locus Allele What is the ratio of phenotypes produced by crossing two grey cats? FBFBFBFBFBFBFBFB FBFWFBFWFBFWFBFW FBFWFBFWFBFWFBFW FWFWFWFWFWFWFWFW FWFWFWFW FBFBFBFB FBFBFBFB FWFWFWFW 1 Black:2 Grey:1 White 10

11 Height inheritance in plants  H T H T = tall stem H T H S = medium stem H S H S = short stem  A farmer crosses 2 heterozygotes. Draw a diagram to show the genotypes and phenotypes of the offspring  A farmer wishes to produce all medium stemmed plants. Which two parents must he choose to cross in order to achieve this? Draw a diagram to explain your answer. 11

12 Test Crosses  Crosses an organism showing the dominant phenotype with a homozygous recessive. These are carried out to identify if the phenotypically dominant organism is heterozygous or homozygous.  If the dominant organism is homozygous all offspring will show the dominant characteristic.  If the dominant organism is heterozygous 50% of the offspring will show the dominant trait and 50% of the offspring will show the recessive trait 12

13 Example: a dog breeder wants to find out if his black lab is a heterozygote or a homozygote.  Black lab – AA or Aa  Golden lab – aa A A aa A a aa AaAa AaAa AaAa aaaa If all the offspring are black the breeders dog is homozygous If only 50% are black the dog is heterozygous 13

14 Test cross Question :  A farmer wants to find out if his fat pig is a heterozygote (FF, Ff = fat and ff = thin). He completes a test cross. All the offspring are fat. What can the farmer deduce about the parent pigs genotype? Draw a Punnett square to explain your answer. 14

15 Multiple Alleles E.g. Blood groups: 3 alleles exist - I A,I B,i  I A and I B are co-dominant  i is recessive to I A and I B  I A I A Group A IAiIAi  I A I B = Group AB  I B I B Group B IbiIbi  ii = Group O 15

16 Multiple allele example  Example: A couple have a child. One parent is blood group AB, the other is O. What are the possible genotypes of their child?  I B i i or I Ai i 16

17 Sex Inheritance  There is 1 pair of sex chromosomes in a diploid cell.  All other chromosomes are known as autosomes.  In humans the sex chromosomes are called X and Y  Girls = XX  Boys = XY  Sex chromosomes are not always homologous (e.g. in a boy the X and Y are not homologous but they do have homologous regions.) 17

18  The X chromosome contains more genes than the Y chromosome. Therefore girls (XX) have 2 copies of every gene; boys only have a single copy of some genes that are carried on the X chromosome but not on the Y.  Example: A gene on X chromosome codes for a certain enzyme. A faulty gene is recessive but leads to a disease called Lesch-Nyhan disease.  X N = normalX n = disease  Possible genotypes:  X N X N = X n Y = X N X n =X N Y =  X n X n = Sex linkage Normal girl Normal boy Sufferer boy Sufferer girl 18


20 Autosomal linkage  When 2 or more genes are located on the same chromosome they tend to be inherited together.  Example  Blood group gene and nail & patella development gene  Both located on chromosome 9  Very close together 20

21 Dihybrid Crosses Looks at the inheritance of 2 genes 21

22  Example: hair colour and eye colour  These genes are on different chromosomes  Where: B = brown eyes b = blue eyes A = brown hair a = blonde hair  If a person had the genotype AaBb they would have brown hair and brown eyes 22

23  Person with genotype AaBb  At metaphase I the chromosomes can line up as  Leading to the following gametes in approximately equal numbers:  This is to do with independent assortment Aa Bb ABabABab aBAbaBAb or ABabaBAb 23

24 Question: What gametes would be made from: 1. aabb 2. Aabb 3. AAbb 4. AABB 5. aaBb ab Ab AB aB Ab ab 24

25 Parents Gametes How to draw out a dihybrid cross  2 people met, one with the genotype AaBb and the other with the genotype aabb. Work out the possible offspring AaBb x aabb AB, Ab, aB, ab ab ab aB Ab ABAaBb Aabb aaBb aabb Brown eyes brown hair Brown eyes blonde hair Blue eyes brown hair Blue eyes blonde hair 25

26 Two people with the genotype AaBb met. Work out the possible offspring.  Parents: AaBb x AaBb  Gametes: AB, Ab, aB, ab AB, Ab, aB, ab ABAbaBab ABAABBAABbAaBBAaBb AbAABbAAbbAaBbAabb aBAaBBAaBbaaBBaaBb abAaBbAabbaaBbaabb 26

27 There are four possible combinations of the two characteristics. Peas can be: 1) round and yellow 2) round and green 3) wrinkled and yellow 4) wrinkled and green 27

28 Phenotype Round, yellow Wrinkled, green Genotype RRYY rryy Gametes all RY all ry F1 all RrYy All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow. X 28

29 PhenotypeRound, yellow Genotype RrYy Gametes RY, Ry, rY, ry F2We really need a punnet square for this… X Round, yellow RrYy RY, Ry, rY, ry 29

30 F ::: Round Yellow Round Green Wrinkled Yellow Wrinkled Green This is the typical ratio expected in a dihybrid cross. 30

31 Guinea pigs, which were homozygous for long, black hair were crossed with ones which were homozygous for short white hair. All the F 1 offspring had short, black hair. (i) Using suitable symbols, draw a genetic diagram to explain this result.[3] (ii) Draw a genetic diagram to show the results of interbreeding the F 1 offspring.[5] (iii) State the ratio of phenotypes expected in the F 2 offspring.[1] [Total : 9] 31

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