5Dominance 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?
6Steps to answering genetics questions 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 questionsDecide on suitable lettersWork out which characteristic is dominant – the offspring phenotype will tell you thisWrite the genotypes of the parentsDraw a Punnett squareAlways put the dominant feature 1stWrite down the phenotypes of each genotype and state the numbers of each type
7Cystic fibrosis F = normal CTFR allele f = faulty CTFR allele CTFR gene codes for a protein channel allowing exit of chloride ions from cellsMutated CTFR protein are not placed in the plasma membrane.This causes a build up of thick mucus in the lungs and other organs.
8Explain what is wrong with each of these statements Draw a genetic diagram to determine the chance of a heterzygous man and homozygous dominant woman having a child with cystic fibrosisExplain what is wrong with each of these statementsA 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.
9Occurs when both alleles are expressed in the phenotype Co-dominanceOccurs when both alleles are expressed in the phenotypeExample: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.
10Co-dominance example: AlleleWhat is the ratio of phenotypes produced by crossing two grey cats?FBFWFBFWGene locusFBFBFBFBFWFWFBFWFWFW1 Black:2 Grey:1 White
11Height inheritance in plants HTHT= tall stem HTHS = medium stem HSHS = short stemA farmer crosses 2 heterozygotes. Draw a diagram to show the genotypes and phenotypes of the offspringA 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.
12Test CrossesCrosses 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
13a a a a A Aa Aa A Aa Aa A Aa Aa a aa aa Black lab – AA or Aa Example: a dog breeder wants to find out if his black lab is a heterozygote or a homozygote.If all the offspring are black the breeders dog is homozygousIf only 50% are black the dog is heterozygousBlack lab – AA or AaGolden lab – aaaaaaAAaAaAAaAaAAaAaaaaaa
14Test 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.
15Multiple Alleles E.g. Blood groups: 3 alleles exist - IA,IB,i IA and IB are co-dominanti is recessive to IA and IBIAIA Group AIAiIAIB = Group ABIBIB Group BIbiii = Group O
16Multiple 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?IBii or IAii
17Sex 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 YGirls = XXBoys = XYSex chromosomes are not always homologous (e.g. in a boy the X and Y are not homologous but they do have homologous regions.)
18Sex linkageThe 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.XN = normal Xn = diseasePossible genotypes:XNXN = XnY = XNXn = XNY =XnXn =Normal girlSufferer boyNormal boyNormal girlSufferer girl
20Autosomal linkageWhen 2 or more genes are located on the same chromosome they tend to be inherited together.ExampleBlood group gene and nail & patella development geneBoth located on chromosome 9Very close together
21Looks at the inheritance of 2 genes Dihybrid CrossesLooks at the inheritance of 2 genes
22Example: hair colour and eye colour These genes are on different chromosomesWhere: B = brown eyesb = blue eyesA = brown haira = blonde hairIf a person had the genotype AaBb they would have brown hair and brown eyes
23Person with genotype AaBb At metaphase I the chromosomes can line up asLeading to the following gametes in approximately equal numbers:This is to do with independent assortmentAaBba BA bA Ba borABabaBAb
24Question: What gametes would be made from: aabb Aabb AAbb AABB aaBb ab
25How 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 offspringAaBb x aabbParentsGametesAB, Ab, aB, ab ababABAaBbBrown eyes brown hairAbAabbBrown eyes blonde hairaBaaBbBlue eyes brown hairabaabbBlue eyes blonde hair
26Two people with the genotype AaBb met. Work out the possible offspring. Parents: AaBb x AaBbGametes: AB, Ab, aB, ab AB, Ab, aB, abABAbaBabAABBAABbAaBBAaBbAAbbAabbaaBBaaBbaabb
27There are four possible combinations of the two characteristics There are four possible combinations of the two characteristics. Peas can be:1) round and yellow2) round and green3) wrinkled and yellow4) wrinkled and green
28X rryy RRYY Phenotype Round, yellow Wrinkled, green Genotype Gametes all RYall ryall RrYyF1All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow.
29X RrYy RrYy RY, Ry, rY, ry RY, Ry, rY, ry Phenotype Round, yellow GenotypeRY, Ry, rY, ryRY, Ry, rY, ryGametesF2We really need a punnet square for this…
30This is the typical ratio expected in a dihybrid cross. F2:::9331RoundYellowRoundGreenWrinkledYellowWrinkledGreenThis is the typical ratio expected in a dihybrid cross.
31(i) Using suitable symbols, draw a genetic Guinea pigs, which were homozygous for long, black hair were crossed with ones which were homozygous for short white hair. All the F1 offspring had short, black hair.(i) Using suitable symbols, draw a geneticdiagram to explain this result.(ii) Draw a genetic diagram to show theresults of interbreeding the F1 offspring.(iii) State the ratio of phenotypes expectedin the F2 offspring.[Total : 9]