Introduction to Genetics: From Proteins to Mendel MENDELIAN GENETICS

Show the inheritance of a trait Monohybrid Crosses Show the inheritance of a trait

Allele Genotype Phenotype Homozygous Heterozygous

BB bb B B b b B = b = BB = Bb = bb = Bb Bb Bb Bb

Dominance 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?

Steps 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 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 1st Write down the phenotypes of each genotype and state the numbers of each type

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.

Explain 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 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.

Occurs when both alleles are expressed in the phenotype 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.

Co-dominance example: Allele What is the ratio of phenotypes produced by crossing two grey cats? FB FW FB FW Gene locus FB FBFB FBFW FW FBFW FWFW 1 Black:2 Grey:1 White

Height inheritance in plants HTHT= tall stem HTHS = medium stem HSHS = 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.

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

a 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 homozygous If only 50% are black the dog is heterozygous Black lab – AA or Aa Golden lab – aa a a a a A Aa Aa A Aa Aa A Aa Aa a aa aa

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.

Multiple Alleles E.g. Blood groups: 3 alleles exist - IA,IB,i IA and IB are co-dominant i is recessive to IA and IB IAIA Group A IAi IAIB = Group AB IBIB Group B Ibi ii = Group O

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? IBii or IAii

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.)

Sex linkage 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. XN = normal Xn = disease Possible genotypes: XNXN = XnY = XNXn = XNY = XnXn = Normal girl Sufferer boy Normal boy Normal girl Sufferer girl

SEX-LINKED DISORDERS

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

Looks at the inheritance of 2 genes Dihybrid Crosses Looks at the inheritance of 2 genes

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

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 A a B b a B A b A B a b or AB ab aB Ab

Question: What gametes would be made from: aabb Aabb AAbb AABB aaBb ab

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 Parents Gametes AB, Ab, aB, ab ab ab AB AaBb Brown eyes brown hair Ab Aabb Brown eyes blonde hair aB aaBb Blue eyes brown hair ab aabb Blue eyes blonde hair

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 AB Ab aB ab AABB AABb AaBB AaBb AAbb Aabb aaBB aaBb aabb

There are four possible combinations of the two characteristics 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

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

X RrYy RrYy RY, Ry, rY, ry RY, Ry, rY, ry Phenotype Round, yellow Genotype RY, Ry, rY, ry RY, Ry, rY, ry Gametes F2 We really need a punnet square for this…

This is the typical ratio expected in a dihybrid cross. F2 : : : 9 3 3 1 Round Yellow Round Green Wrinkled Yellow Wrinkled Green This is the typical ratio expected in a dihybrid cross.

(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 genetic diagram to explain this result.[3] (ii) Draw a genetic diagram to show the results of interbreeding the F1 offspring.[5] (iii) State the ratio of phenotypes expected in the F2 offspring.[1] [Total : 9]