Learning question: How can genetic diagrams be used to solve problems?

What do these two famous

people have in common?

Title: Using genetics Thursday 23rd January 2014

Homework: Learning package 6 (apart from 1(c) ) for Monday 27th January

Learning Outcomes

• Explain the terms allele, locus, phenotype, genotype, dominant, codominant and recessive;

• Explain the terms linkage and crossing-over;

Starter• Complete the activity sheet on genetics.

• There are three parts to this activity:– Vocabulary – Genetic diagrams– Codomiance

• You will be tested on all three areas in your examination!

Progress tracker

0-5 5-10 10-15

Monohybrid inheritance• Mendel’s First Law

– principle of segregation

“The alleles of a gene exist in pairs but when gametes are formed, the members of each pair pass into different

gametes, thus each gamete contains only one of each allele.”

Genetic Cross conventions

• Use symbols to represent two alleles

• Alleles of the same gene should be given the same letter– Capital letter represents the dominant allele– Small letter represents the recessive allele

• Choose letters where the capital and small letter look different– The examiner needs to be in no doubt about what you

have written

Inheritance of height in pea plants• Laying out the cross

– P phenotype– P genotype– Gametes– F1 genotype– F1 phenotype– F1 self-fertilised– Gametes– Random fertilisation– F2 genotypic ratio– F2 phenotypic ratio

Inheritance of height in pea plants

• Follow out the following cross to the F2 generation– Homozygous tall pea plant with a homozygous dwarf pea

plant• Write out the genotypic and phenotypic ratios from the

F2 generation

gene Allele relationship Symbol

Height of pea plants

Tall Dominant T

dwarf recessive t

Pupil Activity – example question(a)In peas the height of the plant is controlled by one gene which has two alleles.

T represents the dominant allele for tall stems.t represents the allele for short stems.  

 True breeding, tall-stemmed pea plants were crossed with short-stemmed pea plants to produce the F1 generation.  (i) State the genotypes of the parents.

(ii) State the phenotype of the F1 plants.  Plants from the F1 generation were crossed to produce the F2 generation of plants.  (iii) State the phenotypes and their expected ratio in the F2 generation. 

Cystic Fibrosis• Cystic Fibrosis is caused by a mutation to a gene on one of

the autosomes.• Mutation

– Changes the shape of the transmembrane chloride ion channels (CFTR protein)

– The CFTR gene is found on Chromosome 7– The faulty gene is recessive

Inheritance of cystic fibrosis• Three possible genotypes

– FF unaffected– Ff unaffected– ff cystic fibrosis

• Remember gametes can only contain one allele for the CFTR gene

• At fertilisation, any gamete from the father can fertilise any gamete from the mother– This can be shown in a genetic diagram

Genetic diagram showing the chances of a heterozygous man and a heterozygous woman having a child with cystic fibrosis.

Phenotype ratio of offspring

• Genotype ratio 1FF: 2Ff: 1ff

• Phenotype ratio 3 unaffected:1cystic fibrosis

• Can also be expressed as– 25% chance of the child having cystic fibrosis– Probability of 0.25 that a child will inherit the disease– Probability that 1 in 4 that a child from these parents will

have this disease.

Mini Plenary1. In tomato plants the allele for red fruit is dominant to the allele for yellow fruit. If a heterozygous tomato plant is crossed with a plant which produces yellow fruit, the expected phenotype ratio of the offspring would be 

A 3 red : 1 yellowB 1 red : 3 yellowC 1 red : 2 yellowD 1 red : 1 yellow

Mini Plenary2. Achoo syndrome is a dominant characteristic in humans which causes the sufferer to sneeze in response to bright light. A woman who is homozygous for the syndrome and a man who is unaffected have children. What proportion of their children would be expected to have Achoo syndrome? 

A 0%B 25%C 50%D 100%

Mini Plenary3. Which term refers to a description of a characteristic of an organism? 

A genotypeB phenotypeC alleleD natural selection

Mini Plenary4. Which term refers to forms of a gene controlling the same characteristic? 

A genotypesB phenotypesC allelesD dominant

Learning Outcome

• Use genetic diagrams to solve problems involving sex-linkage and codominance.

Sex-Linkage

• Sex-linked genes are genes whose loci are on the X or Y chromosomes

• The sex chromosomes are not homologous, as many genes present on the X are not present on the Y.

• Examples– Haemophilia– Fragile X syndrome– Red green colour blindness

Sex Chromosomes

Factor VIII and Haemophilia

• Haemophilia is caused by a recessive allele of a gene that codes for a faulty version of the protein factor VIII– XH normal allele– Xh haemophilia allele

Remember, males are XY, females are XX

Possible genotypes and phenotypes

Applying your knowledgeWrite out the genotypes for these phenotypes in hemophilia

• Affected male_____________• Normal male ______________• Affected female ____________• Normal female ____________• Carrier female ____________

Inheritance of Haemophilia

Note! The stages for writing out a genetic diagram is the same

Pedigree for a sex linked recessive disease Write out the

genotypes for as many people in the

family tree as possible.

Mini-plenary• Collect a “connect 4” board and question sheet.

• In pairs, test each others knowledge of genetics and try to win the game!

Codominance

• Codominance describes a pair of alleles, neither of which is dominant over the other.

• This means both have an effect on the phenotype when present together in the genotype

Codominance example

• Flower colour in plants– CR red– Cw white

• Genotypes– CRCR red flowers– CRCW pink flowers– CWCW white flowers

• Write out a genetic cross between a pure breeding red plant and a pure breeding white plant.

• Carry out the cross to the

F2 generation.– Write out the genotype and

phenotype ratio for the F2 generation

Revision Question

• Coat colour in Galloway cattle is controlled by a gene with two alleles. The CR allele produces red hairs and therefore a red coat colour. The Cw allele produces white hairs.

• A farmer crossed a true-breeding, red-coated cow with a true-breeding white-coated bull. The calf produced had roan coat colouring (made up of an equal number of red and white hairs).

• Explain the result and draw a genetic diagram to predict the outcome of crossing two roan coloured animals.

Inheritance of A, B, AB and O blood groups

• Human blood groups give an example of codominance and multiple alleles

–There are 3 alleles present• IA

• IB

• Io

• IA and IB are codominant• Io is recessive

• Remember each human will only have two alleles

Blood Groups

Genotype Phenotype

IAIA Blood Group A

IA Io Blood Group A

IAIB Blood Group AB

IBIB Blood Group B

IB Io Blood Group B

Io Io Blood Group o

Inheritance of blood groups

• Carry out genetic crosses for the following examples. – Two parents have blood groups A and B, the

father is IAIo and the mother is IBIo

– Father has blood group AB and the mother has blood group O

– Mother is homozygous blood group A and the father is heterozygous B.

Progress tracker

0-5 5-10 10-15

Learning Outcome

• Describe the interactions between loci (epistasis).

• Predict phenotypic ratios in problems involving epistasis.

Dihybrid Inheritance

• Monohybrid cross– Inheritance of one gene

• Dihybrid cross– Inheritance of two genes

Example – dihybrid cross

• Tomato plants– Stem colour

A purple stem a green stem– Leaf shape

D cut leaves d potato leaves

• NOTE– In the heterozygote AaDd due to independent assortment

in meiosis there are 4 possible gamete combinationsAD Ad aD ad

Crosses

• Cross a heterozygous plant with a plant with a green stem and potato leaves

• Cross two heterozygous tomato plants

Dihybrid Inheritance

• A woman with cystic fibrosis has blood group A (genotype IAIo). Her partner does not have cystic fibrosis and is not a carrier for it. He has blood group O.

• Write down the genotypes of these two people.• With the help of a full and correctly laid out genetic

diagram, determine the possible genotypes and phenotypes of any children that they may have.

Autosomal linkage

• Each Chromosome carries a large number of linked genes

• If two genes are on the same chromosome then independent assortment can not take place.

• The genes are transmitted together and are said to be linked.

Linked Genes

• Where linked genes are involved the offspring of a dihybrid cross will result in a 3:1 ratio instead of the 9:3:3:1 ratio.

• Example:– In peas, the genes for plant height and seed

colour are on the same chromosome (i.e. linked)

Learning Outcome

• Describe the interactions between loci (epistasis).

• Predict phenotypic ratios in problems involving epistasis.

Flower colour in sweet pea

• Flower colour– Colourless precursor of a pigment C– Gene that controls conversion of this pigment to purple

P– Both dominant alleles need to be present for the purple

colour to develop• Cross

– Cross two white flowered plants with the genotypes CCpp and ccPP

– Follow this cross through to the F2 generation

Interactions of unlinked genes

• A single character maybe influenced by two or more unlinked genes.

• E.g. determination of comb shape in domestic poultry– Dominant allele P pea comb– Dominant allele R rose comb– Two dominant alleles walnut comb– No dominant alleles single comb

Genetic Crosses

• Carry out a genetic cross between a true-breeding pea comb and a true breeding rose comb

• Follow this cross through to the F2 generation

Inheritance of coat colour in mice

• Wild mice have a coat colour that is referred to as “agouti”.– Agouti (A) is dominant to black (a)– C is a dominant gene required for coat colour to

develop– A homozygous recessive cc means that no

pigment can be formed and the individual is albino

Inheritance of coat colour in mice

• Carry out a cross between a pure-breeding black mouse (aaCC) and an albino (AAcc)

• Follow this cross through to the F2 generation.

Epistasis• This is the interaction of different gene loci so that one gene

locus masks or suppresses the expression of another gene locus.

• Genes can– Work antagonistically resulting in masking– Work complementary

Epistasis ratios

• 9 : 3 : 4 ratio– Suggests recessive epistasis

• 9 : 7 ratio– Suggests epistasis by complementary action

• 12 : 3 : 1 ratio or 13 : 3 ratio– Suggests dominant epistasis

Predicting phenotypic ratios

• Read through pages 132 and 133 of your textbook– Answer questions 1 – 7

– Complete the stretch and challenge question on “eye colour in humans”

• Read through and complete the worksheet provided for you on epistasis

Learning objectives

(e) use genetic diagrams to solve problems involving sex linkage and codominance;

(f) describe the interactions between loci (epistasis). (Production of genetic diagrams is not required);

(g) predict phenotypic ratios in problems involving epistasis;

Learning objectives

(e) use genetic diagrams to solve problems involving sex linkage and codominance;

(f) describe the interactions between loci (epistasis). (Production of genetic diagrams is not required);

(g) predict phenotypic ratios in problems involving epistasis;

Genetics

• Genetics is the study of inheritance

• Allele – different varieties of the same gene

• Locus – position of a gene on a chromosome

Genetics

• Dominant– An allele whose effect is expressed in the

phenotype if one copy present

• Recessive– An allele which only expresses as a homozygote

• Co-dominant– Both alleles have an effect on the phenotype

Genetics

• Genotype –genetic constitution of the organism

• Phenotype–appearance of character resulting from

inherited information

• Homozygous– Individual is true breeding– Possesses two alleles of a gene e.g. RR or rr

• Heterozygous– Two different alleles for a gene e.g. Rr