Mendelian Genetics

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Terms

Gene Allele Trait Dominant Recessive Homozygous Heterozygous Genotype Phenotype P and F1 and F2

Definitions

Unit of hereditary One of 2 or more forms of a gene at a given locus Genetically inherited characteristic of organism, varies amongst

individuals Allele that is expressed in heterozygotes Allele that is only expressed in homozygotes Carries two copies of the allele Carries different allelic forms of a given gene Organism’s hereditary make-up Physical characteristics of an organism Patrial generation, first and second filial generation

Mendel – why so famous?

Worked with peas Used pure-breeding varieties Came up with idea of ‘gene’ 20 years before

chromosomes were discovered

Law of segregation

Of the two genes controlling each characteristic, only one is present in each gamete.

During meiosis the two genes are separated.

Independent assortment

The segregation of one pair of alleles does not affect the segregation of another pair.

There is a random arrangement of parental chromosomes at metaphase of meiosis.

Monohybrid cross

Carry out a cross for a dominant and a recessive homozygote where P is for pink flower and p is for white

Cross the F1 and give the ratios of the F2.

P P

p

p

Test Cross

A cross carried out to determine an organism’s genotype, by mating it with a homozygous recessive organism.

Show how a test cross works

Dihybrid Cross

Carry out a cross for a dominant and a recessive homozygote where Y is for yellow and y for green, and R for round and r for wrinkled.

Cross the F1 and give the ratios of the F2.

Dominance

Incomplete dominance

Co-dominance

Lethal Alleles

Incomplete dominance

Action of one allele does not completely mask the action of the other.

Neither allele has dominant control over the trait.

Heterozygous offspring is intermediate in phenotype

Eg snapdragons

P1: RR (Red) x rr (white) F1: Rr (pink) F2: ? (You determine the ratios)

1red:2pink:1white R r

R

r

Co-dominance

Both alleles in heterozygous organism contribute to the phenotype.

Both alleles are independently and equally expressed.

Eg Human Blood Group AB

P1: AA (type A) x BB (type B) F1: AB (type AB)

Eg Coat colour in horses and cattle

P1: CRCR(red) x CrCr(white) F1: CRCr(roan) F2: ? You determine the ratios

1 Red: 2 Roan: 1 white

Roan is a blend of both white hairs and red hairs

CR Cr

CR

Cr

Lethal Alleles

Mutations of a gene that produce a non-functional gene product and affect the organisms survival.

If dominant, may kill in single dose If recessive, kills when homozygote

Eg Manx cat

MM (normal tail) MML (manx - no tail) MLML (lethal – deformity of spine in embryo)

Carry out a cross for two heterozygotes. What is the phenotypic ratio?

Eg Yellow mice

YY (lethal – yellow – terminates at blastocyst stage) Yy (yellow) yy (not yellow)

Again, what is the phenotypic ratio for a cross of heterozygotes?

Eg Huntington’s disease

A dominant allele is lethal. Nerve cell death in brain causing jerky

involuntary movements and dementia. Why does it persist in the human population? Shows in adults 30-40 years

Multiple alleles

More than one allele possible at a gene locus

Blood groups

There are three different alleles:

– A, B and O

The alleles code for making the enzyme that hold the sugars together that make the different antigens on the RBC.

O is nonfunctioning (recessive)

A is A antigen (dominant) B is B antigen (dominant)

A and B antigens can act with other antibodies so must be matched for transfusion.

Frequency in NZ

Phenotype Genotype Frequency in NZ %

O OO 49

A AA, AO 40

B BB, BO 9

AB AB 2

Determine the blood typesCross

Parental genotype

Ratio F1 genotype

Ratio F1 Phenotype

1 ABxAB 1AA:2AB:1BB 1A:2AB:1B

2 OOxOO

3 ABxAO

4 AAxBO

5 AOxOO

6 BOxOO

Dilemma

If a mother is type A and has a baby type B, can the father be type O?– Explain your answer.

You can now do the

self check for this section.

Gene-gene interactions

When a characteristic is influenced by more than one gene at two different loci or even on different chromosomes altogether.

Epistasis (standing upon)

Involves two non-allelic genes (different loci) Action of one gene masks or alters

expression of other genes Three forms – collaboration, complementary,

supplementary

Eg Albinism

Occurs in rodent that are homozygous recessive for colour even if they have alleles for agouti or black fur.

The gene for colour is epistatic

gene 1 gene 2

A B Ccoat colour show one colour/another colour

Collaboration

Ratio 9:3:3:1

(Although the ratio is typical, it is unusual that some of the phenotypes may not have been shown in the parents)

Four different phenotypes depending on the presence or absence of certain genes

Eg comb shape in chickens

P_R_ walnut

P_rr pea

ppR_ rose

pprr single

Carry out a cross for two heterozygotes – PpRr x PpRr

Supplementary Genes (Epistasis)

Ratio 9:3:4 A dominant allele at one locus is necessary

for the expression of alleles at another Typically three phenotypes Carry out a cross for two heterozygotes –

CcBb x CcBb

Eg Coat colour in mice

Gene C controls the production of melanin Gene B indicates whether the colour is black or brown Without the production of melanin, there will be no colour.

gene C gene B

enzyme 1 enzyme 2

no pigment melanin produced Black Brown

C_B_ BlackC_bb Browncc__ No colour

Complementary Genes

Ratio 9:7 Development of a characteristic requires the

presence of at least one dominant allele at both of 2 loci

Typically there are two phenotypes Carry out a cross for two heterozygotes –

PpQq x PpQq

Eg Purple pigment in sweet pea flowers

Gene P makes white intermediate Gene Q converts white to purple

gene P gene Q

enzyme 1 enzyme 2

Colourless Colourless Coloured precursor intermediate

product

(white pp_ _) (white P_ _ _) (purple P_Q_)

Duplicate genes

Ratio 15:1 A characteristic is developed if EITHER or

BOTH of the dominant alleles at two loci is/are present.

Carry out a cross for two heterozygotes – AaBb x AaBb

Eg Fruit width in Shepherd’s Purse

Gene A and B code for two different enzymes which can form wide fruit.

gene A gene B

enzyme A enzyme B

Substance Active Substance

X Product Y

wide (A_B_,A_bb, aaB_)narrow (aabb)

Practice

Self check page 111/112 Create a table to compare

Exam questions Study book pg 31 Qb

Type of interaction

F2 phenotype ratio

Number of phenotypes

Example organism

Example trait

A women who owned a purebred female albino (lacking pigments) poodle (an autosomal recessive phenotype) wanted white puppies, so she took the dog to a breeder, who said he would mate her female with an albino stud male, also from a pure stock. When six puppies were born they were all black, so the women sued the breeder, claiming that he replaced the stud with a black dog, giving her six unwanted puppies. You are called in as an expert witness, and the defence asks you if it is possible to produce black offspring from two pure-breeding recessive albino parents.

(a) Discuss what evidence you would give by explaining what gene-gene interrelationship is involved in each of the parents and using appropriate allele symbols, draw biochemical pathways to obtain an albino phenotype and a black phenotype.

Clear well-labelled diagrams may be used to help you answer this question. (b) Explain the expected possible F2 phenotypes ratios if two of the

black puppies were allowed to interbreed.

Exams 4U 2007 Q5

It is definitely a form of epistasis (that is – there is obviously more than one gene involved.

We know of 3 types of epistasis; collaboration, supplementary, complementary

Use the process of elimination There are only 2 phenotypes, therefore it is not

collaboration There is no intermediary product, therefore it is not

supplementary Both genes are required to create colour, therefore it

is complementary

So, both parent dogs are claimed to be pure breeding – therefore homozygous– What are our options for this with 2 genes? – – AABB, aabb, but also AAbb and aaBB

Note a P1 cross of AABB x aabb and a P1 cross of AAbb x aaBB both make the F1 generation AaBb, which produces the complementary ratio of 9:7 in the F2

Pleiotropy

A single gene may produce a product that can influence a number of traits in the phenotype.