1. Read through core knowledge. What vocab do you need to learn?
Mendelian Genetics
Read through core knowledge.
What vocab do you need to learn?

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

3. 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

4. Mendel – why so famous? Worked with peas Used pure-breeding varieties
Came up with idea of ‘gene’ 20 years before chromosomes were discovered

5. Law of segregation
Of the two genes controlling each characteristic, only one is present in each gamete.
During meiosis the two genes are separated.

6. 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.

7. 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

8. 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

9. 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.

10. Incomplete dominance Co-dominance Lethal Alleles

11. 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

12. Eg snapdragons P1: RR (Red) x rr (white) F1: Rr (pink)
F2: ? (You determine the ratios)
1red:2pink:1white R r

13. Co-dominance
Both alleles in heterozygous organism contribute to the phenotype.
Both alleles are independently and equally expressed.

14. Eg Human Blood Group AB
P1: AA (type A) x BB (type B)
F1: AB (type AB)

15. 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

16. 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

17. 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?

18. 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?

19. 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 years

20. More than one allele possible at a gene locus
Multiple alleles
More than one allele possible at a gene locus

21. 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.

22. 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.

23. Frequency in NZ Phenotype Genotype Frequency in NZ % O OO 49 A AA, AO40 B
BB, BO 9 AB 2

24. Determine the blood types
Cross
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

25. 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.

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

27. 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

28. 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  C
coat colour show one colour/another colour

29. 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

30. 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

31. 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

32. 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_ Black
C_bb Brown
cc__ No colour

33. 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

34. 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_)

35. 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

36. 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)

37. 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

38. 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.

39. 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

40. 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

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