1. Introduction to Genetics: From Proteins to Mendel
MENDELIAN GENETICS

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

3. Allele
Genotype
Phenotype
Homozygous
Heterozygous

4. BB bb B B b b
B =
b =
BB =
Bb =
bb = Bb Bb Bb Bb

5. 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?

6. 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

7. 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.

8. 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.

9. 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.

10. 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

11. 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.

12. 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

13. 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

14. 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.

15. 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

16. 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

17. 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.)

18. 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

19. SEX-LINKED DISORDERS

20. 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

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

22. 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

23. 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

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

25. 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

26. 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

27. 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

28. 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.

29. 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…

30. 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.

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 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]