Genes L10

Question 1

Characteristics of a monohybrid cross

Answer 1

• Monohybrid cross:

• One gene locus
• > 2 alleles
• F2 -> 3:1 phenotypic ratio
• Mendel’s 1st law: Principle of segregation

Question 2

Characteristics of a dihybrid cross

Answer 2

• Dihybrid cross:
- Two gene loci
 2 alleles per locus
- F2 -> 9:3:3:1 phenotypic ratio

Question 3

What is a punnet square used for?

Answer 3

• Punnet squares:
 Analyse genetic crosses
 Predict genotypic & phenotypic ratios

Question 4

What is the Sutton-Boveri theory?

Answer 4

• Sutton-Boveri theory: 1903
Chromosome theory of inheritance:
 Parallels -> Mendel’s laws & chromosomal behaviour -> meiosis & fertilisation.

Question 5

Where are genes located?

Answer 5

• Genes -> located at specific regions of chromosomes

Thomas Hunt Morgan , 1910-14.

Question 6

• How does the behaviour of chromosomes -> meiosis -> explain Mendel’s laws?

Answer 6

• How does the behaviour of chromosomes -> meiosis -> explain Mendel’s laws?
 1st law -> Alleles of single gene randomly & equally segregate into gametes.
Each gamete -> One of the 2 alleles
-> Produced with equal probability
 2nd law -> Alleles from different genes randomly segregate into gametes.
 Each possible combination alleles -> produced at equal frequency in gametes

Question 7

• Summary of Mendel’s theory of inheritance.:

Answer 7

• Summary of Mendel’s theory of inheritance.:
- Distinct, particular characteristics coded for by genome
- Genes -> alternative forms -> alleles
- Members of gene pairs segregate -> gametes
Each gamete -> 1 of 2 genes (1st law)
- Fusion of gametes -> fertilisation -> restores pair of genes -> random.
- Different genes assort independently in gametes
2nd law

Question 8

Name the classes of allele

Answer 8

Wild
Loss-of-function (LOF)
Gain-of-function (GOF)

Question 9

Characteristics of wild alleles?

Answer 9

• Wild:
  >Functional protein
  >Constant regualar changes -> direction -> compact

Question 10

Characteristics of LOF alleles?

Answer 10

• Loss-of-function: (LOF)
  >Non-functional protein
  >Most common
  >Irregular changes -> direction

Question 11

Characteristics of GOF alleles?

Answer 11

• Gain-of-function: (GOF)
  >Protein -> new / enhanced function
  >Less common
  >Less frequent regular changes -> direction -> wide-disperses/longer between changes

Question 12

Explain dominance of LOF & GOF alleles

Answer 12

• LOF alleles -> recessive
50% normal gene product -> sufficient for WT function
• GOF alleles -> dominant

Question 13

What is incomplete dominance? Give an example

Answer 13

• Incomplete dominance:
 Heterozygote phenotype -> intermediate between 2 homozygote types.
Eg. Familial hypercholesterolaemia
Mutation -> LDL receptor

Question 14

Describe the scale of dominance

Answer 14

•	Scale of dominance:
-	Scale 
-	Location of genotype on scale -> dominance of alleles
	0-10
	0 -> A1 -> dominant
	0-5 -> A1 -> incompletely dominant 
	5 -> No dominance 
	5-10 -> A2 -> incompletely dominant
	10 -> A2 -> dominant

Question 15

Name the types of co-dominance

Answer 15

Self tolerance
Universal recipients
Universal donors

Question 16

What is codominance?

Answer 16

 Heterozygotes illustrate phenotype -> both alleles

Question 17

Examples of self tolerance in blood groups

Answer 17

AA / AO / BB / BO

Question 18

Examples of Universal recipients in blood groups

Answer 18

AB

Question 19

Examples of Universal donors in blood groups

Answer 19

OO

Question 20

Explain what multiple alleles are

Answer 20

•	Multiple alleles:
	Possibility -> more than 2 alleles -> gene 
	Dominance series:
A > at >a
>> A dominant over at & a
>> at dominant over a

• Dominance / recessiveness
 Relationship between 2 alleles
 Not fixed property of allele

Question 21

Explain the dominance series

Answer 21

 Dominance series:
A > at >a
» A dominant over at & a
» at dominant over a

Question 22

Explain dominance & recessiveness within alleles

Answer 22

• Dominance / recessiveness
 Relationship between 2 alleles
 Not fixed property of allele

Question 23

What is pleitropy

Answer 23

• Pleitropy:

 One gene -> influence more than one trait.

Question 24

What is pleitropy & give example

Answer 24

• Pleitropy:
 One gene -> influence more than one trait.
Eg. Primary ciliary dyskinesia (PCD)
»Dynegin -> protein -> cilia & flagella function
If mutant
 Respiratory problems -> Failure to clear airways
 Infertility -> Non-motile sperm
&raquo_space; 50% -> PCD -> situs inverus

Question 25

Describe what sitius inverus is

Answer 25

• Situs inverus:
 Reversal -> normal L/R location of major visceral organs
Defective nodal cilia -> early embryogenesis.

Question 26

Describe sickle-cell anaemia in relation to pleitropy

Answer 26

• Sickle-cell haemoglobin & malaria:
 Pleiotropy -> HbS allele -> resistant
 HbA HbS -> advantage over HbA HbA & HbS HbS -> heterozygote advantage

Question 27

Explain lethal alleles

Answer 27

•	Lethal alleles:
	Cause skewed phenotypic ratios 
	Dominant allele 
	Phenotypic ratio 2:1 instead of 3:1 -> heterozygotes – (AYA & AYA)
AYAY genotype -> not produced -> death 
 3 combinations produced:  AYA, AAY, AA
 2 phenotypes -> 2 x (AYA) & (AA)
>>3 produced -> 2:1
	Pleiotropy

Question 28

Describe an example of recessive lethal mutations

Answer 28

• Achrondroplasia:
 Dominant GOF mutation -> growth factor receptor (FGFR3)
–> 99% -> same amino acid substitution
 Overactive mutant
Premature replacement -> cartilage -> bone
&raquo_space;No cartilage formation
 Homozygous babies -> stillborn / infancy death
»Recessive lethal

Question 29

 Tom & Tina heterozygous -> recessive allele (c) -> cystic fibrosis. Have one child not affected & planning 2nd.

i) What is the probability 2nd child affected?
ii) If they have 2 more children, what prob both will be affected?
iii) What is the probability unaffected first child is a carrier?

Answer 29

Heterozygotes -> 1:2:1 genotypic ratio
-> 3:1 phenotypic ratio
i) 1 in 4 ->1/4
ii) Prob -> 1 child = ¼
2 children = ¼ x ¼ = 1/8
iii) Genotypic ratio -> 1:2:1 -> 1 dominant -> 2 heterozygous -> 1 recessive
-> 1 non-affected/non-carrier : 2 non-affected carriers : 1
affected
2 in 4 -> 2/4 = 1/2 .