Genes L10 Flashcards
Characteristics of a monohybrid cross
• Monohybrid cross:
- One gene locus
- > 2 alleles
- F2 -> 3:1 phenotypic ratio
- Mendel’s 1st law: Principle of segregation
Characteristics of a dihybrid cross
• Dihybrid cross:
- Two gene loci
2 alleles per locus
- F2 -> 9:3:3:1 phenotypic ratio
What is a punnet square used for?
• Punnet squares:
Analyse genetic crosses
Predict genotypic & phenotypic ratios
What is the Sutton-Boveri theory?
• Sutton-Boveri theory: 1903
Chromosome theory of inheritance:
Parallels -> Mendel’s laws & chromosomal behaviour -> meiosis & fertilisation.
Where are genes located?
• Genes -> located at specific regions of chromosomes
Thomas Hunt Morgan , 1910-14.
• How does the behaviour of chromosomes -> meiosis -> explain Mendel’s laws?
• 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
• Summary of Mendel’s theory of inheritance.:
• 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
Name the classes of allele
Wild
Loss-of-function (LOF)
Gain-of-function (GOF)
Characteristics of wild alleles?
- Wild:
>Functional protein
>Constant regualar changes -> direction -> compact
Characteristics of LOF alleles?
- Loss-of-function: (LOF)
>Non-functional protein
>Most common
>Irregular changes -> direction
Characteristics of GOF alleles?
- Gain-of-function: (GOF)
>Protein -> new / enhanced function
>Less common
>Less frequent regular changes -> direction -> wide-disperses/longer between changes
Explain dominance of LOF & GOF alleles
• LOF alleles -> recessive
50% normal gene product -> sufficient for WT function
• GOF alleles -> dominant
What is incomplete dominance? Give an example
• Incomplete dominance:
Heterozygote phenotype -> intermediate between 2 homozygote types.
Eg. Familial hypercholesterolaemia
Mutation -> LDL receptor
Describe the scale of dominance
• 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
Name the types of co-dominance
Self tolerance
Universal recipients
Universal donors
What is codominance?
Heterozygotes illustrate phenotype -> both alleles
Examples of self tolerance in blood groups
AA / AO / BB / BO
Examples of Universal recipients in blood groups
AB
Examples of Universal donors in blood groups
OO
Explain what multiple alleles are
• 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
Explain the dominance series
Dominance series:
A > at >a
» A dominant over at & a
» at dominant over a
Explain dominance & recessiveness within alleles
• Dominance / recessiveness
Relationship between 2 alleles
Not fixed property of allele
What is pleitropy
• Pleitropy:
One gene -> influence more than one trait.
What is pleitropy & give example
• 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
»_space; 50% -> PCD -> situs inverus
Describe what sitius inverus is
• Situs inverus:
Reversal -> normal L/R location of major visceral organs
Defective nodal cilia -> early embryogenesis.
Describe sickle-cell anaemia in relation to pleitropy
• Sickle-cell haemoglobin & malaria:
Pleiotropy -> HbS allele -> resistant
HbA HbS -> advantage over HbA HbA & HbS HbS -> heterozygote advantage
Explain lethal alleles
• 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
Describe an example of recessive lethal mutations
• Achrondroplasia:
Dominant GOF mutation -> growth factor receptor (FGFR3)
–> 99% -> same amino acid substitution
Overactive mutant
Premature replacement -> cartilage -> bone
»_space;No cartilage formation
Homozygous babies -> stillborn / infancy death
»Recessive lethal
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?
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 .
