Genetics & Inheritance Flashcards
State three causes of genetic variation
- Mutation
- Crossing over
- Independent segregation
- Random fusion of gametes / fertilisation
Explain how a single base substitution causes a change in the structure of a polypeptide.
- Change in sequence of amino acids
- Change in (position of) hydrogen, ionic, disulfide bonds
- Alters tertiary structure
Describe the process of crossing over and explain how it increases genetic diversity.
- Homologous pairs of chromosomes associate
- bivalent forms
- Chiasmata form
- Equal lengths of (non-sister) chromatids exchanged
- Producing new combinations of alleles
THIS IS ‘RARE’ = Unequal chance of recombinant alleles forming.
Give two differences between mitosis and meiosis.
Mitosis given first
- One division, two divisions in meiosis;
- (Daughter) cells genetically identical, daughter cells genetically different in meiosis;
- Two cells produced, (usually) four cells produced in meiosis;
- Diploid to diploid/haploid to haploid, diploid to haploid in meiosis;
- Separation of homologous chromosomes only in meiosis;
- Crossing over only in meiosis;
- Independent segregation only in meiosis;
What is meant by a genome?
all the ‘genes’ in a cell
What is a ‘gene pool’?
All the alleles in a population.
How do multiple alleles of a gene arise?
- mutations;
- at different positions within the same gene;
What is meant by a recessive allele?
Only expressed (in the phenotype) when homozygous
Define dominant allele
Always expressed within the phenotype
Define codominance
Both alleles (equally) expressed in the phenotype
What is meant by the term phenotype
expression of its genetic constitution and its interaction with the environment
Hardy Weinberg Equations
- P + q = 1.0
(P=dominant, q=recessive)
2.P^2 + 2Pq + q^2 = 1.0
(Pq= heterozygous)
What does Hardy Weinberg’s equation predict?
the allelic frequency from one generation to the next will remain constant if there is no
* migration
* gene mutation
* selection pressures
*should be large population and random mating
Expected offspring phenotype ratio from heterozygous parents:
Monohybrid
Codominance
M= 3 : 1
C= 1:2:1
Monohybrid
- inheritance of a single gene, determines single characteristic
Monohybrid multiple alleles
Codominance
Rules for Dominant alleles
- Affected offspring MUST have at least one affected parent.
- Unaffected parents ONLY have unaffected offspring.
- If both parents are affected and have an unaffected offspring, both parents must be Heterozygous
Rules for recessive alleles
Unaffected parents can have an affected offspring (if they are Heterozygous)
In genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios.
Suggest two reasons why.
- Small sample size;
- fertilisation of gametes is random;
- crossing over;
Define independent segregation
- homologous chromosomes attach to spindle fibres and pair up side by side
- maternal and paternal chromosomes reshuffle in any combination
Sex linked characteristics
- unaffected mum (het) has affected son
unaffected father has affected son - affected father has unaffected daughter (if mum homo D)
Not sex linked
dad and daughter:
-if daughter affected (Homo R)
- dad not
-recessive characteristic
-not SL
mum and son:
- if son affected
- mum not (homo r)
-dominant characteristic
-not SL
Dihybrid inheritance
Phenotype inherited as a result of 2 different genes, so 2 characteristics
can be on different chromosomes or linked on same
Dihybrid ratio
9:3:3:1
Autosomal linkage (dihybrid)
-chromosomes not directly in determining sex of organism
independent segregation and crossing over can randomly produce recombinant offspring
offspring displaying new combo of alleles from either parents (4:4:1:1)
Linkage group definition
set of genes on same chromosome which tend to be inherited together (dont indep seg)
What happens when 2 genes are autosomal linked
-Don’t follow dihybrid ratio
-follow monohybrid ratio (3:1)
-offspring tend to produce parental phenotypes
-recombinant phenotypes only evident if crossing over occurs in meiosis 1 and would be expected to appear in low numbers
genes closer together- less likely to be separated by crossing over
Epistasis
interaction between 2 genes where one allele affects expression of other, leading to graduation of phenotype
Epistasis ratios
15:1
9:7
9:4:3
add to 16 but not 9:3:3:1
