Unit 7 - Inherited Change Flashcards
Gene
- A section of DNA that codes for a specific sequence of amino acids which makes up the primary structure of a protein
- Alternatively codes for functional RNA (rRNA or tRNA)
Allele
- An alternative form of a gene
- Occupies the same locus as a gene
Genotype
All of the alleles that an organism carries on a chromosome
Phenotype
The observable characteristics of an organism as a result of its genotype and environment
Dominant
Only a single allele has to be present for this characteristic to be expressed
Recessive
The characteristic is only displayed if no dominant allele is present
Locus
The position occupied by a gene on a chromosome
Homozygous
The 2 alleles of a characteristic are identical
Heterozygous
The 2 alleles of a characteristic are different
Monohybrid Inheritance
Transmission of one gene from parents to their offspring
F₁ Generation
- The offspring from a cross between individuals with homozygous dominant and homozygous recessive genotypes
- E.g. TT and tt
- All the F₁ generation in a monohybrid cross will be heterozygous and express the dominant trait
F₂ Generation
- The offspring from a cross between 2 heterozygous individuals from the F₁ generation
- The offspring in the F₂ in a monohybrid cross will exhibit a 3:1 ratio of of dominant to recessive phenotypes
- 1 homozygous dominant, 2 heterozygous, 1 homozygous recessive
Dihybrid Inheritance
- Simultaneous inheritance of two genes controlling separate characteristics
- Hair colour and eye colour
F₁ Generation for a Dihybrid Cross
All have an identical genotypes and therefore identical phenotypes
F₂ Generation for a Dihybrid Cross
9:3:3:1 ratio of
Reasons for unexpected ratios in a Dihybrid Cross
Random Fertilisation:
- Gamete fusion is a chance process
- Small sample sizes can lead to unexpected ratios but large samples minimise random effects
Linked Genes
- Close together on the same chromosome so during crossing over of non-sister chromatids they are crossed together
- As a result they are inherited together which decreases variation
Codominance
- 2 alleles are equally expressed in an organism
- Results in a blend of the 2 characteristics or both characteristics being seen together
Multiple Alleles
- Genes that exist in more than 2 allelic forms
- An individual can only have two alleles of a specific gene at any one time
- E.g Blood type alleles exist as IA, IB and IO
Blood Type Inheritance
- IA and IB are codominant and are both expressed when inherited together
- IO is recessive
Sex Linkage
The inheritance of genes located on the X or Y chromosomes
Why are some sex linked conditions more common in men?
- Men’s sex chromosomes are XY
- Most sex linked genes are X-linked
- The lack of a 2nd X chromosome means that if a man carries a faulty allele on their X chromosome is has to be expressed as it can’t be masked by another X chromosome
Autosomes
Chromosomes that don’t determine the sex of an organism
Autosomal Linkage
When genes on an autosome are linked
How does Autosomal Linkage impact Variation?
Decreases variation because:
- Non random association of alleles at different loci
- Phenotypic ratios observed in dihybrid crosses are unexpected compared to those that arise from independent assortment
- Parental allele combinations are preserved across generations
Epistasis
Interactions between alleles where the expression of genes at a different locus is masked or supressed by another allele
Hypostatic Gene
The gene whose expression is blocked by another gene
Epistatic Gene
The gene whose allele affects the expression of the hypostatic gene
Dominant Epistasis
- A dominant allele at one gene locus masks the expression of alleles at a second gene locus
- Results in a 12:3:1 phenotype ratio
Recessive Epistasis
- 2 recessive alleles at one gene locus will mask the expression of alleles at a second gene locus
- Results in a 9:3:4 phenotype ratio
Chi Squared Equation
x² = ∑(O-E)²/E
- O = Observed phenotype number
- E = Expected phenotype number
- ∑ = The sum of
