Mechanisms of Hereditary Flashcards
What 3 questions did Mendel want to answer?
- What is inherited?
- How is it inherited?
- What is the role of chance?
Blended Inheritance
Information from parents is irreversibly linked, there is no way for it to be separated out again.
Discrete Inheritance
Information from parents is passed down and maintained separately in an individual
Monohybrid Cross
Cross between 2 individuals that differ in only 1 trait
True-Breeders Definition
Homozygous organisms who’s phenotype is always passed on to their offspring
Advantages of using Pea plants
- Short generation time (1 yr per offspring)
- Individuals produce lots of offspring
- Hermaphroditic (i.e. can self-fertilise)
- Clear-cut binary qualitative traits (e.g. yellow OR green)
Reciprocal Crosses
2 Monohybrid crosses with the same trait being investigate, but the parent phenotypes switch in the second cross (i.e. if dad was purple in first cross, mom will be purple in second cross)
Results of Mendel’s first experiments
- In F2 gen, sometimes 1 of the parental phenotypes that disappeared would reappear
- Reappeared in ration of ±1:3
- This disproved blended inheritance
Conclusions from Mendel’s first experiments
- Some phenotypes are more dominant over others
- 2 units of inheritance in every trait in an individual - 1 from father, 1 from mother. These are maintained in the individual.
Mendels 1st Law
Principle of Segregation
- The 2 alleles for each trait segregate during gamete formation and then unite randomly at fertilisation (1 from each parent)
The Product Rule
When event are completely independent of each other (gamete formation)
- Probability of 2 independent event occurring together is the product of the probability that each event will occur on its own
i. e. prob of throwing 2 6’s in a row = 1/6 x1/6 = 1/36
The Sum Rule
Phenotypic classes (gamete fusion)
- Probability of 2 mutually exclusive events occurring is the sum of their individual probabilities
i. e. prob of throwing a 5 OR a 6 in a single dice roll: 1/6 +1/6 = 2/6
Test Cross
Cross an individual with an unclear genotype with a homozygous recessive individual
- If all F1 gen are dominant phenotypes then mystery individual must by homozygous dominant
- If 50-50 then must be heterozygous
Dihybrid Crosses
Crosses with 2 different traits
Recombinant phenotypes
New combinations of phenotypes that don’t correspond to either of the parents
Mendels 2nd Law
Independent assortment
During gamete formation, different pairs of alleles segregate independently of one another
Chi-squared test
- Goodness of fit test
- Can be used to determine whether results we observe in a cross are consistent with Mendelian inheritance
- test statistic:
x^2 = sum [(O-E)^2/E] - Larger x-squared is, the more your data deviates from expected pattern
- reject null hypothesis if p-val(from table) <=0,05
5 Causes of Non-Mendelian Inheritance
- Dominance relationship between pairs of alleles is not complete
- Lethality: Some allele combination are unfavourable
- Interaction between genes - if phenotype is not determined by a single gene
- Sex-linkage - gene carried on sex chromosome
- Linkage & Recombination - 2 genes on the same chromosome
Incomplete Dominance
The inherited phenotype is an intermediate of the 2 parental phenotypes ( brown cow from white and black parents)
Co-Dominance
2 alleles are equally dominant over each other. Both parental phenotypes are expressed
Non-Mendelian Inheritance: Incomplete Dominance
- Because the dominant phenotype is expressed in homo/heterozygous genotypes, 1 allele is usually the same as 2 (i.e. both states produce enough enzyme to fulfil its function)
- In cases of incomplete dominance, 1 allele is no longer the same as having 2. It doesn’t quite produce the same amount of enzyme/function as a homozygous individual would.
NB: There is no deviation from Mendelian genetics, it is just expressed differently!
Non-Mendelian Inheritance: Lethality
- If you get a 2:1 ratio in F1, then it is lethal to be homozygous for the dominant allele
- If you get all 1 phenotype in F1, then it is lethal to be homozygous for the recessive allele
Non-Mendelian Inheritance: Complementary Gene action
- 2 genes work together to control the outcome.
- 9:7 ratio is proof of complementary gene action
Complementation Tests
- Determine whether 2 mutants (with the same phenotype) are defective in the SAME gene or in DIFFERENT genes.
- Cross 2 mutants together, if the mutant phenotype is present in F1 then they are defective in the SAME gene
- If you cross 2 mutants together and see wild-type phenotype in F1, then they are defective on DIFFERENT genes
Complementation Groups
Mutants that are defective in the SAME gene
i.e. they DON’T complement each other
Non-Mendelian Inheritance: Recessive Epistasis
- ‘Epistasis’ = interaction
- Homozygosity for a recessive allele at 1 locus masks genotype at a second locus
- Ratio 9:3:4
e. g. 2 pigment alleles for fur color in dogs (black & brown), different gene regulates deposition. If homozygous recessive for deposition then pigment is produced but can’t be deposited = yellow fur.
Non-Mendelian Inheritance: Dominant Epistasis
- Dominant alleles at one locus masks genotype at a second locus
- Ratio 12:3:1
Non-Mendelian Inheritance: Chromosomal Theory of Inheritance
- There are 2 copies of each chromosome in every cell & 2 copies of each gene
- During Meiosis homologous chromosomes line up and separate to different gametes (obeys the laws of segregation)
- This process is independent of the separation of other homologous chromosomes (law of independent assortment)
Hemizygous Individual
Male with a gene carried on the X chromosome
Non-Mendelian Inheritance: Linkage & Recombination
- 2 genes that are on the same chromosome won’t segregate in independent assortment
- To test for linkage: do a test cross, if all 4 phenotypes are there in equal numbers then they assorted independently
- There will be much more parental phenotypes if the genes are on the same chromosome
Why are humans terrible organisms. for genetic analysis?
- Few offspring
- Long generation
- Resource intensive
- Ethics - controlled crosses
- Can’t self-fertilise
What is a pedigree analysis?
Looking back in time at phenotypic data to determine the nature of human genetic disorders.
Dominant vs Recessive in Pedigree Analysis
- If a disorder skips a generation then it is recessive.
- If a disorder is dominant it will be in every generation
Autosomal vs X-linked disorders in Pedigree Analysis
- If it affects males and females equally it is autosomal
- If a diseased male passes on the disease to his son then it HAS to be autosomal (Father doesn’t give X-chromosome to son, so can’t be x-linked)
Dosage Compensation Definition
The equalisation of X-linked gene products in the 2 sexes
Strategies for Dosage Compensation
- Hyper activation of X in males. Increased transcription from X chromosome (happens in fruit flies).
- Random inactivation of 1 X chromosome.
- Happens in mammals
- Means that a recessive phenotype might be expressed in a female because the dominant allele could be shut off.
How is dosage compensation achieved?
Through large scale modification of chromatin structure (chromatin remodelling). Nucleosome positioning and chromatin condensed structure blocks transcription factors from binding to DNA
Sex Determination in Mammals
- Y chromosome determines sex.
- Default is for undifferentiated gonad tissue to differentiate into ovarian tissue.
- SRY (sex determining region of Y) gene must be present for it to differentiate into teste tissue.
