Lecture 4: Mendelian Genetics II Flashcards
Dihybrid Crosses - The Experiments
- Monohybrid. vs Dihybrid
Monohybrid is a heterozygote for a single gene, (e.g. A/a)
Dihybrid: double heterozygote (e.g. A/a, B/b)- hybrid from parents with 2 different traits
* if the genes are on different chromosomes: A/a ; B/b
* if the genes are on the same chromosome: AB/ab
* if the location of the two genes is not known: A/a * B/b
Reminder:
A and a are two alleles (forms/copies) of the same gene
Reminder: Parental are pure homozygous lines
Mendel did not know the location of the units of inheritance
Reminder: Dihybrid are heterozygote
round and yellow are the dominant phenotypes, R and Y are the dominant genes
Mendel’s Second Law - The Law of Independent Assortment
“Gene pairs on different chromosome pairs assort independently at meiosis.”
Dihybrid Crosses - The Explanation
In a F1 x F1 self cross - male and female gametes will be in the same proportions and the four female gametes will be fertilised randomly by the four male gametes
Mendel’s two laws of equal segregation and independent assortment
Using a Testcross to Verify the 1:1:1:1 Gametic Ratio Produced from a Dihybrid
Proportions of progeny correspond to proportions of gametes produced by the dihybrid.
Remember: The tester (r/r ; y/y) contributes only recessive alleles - allows gametes produced by individual of unknown genotype to be determined from the progeny resulting from the cross
Determining the Probability that the Difference Between Observed and Expected Values are Due to Chance
The goodness of Fit Chi-Square (χ2) Test
- Looks like a 3:1 ratio. Is it just by chance that these numbers deviate from a true 3:1 ratio?
- Using the chi-square test to determine if
observed=expected. - be sure to work with numbers of individuals, not proportions or percentages
- The calculated chi-square value is compared with theoretical values with the same degrees of freedom (df; see Table, next slide)
*for a goodness of fit chi-square test, df = n − 1, where n is the expected number of phenotypes
No significant difference
0.05 probability (P) level
A 0.05 probability (P) level is the typically accepted cutoff value
- P > 0.05 suggest chance is responsible for the deviation seen between expected and observed values.
- P < 0.05 suggest that chance is not responsible, and a significant difference exists between the expected and observed values.
Relating Genetic Crosses and Mendel’s Laws with Chromosome Behaviour in Meiosis
- Crossing over occurs in Prophase I (if it occurs)
- Mendel’s law of independent assortment results from the independent separation of the chromosomes at anaphase 1
Relating Genetic Crosses and Mendel’s Laws with Chromosome Behaviour in Meiosis
Mendel’s First Law:
“The two members of a gene pair (alleles) segregate from each other into the gametes; so half the gametes carry one member of the gene pair and the other half of the gametes carry the other member of the gene pair.”
Mendel’s Second Law:
“Alleles on different chromosome pairs assort independently at meiosis.”
**random and independent alignment of chromosomes in Metaphase I
**random and independent distribution of chromosomes in Anaphase I
Recognising Equal Segregation and Independent Assortment; 2.
What can we use Mendel’s Laws to predict;
- progeny from known parental genotypes
*establish stocks for plant and animal breeding or basic research
*likelihood of inheriting a medical condition or disorder caused by a single gene - parental genotypes from progeny phenotypes
*inheritance patterns of particular traits (e.g. dominant, recessive)
Single Gene Inheritance - Human Genetics
- Cannot do controlled crosses with humans
- Use pedigree analysis (family tree) to infer single-gene inheritance
+++family member with a distinct phenotype
Human Mendelian Genetics - Autosomal Recessive Traits:
Characterised by = 4? PKU example =4?
- Autosomal recessive inheritance is characterised by:
*both parents being heterozygotes
*appearance in the progeny of unaffected individuals
*both male and female progeny affected
*may skip generations
e.g. Phenylketonuria (PKU)
*defective gene encoding the enzyme phenylalanine hydrolase
*converts phenylalanine to tyrosine
*phenylpyruvic acid is synthesised from phenylalanine instead
*neurological damage, mental retardation
Human Mendelian Genetics - Autosomal Recessive Traits
Expected a 3:1 ratio, not a 1:1 ratio… HOWEVER: sample size is small
To see a 3:1 ratio, would need around 20 progeny(!)
Human Mendelian Genetics - Autosomal Dominant =3 , example?
Autosomal dominant inheritance is characterised by:
1*appearance in every generation
2 *both male and female progeny affected
3 *affected parent has a 50% chance of passing allele to each child
*e.g. Huntington disease
Human Mendelian Genetics - Autosomal Dominant Traits.Ear Wax experiment:
Ear Wax experiment:
2 types-yellow wet wax
-grey dry crumbly wax
Gene responsible is called the
ABCC11
1 bp mutation G/A G=wet- dominant trait A=dry
*Autosomal dominant trait
*Caucasian/east African- wet trait
*East asia – dry trait
*Trait has been used to track human migration patterns
Locus,
Locus = specific place on a chromosome occupied by an allele
Heterozygote: an individual organism possessing two different alleles at a locus.
Homozygote: An individual organism possessing two of the same alleles at a locus.
Characteristic/character: attribute or feature possessed by an organism.
Genotype: a set of alleles possessed by an individual organism
Phenotype: trait/ the appearance or manifestation of a characteristic
Allele: one of 2 or more alternative forms of a gene.
Gene: an inherited factor (region of DNA) that helps determine a characteristic.
