Lecture 6: Mendelian Genetics Flashcards
What happens when you breed together two ‘pure breed’ (homozygous) strains with a single contrasting trait?
One of the contrasting character traits disappears entirely in the F1 (offspring of P1 generation [parents]) generation.
What happens if you cross F1 heterozygous generation back on itself (self-pollination/-fertilisation)?
(F2 = Selfed offspring of F).
F2 generation will display both contrasting traits, but always in an approximately 3:1 ratio, favouring the trait seen in the F1 generation.
What are Mendel’s three postulates of inheritance?
• Traits were ‘unit factors’ that exist in pairs.
• In the pair of unit factors for a single characteristic in an individual, one unit factor is dominant and the other is recessive.
• The paired unit factors segregate (separate) independently during gamete formation.
Unit factor = alleles (dominant /recessive)
What is a Punnett square?
A square diagram that is used to predict the genotypes of a particular cross or breeding experiment.
It allows the genotypes and phenotypes resulting from a cross to be visualised easily.
How do Mendel’s postulates translate with modern understanding?
In a diploid individual, genes have two alleles, each on a chromosome of the homologous pair.
In the pair of alleles for a single characteristic in an individual, one allele is dominant and the other is recessive.
The two alleles of a single gene separate independently during gamete formation.
What is genotype?
The genetic makeup of an individual.
Define phenotype.
The physical expression of the genetic makeup.
What are alleles?
Alternative forms of a single gene.
What is a homozygous individual?
When the alleles for a trait in an individual are the same.
What does it mean when an individual is heterozygous?
When the alleles for a trait differ.
What must be the genetic makeup of an individual to express a recessive phenotype?
Homozygous genotype.
How do you determine whether a dominant phenotype of an individual is homozygous or heterozygous?
You perform a testcross.
What is a testcross?
A cross between an individual displaying a recessive trait and one displaying a dominant trait to determine whether or not the dominant trait is heterozygous.
OR
A cross between a parent with the dominant phenotype with a parent showing the recessive phenotype, the phenotype of the F1 testcross tells us the genotype of the parent showing the dominant trait.
What is Mendel’s fourth postulate?
• Traits assort independently during gamete formation.
• All possible combinations of gametes will form with equal frequency.
How does Mendel’s fourth postulate relate to chromosome behaviour in meiosis?
The chromosome theory of inheritance proposed that the separation of chromosomes during meiosis could be the basis for Mendel’s principles — as long as the two characteristics are located on different chromosomes.
OR
Boveri and Sutton’s chromosome theory of inheritance states that genes are found at specific locations on chromosomes, and that the behaviour of chromosomes during meiosis can explain Mendel’s laws of inheritance.
+ Independent assortment explains Mendel’s ratio (9:3:3:1) from the dihybrid cross.
