BIOL 2500 - Topic 3 (part 4)

Mendel’s first law

Mendel’s law of segregation (aka the law of equal segregation), indicating that each gamete will only contain one member of the gene pair (either the one from dad or the one from mom)

What stage does the law of segregation come into play in meiosis?

Anaphase I, when the homologous chromosomes are being separated by the spindle fibers

Mitosis in haploid organisms vs. mitosis in diploids

They occur similarly

Can meiosis occur in haploids?

1.) Yes, it can, but it requires the fusion of 2 strains to create a temporary diploid cell

2.) It only takes place during a short part of the haploid organisms life cycle

Meiosis in haploid yeast

1.) Two haploid cells of opposite mating types fuse together, resulting in the formation of a diploid meiocyte

2.) The meiocyte will then undergo meiosis to produce haploid sexual ascospores

Alleles at the molecular level

They are different from each other in their DNA sequences, either by a single base-pair or a few nucleotides within the gene

What causes differences in alleles

1.) Mutations

2.) Recombination

Dihybrid crosses

Looks at two traits and how they are simultaneously inherited

Mendel’s dihybrid crosses

1.) He started with pure-breeding parents, either AABB + aabb OR AAbb + aaBB

2.) The resulting F1 generation are the heterozygous for both traits —> AaBb

A/a ; B/b

This indicates that the two genes are on separate chromosomes

AB/ab

This indicates that the two genes are on the same chromosome (aka syntenic)

A/a • B/b

This indicates that we do not know if the two genes are syntenic or not

General annotation for unlinked genes

AaBb (unlinked meaning on separate chromosomes)

Mendel’s 2nd law

Law of independent assortment

Law of independent assortment

Unlinked or distantly linked segregating gene pairs assort independently at meiosis (i.e. the inheritance of 2 separate traits is independent)

When does the law of segregation occur in meiosis?

Metaphase I

What did Mendel’s dihybrid experiment look at

1.) Seed colour: Y and y

2.) Seed shape: R and r

What did Mendel’s dihybrid experiment start with

He crossed two pure-breeding parents (RRyy and rrYY), such that each gamete will have one allele from each gene

Mendel’s dihybrid experiment F1 generation

They all produced one phenotype and they were all heterozygous (RrYy)

What did Mendel’s F1 from his dihybrid experiment show us

1.) It showed us which alleles were dominant (whichever phenotype was presented)

2.) It also showed that the dominance of one character was independent of the dominance of the other

Mendel’s dihybrid experiment F2 generation

1.) He crossed the heterozygous F1 plants to produce the F2 generation, which gave him four different phenotypes

2.) Two parental phenotypes and 2 recombinant phenotypes (i.e. different from the parents)

Resulting F2 phenotypic ratio of Mendel’s dihybrid experiment

9:3:3:1 (which is a typical pattern for Mendelian dihybrid crosses)