What did Darwin not know about evolution?
He did not understand how heritability worked, and that DNA contributed to it.
What are some problematic theories of inheritance from the mid 1800’s?
- Blending inheritance (mix of parents traits in offspring, doesn’t explain looking like a grandparent but not a parent). If this was the case, variation would be reduced over time.
- Inheritance of acquired traits (favourable traits from parents being passed on to offspring).
What was the basis of Mendel’s research?
- Tested hypothesis of blending versus particulate inheritance.
- Used true-breeding varieties of peas (aa or AA).
What were the phenotypes of peas in Mendel’s experiments?
-Seed colour
- Seed shape
- Pod colour
- Pod shape
- Flower colour
- Flower position
- Plant type/size
What was the main objective of Mendel’s experiments?
To determine whether statistical patterns in the occurrence of the contrasting traits persisted by means of crossing.
What three strategies did Mendel use when observing peas?
- True-breeding strains (aa, AA)
- Focused on a single trait at a time
- Counted every individual pea
What is the P0 generation?
The parental generation, or the first generation of crossed peas. Always true-breeding (aa or AA).
What is the F1 generation?
The first filial generation, always showing the dominant phenotype because it masks the recessive phenotype. Always heterozygous (Aa).
What is the difference between dominant alleles and recessive alleles?
Often the dominant allele codes for a functional protein, the recessive allele does not. Ex. pea colour is determined by the presence of a functional enzyme that breaks down chlorophyll, so AA or Aa will break down chlorophyll making the pea yellow.
What did the F2 generation show?
Some recessive traits returned (aa), some were dominant. This suggests that over generations, missing phenotypes came back.
What does diploid mean?
Each organism has two alleles of each gene.
What is a homozygous diploid?
The two alleles in a gene are the same, therefore they only produce one type of gamete (aa or AA).
What is a heterozygous diploid?
The two alleles in a gene are different, therefore it produces two types of gamete (Aa).
What is the expected phenotypic ratio of dominant to recessive when crossing two heterozygotes?
3:1 ratio, or 3/4 dominant and 1/4 recessive.
What is the expected genotypic ratio of AA, Aa, and aa when crossing two heterozygotes?
1:2:1 ratio.
What do phenotypic and genotypic ratios explain?
They demonstrate probabilities, so it is not a guaranteed outcome. Each individual pea has the same chances of these ratios.
What is a test cross?
When you mate aa with either AA or Aa. Used to determine if the dominant phenotype is either a homozygote (AA) or a heterozygote (Aa).
If a dominant phenotype is heterozygote in a test cross, what phenotypic ratio will the offspring show?
1:1 ratio of dominant and recessive phenotype. Aa, Aa, aa, aa.
If a dominant phenotype is homozygote in a test cross, what phenotypic ratio will the offspring show?
We will see all dominant phenotype. Aa, Aa, Aa, Aa.
What were the three parts of Mendel’s hypotheses?
- Adult plants carry two copies of factors (genes) that govern the inheritance of a characteristic. Ex. wrinkled versus smooth peas.
- If an individual’s pair of genes consists of different alleles, one allele is dominant over the other, and it will mask the recessive trait.
- The pair of alleles that make up a trait separate as gametes (in meiosis), half of the gametes (haploid) carry one allele, the other haploid carries the other allele. Diploid organisms get one allele from each parent (random combination during fertilization).
What is the product rule?
- The statistical probability of two independent events occurring sequentially.
- Individual probabilities are multiplied.
- Ex. coin flip probabilities
- Heads = 1/2
- Tails = 1/2 - Two heads = 1/2 x 1/2 = 1/4 (like random union of gametes)
- Questions use “and”.
What is the sum rule?
- The statistical probability of an outcome that can be achieved by two or more exclusive events. Sum if their individual probabilities.
- Individual probabilities are added.
- Ex. probability of a heads and tails in two tosses
- First probability - heads then tails (1/4)
- Second probability - tails then heads (1/4) - Total probability - 1/4 + 1/4 = 1/2
Practice question - In the mating Aa x Aa, what is the probability that, (i) the pea nearest to the stem is green and (ii) among four seeds in a pod, exactly one is green?
- First probability (i) - 1/4 x 3/4 x 3/4 x 3/4 (seed colour) = 27/256
** Three other pods with different placement of green pea, all same probability (27/256). - (ii) Add all four pod probabilities (four possible ways that one seed can be green, but all are mutually exclusive)
- 108/256 = 42% probability
What is a monohybrid cross?
A cross between two heterozygotes for a single gene.
