L5 - Mendelian genetics & monohybrid punett squares

Trait

Specific feature/characteristic

• e.g. Height, lactose intolerance, attached earlobes, hair texture

Blending theory

• developed in the 1800s that believed characteristics of parents were blended and presented in the offspring

Why were pea plants chosen?

1. Self fertilizing plants

   • 1 plant contains both male and female gametes = control which parent produces offspring

2. Distinct qualitative traits w 2 distinguishable forms for easy observation. e.g. round or wrinkly seeds

3. Pea plants are true breeding plants = offspring still exhibit the same traits after generations

   • e.g. plant w purple flowers = offspring ALWAYS w purple flowers

What were the 3 conclusions that Mendel made from the experiments?

1. blending theory was incorrect, if it was correct the flower colour should’ve been light purple (white x purple)

2. Law of segregation: offspring inherit ONE allele from each parent

3. Principle of dominance

What is the law of segregation?

• Law of segregation = during meiosis, gametes will only receive one of the 2 alleles for each gene

  • Pea plant flower colour gene: Purple allele & white

• Offspring inherit 1/ 2 alleles from each parent bc chromosomes segregate randomly during gamete formation

  • which sperm fertilizes which egg = random

What is the principle of dominance?

• traits are determined by the pair of alleles

• There are dominant & recessive alleles

• IF the pair is: DOM + REC → only the dom trait will be expressed

  • rec allele never expressed when dom allele is present

Homozygous

• inherited same version of alleles from each parent

  • e.g. BB or bb

Heterozygous

• inherited different version of alleles from each parent

  • e.g. Bb or bB

Genotype

• allele combination for any trait found in an organism

  • Bb x BB

Phenotype

• an individual’s observable traits

  • e.g. height , eye colour, blood type

Notation: Dom & Rec alleles

• alleles are rep. by letters

Dom: CAPITALS → BB

rec: lower case → bb

Punnett square

• dom alleles are listed first