Lecture 4Early ideas about genetics

• Constancy of species • Direct transmission of traits • Blending Theory of Inheritance • Parents hereditary material blends in

offspring 1. Individuals eventually reach uniform

appearance 2. Once blended, original traits are not

separable

Inconsistencies with this Theory

1. Individuals in population do not reach uniform appearance

2. Some traits skip one or several generations

Until:Gregor Mendel in 1850Experiments with garden peas, – Pisum sativum

• Hybridized true-breeding plants in genetic crosses;

• true-breeding parental plants of crosses = P generation

• hybrid offspring of the P generation = F1 generation

• F1 plants self-pollinate to produce F2 generation

• Mendel discovered that traits are inherited in discrete units

• 7 different traits, • studying 22 strains of peas • using large sample sizes, and he

always saw a ~3:1 ratio in the F2 generation

• Mendel realized that these results were explainable if three things were true. He hypothesized that:

1. Every trait is controlled by two "heritable factors". 2. If the two alleles differ, one is dominant and one is

recessive, Dominant traits mask the appearance of recessive traits.

3. Alleles are randomly donated from parents to offspring

- the factors (alleles) separate when the gametes are formed by meiosis, allowing all possible combinations of factors to occur in the gametes.

• Mendel's Law of Segregation - The two factors (alleles)

separate when the gametes are formed, and only one factor (allele) is present in each gamete.

Monohybrid cross

• Geneticists use letters to represent alleles

• A capital letter = Dominant trait, a lowercase letter = a recessive trait.

• The same letter is used to indicate both alleles

• = Widow's peak: W = widow's peak, w = continuous hairline (which are you?)

• = Freckles: F = Freckles, f = no freckles (which are you?)

• = Earlobes: E = unattached, e = attached (which are you?)

Dihybrid Cross• Mendel also did a type of cross where

two traits were followed at one time • AA or Aa = purple; aa = white• BB or Bb = tall; bb = short• Important thing that Mendel noticed

from this type of cross was that traits (like flower color, height) are inherited independently

• Mendel's Law of Independent Assortment

• AaBb X AaBb

Some head lice are now resistant to all commonly used pesticide shampoos used to kill them. Which is

the better explanation for this observation?

a. Frequent contact with pesticide shampoos caused mutations that allowed the lice to become resistant so resistant lice became more and more common

b. Some lice carried genes for resistance to the pesticide before contact with the shampoos and their numbers increased after lice shampoos became common

• A class of 100 students was tested for the ability to taste phenylthiocarbamide (PTC). It was found that 50 students could taste it very strongly (genotype TT), 20 students could taste it weakly (genotype Tt), and 30 students could not taste PTC at all (genotype tt). What is the frequency of the recessive allele, t, in the population?

a. 0.40b. 0.50c. 0.60d. 0.80