How are traits inherited?

Darwin's dilemma

• Darwin found a lot of evidence supporting his theory of evolution by natural selection, but one thing troubled him.

• He knew that species occasionally would appear with advantageous traits, but feared those traits would get blended away.

• Consider a giraffe that is born with a longer neck.

• It would eat better than other giraffes, and would have a better than average chance to mate.

• But who would it mate with?• He thought the kids would be

half-way between both parents.

• Who would the kids mate with?

• Darwin died wondering about this paradox.

Blending vs. Particulate Inheritance

• Ironically, the answer was available in a journal in Darwin's library, but he never read that article.

• Gregor Mendel, an Austrian monk, published his data on pea plants providing evidence that blending inheritance was incorrect. 

Mendel’s particulates factors were genes.

• Mendel determined that hereditary factors occurred in discrete pairs, with one of each coming from each parent.

• 40 years later, we discovered that these ‘factors’ (now called genes) were carried on chromosomes.

Once the connection was made between genes and inheritance, Darwin's theory of evolution by natural selection was greatly

strengthened. 

Incidentally, the combination of Darwinian evolution and population

genetics is called the New Synthesis.

Mendelian Genetics

• Mendel knew that his 'factors' were discrete and non-blending. 

• He also knew much more about the behavior of these units of inheritance.

• So let’s revisit his peas!

Law of Segregation

• Mendel's First Law (Law of Segregation): Mendel determined that each individual has two copies of each gene (e.g., Pp).

• These copies are called alleles.  If both alleles are the same, then the individual is homozygous (e.g., PP or pp). 

• If the two alleles are different, then the individual is heterozygous (e.g., Pp).

• When an individual creates gametes (sex cells: egg or sperm in humans, egg or pollen grain in plants), only one of each allele is packaged in the gamete. 

• Mendel determined that which allele appears in the gamete is random, with each allele having a 50% chance.  This rule is the Law of Segregation.

Flower color

• Pea flowers are either purple or white.

• Peas fertilize themselves, so

• white white and purple purple.

• called true-breeding• But…

• …if you cross a true-breeding purple with a true-breeding white…

• …all of the offspring have purple flowers.

• Hence Mendel said that purple was dominant to white.

• PP: purple• pp: white• Pp: purple!

Terms to understand

• gene: stretch of DNA that codes for a particular trait. (e.g., flower color)

• allele: a particular variant of a gene (e.g., purple)

• genotype: what alleles an individual has for a particular trait or set of traits (e.g., Pp)

• phenotype: the expression of the genes; what the individual looks like (e.g., purple)

• dominant trait: an allele that is expressed no matter what the other allele is (e.g., purple flower color being dominant to white flower color in pea plants)

• recessive trait: an allele that is only expressed if it is the only allele present (i.e., both alleles are the same) (e.g., white flower being recessive to purple flower color)

Terms to understand

• homozygous: has 2 copies of the same allele for a given trait (e.g., PP or pp)

• heterozygous: has 1 copy of each of two alleles for a given trait (e.g., Pp)

• F1 generation: the kids of the parents

• F2 generation: the grandkids of the parents (kids of F1)

• gamete: sex cell (egg or sperm); only has ONE allele for each gene since it only has one homologous chromosome (either the one you received from Mom or the one you received from Dad)

• True-breeding: homozygous for the trait.

Forming gametes

• How many different gametes can PP make?

• 1• P• How many different

gametes can Pp make?

• 2• P or p

• When forming gametes, you always need one allele for each gene.

• How many different gametes can PPTt make?

• 2• PT or Pt

Determining the number of different gametes possible

• AaBBCc?• 2 x 1 x 2 = 4• AaBbCC?• 2 x 2 x 1 = 4• AaBbCcDd?• 2 x 2 x 2 x 2 = 16• AAbbCCddEE?• 1 x 1 x 1 x 1 x 1 = 1• What is it?• AbCdE

• Which of the following gametes can this parent (AABbCCDdeeFf) make? a. AAbCEf b. ABCDEF c. abcdef d. ABCdef

• d is the answer.• What is the chance of

that parent producing that gamete?

• 1/8 Why?

Determining the number of different alleles

• AaBBCc?• 2 + 1 + 2 = 5 alleles• AaBbCC?• 2 + 2 + 1 = 5 alleles• AaBbCcDd?• 2 + 2 + 2 + 2 = 8• AAbbCCddEE?• 1 + 1 + 1 + 1 + 1 = 5

How many different genes are shown at right?

• 3, 3, 4, and 5 (top to bottom)