Ch. 3 Genetics

Basic Principles of Heredity

What did Gregor Mendel discover and how?

• basic principles of heredity

• by breeding garden peas in planned, quantitative experiments

What was Gregor Mendel unaware of?

• role of chromosomes in heredity

• concept of a gene

• Darwin natural selection as mechanism of evolution

What were the advantages of garden peas for genetic study?

• short generation time

• easy to control breeding and prevent self-fertilization

• many purebred varieties available

How did Mendel compose an accurate experiment?

• chose traits that exist in 2 easily differentiated groups

• used careful statistical analysis

• conducted hypothesis-driven breeding experiments

Gene

an inherited factor/region of DNA that helps determine a characteristic

Locus

specific place on a chromosome occupied by an allele

Genotype

set of alleles possessed by an individual

Heterozygote

an individual organism possessing 2 different alleles at a locus

Homozygote

an individual organism possessing 2 of same alleles at a locus

Phenotype

expression of genotype interacting with other genes and the environment

appearance or manifestation of a characteristic

Can an organism inherit a phenotype?

No, only genotypes can be inherited

• cannot inherit expression of the genotype, only the nucleotide sequence (genotype itself)

Monohybrid

follows one character

Characteristic/Character

attribute/feature possessed by an organism

How many possible traits can characters in a monohybrid experiment have?

only 2 possible traits (either/or)

Describe the parent (P) generation of the monohybrid experiment.

true breeders: homozygous for 1 of the possible traits

Describe the F1 offspring of the monohybrid experiment

express only 1 of the P generation phenotypes

Describe the F2 offspring of the monohybrid experiment

express both P generation phenotypes, in 3:1 ratio

Reciprocal crosses

switching which trait is carried in the male gamete

When Mendel performed reciprocal crosses in his monohybrid experiment, what were the results as compared to the original experiment?

same results: 3:1 ratio in F2 generation

Mendel tested 7 different characters in his Monohybrid experiments, what were the results?

all had 3:1 ratio in F2 generation for all characters

What did Mendel conclude about alleles?

• account for variations in inherited characters

• organisms possess 2 alleles for each character

What is the allele seen phenotypically in F1 offspring denoted by? What is the other allele denoted by?

• uppercase letter (seen phenotypically in F1)

• lowercase letter (other allele)

Different alleles for a particular gene occupy the same _____ on _______ chromosomes.

• same locus

• homologous chromosomes

What was Mendel’s conclusion about the Law of Separation?

The 2 alleles separate when gametes form, and 1 allele goes in each gamete

• fertilization produces a zygote with 2 alleles

If 2 alleles at a locus differ, which allele determines the organism’s appearance, and which has no noticeable effect?

• dominant allele = determines appearance

• recessive allele = masked/suppressed

How did the Round allele (R) code for the pea to become round?

codes for an enzyme that converts starch from a linear structure to a branched form

How did the Wrinkled allele ( r ) code for the pea to become wrinkled?

mutated so codes for a non-functional form of the enzyme = starch not converted = accumulation of sucrose in seed

High sucrose causes young pea to absorb water and swell = mature pea loses water = wrinkles

What is the phenotype of a pea with rr genetype and why?

wrinkled because no enzyme

What is the phenotype of a pea with Rr genetype and why?

Round because has functional enzyme

• single R allele is sufficient to produce enough functional enzyme to prevent water absorption and swelling

What is the phenotype of a pea with RR genotype and why?

round because has functional enzyme

What did Mendel conclude about the probability of how each allele is passed on to the gamete, when the two alleles separate?

When the 2 alleles separate, they are passed on with equal probability to each gamete

How will an F1 Rr plant pass on it’s alleles to the gametes, according to Mendel’s conclusions?

Rr will pass R allele to 50% of gametes and will pass r allele to 50% of gametes

• equal probability

Backcross

Cross F1 genotype with either of the P parental genotypes

How can you tell the genotype of an individual with a dominant phenotype?

Perform a testcross

Testcross

cross an unknown dominant genotype (AA or Aa) with a homozygous recessive genotype (aa).

Rules of Probability

the likelihood of the occurence of a particular event

Rule of Multiplication

the probability that 2 or more independent events will occur together is the product of their individual probabilities

Rule of Addition

the probability that either 1 or 2 or more mutually exclusive events will occur is the sum of their individual properties

wild type

in animals, the most common allele for a character

What is the wild type allele designated by?

+ in superscript after letter: cn⁺

In animals, what does a slash (/) between alleles in diploid organisms indicate?

designates genotypes

• cn⁺/cn or +/cn

What was the conclusion of Mendel’s dihybrid cross?

Principle of independent assortment

• alleles at different loci are independent of one another

What did Mendel cross in his Dihybrid cross and why?

round & yellow x wrinkled & green (RY x ry)

• wanted to know if these traits are passed on together or separately

What is the mechanism behind the law of independent assortment?

independent separation of homologous chromosome pairs during meiosis

For cross RrYy x RrYy, what is the probability of an offspring with genotype RRyy?

1/16

• Rr x Rr = RR? ½ R x ½ R = ¼ RR

• Yy x Yy = yy? ½ y x ½ y = ¼ yy

• RR ¼ x yy ¼ = 1/16