Ch 14: Mendel and the Gene Idea

Genes

discrete units of DNA that encode for certain characteristics

• ex. ear size

• “lives” on a specific chromosome and at a specific spot

  • locus

• for each gene, you receive one allele from mom and one from dad

Traits

variations of a character

• ex. small vs large ears are traits of ear size

Alleles

• can be dominant or recessive (dominant determines the trait)

• alleles are on homologous chromosomes

Genotype

the specific combination of alleles that the organism has for a gene

• homozygous dominant

• homozygous recessive

• heterozygous

Phenotype

the observable physical characteristics that an organism has due to the combination of alleles

• homozygous dominant or heterozygous: displays dominant gene

• homozygous recessive: displays recessive gene

Model Organism

a species that is often used in scientific experiments

• easy to breed and keep in laboratory

• short lifespan (generation time)

• produces lots of offspring

• experimental advantages (few chromosomes, easily identifiable physical characteristics)

• ex. flowering plants, fruit flies, crickets, fish, mice

Gregor Mendel

“father of genetics”

• monk in Austria in second half of 1800s

• conducted genetics experiments on pea plants (model organism)

  • his work was virtually unknown until early 1900s

The Law of Segregation

• the two alleles for a gene separate during Meiosis I and end up in different gametes

  • each homologous has one allele, homologs are separated in Meiosis I

• no gamete will have two alleles for the same gene

  • in males/females, each sperm/egg will get only one of the two alleles present in somatic cells

How to use a Punnett Square

• must know genotype of parenys

• must be able to determine types of gametes produced by the parents

  • mom’s gametes on top

  • dad;s gametes on side

• offspring genotype in boxes

• offspring phenotype determines from genotype

Test Cross

• allows us to determine the genotype of an individual that displays the dominant phenotype

• cross dominant phenotype individual with an individual that is homozygous recessive

Dihybrid Crosses

• involves two traits

• they sort independently on separate chromosomes

  • law of independent assortment: in metaphase, each pair of alleles segregates independently of other pairs of alleles

• consequence: Punnett square will have 4 gamete entries per parent & 16 possible offspring combinations

The FOIL Method

determines the type of gametes an organism can produce

Mendel’s Laws

mendelian genetics apply to traits that follow Mendel’s laws

• gene has only two alleles

• law of segregation

• law of independent assortment

• law of dominance

  • one allele is completely dominant over the other allele

Exceptions to Mendelian Genetic Patterns

• alleles are not completely dominant or recessive

• a gene has more than two alleles

• a single gene produces multiple phenotypes

• more than one gene affects the phenotype

Dominant Alleles and Phenotype

• chemical differences

• we call one “dominant” because we can see the affect in the phenotype

• the dominant allele does not suppress the recessive allele

• it is the expression of the alleles that is different

  • the protein produced by the recessive allele is oftentimes non-functional

Degrees of Dominance: Complete Dominance

one allele is completely dominant over the other

• ex. flower color in Mendel’s pea plants

  • purple allele is completely dominant over white

  • RR and Rr have same phenotype: purple

Degrees of Dominance: Incomplete Dominance

neither allele is completely dominant over the other

• offspring of two true-breeding parents have phenotype in between that of the parents

  • homozygous dominant genotype and heterozygous genotype do NOT have same phenotype

• ex. snapdragons: C^RC^R produces red pigment; C^WC^W produces white pigment; C^RC^W produces pink pigment

Degrees of Dominance: Codominance

each allele affects the phenotype in separate, distinct ways

• offspring of true-breeding parents display both phenotypes

• ex. BB chickens are black; WW chickens are white; BW chickens are checkered (both black and white)

Frequency of Dominant Alleles

• dominant alleles are not always more common than recessive alleles

  • i.e., higher frequency in a population

• many recessive alleles actually occur at a greater frequency than dominant alleles

  • ex. polydactyly in humans (extra fingers/toes); dwarfism in humans

Multiple Alleles

many genes have more than two alleles

• ABO blood groups in humans have multiple alleles and codominance

• three alleles: I^A, I^B, and i

• four possible phenotypes, six possible genotypes: blood type A (I^AI^A or I^Ai), blood type B (I^BI^B or I^Bi), blood type AB (I^AI^B), blood type O (ii)

Rh Factor in Blood

refers to a different antigen on the blood (not A, B, or O antigen)

• trait is assigned + or - suffix to blood type

  • A+, B-, etc.

• presence of the antigen is dominant to absence of the antigen

• genotypes (phenotypes0:

  • ++ (Rh positive

  • +- (Rh positive)

  • -- (Rh negative)

• trait is inherited via simple mendelian genetics

Pleiotropy

when one gene affects more than one phenotypic trait

• ex. frizzle gene in chickens

  • chickens with the frizzle allele have frizzy feathers

  • this allele also causes changed to their metabolism

Polygenic

multiple genes affect a single trait

• “many genes”

• human examples: height, skin color, hair color, eye color

Epistasis

one gene alters the affect of a second gene

• ex. yellow labs

Lethal Dominant Alleles

• lethal disorders due to dominant alleles

• homozygous recessive does not have the trait

• heterozygous genotype has trait

• homozygous dominant genotype is fatal

Pedigree Analysis

• allows tracking of genetic traits in family trees through multiple generations

• males are squares: ▢

• females are circles: ◯

• individuals affected are shaded: ⬜

• horizontal line: mating

• vertical line: offspring