ch12

Chromosomes--->Genes---->Proteins---->Characters

w/different traits alleles

• Random Fertilization
• Crossing over
• Independent assortment

Alleles

• -DNA copying mistakes
• -mutagens

evolution, disease, and genetics

transmission of traits from one generation to next

scientific study of heredity

he crossed tobacco strains to produce hybrids that differed from both parents

• he crossed 2 varieties of garden pea, Pisum sativa
•    -crossed 2 true-breeding strains
•    -1st generation resebled only 1% strain
•    -2nd generation resebled both (grandparents)

following one character

heritable feature that varies among individuals

variant of a character

repeated generations have consistent traits, (self-fertilization)

offsprng of two different true-breeding varieties (cross-fertilization)

P generation

F1 generation

F2 generation

F3 generation

only 2 variations (traits) of a single character

• of pea strains for 7 different character
•     -each character had 2 variants

• -First filial generation
• -offspring produced by crossing 2 true-breeding strains
• -for every trait mendel studied, all F1 plants resembled only 1 parent 
•        *referred to trait as dominant
•        *Alternative trait was recessive
• -No plants with characteristics intermediate btw the 2 parents were produced

• -Second filial generation
• -Offspring resulting from self-fertilization of F1
• -Although hidden in F1, recessive trait had reappeared among some F2
• -Counted proportions of traits
•      *always found about 3:1 ratio of dominant: recessive

F2 

• -3/4 plants w/ dominant form
• -1/4 plants w/ recessive form
• - dominant to recessive ratio was 3:1

1:2:1

• -1true-breeding dominant plant
• -2 not-true-breeding dominant plants
• -1 true breeding recessive plant

His plants did not show intermediate traits (each trait is intact, dicrete, not blended)

For each character, one trait was dominant, other recessive

Pairs of alternative traits examined were segregated among progeny of a particular cross

Alternative traits were expressed in F2 generation in raio of 3/4 dominant to 1/4 recessive

2 of the same allele

both alleles for a gene are identical (TT, tt)

gene has 2 differnt alleles (ex Tt)

phenotype observed only when two copies of allele (homozygous) (tt)

phenotype observed when homozygous or heterozygous (TT, Tt)

physical, biochemical, physiological traits of an individual based on genotype

Two versions of each chromosome 

• -genes for characters at specific loci on chromosome
• -forms of genes = alleles

Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization 

physical basis for allele segregation is the behavior of chromosomes during meiosis

States that ..

• -genes are located at specific positions on chromsomes
• -behavior of chromosomes during meiosis and fertilization account for inheritance patterns

Chromosome undergo segregation andindependent assortment during meiosis to account for mendel's observations, conclusions and 5-element model

mating of parental varieties differing in two characters

1. do the traits of two characters remain together: dependent Assortment

2. Do the traits of two characters separate: independent Assortment

each pair of alleles for a given character segregates independently of the other pair during meiosis 

='s Law of Independent Assortment

different chromosomes

0 to 1

• -Event certain to occur: probability of 1
• -Event that is certain not to occur: probability of 0
• -The probabilities of all possible outcomes for an event must ass up to 1

Ss *Ss: probability of ss offspring = 1/4

Yy *Yy: probability of yy offspring = 1/4

Aa *Aa: probability of aa offspring = 1/4

1/4 ss * 1/4 yy * 1/4 aa = 1/64

to determine genotype of individual w/ dominant phenotype

-Cross individual w/ unknown genotype (ex B_) w/ a homozygous recessive (bb)

-phenotype ratios among offspring are different, depending on genotype of unknown parent 

displays the dominant phenotype

2 copies aa are present

Dominant does not imply normal or more common than a recessive

not necessarily specifies by dominant alleles

Many inherited disorders in humans are controlled y one gene

monohybrid

• -2 recessive alleles are needed to show disease
• -Heterozygous parents
• -probability of inheritance increases w/ inbreeding, mating btw close relatives

• -One dominant allele is needed to show disease
• -Dominant lethal alleles are usually eliminated from the population

Over 10,000 monogenic human genetic disorders, dominat or recessive traits

Majority are recessive

relatively harmless conditions (free/attached earlobe) to severe (HD, CF, hemophillia etc)

are born to normal heterozygous parents

• -A Recessive Disorder
• -Most common lethal genetic disease in US

an excessive secretion of very thick mucus from lungs, pancreas, and other organs

1/25* 1/25 = 1/625

Cc * Cc ='s 1/4

a form of dwarfism and Dominant Disorder

-only heterozygotes display phenotype

-homozygous dominant genotype is embryonic lethal

• -Recessive alleles have normal heterozygote carriers
• -Dominant alleles cannot be carried by heterozygotes w/o affecting them
• -Many lethal dominant alleles result from mutations in a sperm or egg that kill embryo

• -each character is controlled by a single gene
• -each gene has only 2 alleles
• -there is a clear dominant-recessive relationship btw the alleles

Most genes do not meet these criteria

heterozygous phenotype is intermediate to homozygous phenotypes

incomplete dominance in humans

• -dangerously high levels of blood cholesterol
• -heterozygotes have blood cholesterol levels about twice normal
• -homozygotes have blood cholesterol levels about five times normal

neither allele is dominant to the other, both are expressed if present

heterozygote shows some aspect of phenotypes of both homozygotes

heterozygote is intermediate in phenotype btw 2 homozygotes

may br more than 2 alleles for a gene in a population (ABO blood types)

Each individual can only have 2 alleles

For any given gene, usually more than two alleles exist

When genes and enviornment interact

• Continuous variation
• Heritability

variation due to both genetic and environmental difference 

• Large range of phenotypes
• Ex. height, weight, intelligence, cancer susceptibility

proportion of the variation w/in a population due to genetic differnce among individuals

average: sum of all phenotypic values divided by number of individuals

occurs when multiple genes are responsible for controlling phenotype

Phenotype is accumulation of contributions by multiple genes

Characters show continuous variation (quantitative)


multiple genes turn into a single character (skin color)

impact of a single gene on more than one character, symptoms

Sickle cell disease exhibits pleiotrophy

Heterozygotes are normal carriers (recessive traits)

Causes disk-shaped rbcs to deform into a sickle shape w/ jagged edges

heterozygotes (Ss): usually healthy, can experience pleiotropic effects in low O2

Pleiotrophy uses a Single gene to produce multiple characters

Polygenic inheritance uses multiple genes to produce a single character (skin color)

more than one effect on the phenotype

because a gene that affects one trait often performs other, unknown function

• This can be seen in human diseases such as CF or sickle cell anemia
•      -multiple symptoms can be traced back to one defective allele

interaction btw genes at two or more loci (phenotype differs from expected if the loci were expressed independently)

•     -R.A. Emerson crossed 2 white varieties of  corn
•           -F1 was all purple
•           -F2 was 9 purple : 7 white