Chapter 11 - Mendel and the Gene Idea

Character

heritable feature that can vary amongst individuals

ex. flower colour

trait

each variant of a character

ex. pink, white, red are colour variants

true-breeding

self-pollination results in only a single trait being expressed over multiple generations (homozygous for the character in question)

hybridization

crossing of two true breeding variants

• involves a fertilization event

homozygote

individual who carries two identical alleles of a gene

heterozygote

individual who carries two different alleles of a gene

phenotype

measurable/observable trait of an organism

genotype

allelic combination or genetic makeup of an individual

mendel’s process

• mendel worked with pea plants and focused on various characteristics

• tracked character expression through hybridization experiments

• deduced two fundamental principles of heredity:

  • law of segregation

  • law of independent assortment

dominant allele

trait exhibits dominance over other alleles (traits) when present

recessive allele

trait which is masked by dominant allele

what does altering gene sequence do

changes or removes the protein produced by that gene

mendel’s model

• for each character, a diploid organism inherits two copies of a gene, one from each parent

• two copies of a gene may be identical or different

• if two alleles at a given locus differ, then the dominant allele determines the organism’s phenotype

• recessive allele is masked in the presence of a dominant allele

law of segregation

two alleles for a heritable character separate from each other during gamete formation and end up in different gametes

• only one allele inherited from each parent, one from sperm and one from egg

• examined by a monohybrid cross

what hypothesis did mendel debunk and how did he do it

blending hypothesis - parental traits are blended in the offspring, creating an intermediate phenotype

• mendel tested this by making an F2 gen of offspring

• he found that traits did not blend

testcross

crossing an individual of unknown genotype with an individual that is homozygous recessive

if:

• offspring are dominant phenotype = unknown individual is homozygous dominant (PP)

• offspring are half dominant and half recessive phenotype, the unknown individual is heterozygous (Pp)

law of independent assortment

• two or more genes sort independently

• each pair of alleles segregates independently of any other pair during gamete formation

linkage

refers to how genes located closer together on the same chromosome are less likely to independently assort

what contribution does a recessive individual make to the phenotype

null contribution

mendel’s hypothesis on independent vs dependent assortment in dihybrid crosses

if dependent assortment occurred, parental alleles remain together producing two types of gametes

• phenotypic ratio 3:1

if independent assortment occurred, 4 possible combinations of alleles in the gametes

• phenotypic ratio 9:3:3:1

• this is what mendel observed

What does probability of 1 and 0 equal in probability laws

Probability of 1 = will occur

Probability of 0 = will never occur

• Probabilities of all possible events (genetic combinations) must add up to 1

independent events

each event (allele) occurs independently of any other event

• each event has the same likelihood of occurring regardless of whether another has occurred

• each gamete has the same likelihood of containing either allele

• probability of a particular allele = 1/2

mutually exclusive events

either event A or event B occurs

• mutually exclusive event probabilities relate to the number of ways alleles can combine to produce a specific genotype

multiplication rule

• the portability of a particular combination of independent events (alleles) occurring is the product of their individual probabilities

• probability of a specific combination of alleles is the product of all of their individual probabilities

ex. each allele in Rr has a ½ chance to show up, so their offspring would each have a combination of 1/4

addition rule

• probability of any one of two or more mutually exclusive events (either/or events) can be determined by adding their probabilities

• a heterozygous genotype only occurs if the dominant allele is provided by the sperm OR the egg and the recessive allele by the other

Types of inheritance patterns

simple phenotypic patterns:

• Monohybrid = 3 dominant : 1 recessive

• Dihybrid = 9 double dominant : 3 dominant/recessive : 3 recessive/dominant : 1 double recessive

complete dominance

one dominant allele completely masks the phenotype of the recessive one

incomplete dominance

neither allele is completely dominant

• heterozygote has an intermediate phenotype (different from the parent)

• NOT blending, alleles remain the same independent units as in complete dominance

• F2 genotype and phenotype ratios are both 1:2:1

• The original phenotypes of the P gen later appear again in the F2 gen

Codominance

both alleles influence the phenotype in separate ways

• produces a combined phenotype where both products are created, not just an intermediate

• genotype and phenotype are both 1:2:1 when 2 heterozygotes are crossed

ex. blood types (ABO)

does recessive = repressed?

no, recessive alleles are not repressed, but masked by dominant alleles

tay-sachs disease

• recessive disease

• recessive allele produces a defective enzyme for metabolism of certain lipids

• lipids accumulate in the brain, resulting in seizures, blindness, motor and mental deterioration, and death within a few years

“heterozygous individuals are unaffected by the disease”

• at the organismal level, the normal allele demonstrates ___

complete dominance, since this a recessive disease, you need 2 recessive alleles to be affected by the disease

“heterozygous individuals have an intermediate activity level for the lipid metabolizing enzyme”

• at the biochemical level, the normal allele demonstrates ___

incomplete dominance, since the individual is affected intermediately by the disease and not completely

“heterozygous individuals produce equal amounts of the normal and defective enzymes”

• at the molecular level, the normal and Tay-Sachs alleles demonstrate ___

codominance since the individual shows equal amounts of healthy and affected enzymes

Polydactyly

extra phalanges (fingers)

• caused by inheritance of dominant allele which has a low frequency in the population

Can genes have more than 2 allelic form?

yes, most genes have more than 2 allelic forms

ex. ABO blood group

• blood type AB

pleiotropy and examples

single gene with more than one phenotypic effect

ex.

• inherited disorders are associated with multiple symptoms (sickle-cell anemia)

• pea gene for flower colour also affects the colour of the seed coat

Epistasis

phenotypic expression of one gene alters that of another gene

ex. labrador retriever coat colour

polygenic inheritance and how it can occur

phenotype is determined by two or more genes

• can be due to one product increasing or decreasing activity of another product

• or can be due to independent effects on a single trait

quantitative characters

genotype is expressed (quantified) on a spectrum based on how much of that product is being created

why are twins different even with the identical genes?

exposed to different environmental factors

multifactorial characters

many factors, both genetic and environmental, collectively influence the phenotype

why is it hard to study human character inheritance and what is one way to overcome this?

• long generation time (20 years)

• few offspring

• hard to force specific mating

one way to overcome this is through pedigree analysis

pedigree analysis, why its useful, and when its used

study of a particular trait(s) over generations

• useful for finding whether a trait is dominant or recessive

• can help determine the chance of a specific genotype/phenotype appearing in offspring

• used in cases of disabling/deadly disease for genetic counseling

what do recessive alleles of recessive disorders code for?

a malfunctioning protein or no protein

carrier

heterozygous individual that are phenotypically unaffected but pass their recessive allele on to their children

why are genetic histories of populations different

due to historic geographic isolation

why is the frequency of alleles for serious disorders typically low

due to a lack of or reduction in reproduction by homozygous individuals

how are alleles retained in the gene pool

by heterozygous carriers

consanguinity and its relation to recessive allele

mating of close relatives

• increases chance of homozygous recessive traits due to shared genetic histories

• increases chance of inherited recessive disorders

sickle-cell disease, cause, symptoms, and stats

• causes hemoglobin to stick together creating protein fibers which deform the red blood cells

• due to single amino acid change in hemoglobin

• symptoms - physical weakness, pain, organ damage, stroke, and paralysis

• 80% of cases arise in sub-saharan africa

why has the allele for sickle-cell disease been maintained in the population

• malaria parasite lives in red blood cells during part of its life cycle, but sickle-cell hemoglobin reduces parasite density

• reduced frequency and severity of malaria causes an advantage to heterozygotes in areas with high cases of malaria

cystic fibrosis causes

• affects a gene which encodes a transmembrane protein which transports chloride ions across the membranes of exocrine epithelial cells

• mutations to the gene create improperly folding proteins which are degraded by the cells

cystic fibrosis symptoms

• increased extracellular chloride results in thicker mucus and retention in the pancreas, lungs, digestive tracts, and other organs

• pleiotropic (multiple) effects - poor nutrient absorption, chronic bronchitis, and recurrent respiratory infections

cystic fibrosis stats and likeliness of death

most common lethal genetic disease in people of European descent (1/2500)

• 1/25 if them being carriers

if untreated, usually death by the age of 5

• but current therapies allow survival up to 30 years old or beyond

Dominantly inherited disorders

• dominant allele is the affected allele

• individuals with the trait are usually heterozygous

• often much rarer than recessive disease

  • lower frequency of the allele compared to recessive alleles of recessive diseases

• disease symptoms aren’t present at birth

• ex. polydactyly and achondroplasia

Huntington’s disease

• example of Dominantly-inherited disease

• results in deterioration of the nervous system

• affects 1/10,000 people

• symptoms don’t appear until 35-40 years old

  • likely to have reproduced and passed on the affected Dominant allele by then

• gene was sequenced in 1993, leading to the ability to screen for the disease

multifactorial disorders

• hereditary component of a disease is polygenic

• lifestyle has a large impact on whether a disease will stop

• ex. heart disease, many forms of cancer, diabetes, alcoholism, and mental disorders such as depression and schizophrenia

genetic counseling

• uses mendelian genetics and pedigree analysis to predict the chance of an affected child or being a carrier

• can be provided to couples with family histories prior to conceiving children

how does screening individuals help with genetic counseling?

• ability to predict the chance of an affected child is greatly increased

• however ability to screen for and alter genetic abnormalities has serious ethical implications