D3.2: Inheritance

Gregor Mendel

austrian monk who explained inheritance via experimentation

-used common pea plant, Pisum sativum

Advantages of Pisum sativum (3)

1. each plant has both sex organs, allows for self-pollination and self-fertilization

2. one trait had only 2 distinct variations

3. short generation time

Pisum sativum Sex Organs

Anthers have pollen (male) and stigma have ovaries (female)

P Generation

parental generation: true-breeding (homozygous)

Cross-pollination

two true-breeding plants are crossed

-pollen from 1 flower dusted on another

-its own anthers are removed and flowers are covered in paper bag to prevent another pollen source

-zygotes develop in embryo within seeds

F1 Generation

self-pollinated after, pollen dusted on carpel of same flower

gave rise to F2 generation

Mendel's Observations

-when two true-breeding (purple and white) plants were crossed, only one trait was expressed (purple)

-when F1 self fertilized, offspring displayed about 3 purple: 1 white

Monohybrid Punnett Grid Procedure

1. Determine key for traits

2. write parental genotypes as a cross

3. make punnett grid, perform the cross (record phenotypes in grid)

4. determine genotype/phenotype ratios (simplify when possible

5. answer question

**ratio of true monohybrid cross (Aa x Aa) is ALWAYS 3:1

Chromosome

DNA organized into genes

Gene

unit of inheritance that codes for a trait via synthesis of proteins

Trait

genetically determined characteristics

Alleles

different "versions" of a gene

diploid organisms have 2 alleles for most genes, one from each parent

Genotype

combination of alleles

Homozygous

BB or bb

same alleles

Heterozygous

Bb

different in nucleotide sequences (AKA hybrid)

Hemizygous

b

only 1 allele

-males are hemizygous for all sex-linked traits

Phenotype

expressed traits or characteristics

-includes both structures (hair texture) and functions (color blindness)

**most are determined by genotype AND environment

Genotype Only Traits

eye color, blood type

Environment Only Traits

scars, languages

Both

skin color, height, personality

Complete Dominance

what most alleles display, where a dominant (A) allele masks a recessive (a) allele

homozygous (AA) and heterozygous (Aa) are phenotypically indistinguishable

recessive (aa) is only expressed in absence of dominant

Recessive alleles may... (3)

1. code for a non-functional protein so that dominant protein can be expressed

2. have their gene expression fixed off so that less/no protein is produced

3. produce a functioning protein that is masked by the presence of the dominant protein

**expression determined by action of proteins

Phenotype Plasticity

organism's capacity to alter their traits by varying gene expression in response to environmental triggers

-changes may be reversible since they are not due to genotypic changes

-beneficial in heterozygous environment

ex. darkening of skin via melatonin production

Freshwater Snails

example of phenotypic plasticity

change shell structure upon exposure to predators (bluegill sunfish)

-construct shells through biomineralization, or building of an organic matric on which to deposit calcium

in presence of predator, they may alter shell structure by changing biomineralization pattern

-compact shells provide better protection, more resistant to being crushed and less prone to exposure

Phenylketonuria (PKU)

genetic condition caused by a mutation in a gene on chromosome 12 that codes for phenylalanine hydroxylase

-phenylalanine hydroxylase converts AA phenylalanine into tyrosine

-accumulation of Phenylalanine (bc no enzyme) is converted into phenylketone resulting in a toxic buildup in blood/urine

leads to brain damage and other intellectual/developmental disabilities

infants w/ PKU appear normal at birth bc mother breaks down Phe during pregnancy

-condition is recessive, carrier displays no symptoms

PKU Treatment (3)

-completely avoid high-protein foods like meat, fish, dairy

-eat low-protein foods like fruits, veggies, low-protein grains and milk made for those w/ PKU

-supplement w/ essential AAs

**patients diagnosed early and follow strict diet live a normal life w/ no bad symptoms

Single-Nucleotide Polymorphisms (SNPs)

mutation of a single base that exists in large proportion of the population (>1%)

-bc SNPs create phenotypic variation, most genes have more than just 1 dominant/recessive allele

typically multiple alleles are in the gene pool, although organisms only inherit 2 alleles per gene

**different alleles of same gene are very similar and only differ by a few bases

Multiple Alleles

create complexity in inheritance patterns

some traits display a dominance hierarchy in phenotypes (allele A > allele B > allele C)

others show blended or combined phenotypes

**notation for multiple alleles uses a capital letter to denote a gene and superscript for the allele

ABO Blood Groups

human RBC have antigens (surface glycoprotein)

-coded by 1 gene w/ 3 alleles (A, B, O)

-A and B are codominant w/ modified antigen structures

-O is recessive and unmodified

Codominance

both alleles are expressed equally in phenotype

-heterozygotes show both variations (ex IA IB leads to AB blood type)

Antibodies for ABO

Antibodies are produced to detect foreign antigens

-when blood types do not match, it coagulates, causing death

AB is universal acceptor

O is universal donor

Incomplete Dominance

genotype is the same but phenotype is an intermediate btwn two alleles (blend)

ex. Marvel of Peru

Marvel of Peru

crossing dark pink flowers (C^P C^P) w/ white ones (C^W C^W) produces offspring w/ light pink flowers (C^P C^W)

-dark pink flowers code for pigment, white ones do not, therefore heterozygotes produces LESS pigment

Sex Determination

Sex is determined by sex chromosomes, rest of chromosomes are autosomes

-XX = females, XY = males

-females are heterozygous, males are hemizygous

X Chromosome

contains for more genes due to its size

Y Chromosome

contains the SRY gene that codes for male sex traits

-in absence of Y, female sex organs develop

therefore sperm determines sex of offspring

Sex-Linked Disorder

genes that code for other traits are also located on sex chromosome

-Y is much smaller and has fewer genes (55 vs 900)

-therefore sex linked genes are usually x-linked bc there are very few genes on y chromosome

X-linked Dominant Traits

more common in women bc they have two chromosomes and can be carriers

X-linked Recessive Traits

more common in men bc they are hemizygous and have only 1 X chromosome

-their x-linked traits cannot be heterozygous and masked by a second allele

-why affected mothers always have affect sons, while affected fathers cannot have affected daughters if mother isnt a carrier/affected

Hemophilia

x-linked recessive condition in which body cannot clot blood

-dependent on a cascade that activates a series of coagulation factors

-most common is hemophilia A

Hemophilia notated as X^H = normal, X^h = affected

*more common in men*

Hemophilia A

mutation in factor VIII

-prevents fibrinogen from becoming fibrin (clot)

-can receive injections of factor VIII to help cure

Pedigree Charts

genetic history of a family mapped over several generations

-males are boxes, females are circles

-shaded = affected, unshaded = unaffected

-horizontal line = mating, vertical line = offspring

-individuals numbered left to right, increasing

Autosomal Dominant Pedigree Characteristics (3)

-both parents are affected, but offspring is not (if parents are both heterozygous, Aa)

-all affected offspring have at least 1 affected parent

-if both parents unaffected (aa), all offspring must be unaffected

Autosomal Recessive Pedigree Characteristics (2)

-both parents are unaffected, but offspring is affected (parents are both heterozygous carriers)

-if both parents are affected (aa), all offspring must be affected

*unions btwn close relatives discouraged bc of inbreeding, decrease genetic diversity and increase recessive disease conditions*

X-Linked Dominant Pedigree Characteristics (4)

-if male is affected, so must his mother and all his daughters

-unaffected mother cannot have affected sons

-more common in females (not definitive)

*impossible to prove x-linked inheritance from pedigree, but possible to disprove*

X-Linked Recessive Pedigree Characteristics (3)

-if female is affected, so must her father and all her sons

-unaffected mother can have affected sons (if carrier)

-tends to be more common in males (not definitive)

Inductive Reasoning

specific to general

-certainty is probable, not guaranteed

"sun has risen every morning, therefore it will rise tmr"

Deductive Reasoning

general to specific

-certainty is guaranteed, if premises are valid

"all mammals have lungs, a dog is a mammal, therefore a dog has lungs"

Variation in Phenotypes

either discrete or continuous variation

Discrete Variation

organism displays one of many distinct traits

-monogenic

Monogenic

controlled at single gene locus (ex. blood type)

Continuous Variation

organism displays a trait somewhere along a spectrum of traits

-polygenic

Polygenic

controlled by more than two gene loci (ex weight)

Polygenic Inheritance

controlled by more than 2 gene loci

-increase in number of gene loci of a trait, increase number of phenotypes

-produces a normal distribution

-most frequent is when equal number of alleles are expressed rather than one extreme or another

ex. skin color occurs at 3 loci w/ 6 alleles

Changes in Phenotypic Distribution/Frequency

-environmental pressures create selection patterns that change phenotypic distribution and frequency

-smaller populations are more susceptible to changes in allele frequency, leading to outliers

Box and Whisker Plot

can be used to display spread of population data

-shows min, max, median, Q1, Q3, range, and IQR

-can assess variability/skewness of data

can also determine statistical outliers

-outlier categorized if greater than Q3 or less than Q1 by more than 1.5x IQR

Segregation

alleles separate during gamete formation so that each gamete receives one allele of a gene (anaphase I and anaphase II)

Law of Independent Assortment

segregation of one set of alleles has no impact on another

-homologous chromosomes line up during metaphase I

increases variety of gamete combinations and phenotypes

Dihybrid Punnett Grids

studies genotype/phenotype combos of two unlinked traits

Unlinked Traits

located on different chromosomes

Dihybrid Cross Procedure (5)

1. make a key

2. record parental genotype/phenotype

3. list potential gamete combos via FOIL

4. perform cross

5. record phenotype ratio

Special Dihybrid Crosses (2)

True Dihybrids (AaBb x AaBb): 9:3:3:1

Test Cross: (AaBb x aabb): 1:1:1:1

Autosomal Gene Linkage

genes are located closely on same chromosome

-tend to move and be inherited together

-will not follow dihybrid Punnett grid predictions bc they do not assort independently

may separate during crossing over to become recombinants

Thomas Hunt Morgan

established current understanding of gene linkage

-used fruit fly, Drosophila melanogaster

Wild Type

occurs commonly in wild, "dominant"

denotated as "+"

Mutant Type

mutated from wild type, "recessive"

denoted as two capital letters (ex "BL")

Sex Linkage

crossed red eyed, wild type (AA) males w/ white-eyed, mutant (aa) females

-all female offspring were red eyed, all males were white-eyed

-believed it was due to eye color gene being on sex chromosome

Recombinants

gene combinations not found in parents

-occurs during crossing over btwn non-sister chromatids

-there frequency is typically lower than that of linked traits, due to chiasmata forming at different/random locations during meiosis

frequency of recombinant is determined by distance btwn 2 linked genes

-will be greater if genes are farther apart bc more possible locations where chiasma can form

*there is a difference btwn phenotype recombinants (different phenotype than either parent) and genotype recombinants, when crossing over occurs and you get allele combinations not found in parental chromosomes (but same phenotype in the end)*

Chi-Squared Test

determines whether genes are linked by looking at frequency distribution of offspring phenotypes (F2)

-unlinked genes have equal chances of being inherited together due to independent assortment

-linked genes will be inherited together and display combinations found in the parents unless crossing over occurs

*parental combinations inherited w/ greater frequency than recombinants*

-Chi-Squared test determines whether difference btwn observed (actual) and expected (theoretical, from dihybrid cross) data is statistically significant

Chi Square Null

there is no significant difference btwn observed and expected, therefore alleles are unlinked

Chi Square Alternate

there is a significant difference btwn observed and expected, therefore alleles are linked