2.3: hereditary

what were the early ideas on heredity

• people knew sperm + egg carry info and pass traits to offspring

• assumed the “Blending Theory”: that traits were a MIX of our parents - like paint, and that their traits would disappear in offspring (no more tall/short , just medium)

• this is not the case, because real genetics are like marbles in a jar: traits are passed as separate genes from each parent (NOT MIXED).

• even a hidden gene could show up again in later generations and that is why we have variation

who was gregor mendel

1854: he studied plants , focused on peaplants as his main model experiment\

• received little attention until rediscovered in 1900s

1. methodical (step by step), used variables, repeated trials 

2. statistics/probability: counted thousands of plants, 1st to apply math to bio inheritance

3. open minded: didnt assume old theories 

what is mendel’s contribution on the laws of inheritance

• didnt know chromosomes were a thing yet

• proved blending theory wrong: traits passed as separate units (genes)

• some traits are dominant others are recessive'

• basis of modern genetics (chromosomes)

didnt know it at the time but proved Segregation: During the formation of gametes, the two alleles for a trait segregate (separate), ensuring that each gamete carries only one allele for each gene.

• Independent assortment: Pairs of alleles for different traits segregate independently of one another, a principle that explains how traits can be combined in new ways

why were pea plants a good model system?

• self breed —→ true breeding lines

• can be cross pollinated

• make many generations fast

• easy to grow in large #’s

what is true breeding?

group of organisms that are genetically uniform and will produce offspring with identical traits

• individuals are homozygous  

what type of variation did Mendel use initially? what type of graph does it use?

discontinued variation

• traits that come in clear separate categories, no in between

• influenced by one gene

• flower color: violet OR white (not light purple)

• pea shape: round OR unwrinkled (not semi-wrinkled)

• bar graph

examples of discontinued variation

blood type, lobed/lobeless ears, eye color, handedness (left OR right)

what did mendels use of discontinued variation show 

showed how traits do not blend, rather they kept their distinctiveness, and could be passed on because of how traits could reappear

what is continued variation? what type of graph does it use?

traits expressed along a SPECTRUM without distinct categories, due to combined influence of multiple genes and environmental factors

• bell curve middle is average, left and right is extreme, rare cases 

examples of continued variation

height, weight, skin color, arm span, intelligence

(T/F): Mendel began with true breeding plants and then cross bred plants with opposite traits 

T

P0/Parental Generation + allleles,  and what were the offspring 

the 2 OG true breeding plants that crossed (violet + white flower)

• one parent - violet - is AA (true breeding dom (A only passed), other - white is aa (a only passed) (true breeding recessive)

• offspring: AA x aa = 100% Aa (imagine punnet square)

F1 generation + alleles , and their offspring 

• offspring of p0

• found that ALL showed only the dominant trait (violet)

• they were hybrid : the offspring of two different purebred parent organisms (heterozygous)

offspring: Aa x Aa = AA, Aa, Aa, aa 1:2:1 genotype

3:1 (3 dom 75%) - 1 recessive (25%)

how did F1 prove blending theory wrong

• blending theory would have thought they mixed and formed (light purple), but white disappeared 

F2 + alleles. what did F2 show?

• offspring of F1

•  AA, Aa, Aa, aa

• recessive trait (white) that disappeared in f1, reappeared in F2 in a 3:1 ratio, proving that traits are not mixed/lost, but REMAINED present in f1

• found that traits could be dominant or recessive 

dominant vrs recessive

A dominant gene is a variant of a gene that expresses its trait even if only one copy is present in an individual's genotype.

A recessive gene is a variant that is only expressed if two copies of the allele are inherited, one from each parent, because the dominant gene's effect is masked

how many alleles do we get from each parent? how many copies in total for each trait?

1 each parent/ 2 total

• 2 alleles in a parent separate into different gametes, so each gamete gets one allele - meiosis 

• AA —> A only gets passed

• aa —> a only gets passed

• Aa —> 50% A 50% a (some eggs have A, others have a)

alleles of same genes sit on ….

the same locus of homos

• the specific, fixed physical location of a gene or other genetic marker on a chromosome

mendel worked with genes that had how many alleles

2

(T/F): many species (humans and dogs), a single gene can have 2+ alleles

T

• example blood type (3 alleles)

• A, B, O

genotype

2 alleles offspring have for each gene

phenotype

expressed trait (what u see)

(t/f): you can always know genotype from phenotype

F

monohybrid cross

a genetic cross that tracks one single trait + the parents in cross are hybrids (hetero Aa x Aa) for that trait

how many possible outcomes does a monohybrid produce

genotype: 1:2:1 (1 homo dom, 1 homo recessive, 2 heterozygous) = 3 total

phenotype : dominant phenotype and the recessive phenotype. = 2 total (3:1 3 dom offspring and 1 rec offspring, where both heteros show domm trait)

• A or a from mom

• A or a from dad

• AA, Aa, Aa, aa = 3:1

what do monohybrid crosses prove?

law of segregation: alleles separate into gametes and recombine to produce (3:1 dom - rec) ratio in F2

if parents are Aa , gametes have an equal chance of getting each allele (t/f)

t

punnet squares predict …

possible genotypes of offspring and probability of each one

what does test cross do

• between an unknown dom (AA or Aa?) and a recessive (aa) to determine unknown genotype

• if all kids dom —> parent was AA

• if any recessive child appears —> parent was Aa

the average for F2 was 3:1, but not exactly, why?

1. natural variation: gametes (A or a) meeting each other is random

2. sample size matters: chance effects have stronger effect when fewer offspring, larger samples average out

3. tiny experimental errors

exceptions to how alleles interact

1. incomplete dominance

2. codominance

3. multiple alleles 

incomplete dominance

neither allele is fully dominant, heterozygote looks like an in between of both parents

snapdragons: CRCR (red) x CWCW (white) = CRCR, 2 CRCW (pink), CWCW

genotype: 1 red: 2 pink: 1 white

• alleles remain separate, but red+ white appears

complete dominance

Aa, dom: fully hides recessive in heterozygote

codominance (also what is the universal blood type and why)

both alleles are fully expressed in the heterozygote, not mixed/hidden, both traits visible at same time

• ex: MN blood group

• LMLM - m only    ,,, LNLN - n only

• heterozygote = LMLN = both M and N antigens appear 

• An antigen (A or B) is any substance that can provoke a response from the immune system, causing it to produce antibodies

• type O is universal blood type because it has no A or B antigens so immune system does not attack

multiple alleles

more than 2 allele versions exist for the same gene in a population

• mendel worked with genes that had only two, buy many genes have more than 2 allele options

• ex: blood type IA, IB, IO combinations: many

dihybrid cross

• a cross between two heterozygous individuals for TWO trait

• one allele from each gene go in one gamete - 4 types of gametes 

• practice this one : 16 squares look at notes 9 : 3: 3: 1

law of independent assortment states

1. alleles for diff genes separate randomly/independently 

2. every possible combination of alleles for every gene is likely to occur (metaphase I)

what are the 3 ways for phenotypes

1. 1 gene = 1 phenotype (ex: phease shape R - round, r - wrinkled)

2. 1 gene = multiple phenotypes (PLEIOTROPY) ex: single gene mutation in hemoglobin in red blood cells, sickle cell amenia, marfan syndrome

3. 2+ genes interacting = many phenotypes (POLYGENIC) ex: height, skin color, body weight…..each gene adds a small effect; creating a wide range of possible outcomes bell curve

epistasis

when 1 gene masks another gene (albinism) aa- mask color 

environmental variance

portion of phenotypic variation among individuals that is due to non genetic, environmental factors 

• nutrition, lifestyle, bringing

• ex: plants growing in rich soil rhan short plants in bad soil

traits are influenced by both genes and environment (multifactorial traits) (t/f)

T: height (genes + nutrition), skin color ( genes + sun exposure)