Bio 161 - Genetics

Genetics is essentially

the topic of patterns of inheritance

→ how you acquire characteristic traits that define you as an organism

How Mendel patterns of inheritance were developed: 1

• Mendel chose Garden Pea/Pisum Sativum to investigate

Why did Mendel chose Garden peas to observe?

• fast growing

• self pollinating (have both male & female organs)

• allow for great deal of control

How Mendel patterns of inheritance were developed: 2

Mendel cross pollinated 1 plant w/another

→ eliminated potential self pollinated by removing pollen producing organs/male organs of one plant

What does diploid (2n) mean?

having two copies of each chromosome

What does haploid (n) mean?

having one copy of each chromosome

In a diploid organism, how many alleles are present for most genes?

Two alleles, one from each parent

What are homologous chromosomes?

Chromosome pairs that contain the same genes in the same locations but may carry different alleles

What is a gene?

a segment of DNA that codes for/determines specific inheritable character

What is an allele

Different versions of the same gene/different variants of gene that define specific traits from/of a character

How does inheritance work?: Preformation theory

posited that miniature fully formed organisms existed in both sperm or egg/germline cells & that these fully formed small humans ie. “homunculuses” would reside inside soerm & egg & simply enlarge over time….lol

How does inheritance work?: Blending Inheritance Theory

suggested the offspring traits are an average mix of paternal traits → when parents mix, intermediate offspring form

How does inheritance work?: Inherit acquired traits/Lamarcism theory

proposes that habits/changes during development of parent’s lifetime would pass on to offspring

ex). if parent uses weights & gets muscular, children would be muscular lol

→ or giraffe parents would stretch necks a lot for food, offspring have longer necks

What is a cross?

mating between two different organisms

What is true breeding (pure line) organisms?

organisms that always produce the same trait when self-fertilized

How is true breeding achieved?

ensuring that an organism is homozygous for the specific traits you want to reproduce

• performing self fertilizations over & over to then true breed independent characteristics traits

What question was mendel trying to answer?

how traits are inherited from parents to offspring

To investigate inheritance, Mendel..1

chose individual characteristics

→ shape of seed

• ensured all seeds of individual plant would always be of some characteristic shape (true breeding)

To investigate inheritance, Mendel utilized..2

• he utilized male plant that gave rise to only round seeds

• & a female plant that only gave rise to wrinkled seeds

What specifically did Mendel do prior to breeding male parent (that produced round seeds) & female parent (which produced wrinkled seeds)

he self fertilized plants over & over to ensure plants derived from round seeds gave round seeds & those derived from wrinkled seeds gave rise to plants that produce wrinkled seeds

Pure Lines/True breeding

Organisms that always produce the same trait when self-fertilized.

• TT always gives tall for ex

• tt always gives short for ex

Important because:

• Mendel knew the parental genotypes with certainty.

Why were pure-line important in Mendel’s experiments?

They allowed Mendel to know the parental genotypes with certainty.

IF Mendel’s experiment had showcased Blending inheritance…

• True breed parents would be cross pollinated

• F1 generation would all produce slightly wrinkled seeds

• Plants would then grow & be allowed to self fertilize → F2 would all produce slightly wrinkled seeds

If Mendel’s pea experiment had showcased Blending Inheritance….

…..then the F1 generation would be a blend/mixture of the 2 parental traits

What is the problem with blending inheritance as showcased in Mendel’s hypothetical experimental results?

• All seeds produced would be slight wrinkled seeds (mixture/blend of parents)

• problem = everything would become a uniform defining trait

What is a character?

distinguishing inherited feature of an organism

ex) seed shape

What is a trait?

specific state or variant of a said character

ex) round or wrinkled for seed shape

Difference between a character & a trait?

a character is a broad, heritable feature of an organism, while a trait is a specific, observable variation or state of that character

From crossing true breeding male (round) parent & female (wrinkled) parent, Mendel’s ACTUAL results showcased

PARTICULAR INHERITANCE

• not preformation

• not blending

• not acquired traits

Mendel’s Results: What is Particulate Inheritance?

pattern of inheritance that shows phenotypic traits can be passed from generation to generation through discrete particles (genes)

→ these cab keep ability to be expressed while not always appearing in descending generations

What is a genotype?

genetic sequence (namely nucleic acid sequence) on chromosomes that encodes discrete particles ie. genes

• allele combination an organism possesses

What is a phenotype?

observable physical trait

Preformation theory is disproven because

cannot explain how offspring could inherit trace traits from both parents or why children resembled granparents

Blending theory was disproven because

this would ultimately lead to single trait observed in a whole population (which is clearly not the case)

Acquired Traits theory disproven because

of experiment of cutting off tail of parental mice → still led to inheritance of tail in offspring

Mendel’s ACTUAL results: started off by…

crossing true breeding P generation → a plant that produced only round seeds crossed w/plant that produced only wrinkled seeds

Mendel’s ACTUAL results: After crossing P generation, what was produced?

F1 generation (1st Filial) that all produced round seeds

• 100% of the \(F_{1}\) progeny exhibited only one of the parental traits, which Mendel defined as the dominant trait.

Mendel’s ACTUAL results: After the F1 generation grew & was allowed to self fertilize…

F2 (2nd filial gen) produced 5474 round seeds & 1850 wrinkled seeds

→ roughly 3:1 ratio

• wrinkled seed trait from grand parent through parent to grandchild!

What did mendel’s results indicate in regards to the recessive allele?

the recessive allele for wrinkled seeds was still present in F1 even if not expressed (phenotype skipped, but genotype retained)

• The other trait (the recessive trait) disappeared completely in the F1 generation but reappeared in the F2 generation

A dominant trait

is the trait at the higher rate

A recessive trait

is the trait at the lower rate

Mendel’s monohybrid crosses….

..successfully chose series of monohybrid crosses to look at one character

→ 2 traits to that character → look at patterns of inheritance

• always a 3:1 phenotype ratio

A character is equivalent to…

equivalent to a gene

A trait is equivalent to..

equivalent to an allele

Example of character vs trait

• certain gene codes for eye color = character

• specific allele on that gene codes for blue or brown eyes = allele

What is a homozygous genotype?

having two identical alleles

• alleles/traits are the same on homologous pair of chromosomes

What is a heterozygous genotype?

having two different alleles

• 2 different (or 2 or more) alleles for every character/gene

• phenotype depends on dominance

What is homozygous dominant?

Having two dominant alleles (TT).

What is homozygous recessive?

Having two recessive alleles (tt).

What is a monohybrid cross?

A cross tracking one gene/trait.

Monohybrid cross summarized: P generation

• P Generation (Parental): True-breeding parents (e.g., AA x aa)

Monohybrid cross summarized: F1 generation

• Generation (First Filial): Offspring of the P generation, all heterozygous ( Aa).

Monohybrid cross summarized: F2 generation

Generation (Second Filial): Offspring of the

self-pollination ( Aa x Aa).

Monohybrid cross summarized: Expected Ratios of F2 self fertilization

• Genotypic ratios: 1 AA : 2 Aa : 1 aa ( 1: 2 : 1)

• Phenotypic Ratio : 3 Amber : 1 brown (3:1)

What is the P generation

the parental generation

What is the F1 generation

the first filial generation (offspring of P)

What is the F2 generation?

the offspring produced when F1 organisms reproduce

In a TT × tt cross, what are all F1 offspring genotypes?

Tt

Why did all F1 offspring show the dominant phenotype?

The dominant allele masks the recessive allele.

What phenotypic ratio appears in the F2 generation of a monohybrid cross?

3:1

What genotypic ratio appears in the F2 generation of a monohybrid cross?

1:2:1

What evidence disproved blending inheritance?

Recessive traits reappeared in the F2 generation.

Possible explanation 3:1 ratio of specific traits

2 alleles per gene

• Male & female gametes from F1 generation each carried exactly one allele for every inherited trait

• Both the male and female gamete independently contribute one allele to the offspring, restoring the paired condition of two alleles in the new zygote.

• 1 : 2 : 1 genotype ratio led to 3:1 phenotype ratio

A heterozygous parent (eg. Aa) produces gametes that are

50% A and 50% a (applies equally to both sperm & egg)

• each carried exactly one allele for every inherited trait

Does the 3:1 observed ratio mean that if there are 4 offspring, 1 will be wrinkled?

No! not always the case

• these ratios/probabilities are based on large #s

• doesn’t inherently mean ratio will hold up in small #s

• in this case we are looking at only 4 offspring, all 4 could be wrong, all 4 could be wrinkled etc

What is the probability (in this case) that 4 offspring will be all wrinkled?

1/256

• (1/4)⁴

What is the probability that 4 offspring will be smooth?

81/256

• 3/4⁴

Albinism is recessive: how can a female albino reindeer have two brown progeny?

male mate is brown, could be homo or heterozygous

Mendel’s Model summarized

1. 2 alleles/gene: chose phenotypic traits that only had 2 alleles & corresponded to 1 gene in a diploid organism

2. established alleles do not blend

3. showed each gamete carried 1 allele of each gene → principles of segregation

4. males & females contribute equally: isn’t dominance based on sex of crossing pair

5. principle of dominance: in many cases there is a dominant allele over recessive allele

What is the principle of segregation?

The two alleles for a gene separate during gamete formation.

• one allele carried in each gamete

During which stage of meiosis does segregation occur?

Meiosis I when homologous chromosomes separate. (anaphase I)

What is the principle of dominance?

One allele can mask another allele.

What is the principle of independent assortment?

Alleles of different genes assort independently into gametes.

During which stage of meiosis does independent assortment occur?

Metaphase I

How many alleles for each gene does a gamete carry?

one allele

What is the probability of getting TT from Tt × Tt?

1/4

What is the probability of getting a dominant phenotype from Tt × Tt?

3/4

What is a Punnett square used for?

Predicting offspring genotypes and phenotypes.

What gametes can Tt produce?

T and t

What gametes does a TT parent produce?

Only T

What gametes does a tt parent produce?

only t

What is TT × tt offspring outcome?

100% Tt (all heterozygous)

Why does TT × tt produce identical offspring?

Each parent produces only one gamete type → only one combination is possible. (ie. Tt)

Dihybrid crosses: What gametes can TtGg produce?

TG, Tg, tG, tg.

when do you see a 3:1 ratio?

In a monohybrid cross with complete dominance (Tt × Tt).

• phenotype ratio

when do you see a 1 : 2 : 1 ratio?

In a monohybrid cross with complete dominance (Tt × Tt).

• genotypic ratio

Why do dihybrid crosses produce more gametes?

Because of independent assortment of two genes.

What is the phenotype ratio of TtGg × TtGg?

9 : 3 : 3 : 1

When do you see a 9:3:3:1 ratio?

When two genes assort independently and both show complete dominance (dihybrid heterozygous cross).

What does 9:3:3:1 represent?

• 9 = dominant for both traits

• 3 = dominant first, recessive second

• 3 = recessive first, dominant second

• 1 = recessive for both

Why is 3:1 simpler than 9:3:3:1?

3:1 = one gene; 9:3:3:1 = two genes independently assorting.

What biological process causes 3:1 and 9:3:3:1 ratios?

Meiosis (segregation + independent assortment).

Dihybrid: What happens in TTGG × ttgg? special case

100% TtGg offspring

Dihybrid: Why does TTGG × ttgg not produce a ratio?

Each parent produces only one gamete type → only one outcome.

What gametes to TTGG & ttgg produce?

• TTGG → TG

• ttgg → tg

3:1 ratio means what type of cross?

Monohybrid (one gene, heterozygous parents)

9:3:3:1 ratio means what type of cross?

Dihybrid (two genes, heterozygous parents, independent assortment)

If all offspring are identical, what kind of cross is it likely?

One or both parents are homozygous (like TT × tt or TTGG × ttgg)

Diploid (2n) = “full deck”

parents, offspring, zygotes

• organisms

Haploid (n) = “half deck”

gametes only