ANT 201: Week 2

Genetics + Inheritance 09/05/23

Hypothesis: a prediction

Prediction: specific outcomes you expect to happen based on your hypothesis

* People used to think you got each gene from your parents which would blend together

—--> Until Mendel’s Pea plant experiment

• Hypothesis: There are two particles that you inherit from each of your parents and one of those particles could mask the other
• Test: Cross pure green with pure yellow, then cross offspring with each other- observed trait frequencies of each generation
• Prediction: If the hypothesis is true, the first offspring generation should be all yellow, and the second offspring generation should follow a ratio: 75% yellow, 25% green

Principles of Inheritance

Law of segregation: genes are inherited in pairs, and these pairs are randomly separated in the sex cells so that each parent passes only one allele to offspring

Law of Independent assortment: genes for different traits are inherited independently of one another

Law of Dominance: when there are alternate forms of a gene, a dominant allele will mask the presence of a recessive allele

Pedigrees

Uses: trace inheritance of traits (especially medical conditions)

carrier= heterozygotes for a recessive condition

• Determines mode of inheritance

* look into how to study pedigree lines

—---> Shaded will always have at least one dominant (AA or Aa) whereas not shaded is always recessive (aa)

Beyond Mendelian Inheritance

Heterozygotes with different phenotypes

• Codominance: two alleles are expressed equally in the phenotype
• Incomplete Dominance: heterozygote’s phenotype is intermediate between homozygous phenotypes

Codominance → ABO Blood Types:

Blood type is determined by antigens present in your red blood cells .3 alleles can be inherited + represented by A, B, and O

• A and B alleles are both dominant- O allele is recessive

Incomplete Dominance → Tay-Sachs

Tay -Sachs disease occurs when an enzyme functions improperly

• The alleles that led to improper enzyme development are recessive

Pleiotropy

• One gene impacts multiple phenotypes (effects)

EX:

Marfan Syndrome

Variant in one gene

• Vision issue
• Joint hypermobility
• Heart disease etc.

Polygenic

• Several genes contribute to one phenotype (effect)

EX:

• Height
• Skin color
• Eye color
• Diseases (Alzheimer’s etc.)

2 allele codes : + = tall —=Short

—> 3 phenotypes: Homozygous Heterozygous

– – = short ++= tall – += intermediate

Epistasis

A gene can mask or interfere with the expression of another gene

* (Similar to polygenic except that it is only one gene)

EX:

Albinism: single gene, recessive inheritance

• A variant in one gene blocks the expression of all other genes usually involved in producing skin color phenotypes

Epigenetics:

Changes to the expression of genes based on environmental factors

• DNA itself isn’t altered
• Changes how or whether genes are expressed

EX:

Methylation: a combination of two genes and inhibits their expression

• Long-term exposure to cigarette smoke changes methylation at many genes including ones that affect blood clotting, tumor suppression, inflammation, and more
• Some epigenetic changes are heritable

EX: Dutch famine → led to smaller babies → 2 generations later, birthweights of descendants still significantly lower than non-descendants

(those who didn’t experience famine)

Phenotype Plasticity:

Genotypes can produce different phenotypes depending on environmental input

• Versus canalization, where have more direct relationship between genotype to phenotype
• Height: 80% genetic 20% nutrients (environment)

TYPES:

• Developmental Plasticity: develops phenotype in response to childhood stressors and can’t be changed in adulthood (ex: height)
• Acclimatory Adjustments: (acclimatization): temporary changes to phenotypes in response to stressors (ex: altitude changes)

- Phenotype changes associated with plasticity are not evolutionary (because they’re not inherited) but the ability of certain traits to be plastic is evolutionary

Genotype + Environment= Phenotype

CONTINUED… 09/07/23

• If an allele frequency has changed = evolution has occurred

Modern Synthesis:

Unification of genetic and evolutionary theories

Defined the main forces of evolution

• Natural selection
• Mutation
• Gene flow
• Genetic drift

Population Genetics: Studying how these forces impact populations

• Genetic differences in parental to offspring → microevolution

Mutation:

• Changes in the genetic code (very low rate)
• Only source of new variation
• Can be deleterious, beneficial, or neutral
• From an evolutionary standpoint, only mutations in gametes matter

Point Mutations

A change in a single nitrogen base

• Synonymous v.s non-synonymous substitutions

Synonymous: that change doesn’t change what it is coded for

Non- Synonymous: leads to the coding of a different allele

↳ TWO TYPES: Missense + Nonsense mutations

Frameshift Mutations

Insertions and deletions of bases [INDELS]

Chromosomal Mutations —------> —----> —----->

Crossing over

Nondisjunction: during miosis, sister chromatids are not separated

Ex: down syndrome (1 gamete had an extra chromosome)

Gene Flow

Movement of alleles between populations

• Not just “migration”

- making two populations genetically similar

Genetic Drift

• Random changes in allele frequencies through time

Bottleneck Effect

• Sudden drastic reduction in population size

[tip a bottle over + the few that fall out populate the next generation]

Founder Effect

- a small portion of the parent's population leaves and forms a new, separate population

Natural Selection

Individuals who have traits better adapted to a specific environment are more likely to survive and pass them onto the next generation

Fitness = the reproductive success of an individual relative to other individuals

*Natural Selection is not “Survival of the Fittest”

• Focuses on survival, not reproduction
• Selection can be weak so less fit still survive

*Herbert Spencer [argued that poverty was due NS (eugenics)]

• Acts on individuals \
• \

Heterozygotes Advantage

Heterozygotes has phenotype different from both homozygous states (D + R)

• Having one of each is beneficial

EX: sickle cell anemia and malaria

Hardy-Weinburg Equilibrium

Evolution= change in Allele frequencies

Freq (a) = Freq (aa) + 0.5 Freq (Aa)

0.4 = evolution has not occurred

• Under certain conditions, equilibrium will be maintained from generation to generation

—> It is nearly impossible for all these factors to occur

Hardy-Weinberg Equilibrium

p² + 2pq + q² = 1 (Added to equal 1 b/c 1 is 100%)

• p²= Frequency of the homozygous dominant genotype
• 2pq= Frequency of heterozygpus genotype
• q²= Frequency of homozygous recessive genotype