Topic 2 - Biol 213

The Core Evolutionary Cycle

Phenotypic variation drives selection, which leads to differential survival or reproduction, but this requires heritable transmission (heritability)

Darwin's Blind Spot

heritable transmission the exact mechanical laws of which (DNA and genes) were completely unknown to Charles Darwin.

Blending Inheritance (1800s Explanation)

The hypothesis that offspring inherit traits that are intermediate or a literal mix of their parents' phenotypes (e.g. a black rabbit and a white rabbit producing grey offspring)

Blending Inheritance hypothesis Problem

It incorrectly predicts that phenotypic variation will reduce or disappear over time, an outcome that is completely contradicted by natural populations

Inheritance of Acquired Characteristics (Lamarckian Inheritance)

The theory that favorable traits an organism acquires during its own lifetime can be physically passed down to its offspring

Inheritance of Acquired Characteristics hypothesis problem

Many direct natural observations explicitly go against it, and very simple traits continue to persist unchanged in ancient, long-standing lineages

Gregor Mendel's Genetic Contribution

He provided the definitive quantitative evidence for a particulate inheritance mechanism, he did not physically discover or isolate the DNA molecule itself

Mendel cross pollinated

contrasting true breeding varieties (hybridization)

Why the garden pea ideal model organism

It possesses a handful of easily categorizable characters, pollination is incredibly easy to manually control, it has a short generation time, and it produces a massive number of offspring

P Generation

The parental generation consisting of true-breeding individuals used at the start of a genetic cross experiment

F1 Generation

The first filial generation, consisting of the hybrid offspring produced directly from the cross of the P generation parents

F2 Generation

The second filial generation, produced when F1 hybrid individuals either self-pollinate or cross-pollinate with other F1 individuals

Mendel's cross-pollination technique

He manually cut off and removed the immature, pollen-producing stamens from a flower to prevent self-pollination, then used a brush to transfer mature pollen from a contrasting variety to its egg-bearing carpel

Phenotype of Mendel's F1 cross (True-breeding Purple x White)

All resulting hybrid plants in the F1 generation displayed 100% purple flowers, the white trait completely disappeared

Phenotypic ratio of Mendel's F2 flower cross

A distinct 3:1 ratio, specifically yielding 705 purple-flowered plants to 224 white-flowered plants (roughly a 3.15:1 ratio

Mendel's conclusion about the "disappearing" white factor

The heritable factor was neither diluted, blended, nor destroyed, it was simply masked as a recessive trait in the F1 generation and emerged completely unchanged in the F2 generation

Modern term for Mendel's "heritable factor"

A gene

flower colour (character)

purple dominant, white recessive

seed colour

yellow dominant, green recessive

seed shape

round dominant, wrinkled recessive

pod shape

Inflated dominant constricted recessive

pod colour

green dominant yellow recessive

flower position

axial dominant terminal recessive

stem length

tall dominant dwarf recessive

Recessive Alleles

represent a structural mutation that results in the complete absence or non-function of that protein

Dominant Alleles

code for fully functional, active enzymes or proteins

Myth: Dominant alleles are always healthier, fitter, or more common

False. The evolutionary fitness or abundance of an allele has absolutely nothing to do with its structural dominance, recessive alleles are often the most common and abundant in a natural population

Allele

Alternative structural versions of a single gene that reside at a highly specific location (locus) on a chromosome

Mendel's Law of Segregation

two alleles present at a single locus separate randomly from each other during the formation of gametes

Cellular mechanism behind the Law of Segregation

separation of homologous chromosome pairs moving to opposite poles during Anaphase I of meiosis

Homozygous

organism that has two identical copies of an allele for a specific gene locus. is true-breeding and produces only one type of gamete

Heterozygous

An organism that has two completely different alleles for a specific gene locus, it is not true-breeding and produces two distinct types of gametes

Probability Basics

Sum rule and product rule

Phenotype

An organism's physically expressed, observable structural or physiological traits

Genotype

An organisms exact genetic or allelic makeup

Monohybrid Crosses

distinguish between phenotype and genotype

Monohybrid Cross Ratios (Pp x Pp)

Phenotypic Ratio is 3:1 (Dominant:Recessive); Genotypic Ratio is 1:2:1 (1 PP : 2 Pp : 1 pp)

Sum Rule

outcome A OR outcome B occurring, you add individual probabilities together

Product Rule

independent outcomes A AND B occurring simultaneously, you multiply their individual probabilities together

Chromosomal Theory of Inheritance (Walter Sutton, 1903)

theory that: 1. Chromosomes occur in matching pairs in diploid organisms; 2. Homologous chromosome pairs separate into gametes during meiosis; 3. The separation of each pair is entirely independent of other pairs

Mendel's Key Finding on Gene Copies in Adults

Adult plants carry exactly two copies of heritable factors (genes) that govern the physical inheritance of a character, such as purple versus white flowers

Mendel's Key Finding on Allele Interaction in Heterozygotes

If an individual's pair of genes consists of two different alleles, one allele acts as dominant over the other, completely masking the recessive allele

Mendel's Key Finding on Gamete Formation (Law of Segregation)

The pair of alleles controlling a character separate via meiosis during gamete formation, half of the haploid gametes carry one allele and the other half carry the remaining allele, ensuring diploid organisms receive one allele from each parent

Test Cross

A mapping method used to determine the unknown genotype of an organism displaying a dominant phenotype by breeding it directly with a known homozygous recessive individual (pp)

Interpreting a Test Cross

If any offspring display the recessive phenotype, the mystery parent must be heterozygous (1:1 dominant to recessive ratio). If 100% of the offspring display the dominant phenotype, the mystery parent is homozygous dominant

Genotypes of an F2 cross from Yellow (Y/Y) x Green (y/y) Parents

A quantitative mixture of all three possible genotypes in a clear 1:2:1 ratio (1 Y/Y : 2 Y/y : 1 y/y)

Phenotypic outcome of a test cross with a Heterozygous Tall plant (T/t) x Dwarf plant (t/t)

A balanced ratio of 1 Tall plant to 1 Dwarf plant

Combining Probability Rules (Problem: Mating Y/y x Y/y, chance of exactly 1 green seed out of 4)

1. Product Rule: The chance of 1 green seed (1/4) AND 3 yellow seeds (3/4 x 3/4 x 3/4) in a specific order is 1/4 x 27/64 = 27/256. 2. Sum Rule: Because the green seed can be in 4 different positions (1st, 2nd, 3rd, or 4th seed), add the probabilities of these 4 mutually exclusive arrangements together: 27/256 + 27/256 + 27/256 + 27/256 = 108/256 approximately 42.19%

Degrees of Dominance

complete, incomplete and codominance

Complete Dominance

dominant allele entirely masks the presence of the recessive allele, making the heterozygous phenotype identical to the homozygous dominant phenotype

Incomplete Dominance

the heterozygous phenotype emerges as a blend resting directly between the two distinct parental phenotypes (e.g., pink flowers from red and white parents)

Codominance

The genetic condition where two different dominant alleles are expressed simultaneously, affecting the phenotype in completely separate, distinct, and fully distinguishable ways

Multiple Alleles

When a single gene locus features three or more potential alleles within a population, such as the human ABO blood group system which features IA, IB, and i alleles

Polygenic Inheritance

When a single physical character is controlled by the additive effects of multiple different genes, leads to more phenotype complexity

Pleiotropy

one single gene has influence over multiple, seemingly completely unrelated phenotypic traits

How Sickle Cell Disease demonstrates Pleiotropy

A single point mutation in the structural HBB gene produces abnormal hemoglobin, but this single defect triggers a massive cascade of widespread symptoms including chronic anemia, internal organ damage, extreme fatigue, physical pain, and heart failure