Biology: Chap 9 Genetics

Number and Size of Gametes

• Males produce numerous, smaller gametes (e.g., sperm).
• Females produce fewer, larger gametes (e.g., eggs).

Sex Determination Systems

• Distinction between sex determination (pathway to becoming male or female) and sex differentiation (steps for development).

XY System

• Most familiar system, found in humans and most mammals.
• Somatic Chromosomes: The first 22 pairs of chromosomes are somatic; everyone has two copies of each.
• 23rd Pair: This pair determines sex (X and Y chromosomes).
• Biological Male:
  • Has one X chromosome (larger, with multiple genes).
  • Has one Y chromosome (smaller, with the SRY gene).
  • SRY gene activates early in embryonic development, triggering a cascade of effects leading to male development.
• Meiosis in Males:
  • X and Y chromosomes act as a homologous pair.
  • 50% of sperm have an X chromosome, 50% have a Y chromosome.
  • The size difference between X and Y sperm is significant enough for separation in livestock industry applications.
• Biological Female:
  • Has two X chromosomes.
  • Lacks a Y chromosome, so the male development cascade doesn't occur; the individual follows the default (female) path.
• Meiosis in Females:
  • All eggs contain one X chromosome.
• Zygote Determination:
  • Sperm with a Y chromosome leads to a biologically male individual.
  • Sperm with an X chromosome leads to a biologically female individual.
• Mutation and Development:
  • Mutations downstream of the SRY gene can lead to a chromosomally male individual developing as female.
  • Example: A mutated testosterone receptor.

Temperature-Dependent Sex Determination

• Found in organisms like alligators; they lack sex chromosomes.
• All chromosomes are somatic.
• Sex is determined by the incubation temperature of the eggs.
• Process:
  • Female lays eggs after mating.
  • The nest is built with vegetation, which ferments and warms the eggs.
  • Warmer temperatures tend to produce more male alligators.
  • Cooler temperatures tend to produce more female alligators.
  • Temperature receptor proteins inside the eggs detect the average temperature.
• Maternal Influence (Hypothesis):
  • The mother alligator may be able to influence the sex distribution of her offspring based on the local population.
  • More sons if the population is mostly female; more daughters if mostly male.

Haplodiploid Sex Determination System

• Found in hymenoptera insects, such as ants, bees, and wasps, which often live in large colonies.
• Example: Honeybees.
• Queen Bee:
  • Female, diploid (two sets of chromosomes).
  • Newly hatched queens kill rival queens.
  • The queen leaves the colony to mate once in her life, storing sperm in a sac for the rest of her life (4-5 years).
• Egg Laying:
  • The queen can choose to inseminate eggs or not.
  • Inseminated eggs develop into diploid females.
  • Un-inseminated eggs develop into haploid males.
• Genetic Implications:
  • Daughters (females) share 50% of their DNA with their mother.
  • Sons (males) also share 50% of their mother's DNA, which has undergone meiosis.
• Grandkids:
  • Grandkids from the queen's daughters have 25% of the queen's DNA.
  • Grandsons produced by the queen directly, having sperm cells that are clones have 50% of queen's DNA.
• Relatedness:
  • Queen is more closely related to her grandsons (through her sons) than to her granddaughters (through her daughters).
• Worker Bees:
  • Sterile females are forced to be sterile by a chemical produced by the queen and put into their food.
  • They perform labor within the colony.
• Male Bees (Drones):
  • 20% of colony in the late season.
  • Their only purpose is reproduction; they cannot even feed themselves.
  • They gather in areas where new queens live.
  • After mating, their reproductive organs are violently removed, and they die.

Classical Genetics (Chapter 9)

Blending Inheritance (Incorrect)

• The outdated idea that offspring are a blend of their parents' traits, with reproductive fluids mixing.
• Example: Offspring is 50% from one parent and 50% from another.
• Not how genetics actually works, though some traits (e.g., height, skin color) may appear to be inherited this way.

Particulate Inheritance (Correct)

• Traits are inherited through discrete bits of information (genes).
• Individuals are collections of independently inherited genes.
• Gregor Mendel described it 150 years ago but was unrecognized until rediscovered 50 years later.
• Mendel's Approach:
  • Trained in mathematics and physics, he used numerical analysis.
  • He counted traits to determine the mode of inheritance.

Specialized Vocabulary

• Important for understanding classical genetics.

Character

• A feature in a population that varies and is heritable to some degree.
• Example: Eye color.

Trait

• The specific difference or variation of a character.
• Example: Blue eyes, brown eyes.

Simple Trait

• Governed by one gene.
• Little to no environmental effect.
• Often binary (present or absent).
• Genetic diseases are easy to understand.

Complex Trait

• Controlled by multiple genes.
• Large environmental impact.
• Example: Diabetes, height.

Mendel's Experiment

Character: Garden Pea Color

• Traits: Yellow or green.

True Breeding Populations

• A group of peas where self-mating consistently produces the same trait (either all yellow or all green).
• P1 Generation (Parental):
  • All yellow peas (true breeding).
  • All green peas (true breeding).

Crossing

• Force a cross between the yellow and green populations.
• F1 Generation (First Filial):
  • 100% yellow peas.
  • Green trait disappears temporarily.
  • F1 plants are called monohybrids (hybrid for only one character).

Monohybrid Cross

• Crossing the F1 plants with each other.
• F2 Generation (Second Filial):
  • Approximately 75% yellow peas.
  • Approximately 25% green peas.

Phenotype

• The trait an organism has (e.g., yellow or green).

Gene

• The genetic factor that controls a character.
• There is a pea color gene, not a yellow pea gene.

Allele

• A variation of a gene.

• Yellow allele or green allele.
  Important: we all have eye colored genes, but different alleles

• True Breeding P1 Organisms:

  • Yellow: Has two copies of the yellow allele.
  • Green: Has two copies of the green allele.

Gamete Production

• Each gamete will have one copy of the color allele.
• F1 Population:
  • Each plant has one yellow allele and one green allele.
  • The plant's phenotype is yellow.

Dominant and Recessive

• Yellow Allele: Dominant.
• Green Allele: Recessive.
• Recessive does not mean weaker, it simply means the dominant allele masks the recessive allele when both are present.

Punnett Square

• A tool to predict the offspring genotypes from a cross.
• Takes one parent's gametes and puts them on top.
• Takes another parent's gametes to the side.
• Then we resolve the results by matching the gametes.
• Next generation's genotype is obtained, helping assess the type of offspring you may have.
  If a small number of offspring or big population, each square represents the percent chance of each genotype occurring
  *This happens with recessive diseases as well, parents (heterozygous) have a 25% chance of the kid getting the disease every time

Genotype

• Homozygous Dominant: Two copies of the dominant allele
• Homozygous Recessive: Two copies of the recessive allele
• Heterozygous: One copy of the dominant allele, one of the recessive

Law of Segregation

• The copies of the genes separate into different gametes.

Increasing Complexity: Two Genes

• Introducing the law of independent assortment.

Two New Characters

Pea color and Flower color

New Traits: Yellow, Green, Purple and White

• The same true breeding populations are made again with the new traits.
  • Yellow peas w/ purple flowers
  • Green peas w/ white flowers

They are crossed, creating an F1 dihybrid, the dihybrid contains a mix of those genes

F2 now crossed producing approximately:

9/16 are Yellow Pea seeds w/ Purple flowers

1/16 are Green Pea seeds w/ white flowers

These are the 2 parental phenotypes

*Recombinants phenotype result in *
3/16 are Yellow seeds w/ white flowers
*
3/16 are Green seeds and purple flowers

These traits can be modeled w/ a capital letter designated as the dominant gene.

Lowercase letter represents recessive gene.

True breeding populations

YYPP shows yellow with purple recessive
yypp shows green and white recessive.

The gametes look like YP and yp after they make a population resulting in the heterozygous for both traits as YyPp.

Independent Assortment genes segregate indepently of each other in gametes.

The 4 different gametes can occur are: YP, Yp, yP, and yp.

The punnet square is 4 by 4 with one parents gametes on the top, one on the side and matching them inside the square.

Independent Assortment Genes

• Genes segregate in the gametes independently of each other.
• Every gamete must get one copy of every gene.

Chromosome

• If independent assortment is occurring, genes are on different chromosome pairs.
• This means 4 chromosomes are needed.
• One chromosome with the black, one blue
• One with the green and the red

Genes

• Each chromosome contains those genes. But those cells has to make the equivalent of gametes.