Genetics Exam 1 Lect 3

Pangenesis

early genetics theory that explained how body produces seeds that are collected in reproductive organs & transferred to offspring during conception

Blending Hypothesis

early genetics theory that explains how offspring inherits traits as permanent average/mix of parents’ physical characteristics (ex: red + white flower = pink flower)

Mendel’s Garden Pea Experiment

tracked simple, observable traits across garden pea plants & discovered how physical traits are passed down through generations

Why did Mendel use garden peas

• several varieties w/ distinct characteristics

• easy to grow + short life cycle

• traits were true-breeding

  • True-Breeding: describes an organism that passes specific physical traits down to every generation because it is homozygous (ex: RR or rr)

• 4/7 traits were on diff chromosomes

Characters

observable characteristics (ex: eye color)

Trait/Variant

describes the specific properties of a character (ex: blue eyes)

Mendel’s Pea Plant Experimental Set Up

• P Generation: cross 2 true breeding plants differing in some trait

• F1 Generation: hybrids that showed the dominant trait & hid the recessive trait

• F2 Generation: F1 generation self-pollinates & creates 3:1 ratio

Particulate Theory

• supported by Mendel’s experiment

• genetic determinants that govern traits are inherited as discrete units, aka genes)

Law of Segregation

every individual has two alleles for each gene, and these two alleles separate randomly during gamete formation so that each gamete receives only one allele

Alleles

• different version of the same gene

  • Homozygous: identical alleles

  • Heterozygous: different alleles

Genotype

specific allelic composition of an individual (ex: Tt or tt)

Phenotype

observable traits of a person

Punnett Square

estimate the possible combinations of genotypes & phenotypes resulting from a cross

Law of Independent Assortment

• the alleles of two or more genes (for diff traits) are sorted into gametes independent of one another 

  • Ex: not all purple flowers have to also be tall

Phenotypic Ratio for Breeding 2 Traits

9:3:3:1

Meiosis x Segregation Law

• homologous chromosomes segregate from each other in meiosis I

  • leads to segregation of alleles into separate gametes

Meiosis x Independent Assortment

combination of alleles from diff genes depend on how chromosomes line up in metaphase & which side of the cell they are on

Other methods to calculate phenotypes/genotypes when >2 traits

multiplication method & forked-line method

Pedigrees

• Boxes=Male

• Circles=Females 

• Shaded in Boxes/Circles: affected by mutation

• No Shading: unaffected

• Half Shaded: presumed carrier/heterozygous

Autosomal Recessive Inheritance Pattern

• Require BOTH COPIES of recessive allele to manifest 

• Often present in alternating generations 

  • 25% of offspring affected

• Look for affected child w/ unaffected parents 

• Ex: cystic fibrosis, sickle cell disease

Autosomal Dominance Inheritance Pattern

• Requires ONE COPY of dominant allele to manifest 

• Often present in consecutive generations 

• Ex: Huntington Disease & Marfan’s Syndrome

What will a pedigree show if there are 2 affected parents (heterozygous)

unaffected offspring

Probability Equation

Number of Times an Event Occurs / Total Number of Events

What does probability accuracy depend on

• depends on size of sample

  • sampling error worse for smaller sample sizes

Random Sampling Error

the natural difference between a population value and a sample estimate that happens purely by chance

Product Rule for Independent Events

• probability that two or more independent events will occur = to the product of their respective probabilities

• Independent Events: occurence of one event doesn’t affect probability of another

What is the probability that a heterozygous couple’s first three children will have

the recessive trait?

• P(recessive trait): 25%

• (1/4) x (1/4) x (1/4) = 1/64 = 1.6%