H Week 15 PART 1: The Human Genome & Mendelian Genetics

The Human Genome & Mendelian Genetics

Click the sound button for audio!

Lecture Goals:

• Explain the link between genotype and phenotype
• Understand the role of genes in heritable traits
• Examine the characteristics of the human genome as a case study of broader mammal/animal kingdom genomes
• Understand how Mendelian Genetics works and how it applies to allele frequencies

Course Competencies: 19. Compare and contrast the inheritance patterns of Mendelian and non-Mendelian traits and use standard statistical methods to predict the outcome of monohybrid and dihybrid crosses. (X)

Presentation Links

Genotype Versus Phenotype

Relationship between Alleles

Parts of a Genome Review

Chromosomal Theory of Inheritance

Mendel's Experiments

MendelianGenetics

Recall: Parts of a Genome

Terms to Remember

• Genome
• A cell's complete "set" of DNA
• Karyotype
• An image of a cell's entire set of chromosomes (entire genome)
• Ordered from largest to smallest
• Chromosomes
• Chromatid that contains unique genetic information
• Made from a single chromatid (before S-phase) or two identical sister chromatids (after S-phase)

Recall: Parts of a Genome

Terms to Remember

• Homologous chromosomes
• A pair of chromosomes that have the same genes but originally come from different gametic cells
• When the zygote first forms...
• One chromosome is from the high-cost gamete (egg)
• Another chromosome is from the low-cost gamete (sperm)
• Found in diploid organisms

Chromosome from mother (egg)

Chromosome from father (sperm)

Recall: Parts of a Genome

Human Chromosome 17

Terms to Remember

• Genes
• Regions of a chromosome that provide the "code" for a particular trait
• Locus (loci)
• Location of a gene
• Allele
• Different versions of the same gene that have different expression

Info about Chromosome 17

Genes

Genes versus Traits

• Genotype
• Nucleotide sequence for a specific allele found on the chromosome
• Example: GTCCACCTAACTAA...
• Phenotype
• Observable trait that comes from the gene
• Example: cyclin-dependent kinase

Relationship between Alleles

Which Trait "Shows"?

Is B a dominant or recessive allele?

• Different alleles of a specific gene may be found on each homologous chromosome.
• Allele dominance - alleles have different levels of expression
• Types of allele dominance:
• Dominant allele - version of the gene that will be expressed if present (will cause the trait)
• Recessive allele - version of the gene that will not be expressed if present with a dominant allele (won't see the trait)
• Codominant allele - version of the gene that will be expressed together if present with another codominant allele (will see both traits)

RrRw

RwRw

RrRr

Genotype

Alleles Rr and Rw are codominant, so when present together, both are expressed!

Relationship between Alleles

Symbols Used for Alleles

• We represent alleles as letters when examining genotype and the resulting phenotype.
• A single letter is used to represent a gene.
• Which letters do we use?
• Use letters that are easy to distinguish in uppercase versus lowercase
• How do we differentiate between alleles?
• Dominant allele - uppercase letter
• Recessive allele - lowercase letter
• Codominant allele - uppercase letter with a subscripted letter

Dominant allele

Recessive allele

AB

Codominant allele

Relationship between Alleles

Symbols Used for Genotypes

• When written, an individual's genotype will have two letters (one allele from each homologous chromosome)
• Types of genotypes:
• Homozygous dominant - both alleles are the dominant allele

• Homozygous recessive - both alleles are the recessive allele

• Heterozygous - one allele is dominant while the other is recessive

• Codominant - both alleles are codominant, so they are expressed together (even if different)

AA

aa

Aa

ABAC

Discussion Post #1-2

• Define the following terms:
• Dominant allele
• Recessive allele
• Codominant alleles
• What is the difference between a homozygous dominant, homozygous recessive, and heterozygous genotype?

Submit your answers to the discussion post!

Mendel's Experiments

Gregor Mendel

• Austrian monk
• Studied horticultural plant breeding in the 1860's, focusing on pea plants
• Discovered "inheritance particles" (aka genes) and the math for how they worked
• His work was ignored by the scientific community!
• Didn't align with the "blending theory of inheritance"
• His work wasn't acknowledged until the 1900's

Click the link to read more about Gregor Mendel

Mendel's Experiments

Peas: The Perfect Test Subject

• Scientific name - Pisum sativum
• Self-fertilize
• Relatively quick life cycle and storable seeds
• Easily observable traits such as...
• Color
• Pea texture
• Pod size

Mendel's Experiments

Peas: The Perfect Test Subject

• Hybridization tests
• Breeding two individuals with known genotypes and then documenting the results
• Able to examine ratios of genotypes and phenotypes across multiple generations
• Can be predicted mathmatically
• Punnett square
• There are even scientific laws associated with punnett squares!

Mendel's Laws

• Law of dominance and uniformity
• Some alleles are dominant over other alleles and they will be the ones expressed in the phenotype
• Known exception: codominance
• Law of segregation
• Two alleles for each gene separate from each other during gametogenesis, so the parent passes down only one allele
• Offspring can inherit only one allele from each parent
• Law of independent assortment
• Alleles of different genes segregate independently of one another during meiosis and are distributed independently of one another in the next generation
• Known exception: linked genes

Mendelian Genetics

Parent 1: Genotype – Ee Phenotype – red

Parent 2: Genotype – ee Phenotype – white

Punnett Squares

• A tool to determine the results of a genetic cross
• Genotype - allele combination of a gene
• Homozygous dominant/ recessive - both alleles are the same (EE, ee)
• Heterozygous - both alleles are different (Ee)
• Phenotype - observable feature resulting from the genotype

Eered

eewhite

Allele Frequencies:2/4 = 50% chance Ee 2/4 = 50% chance ee

Eered

eewhite

Click to view the completed punnett Square!

Note: This audio is longer than usual (~5 minutes) since I walk through how to complete a punnett square.

Mendelian Genetics

Parent 1: Genotype – rr Phenotype – white

Parent 2: Genotype – RR Phenotype – yellow

Practice Punnett Square #1

• You want to cross a pair of roses.
• You cross a white rose (homozygous recessive) with a yellow rose (homozygous dominant).
• What are the potential phenotypes (petal color) of the offspring?

Mendelian Genetics

Practice Punnett Square #2

Parent 1: Genotype – Rr Phenotype – yellow

Parent 2: Genotype – Rr Phenotype – yellow

• Now you take the offspring from the previous question and cross them.
• What are the potential genotypes of the offspring? Phenotypes (petal color)?

RRyellow

Rryellow

Allele Frequencies:1/4 RR 2/4 Rr 1/4 rr

Rryellow

rrwhite

Click to view the answers

Mendelian Genetics

Practice Punnett Square #3

Parent 1: Genotype – ee Phenotype – smooth

Parent 2: Genotype – Ee Phenotype – rough

• Now we're going to cross pea plants, just like Mendel!
• You cross a smooth seed (homozygous recessive) with a rough seed (heterozygous).
• What are the potential genotypes of the offspring? Phenotypes (seed texture)?

Eerough

Eerough

Allele Frequencies:2/4 Ee 2/4 ee

eesmooth

ee smooth

Click to view the answers

Mendelian Genetics

Practice Punnett Square #4

Parent 1: Genotype – FF Phenotype – purple

Parent 2: Genotype – ff Phenotype – white

• You cross a purple flower (homozygous dominant) with a white flower (homozygous recessive).
• What are the potential genotypes of the offspring? Phenotypes (petal color)?

Ffpurple

Ffpurple

Allele Frequencies:4/4 Ff

Ffpurple

Ffpurple

Click to view the answers

Mendelian Genetics

Parent 1: Genotype – FrFr Phenotype – red

Parent 2: Genotype – FyFy Phenotype – yellow

Practice Punnett Square #5

• Last practice problem!
• You cross a red flower (homozygous dominant) with a yellow flower (homozygous dominant).
• What are the potential genotypes of the offspring? Phenotypes (petal color)?

Fr

Fr

FrFyred & yellow

FrFyred & yellow

Fy

Allele Frequencies:4/4 FrFy

FrFyred & yellow

FrFyred & yellow

Fy

Both alleles are dominant (codominant). So, when they're both present, you will see both traits together!

Click to view the answers

Discussion Post #3

• You decide to cross a blue flower (heterozygous) with a white flower (homozygous recessive). Blue is the dominant allele for petal color.
• What are the potential genotypes and phenotypes of the offspring?
• Draw a Punnett Square to show your work!

Submit your answers to the discussion post!

Chromosomal Theory of Inheritance

Re-Examining Mendel's Work

• Still follows Mendel's Laws
• Chromosomes (instead of "particles") of inheritance
• Determined by Theodor Boveri in 1902
• Observed that sea urchin zygotes don't develop if you remove the chromosomes

Chromosomal Theory of Inheritance

Theory Details

• During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
• Chromosome sorting from each homologous pair into pre-gametes appears to be random.
• Each parent synthesizes gametes that contain only half their chromosomal complement.
• Even though male and female gametes (sperm and egg) differ in size and morphology, they have the same number of chromosomes, suggesting equal genetic contributions from each parent.
• The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.

How We Study Genetics Now

PCR: Polymerase Chain Reaction

Recommended Textbook Reading

OpenStax Biology 2e: Chapter 12

Not required, but highly recommended (it can be very helpful to see the material again but phrased a little differently!)