H Week 15: The Human Genome & Mendelian Genet

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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)

• 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

Lecture Goals:

The Human Genome & Mendelian Genetics

Parts of a Genome Review

MendelianGenetics

Chromosomal Theory of Inheritance

Mendel's Experiments

Relationship between Alleles

Genotype Versus Phenotype

Presentation Links

• 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)

Terms to Remember

Recall: Parts of a Genome

Chromosome from father (sperm)

Chromosome from mother (egg)

• 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

Terms to Remember

Recall: Parts of a Genome

Info about Chromosome 17

Human Chromosome 17

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

Terms to Remember

Recall: Parts of a Genome

Genes versus Traits

Genes

• 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

Genotype

RwRw

RrRw

RrRr

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

Is B a dominant or recessive allele?

Which Trait "Shows"?

Relationship between Alleles

• 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)

Codominant allele

Recessive allele

Dominant allele

Symbols Used for Alleles

Relationship between Alleles

AB

• 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

Symbols Used for Genotypes

Relationship between Alleles

ABAC

Aa

aa

AA

• 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)

Gregor Mendel

Mendel's Experiments

Optional Reading: 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

Peas: The Perfect Test Subject

Mendel's Experiments

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

Peas: The Perfect Test Subject

Mendel's Experiments

• 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

*Note: This audio is a little long (~5 min) since I walk through how to complete a punnett square.

Allele Frequencies:2/4 Ee 2/4 ee

eewhite

Eered

eewhite

Eered

Click to view the completed punnett Square!

Parent 2: Genotype – ee Phenotype – white

Parent 1: Genotype – Ee Phenotype – red

Punnett Squares

Mendelian Genetics

• 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

Parent 2: Genotype – RR Phenotype – yellow

Parent 1: Genotype – rr Phenotype – white

Practice Punnett Square #1

Mendelian Genetics

• You want to cross a pair of roses.
• Specifically, you cross a white rose (homozygous recessive) with a yellow rose (homozygous dominant).
• What phenotype (petal color) would you get for each offspring?

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

rrwhite

Rryellow

Rryellow

RRyellow

Click to view the answers

Parent 2: Genotype – Rr Phenotype – yellow

Parent 1: Genotype – Rr Phenotype – yellow

Practice Punnett Square #2

Mendelian Genetics

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

Allele Frequencies:2/4 Ee 2/4 ee

ee smooth

eesmooth

Eerough

Eerough

Click to view the answers

Parent 2: Genotype – Ee Phenotype – rough

Parent 1: Genotype – ee Phenotype – smooth

Practice Punnett Square #3

Mendelian Genetics

• 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 genotypes of the offspring? Phenotypes (seed texture)?

Allele Frequencies:4/4 Ff

Ffpurple

Ffpurple

Ffpurple

Ffpurple

Click to view the answers

Parent 2: Genotype – ff Phenotype – white

Parent 1: Genotype – FF Phenotype – purple

Practice Punnett Square #4

Mendelian Genetics

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

Allele Frequencies:4/4 FrFy

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

FrFyred & yellow

FrFyred & yellow

FrFyred & yellow

FrFyred & yellow

Fy

Fy

Fr

Fr

Click to view the answers

Parent 2: Genotype – FyFy Phenotype – yellow

Parent 1: Genotype – FrFr Phenotype – red

Practice Punnett Square #5

Mendelian Genetics

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

Re-Examining Mendel's Work

Chromosomal Theory of Inheritance

• 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

Theory Details

Chromosomal Theory of Inheritance

• 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.

PCR: Polymerase Chain Reaction

How We Study Genetics Now

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

OpenStax Biology 2e: Chapter 12

Recommended Textbook Reading