Genetics Study Guide

Genetics Overview

• Optional genetics review session on Friday at 9 AM.
• Additional practice questions are available.

Important Dates

• The last day to drop courses is approaching (check email or student success advisor for the exact date).
• Students with failing grades have been contacted.
  • 77 is considered a passing grade, while 76 is not.

Nucleotides and DNA

• A nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
• DNA base pairs:
  • Adenine (A) pairs with Thymine (T) - "Apples in Trees".
  • Cytosine (C) pairs with Guanine (G) - "Cars in Garages".
• Nucleotides form DNA, which has a double helix structure.

Chromatin vs. Chromosomes

• Histones are proteins that provide structural support for chromosomes.
  • DNA wraps around histones for storage and protection.
  • This wrapping compacts the DNA to fit within the nucleus.
• Nucleosome: DNA wrapped around a histone, resembling "beads on a string".
• Chromatin: Condensed nucleosomes tightly wrapped together.
• Chromosome: Highly condensed chromatin, analogous to "luggage".

Genes and Genomes

• Genome: A complete set of genes in a cell or organism (like a complete set of cookbooks).
• Chromosome: One "cookbook" in the series.
• Locus: The location of a gene on a chromosome (l o c = location), e.g., the section on bread.
• Gene: A recipe (e.g., sourdough).
• Allele: A specific version of the recipe (e.g., New York Times bestseller sourdough recipe).
• Allele is a specific version of a gene that determines how the gene will be expressed.

Transcription

• Transcription: DNA is transcribed into messenger RNA (mRNA) in the nucleus.
• mRNA copies DNA using an enzyme.
• Uracil (U) replaces Thymine (T) in RNA.
  • T, G, C = DNA
  • U, G, C = RNA
• DNA never leaves the nucleus; it writes the message.
• mRNA leaves the nucleus and goes to the ribosome.

Translation

• Translation: RNA (nucleic acid) is translated into amino acids.
• Types of RNA:
  • mRNA (messenger): the script.
  • rRNA (ribosomal): the factory for translation, creates new proteins.
  • tRNA (transfer): transfers one amino acid.
• Process:
  • Occurs on the ribosome.
  • Three nucleotides = one amino acid.
• AUG: the start codon.
• tRNA carries an amino acid, which is left behind.
• A string of amino acids forms a protein.
• Amino acids attract or repel each other, determining protein folding and shape.
• The human genome project is a project to define all constituent genes in the human genome.

Cell Division: Mitosis

• Mitosis: "My Two Sisters" - produces two identical daughter cells.
• A parent cell replicates its DNA and divides into two identical daughter cells.
• Diploid (46 chromosomes or 23 pairs) to diploid.
• Replaces worn-out cells.
• No genetic information is lost.

Cell Division: Meiosis

• Meiosis: Cell division for sperm and egg cells, resulting in half the genetic information (haploid copy).
• Goal: Pass genetic information from one generation to the next.
• Uses "sperm donor" and "egg donor" terminology.
• "Me is We" - requires two people.
• Produces four daughter cells with half the number of chromosomes (haploid - 23).
• 23 egg + 23 sperm = diploid.

Genetic Concepts

• Single Gene Trait vs. Polygenic Gene Trait:

  • Single Gene Trait: One gene with two possible phenotypes (physical appearances).
    • Example: Widow's peak (either present or absent).
  • Polygenic Gene Trait: Multiple genes involved, multifactorial environment.
    • Example: Height, mental illness.
    • Multifactorial: genetic component + environmental factors.
    • Examples: cleft palate, clubfoot, cardiovascular health, congenital hip dislocation, congenital heart disease, urinary tract malformation, diabetes, hypertension, coronary artery disease, cancers, mental illness.

Mutations

• A change that can be passed to future generations.
• Causes: Extra or missing nucleotides.
• Results: No change or a new variation.
• Examples: Cancer, BRCA1, BRCA2.

Chromosomes, Locus, and Allele

• Homologous Chromosomes: Same (homo) - one from egg donor, one from sperm donor.
• Locus: Location (LOC) where the gene is found on the chromosome (like a parking spot).
• Allele: Specific variation of a gene.
  • Alleles play a huge role when looking at inheritance patterns.
• Capital letter = dominant, lowercase = recessive.
• Homozygous: Two identical genes (BB or bb).
• Heterozygous: Mixed, not identical (Bb).
• Genotype: Allele based on the code (genetic code).
• Phenotype (pH): Physical appearance.
  • Example: Brown eyes (B) is dominant, blue eyes (b) is recessive.
  • BB = brown eyes
  • Bb = brown eyes
  • bb = blue eyes
  • With the client being homozygous recessive, the phenotype would be blue in this example..

Inheritance Patterns: Autosomal Dominant

• No carriers.

• Requires only one mutated gene to inherit the condition.

• One dominant allele is needed to express the condition (BB or Bb).

• Examples:

  • Huntington's disease
  • Marfan syndrome

• Example:

  • If an egg donor is dominant, and a sperm donor is not, then work through the Punnett square.
  • \begin{array}{|c|c|c|} \hline & a & a \ \hline A & Aa & Aa \ \hline a & aa & aa \ \hline \end{array}
  • Probability of having Marfan syndrome: 50% (two out of four).
  • Probability of being unaffected: 50%.

• It'll tell you in the problems your what the sperm and egg is and then you have to figure out if it's big a a. So that was the example for Marfan.

Inheritance Patterns: Autosomal Recessive

• Need two copies of the recessive gene (two homozygous) to have the condition (bb).
• A heterozygous (Bb) is a carrier (does not have the condition but can pass it on).
• Autosomal dominant has no carriers, but autosomal recessive does.
• Mnemonic: "Can't Pass Sick Traits Properly" (Cystic fibrosis, PKU, Sickle cell, Tay-Sachs).
• If you are carrier for cystic fibrosis and you marry someone else, who also is a carrier, then your offspring could have it.
• If it's autosomal dominant it'll always tell you like a, and then auto recessive is little a
• To have it, it is either AA or big A, little a is presents
• There's no carriers with autosomal dominant.
• Now we're talking about autosomal recessive, So now there's another layer to this.

Autosomal Recessive Traits

• If they tell you that egg donor and the sperm donor are both hetero, then it's big a little a, big a little a, and you do your square, so then you have no carriers.
• If its recessive, so if you have a little one, you're affected. This person has the disease.
• Knowing the difference of possibility vs. they have it.
• Affected - is it possible that this child has it? - is their little a?
• Having the condition - 25%.(the questions will ask this).
• Cystic Fibrosis:
  • Defective gene causes secretions to become thick and sticky, causing plugs.
  • Respiratory: Mucus plug.
  • GI: Blocks pancreas (digestive enzymes), malabsorption.
  • Reproductive: Infertility due to lack of sperm/semen and clogged mucus in women.
• Sickle Cell Anemia:
  • Red blood cells are crescent-shaped, rigid, and sticky, slowing/blocking blood flow.
• Tay-Sachs:
  • Absence of an enzyme that breaks down fatty substances, leading to toxic buildup in the brain and spinal cord, which build up toxic levels in the brain and spinal cord.
• PKU:
  • Change in the gene, causes a dangerous buildup from protein or aspartame artificial sweeteners.

Inheritance Patterns: X-Linked Recessive

• Females can be carriers of X-linked disorders.
• There is another rule, that is a person born with XX chromosomes follows the typical recessive pattern. In other words one need two copies to be affected.
  • With only one copy, you area carrier for the condition.
  • When an XY chromosome/male only needs one one affected X to be affected.
• Examples:
  • Hemophilia A and B
  • Red-green color blindness
• Examples: what is the offspring of XX, or XY to be
• Even though it's a little x y, to differentiate it when you're taking your test, I recommend that you put an XH as in has the disease so you can keep it straight.
• If we have X and then X with a with a H, we're always gonna wanna choose the healthy x.
• X linked recessive is an inheritance pattern, just like recessive where you need to have the still lower case, but there's an extra rule the rule is now that we now have to look whether the sex is XY, which is male, or XX female.
• Even though there are two x's, your body is going to choose the healthy x.
• Remember, these questions, they're not gonna tell you, like, you have an ex linked recessive to figure it out it's literally just gonna say, like, you have a sperm donor who has hemophilia, and you have to know that that's an ex linked recessive. H is for has

Inheritance Patterns : X-Linked Recessive Steps on what to do

• You need to be able to interpret what they tell you: remember with hemophilia there are never any carriers.
• Remember, if we have ex and then ex with a with a H, we're always gonna wanna choose the healthy ex.
• Levels are very intense.

X-Linked Recessive Traits

• Hemophilia: A coagulation factor deficiency resulting in lifelong bleeding disorders.
• Red-Green Color Blindness: Decreased ability to see colors.
• Duchenne Muscular Dystrophy: Condition that weakens skeletal and heart muscle and gets worse over time.

General Steps for Inheritance Pattern Questions

1. Identify the keywords.
2. Define the definitions as they are very important.
3. If it's autosomal or X-linked.
4. Do the square (Punnets square).
5. Select the answer from the question.

Additional Information

• Polycystic kidney disease is autosomal dominant (for the quiz).

Defining Karyotype

• A karyotype is an image of an individual's complete set of chromosomes.
• Chromosome #23 is where we see XX female or XY male.

Defining Error: Non-Disjunction Error

• Non-Disjunction Error: Error in meiosis, extra or missing genetic material.
• Trisomy 21: There are three of one chromosome (where there should be 2).
• Monosomy: You only have one of any chromosome.

Errors: Trisomy 21 - Down Syndrome

• Down Syndrome is an example of Trisomy 21. *This is the only one where they're affected cognitively.
• Physical growth delays, developmental delays.
• Facial features, flattened face and nose.
• Increases the risk of diabetes, heart defects, and leukemias.

Errors: Klinefelter's Disease

• Klinefelter's Disease: Trisomy XXY (always referred to as male).
• Example of a Non Disjunction Error and Trisomy.
• Relatively tall, above average poor motor coordination.
• Smaller testicles, penis, infertility, absent or delayed puberty.
• Feminization, less body/facial hair, broader hips, gynecomastia.

Errors: Turners Syndrome

• Turner's Syndrome: Monosomy XO (always female).
• Missing either X's or Y.