Genetics: Inheritance, Genes, and Alleles

Inheritance

• Offspring inherit chromosomes from their parents.
• Genes on the chromosomes passed from parents to offspring determine traits.
• Genes come in different versions called alleles for each trait.
• Alleles are the same or contain different information.
• Parents with brown eyes and hair can have children with blue eyes and light hair if they carry recessive genes they don't express, but can pass on.
• Organisms inherit two copies of each gene for each trait, one from each parent.
• Homologous chromosomes have the same kinds of genes, but alleles may differ.
• One set of chromosomes comes from each parent.

Genes and Alleles

• A gene is a DNA segment coding for a protein which determines a trait.
• An allele is a particular form of a gene.
• A dominant allele is always expressed when present, symbolized by an upper-case letter.
• A recessive allele is only expressed when the dominant allele is absent, symbolized by a lower-case letter.
• Genotype is the genetic makeup of an individual, like TT, Tt, or tt.
• Phenotype is the physical appearance of an individual, like curly hair or brown eyes.
• Homozygous individuals have two identical alleles for a trait.
• Homozygous dominant individuals have two dominant alleles (TT).
• Homozygous recessive individuals have two recessive alleles (tt).
• Heterozygous individuals have two different alleles (Tt).

Phenotype and Genotype

• Genotype determines phenotype.
• The dominant trait shows up in the phenotype if any dominant alleles are present in the genotype.
• TT results in the expression of the dominant trait.
• Tt also results in the expression of the dominant trait.
• tt results in the expression of the recessive trait.
• An organism's appearance can differ from its genetics.
• Phenotype refers to appearance or trait, like blue skin.
• Genotype refers to genetic makeup, like BB, Bb, bb.
• Organisms with different genotypes (BB vs. Bb) can have the same phenotype (appearance).

Punnett Squares

• Punnett squares are tools to show possible offspring genotypes.
• Alleles from each parent are written on top and the side of the square, with the dominant allele usually written first.
• The possible alleles are written for each possible match.
• Probability is calculated, and it can be written as a ratio.
• Genotype probability calculation:
  • Homozygous dominant %
  • Heterozygous %
  • Recessive dominant %
• Phenotype probability calculation:
  • Dominant trait expressed %
  • Recessive trait expressed %
• Probability is the number of times an event is expected to occur divided by the number of opportunities for the event to happen.
• Probability is expressed as a fraction, decimal, or percentage, representing the likelihood of a specific event.
• Punnett Squares only provide the most likely results or probability of a genetic cross.

Gregor Mendel

• Mendel crossed a homozygous dominant purple plant with a homozygous dominant white plant.
• The F1 generation was 100% purple plants, which were carriers for the recessive white gene but did not express it.
• The F2 generation had 75% purple (dominant) and 25% white (recessive), a 3:1 ratio.

Mendel's Laws

• 1st law of dominance: When Mendel crossed two P plants, one of the traits disappeared in the F1 plants; The missing trait reappeared in the F2 plants in a 3:1 ratio.
  • One trait is dominant because it masked or dominated the other trait.
  • One trait is recessive because it hides behind the dominant one and can only be seen when the plant has two recessive copies of the trait.
• Some traits mask others.
  • Purple and white flower colors are separate traits that do not blend.
  • Purple crossed wth white does not make light purple; Purple overpowers white
• Dominant allele:
  • Functional protein
  • Affects characteristics
  • Masks other alleles
• Recessive allele:
  • No noticeable effect
  • Allele makes a non-functioning protein
• 2nd law of segregation:
  • Alleles segregate or separate during the formation of gametes (meiosis homologous pairs separate.)
  • Parents only pass on one allele from each of their gene pairs to their offspring and it is a 50% chance of which one.
• 3rd Law of independent assortment:
  • Mendel crossed plants that differed in two characteristics (flower color/seed color), Data showed that the traits do not always appear together (purple flowers don't always appear with green seed colors); They are independent
  • Alleles for different characteristics are distributed to gametes independently or randomly.
  • Genes for separate traits are passed on independently of one another, so traits do not have to be inherited together.
• Dominant phenotypes can have either BB or Bb genotypes.
  • Performing a testcross will determine the unknown genotype.
• A testcross is performed with a known recessive genotype.
  • The unknown must be heterozygous if any recessive offspring appear.
  • The unknown must be homozygous if no recessive offspring appear.

Di-hybrid Crosses

• More than one trait can be passed on at the same time.
• The dihybrid punnett square can determine the outcome of two traits at once.(blonde hair and brown eyes)
  • Step 1: Determine the possible gametes of the parents.
    • SsBb will pass one s and one b to offspring, but there are different combinations.
  • Brown Cat with short tail: SsBb
  • S-short tail, s - long tail, B- brown fur, b - white fur
  • Determine how many combinations of these traits are possible with di-hybrid crosses.
  • Step 2: Make a 4x4 punnett square and put gametes for one parent (mom) on top and gametes for the other parent (dad) on the side (because there are now 4 alleles).
  • Step 3: fill in each box of the Punnett squares and recombine the like letters, then determine each genotype.
  • Figure out the different possible phenotypes to figure out the percentages of each possible outcome.
  • When crossing two heterozygous individuals the ratio always comes out to be 9:3:3:1

Non-Mendelian Inheritance

• Mendel worked with a simple system with peas which are genetically simple.
• Most traits are controlled by a single gene with 2 version.
• One completely dominant (A) and one recessive (a); This is not usually that simple
• Incomplete dominance: neither allele is dominant so that heterozygous phenotype is a blending of traits; Hybrids have in between appearance.
  • RR=red -> RR, rr=white -> WW, Rr=pink -> RW
  • Use a different letter to represent that neither allele is dominant and offspring will look different than either of the parents.
  • Parents: Red flowers(homo) x White flowers(homo)
    • F1: 100% pink flowers
    • F2: 25% red 50% pink 25% white, 1:2:1 ratio
  • Each genotype has a specific appearance (phenotype); one can tell what the genotype will be from the phenotype and vice versa just from looking at it.
• Codominance: both alleles are dominant, so both traits are visible in the heterozygous phenotype.
  • Black chicken x White chicken = not gray = black and white, equal dominance
  • examples: roan cows/horses, speckled chickens
• Multiple allele genes: genes that have more than two options for appearances; example: blue, brown, green, gray, hazel eyes

Blood Types

• Genetics of blood types:
  • Phenotype A: genotype IAIA or IAi; Type A antigens on the surface of red blood cells.
  • Phenotype B: genotype IBIB or IBi; Type B antigens on surface of RBC.
  • Phenotype AB(codominant): genotype IAIB; Both type A and type B antigens on surface of RBC.
  • Phenotype O: genotype ii; No antigens on RBC.
  • "I" represents how the blood type works and what is on the surface on the antigen
  • IAIA or IAi: phenotype produces, A; Antibodies produced in the blood, Anti-B antibodies; Donation status,
  • IBIB or IBi: phenotype produced, B; Antibodies produced in the blood, Anti A antibodies; donation status,
  • IAIB phenotyp produced, AB; Antibodies produced in the blood, no antibodies; donation status, universal recipient, can receive any type of blood.
  • ii: phenotype produced, O; Antibodies produced in the blood, anti-A anti-B antibodies; donation status, universal donor, can donate to all blood types.

Polygenic Inheritance and Environment

• Many genes; one traits
  • Polygenic inheritance
  • Traits that are controlled by two or more genes
    • Additive effect of many genes
      • example(skin color)
• Human skin color= polygenic - AaBbCc x AaBbCc - Can produce a wide range of shades - Most children+ intermediate skin color - Some can be very light and very dark - People with all recessive aabbcc have very light skin - As you start adding more dominant alleles, you get darker shades of skin - AABBCC - very dark skin: Commonly found in skin color (polygenic)- most people fall in the middle of the scale
• Coat color in other animals
  • Epistasis- one gene affects another
  • 2 genes E,e and B,b
    • E: color e: no color
    • How dark color will be B:Black b:brown
    • Dogs that have gene of ee(no color) will always be yellow
    • Dogs that have gene of E(color) will allow the B/b gene to show through
• Environment effect on traits
  • Phenotype is controlled by both environment and genes
  • Human skin color is influenced by genetics and sun
  • Hydrangeas are influenced by soil pH
• Dominant traits don't always mean the most common
  • Polydactyly (when one has more than five fingers or toes)is coded by dominant allele
  • DOminant doesn't always mean in is better or more common, it just means that it will show Gene for the usual 5 fingers is actually recessive; Recessive allele far more common than dominant in this case (how many fingers we have)

Genetics of Sex and Genetic Disorders

• Women and men are very different, but just a few genes create that difference.
• In mammals = 2 sex chromosomes XY/XX
• Sex chromosomes have other genes on them too
  • especially the x chromosomes
    • Hemophilia in humans (blood doesn't clot)
    • Duchenne muscular dystrophy in humans (loss of muscle control)
    • Red-green color blindness (see green and red as shades of grey)
• When making a punnett square for X and Y chromosomes there is a different way to write them
  • HH -> X^H Y
  • Hh -> X^H X^h
• Genetic Traits and Disorders
  • Genes controlling human traits follow a variety of inheritance patterns
    • Genetic disorders: are diseases or conditions that have a genetic basis.
    • Single allele traits: controlled by a single allele.
    • Control more than 200 human traits and have a simple dominance recessive pattern.
• Pedigree: a family record or tree that shows how a trait is inherited over several generations.
  • It follows specific rules; Shows patterns of inheritance or predictable ways phenotypes are passed from one generation to the next.
  • Carriers: are heterozygous who carry a copy of the gene but do not show the trait
• Pedigree rules:
  • Circle-female
  • Square-male
  • Shaded-afflicted with trait
  • Half shaded/dot - carrier
  • / or X - deceased
  • Horizontal line- mating “marriage line"
  • -"sibling line"
  • • vertical line- children
  • diagonal lines - twins
  • order of birth is from left to right
• Autosomal dominant: gene is dominant on an autosomal chromosome (1-22)
  • The trait always shows when the allele is present, so there is no such thing as carriers.
  • The trait usually appears in every generation: (a way you can see if it is autosomal dominant)
• Autosomal recessive
  • Autosomal recessive-gene is recessive and on an autosomal chromosome (1-22)
  • The trait only appears when two alleles are present, so there can be carriers.
  • Traits often skip several generations or show up seemingly out of nowhere. X linked recessive:
  • X linked recessive - gene is recessive and is located on the X chromosome.
  • -The trait only appears when every X has the allele, so there can be carriers.
  • -Men are usually more afflicted with the diseases because they only have one X chromosome that either has the disease or doesnt.
  • -Women have another x chromosome that might be dominant and cancel it out.